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One-Dimensional Semiconductor Nanostructures Fabrication

and Field-Emission Properties

K F HUO12 G X Qian2 P K Chu2

1School of Materials and Metallurgy Wuhan University of Science and Technology Wuhan 430081 China

2Department of Physics and Materials Science City University of Hong Kong Hong Kong China

CONTENTS

1 Introduction

2 Synthesis of 1 D Semiconductor Nanostructures 3 FE Properties of 1 D Semiconductor

Nanostructures 4 Conclusion and Outlook

Glossary References

1 INTRODUCTION

Field emission (FE) also known as field electron emission or electron field emission is a quantum mechanical tunneling phenomenon in which electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field [1 2] The electrons obeying Schrodingers wave equation exhibit a wave-like behavior If a high electric field is applied to the surface of a metal or a highly doped semiconductor surface the surface potential barrier is reduced such that electron waves at the Fermi-level in the materials can quantum mechanically tunnel through the surface and be emitted into vacuum at room temperature This type of electron emission owing to quantum tunneling is referred to as FE which is succinctly illustrated in Figure 1 In comparison with tllermionic emisshysion in which electrons escape from hot fIlaments via heating to overcome the surface work function (potential barrier between the Fermi energy level (EF) of the materials and vacuum level) of the materials FE occurs at room temperashyture from a cold cathode under an electric field It is thus also called cold emission It is quite apparent that a field emitter should be more power efficient than a thermionic emitter that requires heating In addition FE electron sources

ISBN 1-58883-169-8 Copyright copy 2011 by American Scientific Publishers All rights of reproduction in any form reserved

have some attractive advantages such as resistance to both temperature fluctuation and radiation high degree of focus ability less power consumption low thermionic noise low energy spread miniature volume and nonlinear exponenshytial current-voltage relationship in which a small change in the voltage results in a large change in the emission current instantaneously [3 4] Hence FE devices (also sometimes called cold cathode devices or vacuum microelectronic devices) can operate at a higher frequency and high radiation environment in a wider temperature range Potential applishycations include flat panel field-emission displays (FEDs) miniaturized microwave power amplified tubes X-ray tubes parallel electron beam microscopy nanolithography highly efficient lamps high-energy accelerators and so on [1-3 5]

The Fowler-Nordheim (F-N) theory is generally used to describe the FE process in which electrons tunnel through the emitter surface under the influence of a high electric field According to the F-N theory the current density emitted from a field emitter depends very much on the applied electric field the work function of the emitter material and the geometrical morphology of the emitter especially the tip geometric shape The F-N equations are described in the following [2 6]

J = (AfJ2pound2Iltp)exp( -Bp32I1E) (1)

I = S X 1 E Vld (2)

In(Jpound2) In(APlp) - BltJj32I1E (3)

where A and B are constants with values of 154 X 10-6 A eY Y-2 and 683 x 103 ey-2i3 Y 11m-I respectively J is the emission current density S is the emitting area I is the emission current V is the applied potential d is the distance between the sample and anode (also sometimes called

Encyclopedia ofNanoscience and Nanotechnolo[J Edited by H S Nalwa

Volume 20 Pages (25-65)

26 Onemiddot Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Ev

EF

$ tcent

ltPeff

Metal Vacuum

o 5 10 15 20 Position (x~A)

Figure 1 Schematic of the potential barrier of electrons at the surface of a metal Curve 1 is the potential barrier without external field and Cmve 2 is the potential barrier under external field where -e2J4x is the image force potential -eEx is the external potential E is the electric field strength and 1gt is the work function

vacuum gap) E is the applied field Ifgt is the work function of the emitting materials and fJ is the field-enhancement factor (also sometimes called field conversion factor or geometricshyenhancement factor)

Strictly speaking the F-N model is valid only for FE from a flat metal surface at 0 K but it has been proven to adequately describe FE from a sharp tip up to temperature of several hundred degrees Celsius [7] As suggested by eqn (1) the FE I-V or J-E relationship is not linear and so a small change in the voltage will result in a large change in the emission current Eqn (2) is generally used to analyze the FE characteristics by plotting In(JE2) vs 1E which is called the F-N plot In general the F-N plot is linear and is usually used to check if electron emission from a field emitter follows the F-N behavior that is the quantum mechanical tunneling process

The theory for FE from semiconductors can in prinshycipal be derived similar to that for metals However there are minor differences owing to the absorb ate states of a semiconducting surface and the fact that an external field applied to a semiconductor may penetrate significantly deeper into the materials [1 8J In the case in which the external electric field penetrates into the interior of an n-type semiconductor and the surface absorbate states can be neglected In(JP) is shown to be a linear function of liE similar to that for a metal emitter However one needs to substitute a quantity x-a for the work function cP in eqns (1) and (3) where X is the electron affinity defined as the energy required in removing an electron from the bottom of the conduction band of the semiconductor to the vacuum level and a denotes the band bending below theEp[l]

There are two important parameters characterizing the FE properties of emitter materials in practical FE measureshyments and FED applications namely the tum-on field (E t)

and threshold field (Elh) which are generally defined as the

electrical field to produce a current density of 10 f1A cm-2

and 10 mA cm-2 respectively It should be noted that these definitions are not unique and different definitions can be found in the literature [9] In general a good field emitter should have a low E to and E

lh good emission stability high

mechanical strength and melting point and excellent elecshytrical conductivity and thermal conductivity According to eqn (1) the emission current density (1) is a strong function of the work function (rIraquo and field-enhancement factor (13) of the field emitter A field emitter possessing a low work function (cP) or a high field-enhancement factor (fJ) or both will produce a high emission current density (1) at a low electric field (E)

The work function rIgt is related to the intrinsic electron properties and surface status of a field emitter It is clear that at a given electric field a material with a lower rIgt can produce a higher electron emission current However not all materials with low rIgt values are ideal as cold cathode field emitters because some other properties may not be suitable For instance the work function of cesium rIgt 18 eV is one of the lowest but the FE stability and lifetime of cesium or cesium-coated cathodes are serious concerns [2 6] Therefore there has been increasing effort on how to enhance the f3 value of emitters in order to reduce the electric field enabling FE The 13 reflects the local enhancement of an electric field at the emitter surface by emitter geometric configuration It could be represented as E bull = f3E = fJ Vd where E locbull is the local electric ficld It istoc 1 l

quite apparent that a high fJ value will produce a high local field enhancement which is favorable for emitting electron at a low E The value of fJ mainly depends on the geometric parameters of the emitter The emitter height (h) and its tip radius of curvature (r) influence fJ most significantly It can be expressed as [6]

13 hr (4)

That is the higher and sharper the emitter center is the larger is the value of fJ Thus materials with an elongated geometry and sharp tips are better field emitters It should be noted that eqn (4) excludes the field-screening effect that the electric field on each emitter is shielded by nearby neighshyboring emitters resulting in reducing the field enhancement at the emitter tip Thus eqn (4) is only suitable for single and low site density emitters On the other hand in some cases the radius r in eqn (4) could be much smaller than the actual radius of the emitter tip because FE may come from a few spots instead of the whole tip [2] As a result the fJ value calculated from the slope of the F-N curve in eqn (3) assuming the work function of emitter material is known may be generally different from that estimated by eqn (4)

Extensive investigation of cathode emitters with sharp tips to achieve high local field enhancement began in the late 1960s and early 1970s when spindt-type cathodes also known as spindt-type field-emitter arrays (FEAs) were developed These were typically molybdenum (Mo) microtip arrays and silicon (Si) microtip arrays High-resolution FEDs based on the technology of spindt-type FEAs have been produced commercially since the beginning of the 1990s [2] However fabrication of uniform metal or semiconductor-tip FEAs

27 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

over a large area is difficult complicated and expensive Furthermore spindt-type PEAs are highly prone to surface contamination and have poor environmental stability Addishytionally the high operating voltage increases the possibility of tip damage owing to ion bombardment Therefore it is necessary to develop more robust low-voltage field emitters possessing better stability higher local field enhancement and lower work function or electron affinity to meet the application demanded by future FE devices

Many different geometrical-shape cathode emitters have been reported Utsumi et al [410] evaluated the commonly used tip shapes of field emitters as shown in Figure 2 and concluded that the best field-emitter tip should be whiskershylike (a) followed by the sharpened pyramid (b) hemishyspheroidal (c) and pyramidal shapes (d) That is to say the ideal field emitters should be robust long and thin whiskershylike structures that are cheap and easy to fabricate and process Since the discovery of carbon nanotubes (CNTs) in 1991 [11] much attention has been paid to explore the use of this ideal one-dimensional (ID) nanomaterials as field emitters because of their high aspect ratio and whisker-like shape giving rising to optimal gcometric field enhancement high electrical conductivity and mechanical strength and remarkable thermal stability [4] The CNT a seamless small carbon tube formed by rolling up a single or several pieces of graphene sheets may have an aspect ratio as high as 1000 which translates to remarkable field enhancement at the nanotube (NT) apex This provides a great opportunity to produce electron emission at a rather low electric field The prospect of using CNTs as electron sources in FE devices has spurred worldwide research efforts since the first report of FE from CNTs at low electric field in 1995 [79] A first CNTshybased FED [12] and a lighting element [13] were presented in 1998 and 2000 respectively and high-brightness luminesshycent elements and X-ray tubes have hit the market [7]

While FE from CNTs is being actively studied there has been a rapidly growing interest in the investigation of other novel 1D nanostructures especially ID semiconductor nanostructures as potential cold cathode field emitters since the late 1990s It has been shown that the FE propershyties of 1D semiconductor nanostructures have characterisshytics close to or even better than those reported for CNTs In addition unlike CNTs in which both semiconductor and metallic CNTs can coexist in the as-synthesized products it is possible to prepare ID semiconductor nanostructures

Best Worstl

Figure 2 Classification and ranking of field emitters with various tip shapes proposed by Utsumi From best to worst (a) rounded whisker which is ideal (b) sharpened pyramid (c) herni-spheroidal and (d) pyrashymidaL For details see Rek [4 10) Reprinted with permission from PO] T Utsumi IEEE Trans Electron Dev 382276 (1991) copy 1991 IEEE

with a unique electronic property JD semiconductor nanoshystructures generally have tile advantage of a lower surface potential barrier than that of metals or CNTs owing to the lower electron affinity This type of emitter may also respond to the external fields such as photons heat and magnetic fields more sensitively Therefore there have been extensive investigations and reports on the fabrication and FE properties of ID semiconductor nanostructures in the past several years

Since a fair amount of recent literature and reviews concerning the FE properties and applications of CNTs are already available [2 4 7 9] here we provide a compreshyhensive review of the state-of-the-art research activitics on the FE properties of ID inorganic semiconductor nanoshystructures other than CNTs covering materials of the IV group II-VI group III-V group and the other transition metal-oxide semiconductors This chapter is organized into three sections The first section briefly introduces the various synthesis techniques to fabricate 1 D nanostructures especially ID inorganic semiconductor nanostructures The FE properties of some important ID semiconductor nanostructures classified into four groups IV group II-IV group III-V group and other metal-oxide semiconductor are discussed in Section 2 Finally the outlook and future research in the field of 1D semiconductor field emitters and their application in FE devices are described

2 SYNTHESIS OF 1 D SEMICONDUCTOR NANOSTRUCTURES

The synthesis of ID semiconductor nanostructures has gained increasing attention in recent years owing to their fascinating chemical dimension-dependent and shapeshydependent properties as well as potential applications as both interconnects and functional components in nanoscale electronic and optoelectronic devices especially FE devices Many techniques and methodologies have been developed to fabricate ID semiconductor nanostructures and a number of reviews have been published on this topic [14--23] Up t6 now a variety of single element and compound semishyconductor nanostructures have been reported They can be classified into various exciting and fundamentally different morphologies such as nanowiresnanorods (NWsNRs) nanocones (NCs) core-shell structures NTs heterostrucshytures nanobeitsnanoribbons (NBs) nanotapes nanodenshydrites hierarchical nanostructures nanosphere assembly nanosprings and so on [14J These structures are schematishycally illustrated in Figure 3

The techniques to fabricate ID semiconductor nanoshystructures are mainly divided into two groups bottom-up and top-down The top-down routes are primarily based on lithography and etching In the bottom-up approach individual atoms ions or molecules are assembled into ID nanostructures by chemical and biological methods Bottom-up growth provides an intriguing strategy to produce 1D nanostructures in terms of materials diversity cost and potential for large-scale production They bode well for not only fundamental research but also for future nanodevice design and fabrication [16]

28 One-Dimensional Semiconductor Nanostructures Iiabricatioll and Field-Emission Properties

(a) (b) (c) (d) (e) (f)

(g) (h) I bullbullbull

(i)

Figure 3 A schematic summary of quasi-1D nanostructures with different morphOlogies (a) NWs and NRs (b) core-shell structures (c) nanotushybules-nanopipes (d) heterostructures (e) nanobelts-NBs (f) nanotapes (g) dendrites (h) hierarchical nanostructures (i) nanosphere assembly and (j) nanosprings Reprinted with permission from [14] S V N T Kuchibhatla et aI Prog Mater Sci 52 699 (2007) copy 2006 Elsevier

The essence of ID nanostructures formation is the anisotropic growth That is crystal growth proceeds prefshyerentially along one direction while the other growth direcshytions are restrained The actual mechanisms of coaxing this type of crystal growth include (1) growth of an intrinsically anisotropic crystallographic structure (2) use of various templates to direct the formation of ID nanostructures (3) introduction of a liquid-solid interface to reduce the symmetry of a seed (4) use of an appropriate capping reagent to control kinetically the growth rates of various facets of a seed and (5) self-assembly of zero-dimensional (OD) nanostructures [1624] Overall they mainly fall into three main categories namely vapor-phase growth solushytion-based growth and tcmplate-directed growth

21 Vapor-Phase Growth

Vapor-phase growth is probably the most extensively explored approach with regard to the fabrication of ID semiconductor nanostructures In a vapor-phase synthesis the starting reactants are gas phase species Diverse experishymental techniques including chemical vapor deposition (CVD) physical vapor deposition (PVD) thermal evaposhyration laser ablation pulse-laser deposition (PLD) molecshyular beam epitaxy (MBE) and sputtering and so on [14-16] have been adopted These vapor-phase growth methods are mainly based on two mechanisms vapor-liquid-solid (VLS) and vapor-solid (VS) growth The main difference

between these two growth mechanisms is whether a foreign catalyst assists or directs the nucleation and growth of tile 1D semiconductor nanostructures

211 VLS Growth The VLS growth mechanism was first proposed by Wagner and Ellis to produce micrometer-sized Si whiskers in 1964 [25] Succinctly speaking it is a catalyst-assisted growth process in which metal nanoclusters or nanoparticles serve as the nucleation seeds (Fig 4) In this process the metal nanoclusters are heated above the eutectic temperature for the metal-semiconductor system of choice in the presence of a vapor-phase precursor of the semiconductor resulting in the formation of liquid droplets of the metal-semishyconductor alloy which serve as preferential sinks for the continued incorporation of the semiconductor component owing to a much higher sticking probability on liquid vs solid surfaces Under supersaturation conditions crystal precipitation occurs at the solid-liquid interface and the ID semiconductor nanostructures begin to grow [26] Normally the ID nanostructure grows along one particular crystalshylographic orientation which corresponds to the minimum atomic stacking energy In most cases the droplet remains at the tip of the ID nanostructure during subsequent growth as suggested in Figure 4 The VLS growth process has been

(a)

Au 81 ~

cluster nucleation fonnation and groth

S Imiddot

Imnowire

(b)

[ J (

AuSi (I)

Au Atomic PerteRtage $1

Figure 4 Schematic ofVLgt growth of Si NWs (a) A liquid alloy dropshylet AuSi is first formed above the eutectic temperature (363degC) of Au and Si The continued feeding of Si in the vapor phase into the liquid alloy causes oversaturation of the liquid alloy resulting in nucleation and directional NW growth (b) Binary phase diagram for Au and Si illustrating the thermodynamics of VLS growth Reprinted with pershymission from [26J G W Lu et at 1 Phys D Appl Phys R837 39 (2006) copy 2006 rop Publishing

29 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emiion Properties

demonstrated by real time observations of Ge NW growth in a high-temperature transmission electron microscope [27] Figure 5 provides the insight and elucidation of the microshyscopic chemical process Similar in situ observation of VLS growth of Si NW by using transmission electron microscopy (TEM) has also been reported by Ross et al [28]

In the VLS growth metal nanoclusters or nanoparticles serve as the catalysts and nucleation seeds which largely determine the diameters of the NWs or NTs Smaller catashylyst nanoparticles produce thinner NWs or NTs It has been demonstrated that Si and GaP NWs with different diamshyeters can be obtained by controlling the diameter of monoshydispersed gold colloids serving as the catalyst [29 30] In general the lengths of NWs or NTs can be controlled by modifying the growth time In the VLS growth a proper choice of the catalyst is crucial The proper catalyst should be a good solvent capable of forming the liquid alloy with the target material and ideally eutectic compounds should be formed An important feature of the VLS growth is that it provides the intellectual underpinning needed for the prediction of good catalysts and synthesis conditions by considering the pseudobinary phase diagram between the metal catalyst and the solid materials of interest [26] For example one can use equilibrium phase diagrams to choose a suitable catalyst that can form a liquid alloy with the ID nanostructures of interest and the synthesis temperature so that the liquid alloy and solid ID nanostructure coexist VLS method offers the possibility to grow well-oriented or patterned NR or NW arrays on a suitable substrate by depositing or patterning suitable eatalysts on the substrates This is often advantageous to subsequent characterization and applications The VLS process that offers another advantage is that 10 semiconductor heterostructures can be achieved at the individual device level in a controlled fashion As shown in Figure 6 both axial heterostructures (Figs 6(c) and 6(e)) in which sections of different mateshyrials with the same diameter are grown along the wire axis [31-33] and radial heterostructures (Figs 6(d) and 6(f)) in which the core-shell and corc-multishell form along the

Figure 5 In situ TEM images recorded during the process of NW growth (a) Au nanoclusters in solid state at 500 C (b) alloying initiates at 800C at this stage Au exists mostly in solid state (c) liquid Au-Ge alloy (d) the nucleation of Ge nanocrystal on the alloy surface (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f) Reprinted with permission from [27] Y Wu et aI I Am Chern Soc 1233165 (2001) copy 2001 American Chemical Society

(Il)

bull Figure 6 NW heterostructure synthesis (a) Preferential reactant incorshyporation at the catalyst (growth end) leads to 1D axial growth (b) A change in the reactant leads to either (c) axial heterostructurc grovlith or (d) radial heterostructure growth depending on whether the reactant is preferentially incorporated (c) at the catalyst or (d) uniformly on the wire surface Alternating reactants will produce (e) axial superlattices or (f) core-multishell structures Reprinted with permission from [33J L J Lauhon et aI PhiL Trans R Soc Lond A 362 1247 (2004) copy 2004 The Royal Society of Chemistry

radial direction [34-36] have been realized by changing the type of vapor precursors used in the VLS growth

The VLS growth has now become a widely used method to produce ID semiconductor nanostructures from a myriad of inorganic materials such as elemental semiconductors (Si and Ge) [37 38] III-V semiconductors (BN GaN AlN GaAs GaP InP and InAs) [39-48] II-VI semiconductors (ZnS ZnSe CdS and CdSe) [49-52] metal oxides (ZnO Sn02 Inp3 CdO and GaP) [53-57] carbides (SiC and B4C) [5859] nitrides (Si3N4) [6061] and 10 heterostrucshytures [31-36 62] The disadvantage ofthe VLS growth may be the possible incorporation of catalysts into the NWs as well as the difficulty to remove such capsules from the tips of the NWs or NTs This may result in changes in the NWs properties However by selecting an appropriate catalyst the effects of contamination on specific properties of the NWs can be minimized Another disadvantage of the VLS growth is that it is difficult to synthesize 10 nanostructures when the vapor precursor is highly toxic or unavailable for the target materials

Recently we have extended this traditional VLS growth by introducing alloy particles as the catalyst Some 10 nanomaterials such as BN NWs and NTs [39 40 63 64] AlN NWs [48] and Si3N4 nanobelts [65] have been syntheshysized by simply nitriding Fe-B Ni-Al and Fe-Si alloy particles at high temperature with NH3-N respectively In the extended VLS growth technique the alloy catalyst not only acts as the medium for component transport from vapor to solid as is the case of traditional VLS growth but also supplies partial component( s) to form the final solid product For instance the components in the BN product come partially from the vapor precursor that is N in BN from NH3-Nz and partially from the catalyst itself that

B in BN from Fe-B catalyst itself This is clearly different from the traditional VLS growth mechanism as schematically illustrated in Figure 7 In other words a chemical reaction between the component(s) from the vapor precursor(s) and the catalyst takes pJace within the

30 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Liqu

Solid(S)shy Solid(S)-

VLSgmwth Extended VLS growth

Figure 7 Schematic diagram contrasting the VLS (left side) and extendshyed VLS grow1h (right side)

liquid catalyst and the resulting products precipitate with a whisker-like morphology when supersaturation is reached Strictly speaking the alloy particle here is not a catalyst but rather a reactant because the partial component in the alloy is consumed after the chemical reaction This is an extension to the traditional VIS growth providing a new possibility to explore novel compound nanostructures This is especially the case when the vapor source is highly toxic or unavailable while the solid catalyst containing the partial target component(s) can be easily obtained

212 VS Growth Some 1D semiconductor nanostructures can also be fabrishycated by simply evaporating a condensed or powder source material(s) and then condensing the resultant vapor phase( s) under certain conditions [66] Such a self-organization process which does not involve a foreign catalyst is referred to as a VS growth The thermal vaporization technique is a simpler process to vaporize the source materials Here the powder source material(s) is vaporized at elevated tempershyature and then the resultant vapor phase(s) condenses in a relatively low temperature zone under certain conditions (temperature pressure substrate etc) to form the desired product(s) In the VS growth mode control of the size of the 1D semiconductor nanostructures is accomplished mainly by tuning the evaporation and substrate temperatures vapor pressure gas flow and pressure However the exact mechanisms governing the VS growth are not completely understood Possible growth mechanisms such as the anisoshytropic growth defect-induced growth (eg through a screw dislocation) and self-catalytic growth have been suggested based on thermodynamic and kinetic considerations and electron microscopy studies [16 19 24] In an anisotropic growth mechanism ID growth can be accomplished by the preferential reactivity and binding of gas phase reactants on specific surfaces (thermodynamic and kinetic parameters)

to minimize the total surface energy However the degree of anisotropy is generally not significantly large and highly anisotropic growth (Le the length-to-diameter ratiogt100) of nanocrystals at or near the thermal equilibrium state is not expected [19] In the dislocation and defect-induced growth models specific defects (for example screw disloshycations) are known to have larger sticking coefficients for gas phase species thus allowing enhanced reactivity and deposition of gas phase reactants at these defects leading

to the formation of rod- or wire-like shape [19 241 The self-catalytic growth works in an analogous way in the VLS growth but differs in that one component of the gaseous atoms may play the role of the catalyst itself [67-70] The major advantage of a self-catalytic process is that it avoids undesired contamination from foreign metal atoms typically used as catalysts in VLS growth Other recently proposed VS growth mechanisms such as oxide-assisted growth mechshyanism [71-73] have also been reported in which the oxide instead of a metal nanoparticle plays a role in initiating the nucleation and growth of NWs or NRs Although the exact mechanisms responsible for the VS growth have not been completely elucidated many materials with interesting morphologies have been fabricated using these methods [74] Wang et al [6675-77] have fabricated NBs of ZnO Sn0

2 In20

3 CdO and so on with rectangular cross sections

and splendid ID ZnD nanostructures such as nanocombs nanohelices nanosprings nanorings and hierarchical nanoshystructures by VS growth

22 Solution-Based Growth

Comparedtovapor-phasegrowthsolution-basedgrowthhas some advantages such as relatively low temperature simple manipulation high productivity low energy consumption and no complex and expensive facility It thus provides a convenient low temperature and environmentally friendly pathway for fabrication especially large-scale production of ID semiconductor nanostructures with desirable shapes and sizes In recent years various solution-based synthetic techniques have been developed for the synthesis of 1D semiconductor nanostructures They include the solvoshythermal-hydrothermal method solution-liquid-solid (SLS) growth and capping agents-surfactants-assisted synthesis as well as template-directed growth

221 Solvothermal-Hydrothermal Method The solvothermal-hydrothermal process is usually performed in a Teflon-lined stainless-steel autoclave in which water or an organic solvent is used as the reaction medium at elevated temperatures above its boiling point in autogeneous presshysure by heating One outstanding characteristic of the solvoshythermal-hydrothermal strategy is that when the starting precursors are exposed to special conditions often in pure solvent some quite unexpected reactions can take place accompanied by the formation of nanoscopic morpholoshygies and new phases which generally cannot be achieved by traditional reactions [78] Solvothermal synthesis of semishyconductor NWs was first demonstrated in 1993 by Heath et al [79] to fabricate Ge NWs in an alkane solvent The solvothermal-hydrothermal method has been demonstrated to be a powerful tool to fabricate various kinds of ID semishyconductor nanostructures such as elemental semiconductor oxide semiconductor II-VI group semiconductor chalcoshygenide semiconductor and even carbide and nitride semishyconductor nanostructures [80-86J

It has to be emphasized that the nucleation crystallizashytion self-assembly and growth mechanism of ID semiconshyductors under solvothermal-hydrothermal conditions are rather complicated and not well understood yet even though

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

27 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

over a large area is difficult complicated and expensive Furthermore spindt-type PEAs are highly prone to surface contamination and have poor environmental stability Addishytionally the high operating voltage increases the possibility of tip damage owing to ion bombardment Therefore it is necessary to develop more robust low-voltage field emitters possessing better stability higher local field enhancement and lower work function or electron affinity to meet the application demanded by future FE devices

Many different geometrical-shape cathode emitters have been reported Utsumi et al [410] evaluated the commonly used tip shapes of field emitters as shown in Figure 2 and concluded that the best field-emitter tip should be whiskershylike (a) followed by the sharpened pyramid (b) hemishyspheroidal (c) and pyramidal shapes (d) That is to say the ideal field emitters should be robust long and thin whiskershylike structures that are cheap and easy to fabricate and process Since the discovery of carbon nanotubes (CNTs) in 1991 [11] much attention has been paid to explore the use of this ideal one-dimensional (ID) nanomaterials as field emitters because of their high aspect ratio and whisker-like shape giving rising to optimal gcometric field enhancement high electrical conductivity and mechanical strength and remarkable thermal stability [4] The CNT a seamless small carbon tube formed by rolling up a single or several pieces of graphene sheets may have an aspect ratio as high as 1000 which translates to remarkable field enhancement at the nanotube (NT) apex This provides a great opportunity to produce electron emission at a rather low electric field The prospect of using CNTs as electron sources in FE devices has spurred worldwide research efforts since the first report of FE from CNTs at low electric field in 1995 [79] A first CNTshybased FED [12] and a lighting element [13] were presented in 1998 and 2000 respectively and high-brightness luminesshycent elements and X-ray tubes have hit the market [7]

While FE from CNTs is being actively studied there has been a rapidly growing interest in the investigation of other novel 1D nanostructures especially ID semiconductor nanostructures as potential cold cathode field emitters since the late 1990s It has been shown that the FE propershyties of 1D semiconductor nanostructures have characterisshytics close to or even better than those reported for CNTs In addition unlike CNTs in which both semiconductor and metallic CNTs can coexist in the as-synthesized products it is possible to prepare ID semiconductor nanostructures

Best Worstl

Figure 2 Classification and ranking of field emitters with various tip shapes proposed by Utsumi From best to worst (a) rounded whisker which is ideal (b) sharpened pyramid (c) herni-spheroidal and (d) pyrashymidaL For details see Rek [4 10) Reprinted with permission from PO] T Utsumi IEEE Trans Electron Dev 382276 (1991) copy 1991 IEEE

with a unique electronic property JD semiconductor nanoshystructures generally have tile advantage of a lower surface potential barrier than that of metals or CNTs owing to the lower electron affinity This type of emitter may also respond to the external fields such as photons heat and magnetic fields more sensitively Therefore there have been extensive investigations and reports on the fabrication and FE properties of ID semiconductor nanostructures in the past several years

Since a fair amount of recent literature and reviews concerning the FE properties and applications of CNTs are already available [2 4 7 9] here we provide a compreshyhensive review of the state-of-the-art research activitics on the FE properties of ID inorganic semiconductor nanoshystructures other than CNTs covering materials of the IV group II-VI group III-V group and the other transition metal-oxide semiconductors This chapter is organized into three sections The first section briefly introduces the various synthesis techniques to fabricate 1 D nanostructures especially ID inorganic semiconductor nanostructures The FE properties of some important ID semiconductor nanostructures classified into four groups IV group II-IV group III-V group and other metal-oxide semiconductor are discussed in Section 2 Finally the outlook and future research in the field of 1D semiconductor field emitters and their application in FE devices are described

2 SYNTHESIS OF 1 D SEMICONDUCTOR NANOSTRUCTURES

The synthesis of ID semiconductor nanostructures has gained increasing attention in recent years owing to their fascinating chemical dimension-dependent and shapeshydependent properties as well as potential applications as both interconnects and functional components in nanoscale electronic and optoelectronic devices especially FE devices Many techniques and methodologies have been developed to fabricate ID semiconductor nanostructures and a number of reviews have been published on this topic [14--23] Up t6 now a variety of single element and compound semishyconductor nanostructures have been reported They can be classified into various exciting and fundamentally different morphologies such as nanowiresnanorods (NWsNRs) nanocones (NCs) core-shell structures NTs heterostrucshytures nanobeitsnanoribbons (NBs) nanotapes nanodenshydrites hierarchical nanostructures nanosphere assembly nanosprings and so on [14J These structures are schematishycally illustrated in Figure 3

The techniques to fabricate ID semiconductor nanoshystructures are mainly divided into two groups bottom-up and top-down The top-down routes are primarily based on lithography and etching In the bottom-up approach individual atoms ions or molecules are assembled into ID nanostructures by chemical and biological methods Bottom-up growth provides an intriguing strategy to produce 1D nanostructures in terms of materials diversity cost and potential for large-scale production They bode well for not only fundamental research but also for future nanodevice design and fabrication [16]

28 One-Dimensional Semiconductor Nanostructures Iiabricatioll and Field-Emission Properties

(a) (b) (c) (d) (e) (f)

(g) (h) I bullbullbull

(i)

Figure 3 A schematic summary of quasi-1D nanostructures with different morphOlogies (a) NWs and NRs (b) core-shell structures (c) nanotushybules-nanopipes (d) heterostructures (e) nanobelts-NBs (f) nanotapes (g) dendrites (h) hierarchical nanostructures (i) nanosphere assembly and (j) nanosprings Reprinted with permission from [14] S V N T Kuchibhatla et aI Prog Mater Sci 52 699 (2007) copy 2006 Elsevier

The essence of ID nanostructures formation is the anisotropic growth That is crystal growth proceeds prefshyerentially along one direction while the other growth direcshytions are restrained The actual mechanisms of coaxing this type of crystal growth include (1) growth of an intrinsically anisotropic crystallographic structure (2) use of various templates to direct the formation of ID nanostructures (3) introduction of a liquid-solid interface to reduce the symmetry of a seed (4) use of an appropriate capping reagent to control kinetically the growth rates of various facets of a seed and (5) self-assembly of zero-dimensional (OD) nanostructures [1624] Overall they mainly fall into three main categories namely vapor-phase growth solushytion-based growth and tcmplate-directed growth

21 Vapor-Phase Growth

Vapor-phase growth is probably the most extensively explored approach with regard to the fabrication of ID semiconductor nanostructures In a vapor-phase synthesis the starting reactants are gas phase species Diverse experishymental techniques including chemical vapor deposition (CVD) physical vapor deposition (PVD) thermal evaposhyration laser ablation pulse-laser deposition (PLD) molecshyular beam epitaxy (MBE) and sputtering and so on [14-16] have been adopted These vapor-phase growth methods are mainly based on two mechanisms vapor-liquid-solid (VLS) and vapor-solid (VS) growth The main difference

between these two growth mechanisms is whether a foreign catalyst assists or directs the nucleation and growth of tile 1D semiconductor nanostructures

211 VLS Growth The VLS growth mechanism was first proposed by Wagner and Ellis to produce micrometer-sized Si whiskers in 1964 [25] Succinctly speaking it is a catalyst-assisted growth process in which metal nanoclusters or nanoparticles serve as the nucleation seeds (Fig 4) In this process the metal nanoclusters are heated above the eutectic temperature for the metal-semiconductor system of choice in the presence of a vapor-phase precursor of the semiconductor resulting in the formation of liquid droplets of the metal-semishyconductor alloy which serve as preferential sinks for the continued incorporation of the semiconductor component owing to a much higher sticking probability on liquid vs solid surfaces Under supersaturation conditions crystal precipitation occurs at the solid-liquid interface and the ID semiconductor nanostructures begin to grow [26] Normally the ID nanostructure grows along one particular crystalshylographic orientation which corresponds to the minimum atomic stacking energy In most cases the droplet remains at the tip of the ID nanostructure during subsequent growth as suggested in Figure 4 The VLS growth process has been

(a)

Au 81 ~

cluster nucleation fonnation and groth

S Imiddot

Imnowire

(b)

[ J (

AuSi (I)

Au Atomic PerteRtage $1

Figure 4 Schematic ofVLgt growth of Si NWs (a) A liquid alloy dropshylet AuSi is first formed above the eutectic temperature (363degC) of Au and Si The continued feeding of Si in the vapor phase into the liquid alloy causes oversaturation of the liquid alloy resulting in nucleation and directional NW growth (b) Binary phase diagram for Au and Si illustrating the thermodynamics of VLS growth Reprinted with pershymission from [26J G W Lu et at 1 Phys D Appl Phys R837 39 (2006) copy 2006 rop Publishing

29 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emiion Properties

demonstrated by real time observations of Ge NW growth in a high-temperature transmission electron microscope [27] Figure 5 provides the insight and elucidation of the microshyscopic chemical process Similar in situ observation of VLS growth of Si NW by using transmission electron microscopy (TEM) has also been reported by Ross et al [28]

In the VLS growth metal nanoclusters or nanoparticles serve as the catalysts and nucleation seeds which largely determine the diameters of the NWs or NTs Smaller catashylyst nanoparticles produce thinner NWs or NTs It has been demonstrated that Si and GaP NWs with different diamshyeters can be obtained by controlling the diameter of monoshydispersed gold colloids serving as the catalyst [29 30] In general the lengths of NWs or NTs can be controlled by modifying the growth time In the VLS growth a proper choice of the catalyst is crucial The proper catalyst should be a good solvent capable of forming the liquid alloy with the target material and ideally eutectic compounds should be formed An important feature of the VLS growth is that it provides the intellectual underpinning needed for the prediction of good catalysts and synthesis conditions by considering the pseudobinary phase diagram between the metal catalyst and the solid materials of interest [26] For example one can use equilibrium phase diagrams to choose a suitable catalyst that can form a liquid alloy with the ID nanostructures of interest and the synthesis temperature so that the liquid alloy and solid ID nanostructure coexist VLS method offers the possibility to grow well-oriented or patterned NR or NW arrays on a suitable substrate by depositing or patterning suitable eatalysts on the substrates This is often advantageous to subsequent characterization and applications The VLS process that offers another advantage is that 10 semiconductor heterostructures can be achieved at the individual device level in a controlled fashion As shown in Figure 6 both axial heterostructures (Figs 6(c) and 6(e)) in which sections of different mateshyrials with the same diameter are grown along the wire axis [31-33] and radial heterostructures (Figs 6(d) and 6(f)) in which the core-shell and corc-multishell form along the

Figure 5 In situ TEM images recorded during the process of NW growth (a) Au nanoclusters in solid state at 500 C (b) alloying initiates at 800C at this stage Au exists mostly in solid state (c) liquid Au-Ge alloy (d) the nucleation of Ge nanocrystal on the alloy surface (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f) Reprinted with permission from [27] Y Wu et aI I Am Chern Soc 1233165 (2001) copy 2001 American Chemical Society

(Il)

bull Figure 6 NW heterostructure synthesis (a) Preferential reactant incorshyporation at the catalyst (growth end) leads to 1D axial growth (b) A change in the reactant leads to either (c) axial heterostructurc grovlith or (d) radial heterostructure growth depending on whether the reactant is preferentially incorporated (c) at the catalyst or (d) uniformly on the wire surface Alternating reactants will produce (e) axial superlattices or (f) core-multishell structures Reprinted with permission from [33J L J Lauhon et aI PhiL Trans R Soc Lond A 362 1247 (2004) copy 2004 The Royal Society of Chemistry

radial direction [34-36] have been realized by changing the type of vapor precursors used in the VLS growth

The VLS growth has now become a widely used method to produce ID semiconductor nanostructures from a myriad of inorganic materials such as elemental semiconductors (Si and Ge) [37 38] III-V semiconductors (BN GaN AlN GaAs GaP InP and InAs) [39-48] II-VI semiconductors (ZnS ZnSe CdS and CdSe) [49-52] metal oxides (ZnO Sn02 Inp3 CdO and GaP) [53-57] carbides (SiC and B4C) [5859] nitrides (Si3N4) [6061] and 10 heterostrucshytures [31-36 62] The disadvantage ofthe VLS growth may be the possible incorporation of catalysts into the NWs as well as the difficulty to remove such capsules from the tips of the NWs or NTs This may result in changes in the NWs properties However by selecting an appropriate catalyst the effects of contamination on specific properties of the NWs can be minimized Another disadvantage of the VLS growth is that it is difficult to synthesize 10 nanostructures when the vapor precursor is highly toxic or unavailable for the target materials

Recently we have extended this traditional VLS growth by introducing alloy particles as the catalyst Some 10 nanomaterials such as BN NWs and NTs [39 40 63 64] AlN NWs [48] and Si3N4 nanobelts [65] have been syntheshysized by simply nitriding Fe-B Ni-Al and Fe-Si alloy particles at high temperature with NH3-N respectively In the extended VLS growth technique the alloy catalyst not only acts as the medium for component transport from vapor to solid as is the case of traditional VLS growth but also supplies partial component( s) to form the final solid product For instance the components in the BN product come partially from the vapor precursor that is N in BN from NH3-Nz and partially from the catalyst itself that

B in BN from Fe-B catalyst itself This is clearly different from the traditional VLS growth mechanism as schematically illustrated in Figure 7 In other words a chemical reaction between the component(s) from the vapor precursor(s) and the catalyst takes pJace within the

30 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Liqu

Solid(S)shy Solid(S)-

VLSgmwth Extended VLS growth

Figure 7 Schematic diagram contrasting the VLS (left side) and extendshyed VLS grow1h (right side)

liquid catalyst and the resulting products precipitate with a whisker-like morphology when supersaturation is reached Strictly speaking the alloy particle here is not a catalyst but rather a reactant because the partial component in the alloy is consumed after the chemical reaction This is an extension to the traditional VIS growth providing a new possibility to explore novel compound nanostructures This is especially the case when the vapor source is highly toxic or unavailable while the solid catalyst containing the partial target component(s) can be easily obtained

212 VS Growth Some 1D semiconductor nanostructures can also be fabrishycated by simply evaporating a condensed or powder source material(s) and then condensing the resultant vapor phase( s) under certain conditions [66] Such a self-organization process which does not involve a foreign catalyst is referred to as a VS growth The thermal vaporization technique is a simpler process to vaporize the source materials Here the powder source material(s) is vaporized at elevated tempershyature and then the resultant vapor phase(s) condenses in a relatively low temperature zone under certain conditions (temperature pressure substrate etc) to form the desired product(s) In the VS growth mode control of the size of the 1D semiconductor nanostructures is accomplished mainly by tuning the evaporation and substrate temperatures vapor pressure gas flow and pressure However the exact mechanisms governing the VS growth are not completely understood Possible growth mechanisms such as the anisoshytropic growth defect-induced growth (eg through a screw dislocation) and self-catalytic growth have been suggested based on thermodynamic and kinetic considerations and electron microscopy studies [16 19 24] In an anisotropic growth mechanism ID growth can be accomplished by the preferential reactivity and binding of gas phase reactants on specific surfaces (thermodynamic and kinetic parameters)

to minimize the total surface energy However the degree of anisotropy is generally not significantly large and highly anisotropic growth (Le the length-to-diameter ratiogt100) of nanocrystals at or near the thermal equilibrium state is not expected [19] In the dislocation and defect-induced growth models specific defects (for example screw disloshycations) are known to have larger sticking coefficients for gas phase species thus allowing enhanced reactivity and deposition of gas phase reactants at these defects leading

to the formation of rod- or wire-like shape [19 241 The self-catalytic growth works in an analogous way in the VLS growth but differs in that one component of the gaseous atoms may play the role of the catalyst itself [67-70] The major advantage of a self-catalytic process is that it avoids undesired contamination from foreign metal atoms typically used as catalysts in VLS growth Other recently proposed VS growth mechanisms such as oxide-assisted growth mechshyanism [71-73] have also been reported in which the oxide instead of a metal nanoparticle plays a role in initiating the nucleation and growth of NWs or NRs Although the exact mechanisms responsible for the VS growth have not been completely elucidated many materials with interesting morphologies have been fabricated using these methods [74] Wang et al [6675-77] have fabricated NBs of ZnO Sn0

2 In20

3 CdO and so on with rectangular cross sections

and splendid ID ZnD nanostructures such as nanocombs nanohelices nanosprings nanorings and hierarchical nanoshystructures by VS growth

22 Solution-Based Growth

Comparedtovapor-phasegrowthsolution-basedgrowthhas some advantages such as relatively low temperature simple manipulation high productivity low energy consumption and no complex and expensive facility It thus provides a convenient low temperature and environmentally friendly pathway for fabrication especially large-scale production of ID semiconductor nanostructures with desirable shapes and sizes In recent years various solution-based synthetic techniques have been developed for the synthesis of 1D semiconductor nanostructures They include the solvoshythermal-hydrothermal method solution-liquid-solid (SLS) growth and capping agents-surfactants-assisted synthesis as well as template-directed growth

221 Solvothermal-Hydrothermal Method The solvothermal-hydrothermal process is usually performed in a Teflon-lined stainless-steel autoclave in which water or an organic solvent is used as the reaction medium at elevated temperatures above its boiling point in autogeneous presshysure by heating One outstanding characteristic of the solvoshythermal-hydrothermal strategy is that when the starting precursors are exposed to special conditions often in pure solvent some quite unexpected reactions can take place accompanied by the formation of nanoscopic morpholoshygies and new phases which generally cannot be achieved by traditional reactions [78] Solvothermal synthesis of semishyconductor NWs was first demonstrated in 1993 by Heath et al [79] to fabricate Ge NWs in an alkane solvent The solvothermal-hydrothermal method has been demonstrated to be a powerful tool to fabricate various kinds of ID semishyconductor nanostructures such as elemental semiconductor oxide semiconductor II-VI group semiconductor chalcoshygenide semiconductor and even carbide and nitride semishyconductor nanostructures [80-86J

It has to be emphasized that the nucleation crystallizashytion self-assembly and growth mechanism of ID semiconshyductors under solvothermal-hydrothermal conditions are rather complicated and not well understood yet even though

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

28 One-Dimensional Semiconductor Nanostructures Iiabricatioll and Field-Emission Properties

(a) (b) (c) (d) (e) (f)

(g) (h) I bullbullbull

(i)

Figure 3 A schematic summary of quasi-1D nanostructures with different morphOlogies (a) NWs and NRs (b) core-shell structures (c) nanotushybules-nanopipes (d) heterostructures (e) nanobelts-NBs (f) nanotapes (g) dendrites (h) hierarchical nanostructures (i) nanosphere assembly and (j) nanosprings Reprinted with permission from [14] S V N T Kuchibhatla et aI Prog Mater Sci 52 699 (2007) copy 2006 Elsevier

The essence of ID nanostructures formation is the anisotropic growth That is crystal growth proceeds prefshyerentially along one direction while the other growth direcshytions are restrained The actual mechanisms of coaxing this type of crystal growth include (1) growth of an intrinsically anisotropic crystallographic structure (2) use of various templates to direct the formation of ID nanostructures (3) introduction of a liquid-solid interface to reduce the symmetry of a seed (4) use of an appropriate capping reagent to control kinetically the growth rates of various facets of a seed and (5) self-assembly of zero-dimensional (OD) nanostructures [1624] Overall they mainly fall into three main categories namely vapor-phase growth solushytion-based growth and tcmplate-directed growth

21 Vapor-Phase Growth

Vapor-phase growth is probably the most extensively explored approach with regard to the fabrication of ID semiconductor nanostructures In a vapor-phase synthesis the starting reactants are gas phase species Diverse experishymental techniques including chemical vapor deposition (CVD) physical vapor deposition (PVD) thermal evaposhyration laser ablation pulse-laser deposition (PLD) molecshyular beam epitaxy (MBE) and sputtering and so on [14-16] have been adopted These vapor-phase growth methods are mainly based on two mechanisms vapor-liquid-solid (VLS) and vapor-solid (VS) growth The main difference

between these two growth mechanisms is whether a foreign catalyst assists or directs the nucleation and growth of tile 1D semiconductor nanostructures

211 VLS Growth The VLS growth mechanism was first proposed by Wagner and Ellis to produce micrometer-sized Si whiskers in 1964 [25] Succinctly speaking it is a catalyst-assisted growth process in which metal nanoclusters or nanoparticles serve as the nucleation seeds (Fig 4) In this process the metal nanoclusters are heated above the eutectic temperature for the metal-semiconductor system of choice in the presence of a vapor-phase precursor of the semiconductor resulting in the formation of liquid droplets of the metal-semishyconductor alloy which serve as preferential sinks for the continued incorporation of the semiconductor component owing to a much higher sticking probability on liquid vs solid surfaces Under supersaturation conditions crystal precipitation occurs at the solid-liquid interface and the ID semiconductor nanostructures begin to grow [26] Normally the ID nanostructure grows along one particular crystalshylographic orientation which corresponds to the minimum atomic stacking energy In most cases the droplet remains at the tip of the ID nanostructure during subsequent growth as suggested in Figure 4 The VLS growth process has been

(a)

Au 81 ~

cluster nucleation fonnation and groth

S Imiddot

Imnowire

(b)

[ J (

AuSi (I)

Au Atomic PerteRtage $1

Figure 4 Schematic ofVLgt growth of Si NWs (a) A liquid alloy dropshylet AuSi is first formed above the eutectic temperature (363degC) of Au and Si The continued feeding of Si in the vapor phase into the liquid alloy causes oversaturation of the liquid alloy resulting in nucleation and directional NW growth (b) Binary phase diagram for Au and Si illustrating the thermodynamics of VLS growth Reprinted with pershymission from [26J G W Lu et at 1 Phys D Appl Phys R837 39 (2006) copy 2006 rop Publishing

29 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emiion Properties

demonstrated by real time observations of Ge NW growth in a high-temperature transmission electron microscope [27] Figure 5 provides the insight and elucidation of the microshyscopic chemical process Similar in situ observation of VLS growth of Si NW by using transmission electron microscopy (TEM) has also been reported by Ross et al [28]

In the VLS growth metal nanoclusters or nanoparticles serve as the catalysts and nucleation seeds which largely determine the diameters of the NWs or NTs Smaller catashylyst nanoparticles produce thinner NWs or NTs It has been demonstrated that Si and GaP NWs with different diamshyeters can be obtained by controlling the diameter of monoshydispersed gold colloids serving as the catalyst [29 30] In general the lengths of NWs or NTs can be controlled by modifying the growth time In the VLS growth a proper choice of the catalyst is crucial The proper catalyst should be a good solvent capable of forming the liquid alloy with the target material and ideally eutectic compounds should be formed An important feature of the VLS growth is that it provides the intellectual underpinning needed for the prediction of good catalysts and synthesis conditions by considering the pseudobinary phase diagram between the metal catalyst and the solid materials of interest [26] For example one can use equilibrium phase diagrams to choose a suitable catalyst that can form a liquid alloy with the ID nanostructures of interest and the synthesis temperature so that the liquid alloy and solid ID nanostructure coexist VLS method offers the possibility to grow well-oriented or patterned NR or NW arrays on a suitable substrate by depositing or patterning suitable eatalysts on the substrates This is often advantageous to subsequent characterization and applications The VLS process that offers another advantage is that 10 semiconductor heterostructures can be achieved at the individual device level in a controlled fashion As shown in Figure 6 both axial heterostructures (Figs 6(c) and 6(e)) in which sections of different mateshyrials with the same diameter are grown along the wire axis [31-33] and radial heterostructures (Figs 6(d) and 6(f)) in which the core-shell and corc-multishell form along the

Figure 5 In situ TEM images recorded during the process of NW growth (a) Au nanoclusters in solid state at 500 C (b) alloying initiates at 800C at this stage Au exists mostly in solid state (c) liquid Au-Ge alloy (d) the nucleation of Ge nanocrystal on the alloy surface (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f) Reprinted with permission from [27] Y Wu et aI I Am Chern Soc 1233165 (2001) copy 2001 American Chemical Society

(Il)

bull Figure 6 NW heterostructure synthesis (a) Preferential reactant incorshyporation at the catalyst (growth end) leads to 1D axial growth (b) A change in the reactant leads to either (c) axial heterostructurc grovlith or (d) radial heterostructure growth depending on whether the reactant is preferentially incorporated (c) at the catalyst or (d) uniformly on the wire surface Alternating reactants will produce (e) axial superlattices or (f) core-multishell structures Reprinted with permission from [33J L J Lauhon et aI PhiL Trans R Soc Lond A 362 1247 (2004) copy 2004 The Royal Society of Chemistry

radial direction [34-36] have been realized by changing the type of vapor precursors used in the VLS growth

The VLS growth has now become a widely used method to produce ID semiconductor nanostructures from a myriad of inorganic materials such as elemental semiconductors (Si and Ge) [37 38] III-V semiconductors (BN GaN AlN GaAs GaP InP and InAs) [39-48] II-VI semiconductors (ZnS ZnSe CdS and CdSe) [49-52] metal oxides (ZnO Sn02 Inp3 CdO and GaP) [53-57] carbides (SiC and B4C) [5859] nitrides (Si3N4) [6061] and 10 heterostrucshytures [31-36 62] The disadvantage ofthe VLS growth may be the possible incorporation of catalysts into the NWs as well as the difficulty to remove such capsules from the tips of the NWs or NTs This may result in changes in the NWs properties However by selecting an appropriate catalyst the effects of contamination on specific properties of the NWs can be minimized Another disadvantage of the VLS growth is that it is difficult to synthesize 10 nanostructures when the vapor precursor is highly toxic or unavailable for the target materials

Recently we have extended this traditional VLS growth by introducing alloy particles as the catalyst Some 10 nanomaterials such as BN NWs and NTs [39 40 63 64] AlN NWs [48] and Si3N4 nanobelts [65] have been syntheshysized by simply nitriding Fe-B Ni-Al and Fe-Si alloy particles at high temperature with NH3-N respectively In the extended VLS growth technique the alloy catalyst not only acts as the medium for component transport from vapor to solid as is the case of traditional VLS growth but also supplies partial component( s) to form the final solid product For instance the components in the BN product come partially from the vapor precursor that is N in BN from NH3-Nz and partially from the catalyst itself that

B in BN from Fe-B catalyst itself This is clearly different from the traditional VLS growth mechanism as schematically illustrated in Figure 7 In other words a chemical reaction between the component(s) from the vapor precursor(s) and the catalyst takes pJace within the

30 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Liqu

Solid(S)shy Solid(S)-

VLSgmwth Extended VLS growth

Figure 7 Schematic diagram contrasting the VLS (left side) and extendshyed VLS grow1h (right side)

liquid catalyst and the resulting products precipitate with a whisker-like morphology when supersaturation is reached Strictly speaking the alloy particle here is not a catalyst but rather a reactant because the partial component in the alloy is consumed after the chemical reaction This is an extension to the traditional VIS growth providing a new possibility to explore novel compound nanostructures This is especially the case when the vapor source is highly toxic or unavailable while the solid catalyst containing the partial target component(s) can be easily obtained

212 VS Growth Some 1D semiconductor nanostructures can also be fabrishycated by simply evaporating a condensed or powder source material(s) and then condensing the resultant vapor phase( s) under certain conditions [66] Such a self-organization process which does not involve a foreign catalyst is referred to as a VS growth The thermal vaporization technique is a simpler process to vaporize the source materials Here the powder source material(s) is vaporized at elevated tempershyature and then the resultant vapor phase(s) condenses in a relatively low temperature zone under certain conditions (temperature pressure substrate etc) to form the desired product(s) In the VS growth mode control of the size of the 1D semiconductor nanostructures is accomplished mainly by tuning the evaporation and substrate temperatures vapor pressure gas flow and pressure However the exact mechanisms governing the VS growth are not completely understood Possible growth mechanisms such as the anisoshytropic growth defect-induced growth (eg through a screw dislocation) and self-catalytic growth have been suggested based on thermodynamic and kinetic considerations and electron microscopy studies [16 19 24] In an anisotropic growth mechanism ID growth can be accomplished by the preferential reactivity and binding of gas phase reactants on specific surfaces (thermodynamic and kinetic parameters)

to minimize the total surface energy However the degree of anisotropy is generally not significantly large and highly anisotropic growth (Le the length-to-diameter ratiogt100) of nanocrystals at or near the thermal equilibrium state is not expected [19] In the dislocation and defect-induced growth models specific defects (for example screw disloshycations) are known to have larger sticking coefficients for gas phase species thus allowing enhanced reactivity and deposition of gas phase reactants at these defects leading

to the formation of rod- or wire-like shape [19 241 The self-catalytic growth works in an analogous way in the VLS growth but differs in that one component of the gaseous atoms may play the role of the catalyst itself [67-70] The major advantage of a self-catalytic process is that it avoids undesired contamination from foreign metal atoms typically used as catalysts in VLS growth Other recently proposed VS growth mechanisms such as oxide-assisted growth mechshyanism [71-73] have also been reported in which the oxide instead of a metal nanoparticle plays a role in initiating the nucleation and growth of NWs or NRs Although the exact mechanisms responsible for the VS growth have not been completely elucidated many materials with interesting morphologies have been fabricated using these methods [74] Wang et al [6675-77] have fabricated NBs of ZnO Sn0

2 In20

3 CdO and so on with rectangular cross sections

and splendid ID ZnD nanostructures such as nanocombs nanohelices nanosprings nanorings and hierarchical nanoshystructures by VS growth

22 Solution-Based Growth

Comparedtovapor-phasegrowthsolution-basedgrowthhas some advantages such as relatively low temperature simple manipulation high productivity low energy consumption and no complex and expensive facility It thus provides a convenient low temperature and environmentally friendly pathway for fabrication especially large-scale production of ID semiconductor nanostructures with desirable shapes and sizes In recent years various solution-based synthetic techniques have been developed for the synthesis of 1D semiconductor nanostructures They include the solvoshythermal-hydrothermal method solution-liquid-solid (SLS) growth and capping agents-surfactants-assisted synthesis as well as template-directed growth

221 Solvothermal-Hydrothermal Method The solvothermal-hydrothermal process is usually performed in a Teflon-lined stainless-steel autoclave in which water or an organic solvent is used as the reaction medium at elevated temperatures above its boiling point in autogeneous presshysure by heating One outstanding characteristic of the solvoshythermal-hydrothermal strategy is that when the starting precursors are exposed to special conditions often in pure solvent some quite unexpected reactions can take place accompanied by the formation of nanoscopic morpholoshygies and new phases which generally cannot be achieved by traditional reactions [78] Solvothermal synthesis of semishyconductor NWs was first demonstrated in 1993 by Heath et al [79] to fabricate Ge NWs in an alkane solvent The solvothermal-hydrothermal method has been demonstrated to be a powerful tool to fabricate various kinds of ID semishyconductor nanostructures such as elemental semiconductor oxide semiconductor II-VI group semiconductor chalcoshygenide semiconductor and even carbide and nitride semishyconductor nanostructures [80-86J

It has to be emphasized that the nucleation crystallizashytion self-assembly and growth mechanism of ID semiconshyductors under solvothermal-hydrothermal conditions are rather complicated and not well understood yet even though

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

29 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emiion Properties

demonstrated by real time observations of Ge NW growth in a high-temperature transmission electron microscope [27] Figure 5 provides the insight and elucidation of the microshyscopic chemical process Similar in situ observation of VLS growth of Si NW by using transmission electron microscopy (TEM) has also been reported by Ross et al [28]

In the VLS growth metal nanoclusters or nanoparticles serve as the catalysts and nucleation seeds which largely determine the diameters of the NWs or NTs Smaller catashylyst nanoparticles produce thinner NWs or NTs It has been demonstrated that Si and GaP NWs with different diamshyeters can be obtained by controlling the diameter of monoshydispersed gold colloids serving as the catalyst [29 30] In general the lengths of NWs or NTs can be controlled by modifying the growth time In the VLS growth a proper choice of the catalyst is crucial The proper catalyst should be a good solvent capable of forming the liquid alloy with the target material and ideally eutectic compounds should be formed An important feature of the VLS growth is that it provides the intellectual underpinning needed for the prediction of good catalysts and synthesis conditions by considering the pseudobinary phase diagram between the metal catalyst and the solid materials of interest [26] For example one can use equilibrium phase diagrams to choose a suitable catalyst that can form a liquid alloy with the ID nanostructures of interest and the synthesis temperature so that the liquid alloy and solid ID nanostructure coexist VLS method offers the possibility to grow well-oriented or patterned NR or NW arrays on a suitable substrate by depositing or patterning suitable eatalysts on the substrates This is often advantageous to subsequent characterization and applications The VLS process that offers another advantage is that 10 semiconductor heterostructures can be achieved at the individual device level in a controlled fashion As shown in Figure 6 both axial heterostructures (Figs 6(c) and 6(e)) in which sections of different mateshyrials with the same diameter are grown along the wire axis [31-33] and radial heterostructures (Figs 6(d) and 6(f)) in which the core-shell and corc-multishell form along the

Figure 5 In situ TEM images recorded during the process of NW growth (a) Au nanoclusters in solid state at 500 C (b) alloying initiates at 800C at this stage Au exists mostly in solid state (c) liquid Au-Ge alloy (d) the nucleation of Ge nanocrystal on the alloy surface (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f) Reprinted with permission from [27] Y Wu et aI I Am Chern Soc 1233165 (2001) copy 2001 American Chemical Society

(Il)

bull Figure 6 NW heterostructure synthesis (a) Preferential reactant incorshyporation at the catalyst (growth end) leads to 1D axial growth (b) A change in the reactant leads to either (c) axial heterostructurc grovlith or (d) radial heterostructure growth depending on whether the reactant is preferentially incorporated (c) at the catalyst or (d) uniformly on the wire surface Alternating reactants will produce (e) axial superlattices or (f) core-multishell structures Reprinted with permission from [33J L J Lauhon et aI PhiL Trans R Soc Lond A 362 1247 (2004) copy 2004 The Royal Society of Chemistry

radial direction [34-36] have been realized by changing the type of vapor precursors used in the VLS growth

The VLS growth has now become a widely used method to produce ID semiconductor nanostructures from a myriad of inorganic materials such as elemental semiconductors (Si and Ge) [37 38] III-V semiconductors (BN GaN AlN GaAs GaP InP and InAs) [39-48] II-VI semiconductors (ZnS ZnSe CdS and CdSe) [49-52] metal oxides (ZnO Sn02 Inp3 CdO and GaP) [53-57] carbides (SiC and B4C) [5859] nitrides (Si3N4) [6061] and 10 heterostrucshytures [31-36 62] The disadvantage ofthe VLS growth may be the possible incorporation of catalysts into the NWs as well as the difficulty to remove such capsules from the tips of the NWs or NTs This may result in changes in the NWs properties However by selecting an appropriate catalyst the effects of contamination on specific properties of the NWs can be minimized Another disadvantage of the VLS growth is that it is difficult to synthesize 10 nanostructures when the vapor precursor is highly toxic or unavailable for the target materials

Recently we have extended this traditional VLS growth by introducing alloy particles as the catalyst Some 10 nanomaterials such as BN NWs and NTs [39 40 63 64] AlN NWs [48] and Si3N4 nanobelts [65] have been syntheshysized by simply nitriding Fe-B Ni-Al and Fe-Si alloy particles at high temperature with NH3-N respectively In the extended VLS growth technique the alloy catalyst not only acts as the medium for component transport from vapor to solid as is the case of traditional VLS growth but also supplies partial component( s) to form the final solid product For instance the components in the BN product come partially from the vapor precursor that is N in BN from NH3-Nz and partially from the catalyst itself that

B in BN from Fe-B catalyst itself This is clearly different from the traditional VLS growth mechanism as schematically illustrated in Figure 7 In other words a chemical reaction between the component(s) from the vapor precursor(s) and the catalyst takes pJace within the

30 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Liqu

Solid(S)shy Solid(S)-

VLSgmwth Extended VLS growth

Figure 7 Schematic diagram contrasting the VLS (left side) and extendshyed VLS grow1h (right side)

liquid catalyst and the resulting products precipitate with a whisker-like morphology when supersaturation is reached Strictly speaking the alloy particle here is not a catalyst but rather a reactant because the partial component in the alloy is consumed after the chemical reaction This is an extension to the traditional VIS growth providing a new possibility to explore novel compound nanostructures This is especially the case when the vapor source is highly toxic or unavailable while the solid catalyst containing the partial target component(s) can be easily obtained

212 VS Growth Some 1D semiconductor nanostructures can also be fabrishycated by simply evaporating a condensed or powder source material(s) and then condensing the resultant vapor phase( s) under certain conditions [66] Such a self-organization process which does not involve a foreign catalyst is referred to as a VS growth The thermal vaporization technique is a simpler process to vaporize the source materials Here the powder source material(s) is vaporized at elevated tempershyature and then the resultant vapor phase(s) condenses in a relatively low temperature zone under certain conditions (temperature pressure substrate etc) to form the desired product(s) In the VS growth mode control of the size of the 1D semiconductor nanostructures is accomplished mainly by tuning the evaporation and substrate temperatures vapor pressure gas flow and pressure However the exact mechanisms governing the VS growth are not completely understood Possible growth mechanisms such as the anisoshytropic growth defect-induced growth (eg through a screw dislocation) and self-catalytic growth have been suggested based on thermodynamic and kinetic considerations and electron microscopy studies [16 19 24] In an anisotropic growth mechanism ID growth can be accomplished by the preferential reactivity and binding of gas phase reactants on specific surfaces (thermodynamic and kinetic parameters)

to minimize the total surface energy However the degree of anisotropy is generally not significantly large and highly anisotropic growth (Le the length-to-diameter ratiogt100) of nanocrystals at or near the thermal equilibrium state is not expected [19] In the dislocation and defect-induced growth models specific defects (for example screw disloshycations) are known to have larger sticking coefficients for gas phase species thus allowing enhanced reactivity and deposition of gas phase reactants at these defects leading

to the formation of rod- or wire-like shape [19 241 The self-catalytic growth works in an analogous way in the VLS growth but differs in that one component of the gaseous atoms may play the role of the catalyst itself [67-70] The major advantage of a self-catalytic process is that it avoids undesired contamination from foreign metal atoms typically used as catalysts in VLS growth Other recently proposed VS growth mechanisms such as oxide-assisted growth mechshyanism [71-73] have also been reported in which the oxide instead of a metal nanoparticle plays a role in initiating the nucleation and growth of NWs or NRs Although the exact mechanisms responsible for the VS growth have not been completely elucidated many materials with interesting morphologies have been fabricated using these methods [74] Wang et al [6675-77] have fabricated NBs of ZnO Sn0

2 In20

3 CdO and so on with rectangular cross sections

and splendid ID ZnD nanostructures such as nanocombs nanohelices nanosprings nanorings and hierarchical nanoshystructures by VS growth

22 Solution-Based Growth

Comparedtovapor-phasegrowthsolution-basedgrowthhas some advantages such as relatively low temperature simple manipulation high productivity low energy consumption and no complex and expensive facility It thus provides a convenient low temperature and environmentally friendly pathway for fabrication especially large-scale production of ID semiconductor nanostructures with desirable shapes and sizes In recent years various solution-based synthetic techniques have been developed for the synthesis of 1D semiconductor nanostructures They include the solvoshythermal-hydrothermal method solution-liquid-solid (SLS) growth and capping agents-surfactants-assisted synthesis as well as template-directed growth

221 Solvothermal-Hydrothermal Method The solvothermal-hydrothermal process is usually performed in a Teflon-lined stainless-steel autoclave in which water or an organic solvent is used as the reaction medium at elevated temperatures above its boiling point in autogeneous presshysure by heating One outstanding characteristic of the solvoshythermal-hydrothermal strategy is that when the starting precursors are exposed to special conditions often in pure solvent some quite unexpected reactions can take place accompanied by the formation of nanoscopic morpholoshygies and new phases which generally cannot be achieved by traditional reactions [78] Solvothermal synthesis of semishyconductor NWs was first demonstrated in 1993 by Heath et al [79] to fabricate Ge NWs in an alkane solvent The solvothermal-hydrothermal method has been demonstrated to be a powerful tool to fabricate various kinds of ID semishyconductor nanostructures such as elemental semiconductor oxide semiconductor II-VI group semiconductor chalcoshygenide semiconductor and even carbide and nitride semishyconductor nanostructures [80-86J

It has to be emphasized that the nucleation crystallizashytion self-assembly and growth mechanism of ID semiconshyductors under solvothermal-hydrothermal conditions are rather complicated and not well understood yet even though

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

30 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Liqu

Solid(S)shy Solid(S)-

VLSgmwth Extended VLS growth

Figure 7 Schematic diagram contrasting the VLS (left side) and extendshyed VLS grow1h (right side)

liquid catalyst and the resulting products precipitate with a whisker-like morphology when supersaturation is reached Strictly speaking the alloy particle here is not a catalyst but rather a reactant because the partial component in the alloy is consumed after the chemical reaction This is an extension to the traditional VIS growth providing a new possibility to explore novel compound nanostructures This is especially the case when the vapor source is highly toxic or unavailable while the solid catalyst containing the partial target component(s) can be easily obtained

212 VS Growth Some 1D semiconductor nanostructures can also be fabrishycated by simply evaporating a condensed or powder source material(s) and then condensing the resultant vapor phase( s) under certain conditions [66] Such a self-organization process which does not involve a foreign catalyst is referred to as a VS growth The thermal vaporization technique is a simpler process to vaporize the source materials Here the powder source material(s) is vaporized at elevated tempershyature and then the resultant vapor phase(s) condenses in a relatively low temperature zone under certain conditions (temperature pressure substrate etc) to form the desired product(s) In the VS growth mode control of the size of the 1D semiconductor nanostructures is accomplished mainly by tuning the evaporation and substrate temperatures vapor pressure gas flow and pressure However the exact mechanisms governing the VS growth are not completely understood Possible growth mechanisms such as the anisoshytropic growth defect-induced growth (eg through a screw dislocation) and self-catalytic growth have been suggested based on thermodynamic and kinetic considerations and electron microscopy studies [16 19 24] In an anisotropic growth mechanism ID growth can be accomplished by the preferential reactivity and binding of gas phase reactants on specific surfaces (thermodynamic and kinetic parameters)

to minimize the total surface energy However the degree of anisotropy is generally not significantly large and highly anisotropic growth (Le the length-to-diameter ratiogt100) of nanocrystals at or near the thermal equilibrium state is not expected [19] In the dislocation and defect-induced growth models specific defects (for example screw disloshycations) are known to have larger sticking coefficients for gas phase species thus allowing enhanced reactivity and deposition of gas phase reactants at these defects leading

to the formation of rod- or wire-like shape [19 241 The self-catalytic growth works in an analogous way in the VLS growth but differs in that one component of the gaseous atoms may play the role of the catalyst itself [67-70] The major advantage of a self-catalytic process is that it avoids undesired contamination from foreign metal atoms typically used as catalysts in VLS growth Other recently proposed VS growth mechanisms such as oxide-assisted growth mechshyanism [71-73] have also been reported in which the oxide instead of a metal nanoparticle plays a role in initiating the nucleation and growth of NWs or NRs Although the exact mechanisms responsible for the VS growth have not been completely elucidated many materials with interesting morphologies have been fabricated using these methods [74] Wang et al [6675-77] have fabricated NBs of ZnO Sn0

2 In20

3 CdO and so on with rectangular cross sections

and splendid ID ZnD nanostructures such as nanocombs nanohelices nanosprings nanorings and hierarchical nanoshystructures by VS growth

22 Solution-Based Growth

Comparedtovapor-phasegrowthsolution-basedgrowthhas some advantages such as relatively low temperature simple manipulation high productivity low energy consumption and no complex and expensive facility It thus provides a convenient low temperature and environmentally friendly pathway for fabrication especially large-scale production of ID semiconductor nanostructures with desirable shapes and sizes In recent years various solution-based synthetic techniques have been developed for the synthesis of 1D semiconductor nanostructures They include the solvoshythermal-hydrothermal method solution-liquid-solid (SLS) growth and capping agents-surfactants-assisted synthesis as well as template-directed growth

221 Solvothermal-Hydrothermal Method The solvothermal-hydrothermal process is usually performed in a Teflon-lined stainless-steel autoclave in which water or an organic solvent is used as the reaction medium at elevated temperatures above its boiling point in autogeneous presshysure by heating One outstanding characteristic of the solvoshythermal-hydrothermal strategy is that when the starting precursors are exposed to special conditions often in pure solvent some quite unexpected reactions can take place accompanied by the formation of nanoscopic morpholoshygies and new phases which generally cannot be achieved by traditional reactions [78] Solvothermal synthesis of semishyconductor NWs was first demonstrated in 1993 by Heath et al [79] to fabricate Ge NWs in an alkane solvent The solvothermal-hydrothermal method has been demonstrated to be a powerful tool to fabricate various kinds of ID semishyconductor nanostructures such as elemental semiconductor oxide semiconductor II-VI group semiconductor chalcoshygenide semiconductor and even carbide and nitride semishyconductor nanostructures [80-86J

It has to be emphasized that the nucleation crystallizashytion self-assembly and growth mechanism of ID semiconshyductors under solvothermal-hydrothermal conditions are rather complicated and not well understood yet even though

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

31 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

this approach has been studied for many years Many factors such as the temperature concentration pH conditions precursors inorganic additives solvents and surfactants together with catalysts and other dynamic parameters have been reported to affect the nucleation and growth behavior of 1 D semiconductor nanostructures thereby making it difficult to clearly identify the driving force and fathom the underlying mechanism of solution-based growth methods

222 SLS Growth The SLS growth mechanism was first proposed by Buhro et al [87] for the synthesis of crystalline NWs of III-V semiconductors at low temperature Figure 8 presents a schematic illustration of the SLS growth mechanism Analshyogous to the well-known VLS growth mechanism the SLS growth process also needs a molten metal catalyst to direct the nucleation and growth of ID semiconductor nanostrucshytures In the SLS growth the catalyst is a low-melting metal such as In Sn Bi and so on The main difference between SLS and VLS is that the precursor of the resulting ID nanoshystructure resulting from the VLS growth comes from the vapor phase whereas the precursor in the SLS growth is from the liquid Now the SLS growth has been developed into a general means to synthesize semiconductor NWs Various II-VI and III-V group semiconductor NWs such as CdTe ZnTe InP InAs GaAs AlxGa1_xAs and InN have been synthesized by this growth method [88-93] It should be pointed out that the complex architecture of ID semishyconductor nanostructures can also be fabricated by the SLS growth process For example branched CdSe PbSe and CdTe NWs as well as CdS-CdSe NR heterostructures have been fabricated by SLS growth [94-98]

Korgel and coworkers developed a supercritical fluidshyliquid-solid (SFLS) method to synthesize defect-free ultra fine Si NWs Here the monodispersed and high melting point Au nanoparticles readily available act as the catalysts The as-synthesized Si NWs have diameters in the range of 4-5 nm narrow diameter distributions (standard deviation within plusmnlO of the mean NW diameter) and large aspect ratios over 1000 [99] This SFLS method has further been adopted to synthesize Ge [100] GaAs [101] and GaP [102] NWs

Solution ILiquidlI I

Solid

Byprooucts II

II Growth direction

(

) I

Metallo-organic Catalyst Crystalline precursors particle semiconductor

Figure 8 Schematic illustration of the SLS growth mechanism Reprinted with permission from (88 F Wang et al Inorg Chern 45 7511 (2006) copy 2006 American Chemical Society

223 Capping Agents-Surfactant-Assisted Synthesis

Capping agents-surfactant-assisted synthesis has been widely explored for the fabrication of ID semiconductor nanostructures such as ZnO [103] Sn0

2 [104] and CeOz

[105] NRs Anisotropic growth of crystals induced by different surface energies is the reason for the formation of most elongated nanocrystals However in most cases the difference in the surface energies (the intrinsic properties of the crystal) is not large enough to cause highly anisoshytropic growth oflong NWs or NRs It is believed that surfacshytants or capping agents can play a significant role in the kinetic control of the nanocrystal growth by preferentially adsorbing and bonding to certain surfaces of the nanocrystal seeds thus inhibiting growth on that surfacc This selective capping effect can induce nanocrystal elongation along a specific direction to form ID nanostructures That is to say the surfactants serve as structural directors This selective capping mechanism has been proven in the fabrication of many nanomaterials For instance Sun et al [106 107] have produced Ag NWs using poly(vinyl pyrrolidone) (PVP) as a capping agent The silver nanocrystals initially formed by reducing AgN03 with ethylene glycol (EG) are heated to -160degC In the presence of PVP most of the silver partishycles are directed to grow into NWs with uniform diameters One possible explanation is that PVP selectively binds to the 1 0 O facets of silver while maintaining the Ill facets to allow growth Using a similar synthetic route ZnO NWs have been fabricated via the hydrothermal reactions of Zn salts in the presence of capping agents or surfactants such as amines [108] and hexamethylenetetramine [109]

The main advantage of capping agents-surfactantshyassisted synthesis may be the possibility to create ID nanoshystructure arrays on suitable substrates at low temperatures For example Greene et al [109] fabricated dense arrays of ZnO NWs on Si substrate in an aqueous solution by using hexamethylenetetramine as a structural director Very recently we have fabricated ultrafine CdS and ZnS NW arrays on Cd or Zn substrates using a similar synthetic route [110111] In this technique the Cd or Zn foil serves as the substrate as well as Cd or Zn source and the CdS or ZnS NWs grow directly on the conducting Cd or Zn foil This growth process ensures that a majority of the NWs in the array are in direct contact with the substrate thereby providing a continuous pathway for carrier transport an important feature demanded by future FE devices based on these materials

A major limitation of this growth method is that most capping agents are chosen via an empirical trial-and-error approach It will therefore be advantageous to develop a library of bond strengths of various chemisorbed capping agents on specific crystal planes [24]

23 Template-Directed Growth Template-directed growth which is a convenient and versashytile method to produce ID nanostructures was pioneered by Martin and Moskovits [112-114] Researchers are now using this method to prepare various ID nanostructures composed of electronically conductive polymers metals

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

32 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

semiconductors carbon and many other materials In this approach the template simply serves as a scaffold within (or around) which building blocks of the ID nanostructures nucleate and grow with its morphology complementary to that of the template The common templates include hard template such as porous anodized alumina oxide (AAO) [113-116] track-etehed polymer film [117 118] mesoposhyrous silica (MCM-41 or SBA-15) [119 120] ID NTs NWs or nanobelts [16 121] even substrate ledge or step [122] and soft templates sueh as mesoscale structures self-assemshybled from organic surfactants or block copolymers [123shy125] and biological macromolecules [126 127] Among the various templates the most common commercially available template is porous AAO formed by anodic oxidashytion of an aluminum sheet in solutions of sulfuric oxalic or phosphoric acid As shown in Figure 9 the pores in the AAO template are arranged in a regular hexagonal lattice The diameter of these nanochannels and the interchannel distance can be easily controlled by the anodization voltage The pore size ranges from 10 nm to 100 ~m and the pore densities can be as high as 1011 pores ern-2 [121] The pores in the AAO template can be filled to form 1D semiconshyductor NWs or NTs by electrochemical deposition sol-gel deposition liquid-phase injection or vapor-phase-based CVD [21 115 121] For example semiconductor NW and NR arrays such as ZnO [128 129] SnOz [130] and Inp3 [131] ZnTe [132] and CdTe [133] have been fabricated by electrochemical deposition using AAO templates By soaking AAO into a zinc nitrate solution mixed with urea and kept at 80nC for 24-48 h followed by thermal heating

Figure 9 Schematic illustration (a) SEM images of top surface (b) and cross section (c) of AAO template (b and c) reprinted with permisshysion from [115J L D Zhang et aI 1 Nanosci Nanotech 8 149 (2008) copy 2008 American Scientific Publishers

ZnO NR arrays can be fabricated by the sol-gel method [134] Mn0l [135] ZrOz [136] Ti02 [137] and multicomshypound oxide NRs [138 139J have also been synthesized by similar processes

Template-based growth of semiconductor NTs or NWs provides an advantage that the diameter density and length of the resulting NR or NW arrays can be easily controlled by varying the parameters of the templates Since the pores in the membranes used have mono dispersed diameters analogous monodispersed NTs or NWs can in principle be obtained Another advantage is the possibility of forming multilayered hollow NT or solid nanocable structures For example Ni-VzOs nH20 nanocable arrays have been synthesized by a two-step approach [140] This technique intrinsically yields products with a polycrystalline nature In addition the quantity of ID semiconductor nanostructures produced in each run is relatively limited

It is also recognized that 1D nanostructures such as NTs NWs or nanobelts themselves are good templates to generate other types of ID nanostructures which are involved not only physically as a scaffold or mold for the fabrication of 1D semiconductor nanostructures but also chemically as sacrificed templates to transform into other 1D nanostructures by reacting with the proper chemicals In one approach the surface of NWs can be coated with conformal sheaths made of different materials or the NTs can be filled with other materials to form coaxial nanoshycables For example aligned CNTs have been used to grow TiOz NT arrays by electrophoretic deposition using Ti02 sol as the growth precursor [141] Coaxial nanocable arrays are first formed with CNTs as the core and TiOo as the shell Subsequent elimination of the original CNT~ leads to the formation of the TiO NTs The NRs or NWs can also serve as a sacrificed template for the fabrication of other 1D nanostructures by reacting with appropriate regents containing other desired elements to form the final products without changing the original 1D nanostructure morphologies For example Gates et al [142] fabricated single-crystalline NWs of AgzSe by chemically reacting the as-prepared Se NWs and AgN03 in an aqueous solution at room temperature Liang et al [143] fabricated highly uniform CdTe and PbTe NWs using ultrathin Te NWs as templates via a low-temperature hydrothermal process Using CNTs as the reaction templates SiC [144] and GaN [145] NWs as well as BN NTs [146 147] can be generated Template-directed synthesis provides an effective route to produce ID nanostructures that are difficult (or impossible) to directly synthesize or fabricate by other methods

3 FE PROPERTIES OF 1 D SEMICONDUCTOR NANOSTRUCTURES

As discussed in the first session ID semiconductor nanoshystructures especially aligned nanostructure arrays are promising field emitters because of their ideal sharp tips and high aspect ratios Such novel electron emission sources allow a variety of new or improved devices with a small size and long life such as FEDs parallel electron beam microsshycopy nanolithography compact microwave amplifiers

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

33 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

and portable X-ray tubes Some review papers have been published [2 6 7] and so in this section we mainly focus on recent progress on the investigation of FE properties and applications of the most widely studied inorganic ID semishyconductor nanostructures such as IV group SI SiC II-VI group ZnO ZnS ZnSe CdS III-V group AlN GaN InN and some typical metal oxides such as CuO SnOz Inz0 3

ITO Ti02 we) and W190

43 bull In the subsequent FE propshy

erties description Eo and E jh which are two important parameters to characterize FE properties of emitter mateshyrials in the practical FE measurements are defined as the electric field to produce a current density of 10 lA cm-2 and 10 rnA cm ~2 respectively In cases where other values are used they will be specifically mentioned

31 Group IV Semiconductors

311 Si Si is the most important semiconducting materials in the microelectronics and integrated-circuit industry Singleshycrystal silicon is an excellent material to produce emitter structures that are both mechanically and electrically consisshytent and robust ID Si nanostructures are of particular interest as field emitters owing to their sharp tips and high aspect ratios well-understood electronic property low work function (36 eV) and the fact that Si-based FE devices are compatible with up-to-date integrated-circuit techniques and have the possibility of direct integration into various monolithic circuits or integrated circuit [148]

One ofthe early fabrication techniques to produce SiFEAs is the orientation-dependent etching (ODE) method to form convex pyramidal structures as shown in Figure 2( d) The fabrication process begins by lithographic definition of a set of dot masks on the (1 0 0) face of a Si wafer over the

points where tips are to be placed In practice however ODE becomes difficult to control once the emitter apex is formed [148] and so this method has seldom been used successfully Up to now the pyramidal silicon FEAs on silicon substrates are formed using different top-down processes such as wet chemical etching [149] patterned reactive ion etching (RIE) [150 151] laser beam irradiation [152] hydrogen plasma etching [153] and a combination of using self-assembled nanomask and anisotropiC plasma etching [154] Figures 10(a)-1O(c) are the scanning electron microscopy (SEM) images ofSi NC arrays fabricated by RIE with 125 20 and 25 02 in the gas mixture of CF4 and 02 respectively [151] These three Si NCs exhibit similar FE characteristics The E

to (defined to be the electric field

required to generate a current density of 01 lA cm- I ) was measured to be about 6 V jlm-1 as shown in Figure 1O(d) Their excellent FE properties are attributed to the high density (9 X 108 cm-2) sharp tips (less than 10 nm) and the well-controlled spacing between Nes so that the fieldshyscreening effect (or antenna effect) is minimized

In addition to the top-down routes 1D Si nanostructures such as NWs [155-162] nanotip arrays [163-165] and NT arrays [166] have been fabricated using bottom-up routes such as thermal evaporation CVD and AAO templateshydirected growth and so on Fang et al [160] fabricated Si NW semisphere-like ensembles via thermal evaporation without using templates and metal catalysts SEM images reveal that the NWs within each semisphere ensemble are well-aligned and evenly distributed (Figs Il(a) and l1(b) ) TEM and high-resolution TEM (HRTEM) images reveal that these arrays are structurally uniform and composed of single-crystalline wires with an average diameter of the NWs of about 40 nm as shown in Figures l1(c) and lIed) The FE current density (J) as a function of the applied field

30 2(

211 15 18

llIl IlO

~5+-~~-r~__~~~-r~~ II a 3 5 6

Applied eleGkiltgt Held VIp ml

Figure 10 SEM images of Si NC arrays fabricated by RIB with 125 (a) 20 (b) and 25 (c) 02 in the mixed gases of CF and 02 and the corshy4

responding FE eurrent density VB electric field curves The inset shows a In(Jpound2)-liE plot Reprinted with permission from [151J H Y Hsieh et aI Nanotechnology 18 505305 (2007) copy 2007 IOP Publishing

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

34 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

j)------------

00

4 bull bull Electric Field (Wpm)

O_OB o1t 012 014 ote 11pound (jill1l1

Figure 11 (a) and (b) low- and high-magnification SEM images of Si NWs assembled into semi spheres (e) TEM image of a Si NW bundle (d) HRlEM image of a single NW (e and f) FEJ-E cUlves and corresshyponding F-N plots of Si semisphere ensembles at the anode-eathode distanees of 100 and 120 Jllll Reprinted with permission from [160] X S Fang ct al Chern Commun 4093 (2007) copy 2007 The Royal Society of Chemistry

(E) is shown in Figure l1(e) at anode-cathode distances (d) of 100 and 120 )lm The relatively smooth and consistent curves indicate the stability of emission from the Si nanoshystructure emitters Eta is found to be -73 V )lm- I and the F-N plot is nearly linear (Fig 11(f)) indicating that FE from the Si nanostructures is controlled by a barrier tunneling quantum-mechanical process The field-enhancement factor (fJ) in this case is estimated to be -424 assuming that the work function (ltraquo of Si is 36 eV

The density of 1D Si nanostructure emitters has an influshyence on their FE properties Zeng et al [157] have fabrishycated Si NWs with high and low density on carbon cloth via VLS growth using silane gas as the silicon source and gold as the catalyst from the decomposition of hydrogen gold tetrachloride (HAuC14 bull 310) The density is controlled by soaking the carbon cloth into HAuCl4 bull 3HzO with different

concentrations Figures 12( a)-12( d) depict Si NWs on the carbon cloth with high and low density respectively The Eta and Eth of the samples with low site density are 03 and 07 V flm-l respectivcly and lower than the one with high site density (05 and 11 V )lm- I

) as shown in Figure 12(e) Calculated from the F-N plot shown in Figure 12(f) the fJ value of the sample with low site density is 61000 whereas that for high density is 25000 Since the lower-density Si NWs have much better FE properties than the highershydensity ones it can be deduced that the screening effect exists in the high-density Si NWs The results suggest that a lower density of Si NWs is absolutely necessary for good FE performance

The FE properties of ID Si nanostructures can be further enhanced by coating other components such as amorphous carbon [153 167] cerium (Ce) [158] amorphous diamond [168] SiC [169] and thin ferroelectric films [170] or by hydrogen (H2) plasma treatment [171] For instance Bai et al [167] fabricated Si nanotips in situ coated with a -3 nm thick amorphous carbon film using a plasma-assisted hotshyfilament CVD process by increasing the methane concentrashytion in the gas precursors In comparison with the uncoated silicon nanotips arrays the coated tips have cnhanced FE properties with E

to and Elb of 16 and 3 V flm-1 respectively

which are even lower than those of well-aligned CNT emitter arrays [172 173] The enhancement in FE is attributed to the uniform coverage of carbon onto the sharp nanotips This in situ carbon-coating process avoids the complexity of postshycoating treatments as in conventional approaches The coated carbon film also has the advantage of preventing surface oxidation and the sample possesses the equivalent merits of carbon nanotip arrays The enhanced FE properties of Si nanotip arrays with amorphous a carbon coating have also been observed by Wang et al [153] They compare the FE properties of four samples Si cone arrays with and without a carbon coating on porous Si and pure Si Studies indicate that the amorphous carbon coating can effectively enhance the FE ability of Si NCs on porous Si It is considered to result from the fact that the amorphous carbon coating reduces the work function of the Si NCs from 415 to 237 eV

1D Si emitters are excellent electron-emitter structures that are both mechanically and electrically consistent and robust They are considered excellent candidates in FE devices Table 1 summarizes the representative FE propershyties of 1D Si nanostructures reported so far along with a brief description of the corresponding synthctic methods

312 Ge Germanium (Ge) is another important member of the group IV semiconductors It has a higher carrier mobility larger exciton radius and hence more prominent quantum size effects in comparison with Si However exploration and invesshytigation of ID Ge nanostructure have been relatively limited and there have been few articles reporting the FE properties of ID Ge nanostructures Wan et at [174] fabricated Ge cone arrays on N+-type (1 OO)Si by high-vacuum electron-beam evaporation Thc FE measurements show that the Eta (defined as the field required to detect a current of 1 lA) is about 15 V lm- I

Li et al [175] synthesized the aligned Ge NW arrays on a Si substrate using an oblique angle deposition The

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

35 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

~M a 041~2

o o Q Q

I)

I High_Uy (SomploA) bull

bull bullbullbull0shyshy

~ o~l

-6

High dnlty (SompleA)

(f)

21-5~1-L8~JL-5--2Jl--~25--3-8~-35--40--~45 1)3 114 os 06 Ij7 08 09 LO Ll

Applied Field (Vpm) lIF

Figure 12 Low- and medium magnifications SEM micrographs of high-density (a b) and low-density (c d) Si NWs grown on carbon cloth by CVD and the corresponding FET-E curves (e) and F-N plots (t) Reprinted with permission from [157] R Q Zeng et alAppr Phys Lett 90033112 (2007) copy 2007 American Institute of Physics

measured FE properties reveal that the Etc (defined as the applied field at a current density of 100 nA cm-2) is about 85 105 and lOA V )tm-1 for different anode-cathode distances of 95100 and 105 )tm respectively The effect of the annealing temperature on the FE properties is also analyzed It is found that increasing the annealing temperature results in a first increase and then a decrease in J value There is an optimal annealing temperature of 550degC where the Etc decreases to 76 V iJl11-1 at 1 lA cm-2bull The gradually increasing maximum J and decreasing EtQ are mainly attributed to the decreased density of Ge NWs As the annealing temperature is further increased to 580degC all the Ge NWs melt into nanoparticles resulting in the lower maximum J and the higher E

to largely

owing to low aspect ratio The measured J values fluctuate between 0103 and 0130 IlA cm-2 within 3000 s at 10 V jIlll-l suggesting good emission stability

313 SiC Silicon carbide (SiC) is one of the most promising electronic ceramics in the microelectronics industry It possesses

superior mechanical physical chemical and electronic properties such as low density high hardness and mechanshyical strength high thermal and chemical stability good thermal conductivity and high fracture toughness SiC exists in many various poly types All the polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms The fundamental structural unit in all SiC poly types is a covalently bonded tetrahedron of four C atoms with a single Si atom at the center Four Si atoms likewise surround each C atom The tetrahedrons are linked via their corners In a common poly type notashytion the letters C H and R are used to represent cubic hexagonal and rhombohedral structures respectively and numerals are used to represent the number of c1osestshypacked layers in the repeating sequence The most common poly types are 3C and 6H 4H 15R and 2H have also been identified but are rarer Other poly types are combinations of these basic structures [176]

The 3C-SiC with a face-centered cubic crystal structure (similar to diamond or zinc blende) is also referred to as J-SiC which is he most frequent crystal structure of the synthesized

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

36 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 1 Key performance parameters of 1D Si nanostructure field emitters

Si emitters Synthesis method E to

(V lm-1) Eh (V lm-1 ) Ref

Carbon-coated Si cone arrays Plasma etching 175 and 252 for different substrates at 1 lA cm-2

NA [153]

AlignedNWs Microwave plasma-enhanced CVD 08 50 [154]

RandomNWs CVD 20 34 at 1 rnA cm-2 [156]

Random on Ccloth VLS growth 03 07 at 1 rnA cm-2 [157]

Cesiated Si NWs VLS and post-growth processing NA 776 plusmn 055 [158]

Taper-like NWs High-temperature annealing 63-73 9-10 [159]

Microsized semisphere ensembles Thermal-evaporation process 73 NA [160]

RandomNWs CVD 74 99 [161]

RandomNWs CVD 27 NA [162]

NCs RIE 60 at 01 lA cm-2 NA [151]

Anodization by Si-Based AAO mask 85 NA [163]

Ion-beam sputtering 13 235 at 1 rnA crn-2 [164]

SiC nanomasks 447-584 NA [165]

NTs Multistep template replication route 51 73 at 1 rnA cm-2 [166]

Plasma-assisted hot-filament CVD 60 30 [167]

SiC-capped nanotip arrays Plasma etching 035 NA [169]

The Eo is defined as the applied electric field required for achieving a FE current density of 10 lA cm-2 (unless otherwise indicated) The Eh is defined as the field at which the emission current reaches 10 rnA cm-2 (unless otherwise indicated) NA is referred to as unknown or not reported in the corresponding reference The

symbols in the following Tables (2-13) are the same as in Table 1

1D SiC nanomaterials j1-SiC has a band gap of about 23 eVat room temperature It is less sensitive to high temperature and should be able to stabilize at temperature exceeding 500dege Moreover it has been reported that j1-SiC NWs exhibit considshyerably greater elasticity and strength than micrometer-size SiC whiskers and bulk SiC [177 178] Hence 1D SiC nanostrucshytures with superior thermal and chemical stability are robust field emitters in vacuum nanoelectronic devices

Several techniques have been developed to synthesize 1D SiC nanostructures such as NWs [179-188] porous NWs [189] NRs [190-192] bamboo-like and needleshyshaped NWs [193 194] nanobelts [195] and NTs [196 197] SiC NWs were synthesized by a reaction between CNTs and SiO or SiI2 [179] or by a two-step process involving first generating SiO vapor and then its reaction with CNTs [180] Later Meng et al prepared SiC NWs by carbothermal reduction of silica xerogel containing carbon nanoparticles [181] The aligned CNTs may be used as templates for the synthesis of aligned SiC NWs [182] This is realized by the reaction between CNTs and SiO at elevated temperature under flowing Ar gas Single-crystalline bamboo-like fJ-SiC NWs with hexagshyonal cross sections were synthesized by thermal evaposhyration of mixed SiO + C + GaN powders in an Ar atmosphere [193] Wei et a1 [195] fabricated ultrathin 3C-SiC nanobelts via a microwave-assisted carbo thermal reduction method using a mixture of Si Si0

2 and charshy

coal Single-phase SiC NTs can also be synthesized by the reaction of CNTs with silicon powder [196] or Si from disproportionation of silicon monoxide [197] Aligned SiC NW arrays on silicon substrates have been fabricated by simple thermal evaporation of ZnS and carbon on a

silicon wafer [198] Deng et a1 [199200] have developed an alternative approach in which needle-shape and treeshylike SiC NWs can be grown from SiC particles even on the surface of a bulk SiC ceramic substrate using iron as the catalyst in a thermal process Recently porousshyaligned SiC NWs have been synthesized on Si substrates via in situ carbonizing aligned Si NW arrays standing on Si substrate with ethanol [189] In addition to NWs or NTs SiC-Si0

2 biaxial NWs aligned SiC-C nanocables

SiC-SiO or SiC-BN nanocables and so on have also been synthesized [201-208]

The FE properties of various 1D SiC nanostructures have been widely investigated Niu et a1 [186] investigated the FE properties of aligned and random SiC NW arrays synthesized by a simple VS reaction of solid carbon and silicon substrate with (for aligned) or free of (for random) ZnS powders in a simple CVD system The mean diameter of the random NWs is about 70 nm while well-aligned NWs with a mean diamshyeter of -10 nm are uniformly distributed on the substrate The measured FE J-E curves indicated that the E

to of 105 V

f1m-1 is obtained from the aligned sample whereas a higher value of 295 V flm-1 is observed with the random sample The aligned SiC NW array shows relatively better FE perforshymance compared to the random sample Clearly the aligned structure has enhanced emitting behavior For the aligned NW arrays the majority of the SiC NWs with a small tip are straight and vertical to the substrate thus contributing to the high emitting efficiency However with regard to the random SiC NWs many electrons emitted from the NWs cannot arrive at the anode and thus the collected emission current density is reduced Furthermore the random sample with a big size exhibits reduced FE efficiency Thus the aligned SiC

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

37 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

NW arrays with a highly ordered structure with very small size have better FE performance

Very recently porous-aligned SiC NWs have been syntheshysized on Si substrates via in situ carbonizing aligned Si NW arrays standing on a Si substrate with ethanol [189] The resulshytant SiC NW arrays have diameters in the range of tens to hundreds nanometers and lengths of about 20 lm inheriting the diameter and length of the mother Si NW arrays The FE measurements disclose that the Eto values of these porous SiC NWs are 2927 and 23 V jUll-I at cathode-anode distances of 300 400 and 500 jUll Examination of the emission stability is performed at an anode-cathode distance of 300 lm while keeping the same applied voltage of 3000 V No obvious degradation can be observed at 057 ttA cm~2 of the current density over a period of 2 h suggesting good FE stability

The FE properties of 1D SiC nanostructures can be further enhanced by coating other materials to form coreshyshell nanostructures For example BN coatings can reduce the Eta of SiC NWs from larger than 10 V lffi-I to lower than 6 V lffi-I [205] The low Eta is owing to a decrease in the effective potential barrier height which is considered to result from the existence of a defect-induced positive space charge Table 2 gives the important results pertaining to the FE properties of 1D SiC nanostructures reported up to date along with a brief description of the corresponding synthetic methods

32 Group -VI Semiconductors

321 ZnO Zinc oxide (ZnO) is an important II-VI compound semishyconductor possessing a wide band gap of 337 eV at room temperature and a large exciton binding energy of 60 meV It can lead to ultraviolet (UV) lasing action based on exciton recombination even at room temperature The wurtzite Zn0 structure has a hexagonal unit cell in which the oxygen anions and Zn cations form a tetrahedral unit Hence the entire structure of wurtzite ZnO lacks central symmetry Combined with its large electromechanical coupling the materials have excellent piezoelectric properties ZnO is also biologically safe and biocompatible and it can be used directly in biomedical applications without the need to form a coating As a consequence ZnO finds wide applications in optics optoelectronics sensors actuators energy biomedshyical sciences spintronics solar cells and so on [209]

The morphology of ZnO nanostructures has been shown to be one of the most diverse among inorganic semiconducshytors Up to now various ZnO nanostructures such as NRs NWs NBs NTs nanosheet nanobows nanonails nanoshycombs nanohelices nanosprings nanorings nanoflower and hierarchical nanostructures have been synthesized [75-77 210-217] Some exciting properties and potential applications based on various ZnO nanostructures for

Table 2 FE performances of 1D SiC nanostructures reported in the literature

Stability testing time SiC emitters Synthesis method Ew (V lm-I) Eh (V Jill1-1) and fluctuation Ref

NWs Aligned Reacting aligned CNTs 07-5 25-35 24h lt3 [182]

withSiO

Random Hot-filament CVD 20 30 180 min lt25 [183J

Random Thermal evaporation 31-35 NJA 120 min lt 15 [184]

Random and aligned Vapor solid reaction 105 (aligned) 295 (random) NJA NJA [186]

Random Thermal evaporation 8 NA 90 min lt20 [187]

Random MOCVD 20 NA 75 h lt2 [188J

Porous Carbonizing aligned Si NW 23-29 NA 1200 s no obvious [189] arrays with ethanol fluctuation

NRs

Random Rapid thermal processing 18 NA NA [190J

Random CVD 13-17 NA NA [191]

Bamboo-like NWs Thermal evaporation 101 NA NA [193J

Needle-shaped NWs Thermal evaporation 5 85 lOOh lt4 [194] process

Nanobelts Microwave-assisted 32 NA NA [195] carbo thermal reduction method

NWsNRs grown on SiC Catalyst-assisted thermal 33 57 NA [200] substrate heating process

BN-coated SiC-NWs VLSprocess 6 NA NA [205]

C-coated SiC-NWs CVD 42 NA NA [206]

SiOz-coated SiC-NWs Heating the NiO-catalyzed 40 (bare SiC) 33 (10 nm NA NA [207] silicon substrate Si02-coated SiC) 45 (20

nm SiO-coated SiC)

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

38 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

example nanogenerators have recently been revealed [212 218-221J

Similar to CNTs ID ZnO nanostructures such as NWs or NRs have small curvature radii and high aspect ratios that is high f3 value Furthermore ID ZnO nanostructures have better thermal stability and oxidation resistance than CNTs thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications [222] Therefore ZnO ID nanoshystructure has been regarded as a good alternative FE electron source in FE devices especially in poor vacuum and harsh environment As a consequence there has been increasing interest in the investigation of fabrication and FE properties of 1D ZnO nanostructures The FE properties of various 1D nanostructured including NRs-NWs [223-239] nanosheets [238J nanofiber arrays [240] NTs [241 242J nanobelts [238 243] nanonails [244] nanoneedles [245-250] nanopins [251] nanopencils [244 252] NCs [253] nanoscrews [254] tetrapodshylike nanostructures [255] and so on have been investigated

Very recently we have developed a simple method to conduct direct and large-area synthesis of ZnO nanosheets nanocombs NWs and nanobelts on conducting brass substrates by annealing a CuObull66Zn034 foil under Ar-Oz at different temperature As shown in Figure 13 ZnO nanosheets (Figs 13(a) and 13b ) nanocombs (Figs 13(c) and 13d) and NWs (Fig 13(eraquo distributed randomly on the substrates were fabricated by annealing CUO66Zn034 foils

Figure 13 Typical SEM images of the as-synthesized products directly grown on Cu-Zn alloy substrates at different temperatures under Ar-Oz (4 02) (a b) 600degC for 1 h (c d) 700C for 1 h (e) 800degC for 1 h and (f) first 500C for 30 min and then 700C for another 1 h Inset in (a) and (c) are the corresponding TEM images of the sample Reprinted with permission from [238] K F Huo et al J Phys Chern C 1115876 (2007) copy 2007 American Chemical Society

under Ar-02 (4 OJ at 600 700 and BOOoe for 60 min respectively The quasi-aligned ZnO nanobelts arrays (Fig 13(t)) could also be fabricated on brass substrate by two-step annealing process First ZnO nanoparticles were formed by annealing a CuObull66Zn034 foil under Ar-Oz (4 OJ at 500degC for 30 min and then the brass foil was further annealed at 700degC for another 60 min In this method the brass foil serves as both the Zn source and substrate for the ID ZnO nanoshystructures and synthesis and assembly of ZnO nanostrucshytures on conducting brass substrate are accomplished in one step Thus good adhesion and electrical contact between the ID ZnO nanostructures and conducting metal substrate can be accomplished Such a configuration constitutes a promshyising field emitter since most of the as-grown 1D ZnO nanoshystructures are in direct contact with the substrate thereby providing a continuous pathway for electron transport from the cathode to ZnO nanostructures FE measurements of such ZnO nanostructures directly grown on brass substrate were carried out using a parallel-plate diode configurashytion in a test chamber maintained at 5 X 10-6 Torr and a sample cathode distance of 200 lm Figure 14(a) depicts the curves of the current density (1) as a function of the applied field (E) and Figure 14(b) displays the F-N plots of In(JE2)-(1E) It is found that the Eto of ZnO nanosheets nanocombs NWs and quasi-aligned nanobelts are about 393830 and 23 V Jlll-l respectively The corresponding applied fields to produce the emission current density of 1 rnA cm-2 are 89 76 68 and 53 V JLm-l respectively The relatively smooth and consistent J-E curves in Figure 14( a) indicate the stability of emission from the ZnO nanostrucshyture emitters The linearity demonstrated by the F-N curves (Fig 14(braquo within the measurement range confirms that electron emission from the ZnO nanostructures follows the F-N behavior Taking the work function of ZnO to be 53 e V the field-enhancement factor f3 is estimated to be about 1600 4208 4611 and 6720 for the nanosheets nanoshycombs NWs and quasi-aligned nanobelts respectively As shown in Figures 13(a)-13(e) the nanosheets nanocombs and NWs are randomly distributed on the brass substrate and have a similar density Hence the sizes and morpholoshygies of these emitters should be the main factors that affect their FE performance Comparing the three samples shown in Figures 13(a) 13(c) and 13(e) the nanosheets have the largest size and the NWs have the smallest tip diameter The nanocombs are composed of a mixture of large backshybones with small teeth Hence the NWs should possess the lowest E and the nanosheets have the highest E whereasto to the nanocombs possess an intermediate E It is consistentbull

to

with the experimental results shown in Figure 14(a) As for the quasi-aligned nanobelts the tip size of the nanobelt is approximately the same as the diameter of the NW The good alignment can further enhance the f3 value Hence the quasi-aligned nanobelts should have even better FE propershyties than the NWs The calculated f3 is 6720 the highest of the four examined samples

The FE properties of ID ZnO nanostructures depend on not only the tip morphology but also on the growth coverage density of the nanostructure A high density of emitters may greatly reduce the field-enhancement effect at the emitter tips owing to serious screening effects while emitters with a low density have few emitting sites which cannot meet

39 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

20

16

N

sect 12 ~ E= 08

04

2 4 6 8 10

E(Vllm)

N~

1 gt

N

~ c

~ 3shy5 -9

E-4

-5

E-S

E -7

-8shy

010 015 020 025 030 035 040 045

1JE(rtmN

Figure 14 (a) FE current density (1) as a function of the applied electric field (E) for the different Zno nanostructures and (b) the correspondshying F-N plots Reprinted with permission from [238] K F Huo et aI J Phys Chern C 111 5876 (2007) copy 2007 American Cnemieal Society

the desired requirement for high current density and highshyemitting points Recently Wang et a1 [234] investigated the FE properties of aligned ZnO NWs with five different densities of 108 86 64 45 and 28 NWs jtm-2bull The FE measurements were carried in a seanning electron microshyscope with an in situ FE measurement system This system provides a novel and reliable way to measure the FE propershyties with the knowledge of the exact emitting distance NW density and the region being tested During the in situ FE measurement the vacuum in the scanning electron microshyscope chamber was 5 x 10-6 Torr The distance between the NW array and W tip is controlled by the fine movement of the scanning electron microscope stage and measured directly from the SEM image This sample-anode distance is kept at 20 lm for all the measurements The SEM images and FE J-E curves of the ZnO NWs with five different densities are shown in Figure 15 The highest-density NWs do not turn out to be a very effective electron emisshysion source The emission is turned on by the electric field around 20 V m-1 and the emission current increases to around 20 IlA when the electric field increases to 25 Vm-1

5

II t i bullbullt

o +--~---jIII~---=---1I-

I

bulli IV

20o 5 10 15 Electric field (VtJm)

Figure 15 (a) Top-view SEM images of the aligned Zno NW arrays with five different densities (NWs lmmiddot-Z

) Curves IV and V are magnified five times for better illustration (b) Corresponding FE I-V charactershyistics and the F-N curves Reprinted with permission from [234] X D_ Wang et aLAdv Mater 191627 (2007)_ copy 2007 Wiley-VCH

This is believed to be caused by a large screening effect owing to the very small space between the NWs Moreover after FE measurement SEM observation shows that the NWs agglomerate with each other (for details see Ref [234]) resulting in reduction in the sharp emitting tips Such an attractive phenomenon may also be responsible for the low-emission efficiency of high-density NWs The highest-emission efficiency is observed from samples II and III with medium densities (86 and 64 Ilm-2) having Etc of 8-10 V Ilm-1 However if the density of the NWs decreases to 45 Jlm-2 or lower the emission efficiency becomes very low only l-2lA at an electric field of 25 V lm- I By taking 52 e V as a ltP value for ZnO the fJ values are calculated to be about 498802860370 and 312 for the aligned ZnO NWs with five different densities of 108 86 64 45 and 28 NWs ~Lm-2 respectively

The FE properties of ZnO nanostructures can be further improved by post treatment such as thermal annealing plasma treatment gas exposure and UV irradiation Zhao et a1 [228] investigated the effects of post-thermal annealing of ZnO NRs in different ambients to optimize their FE behavior It is demonstrated that the FE propshyerties of the ZnO NRs are considerably improved after annealing in 02 but become worse after annealing in air or NH3 The Eta (defined as the E corresponding to the J of 01 A cm-2) of the as-grown ZnO NRs and the samples further annealed in 0Z air and NH3 at 500middotC for 1 hare 4441 85 and 88 V )Lm- I respectively The photolumishynescence and Raman spectra show that annealing in oxygen reduces the oxygen vacancy concentration improves the crystal quality lowers the work function and increases the

25

40 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

conductivity of the ZnO NRs thereby rendering improved FE properties However it has also been pointed that the FE current is suppressed after O

2 exposure [236] The FE

properties of the as-prepared 10 ZnO nanostructures can also be enhanced by further annealing under Hz atmosphere or by hydrogen plasma treatment [231 256] The enhaneed FE properties are ascribed to the improved electrical conductivity arising from hydrogen doping Moreover the FE current of 10 ZnO nanostructures can be influenced by UV illumination [236] Upon UV illumination the FE current from the ZnO NWs increases immediately and can eventually rises by about two orders of magnitude When the UV is turned off it takes much longer for the emisshysion current to fall to its original value which is owing to the change in the carrier concentration and tunneling width upon ionosorption of the respective gas molecules because photo excited electrons participate in the field-tunneling process The high FE current sensitivity of ZnO NWs to gas exposure and UV illumination can be utilized for new applishycations such as FE-based gas and UV sensors [236]

Tremendous progress has already been made pertaining to the FE properties and applications of ZnO nanostrucshytures in the past few years There have been over 100 papers published on this topic recently and Table 3 summarizes some important results reported on the FE properties of 1D ZnO nanostructures so far along with a brief descripshytion of the corresponding synthetic methods

32~2 ZnS Zinc sulfide (ZnS) has two types of crystal structures hexagshyonal wurtzite ZnS and cubic zinc bIen de ZnS having wide band gaps of372 and 377 eV respectively In comparison with the wurtzite structure the zinc bIen de structure is more stable at room temperature [257 258] ZnS is well known for its photoluminescence and electroluminescence propershyties and have many applications in flat-panel displays elecshytroluminescent devices infrared (rR) windows sensors and photocatalysts [259-261] Generally it is considered that a material with a lower (j) value can produce a higher electron emission current at a specific field Although the (j) value of ZnS (70 eV) is larger than that of Si (36 eV) SiC(40 eV) or ZnO (53 eV) it has been reported that the FE propershyties of 10 ZnS nanosiructures are comparable to or even better than those of many other 10 nanostructures because of their high aspect ratios and unique geometry structures

Fang et al [262] have synthesized ultrafine ZnS nanoshybelts by controlling the evaporation and agglomeration rates during synthesis using commercial ZnS C and S powders as precursors A typical width of the nanobelts is in the range of 5-30 nm and the mean is 10-20 nm The nanobelt thickness is likely to be only several nanometers The FE measurements show that is about 347 V Jlm- 1

and J is 115 rnA cm- 2 at a macroscopic field of 55 V Jlm-1bull

The f3 value is calculated to be over 2000 The excellent FE characteristics are attributed to the specific ultrashyfine nanobelt shapes ZnS nanobelt arrays have also been fabricated in situ on Zn metal foils by solvethermal reacshytion and subsequent heat treatment [263] These nanobelts are 300-500 nm wide up to 4 lID long and about 30 nm thick They are aligned in a dense array and approximately

perpendicular to the substrate surface When the distance between the anode and sample is 300 lm the measured E is 38 V lm-I

Such a low E to for ZnS nanobelt arrays can be assigned to the sharp corners and edges of the nanobelts suitable emitter density well-aligned morphologies and good electrical contact with the substrate

We have recently fabricated quasi-aligned ultrafine ZnS NW arrays on zinc substrates via a simple hydrothermal method [111 264] In a typical synthesis 038 g of thiourea and 024 g of sodium dodecyl sulfate (SOS) are added to a 40 ml solution composed ofethylenediamine (en) and deionshyized water (~O) with the volume ratio of 11 After stirring magnetically for 30 min a transparent mixture solution is obtained and then transferred to a 50 ml Teflon-lined stainshyless-steel autoclave The zinc foil is subsequently immersed in the solution and reacts for 10 h at 180degC Herein the Zn foil serves as both the Zn source and substrate and direct synthesis and assembly of ZnS NWs on an electrishycally conductive Zn substrate are accomplished in one step The SEM and TEM images and corresponding X-ray energy-dispersive spectroscopy (EDS) of the as-prepared product are shown in Figures 16(a)-16(c) revealing that quasi-aligned NW arrays are grown on the Zn foil with an ultrafine diameter of about 5-15 nm and length up to micrometers The HRTEM image (Fig 16(draquo discloses that the ZnS NWs are single crystalline with growth along the [0 0 1] direction Figure 16( e) depicts the plot of the current density (J) as a function of the applied electric field (E) According to the J-E plot E to is about 54 V Jlm-- 1bull

When the applied electric field is increased to 142 V Jlmt a current density of 1 rnA cm-z is obtained The linearity of the F-N curve within the measurement range demonstrated in the inset of Figure 16( e) suggests that electron emission from ZnS NW arrays follows the F-N behavior The ~ value calculated from a slope of the fitted straight line is about 5790 assuming that ifJ value is 70 eV for ZnS

Although the reported FE properties of 1D ZnS nanoshystructures are comparable to or even better than those of many other 10 nanostructures it should be emphasized that studies on the FE properties of 1D ZnS nanostructures have hitherto been rather limited Table 4 summarizes the recent work on the FE properties of ZnS nanobelts [262 263 265] NRs-NWs [264-269] and branched ZnS NT-In NW core-shell heterostructures [270]

323 CdS Cadmium sulfide (CdS) is a direct band-gap II-VI semiconshyductor having a band gap of 24 e V It is considered an excelshylent optoelectronic material because its band gap is in the visible region and has important applications in nonlinear optical devices flat panel displays light-emitting diodes (LEDs) photoelectrochemistry and so on [271 272] In the past few years various 1D CdS nanostructures and arrays such as NWs-NRs [273 274] nanobelts NTs [276] nanocombs [277] and CdS-C nanostructures [278] have been synthesized However there have been few reports on the FE properties of ID CdS nanostructures [279-282] although CdS has a low work function of 42 e V which is favorable for FE in comparison with ZnS (70 eV) and ZnO (53 eV)

41 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 3 ID ZnO nanostructures with various shapes and corresponding FE properties

Stability testing time ZnO emitters Synthesis method E to (V fim ) Eth (V flilll) and fluctuation Ref

lVWs

Random Thermal oxidation of brass alloy 25 NA 120 min lt6 [224]

Aligned Thermal evaporating 62 at 01 flA cm-2 15 at 1 mA cm-2 NA [225J

Aligned Olemical reaction in a dilute 2 at 01 rnA em-2 5atlmAcm-2 NA [226] solution

Aligned Solvothermal route 53 NA NA [227J

Random Thermal evaporation NA 646-1877 at 1 NA [231] mAcm-2

Aligned VLSgrowth 083 at 025 flA NA NA [232] cm-2

NWs grown on C cloth Carbon-thermal vapor transport 07 at 1 mAem 2 NA NA [233]

Aligned Solution-phase growth 71-167 13-275 NA [237]

Nanosheet Thermal oxidation of brass alloy 39 NA NA [238]

Nanoeomb Thermal oxidation of brass alloy 38 NJA NA [238]

Gallium-doped Vapor-phase transport 24 atO1 flAem-2 NA NA [240] nanofiber arrays

lvTs

Aligned Solution-based growth 7 at 01 flAcm- 2 NA 24h lt10 [241]

Aligned template-based CVD 73 at 01 flA cm- 2 NA NA [242]

Nanobelts

Aligned Thermal oxidation of brass alloy 23 NA NA [238]

Random Molten-salt-assisted thermal 13 29 NJA [243] evaporation

Nanonails Thermal evaporation 79 NA NA [244]

Nanopencils Thermal evaporation 72 NA NA [244]

Nanoneedle

Aligned Solution-phase growth 42 72 NA [249]

Aligned Template-free electrochemical 189 NA NA [250] deposition

Nanopins Vapor transport 192 at 01 flA cm-2 NA NA [251]

NCs Thermal evaporation 48 NA NA [253]

Nanoscrews Vapor-phase self-catalyst growth 36 NA 30 min lt 10 [254]

Tetrapod-like nanostructures

Vapor transport NA 115 at 18mAem-2 36 h lt2 [255]

Lin et al [280] have controllably synthesized CdS nanoparticles quasi-aligned NWs and well-aligned NWs on Si substrates via the noncatalytic and template-free metal~organic chemical vapor deposition (MOCVD) process by changing the reaction temperature from 350 to 5500c Cd(S2CNProP2)2 is used as the precursor To invesshytigate the effects of the morphologies of nanostructures on the FE properties the electron-emission properties of these nanostructures are measured with a 100 jJm gap between the anode and cathode Both quasi-aligned and well-aligned NWs exhibit FE characteristics with Eo (at the current density of 01 lA cm-2

) of 89 and 78 V JlIll-1 and Eth (at the current density of 1 rnA cm-2) of 144 and 122 V lID- respectively However no apparent FE is observed from the nanoparticle film sample The highEto of CdS NWs is ascribed to the thick CdS buffer layer between the roots of the NWs and substrate surface and thus the stronger electric fields are needed to push electrons through this buffer layer

We have recently reported the solvothermal fabrication and FE properties of CdS NW arrays on the Cd substrate without using a template and catalyst [110] Figure 17 displays the SEM (a b) TEM (c) and HRTEM (d) images of the synthesized CdS NW arrays on a Cd foil The diamshyeter of the CdS NWs is about 20-40 nm and they coalesce together at the ends The TEM image in Figure 17(c) reveals that the NWs retain the wire-like array geometry even after they have been scraped off from the substrate and have undergone ultrasonic treatment for 30 min suggesting that the as-grown NWs have good alignment The HRTEM and selected area electron diffraction (SAED) results shown in Figure 17( d) confirm that the NWs are single crystalline with growth along the [0 0 1] direction FE measurements are performed in order to study the electron-emission properties of the CdS NW arrays fabricated on Cd foils Figure 17 ( e) is the curve of current density J as a function of elecshytric filed E The F-N plot of In (1E2) vs 1E is shown in

42

~ ~ -8 0 --J 10 006008010012014018018020

- liE (mIV)

05

00

One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

(c) en

Zn S

Zn

IL Ao M ~~ 023 4 507 8 9

Energy (keV)

30-=-===-==--==-c=-----

---S5251~~~-1 (e)

sect -60 bull middot 20 10 middotmiddot middotE sect-7G ~ bullbull

o 15 1 -7$ ~

E (Vlj1m)

Figure 16 (a) SEM image (b) TEM image (e) EDS and (d) HRTEM image of the ultrafine ZnS NW arrays (e) FEJ-E curve and (f) corresponding F-N plot Inset in (a) is the high-magnification SEM image Reprinted with permission from [264] O X Qian et aI J Nanosci Nanotech 93347 (2009) copy 2009 American Scientific Publishers

Figure 17(f) The measured E to is about 134 V Jllll~l at a sample-anode distance of 200 j1m The good linearity in the F-N plot (Fig 17(f)) within the measurement range implies that electron emission from the CdS NW arrays follows the F-N behavior The f3 calculated from the slope of the fitted straight line is about 820 assuming that (Jgt value is 42 e V for CdS In comparison with the FE characteristics of CdS NW arrays reported in Ref [280] the f3 value of our CdS NW arrays is lower resulting in a little higher Eo It may be ascribed to the large screening effect owing to coalescence of the NWs at their ends Table 5 is the summary of the FE properties of 1D CdS nanostructures and their correshysponding synthesis methods reported so far

324 ZnSe Zinc selenide (ZnSe) has a room temperature bulk band gap of 270 eV and large exciton binding energy of 21 meV [283J Recently ZnSe has received much interest and is being

widely investigation for applications to LEDs laser diodes photodetectors and so on [284 285] However investigation of the FE properties of ZnSe is relatively scarce although it has a low work function of 484 [286] Zhao et al [287] studied the FE characteristics of ZnSe NB arrays grown directly on Zn foils at 140degC via a solvothermal process and subsequent heating in N at 300degC for -5 h The zinc foil and Se powder z served as the zinc and Se sources respectively Ethyleneshydiamine (en) is the solvent and NzH4

bull HzO is the reducing agent The FE measurements show that the E to of the ZnSe nanobelt arrays producing a current density of 01 j1A cm~2 is 50 V Jlml The calculated f3 value is of 1382 assuming that (Jgt value is 484 eV for ZnSe Moreover the stability examishynation discloses that the current fluctuation with time at a fIXed voltage (500 V~l kV dc mode) is as low as plusmn8 at least during the first 30 min though high-voltage (2 kV) FE can damage some areas in the sample These results demonstrate that lD ZnSe nanostructures are good candidates for elecshytron emitters and have potential applications in FE devices

43 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 4 FE properties of representative lD ZnS nanostructures

Stability testing time ZnS emitters Synthesis method E

to (V lID-I) (VIID-I) and fluctuation Ref

Nanobelts

Random Thermal evaporation 347 55 at 115 rnA cm-2 NA [262]

Aligned Solvothermal reaction 38 NA NA [263]

Random Thennalevaporation 355 NA 6h lt3 [265]

NRsINWs

Aligned Solvothennal reaction 54 NA NA [264]

Aligned Thermal evaporation 541 at 01 fJA cm-2 NA NA [266]

Aligned Epitaxially grown on ZnP 2

crystals 372 NA NA [267]

Random Vapor-phase deposition NA 117 at 1 rnA cm-2 NA [268]

Random Radio frequency magnetron

sputtering technique 29-63 at 245 fJA

cm-2 NA NA [269]

Branched ZnS NT-In NW core-shell Carbotherrnal CVD technique 543 745 at 11 rnA cm-2 NA [270] heterostructures

33 Group III-V Semiconductors

331 AIM Aluminum nitride (AlN) an important member of the group III-V compound semiconductors with the highest band gap of about 62 e V has excellent thermal conductivity good elecshytrical resistance low dielectric loss high piezoelectric response and ideal thermal expansion (matching that of silicon) AIN nanostructures especially 1D AlN nanostructures have been shown to be very promising field emitters because of their large aspect ratio excellent mechanical and thermal properties and very small ( lt025 e V) or even negative electron affinity [288 289] implying that electrons excited into the conduction band can be freely emitted into vacuum

Since AlN nanostructures are easily hydrolyzed and oxidized in an aqueous solution 1D AlN nanostructures are generally produced by vapor-phase growth mostly utilizing the direct reaction between Al and NH at high temperature Zhao et al [290] have fabricated AlN nanoneedle arrays with an average tip diameter of about 15 rIm on a Si substrate by the vaporshyphase reaction between AI from vaporized AI powders and NH3 at 875degC The emission current density (1) observed from the AlN nanoneedles is as high as 47 rnA cm-2 at the field of

Table 5 FE properties of ID CdS nanostructures

99 V 11m-I and theE to

defined as the field where the emission current density is distinguished from the background noise is about 31 V JUIl-1bull The estimated f3 value is 748 and mueh lower than the theoretical value of a single nanoneedle of 3 X 1()4 It is ascribed to the great field-screening effect Wu et al [48] synthesized AlN NWs by the extended VLS growth in which the reaction occurred between metallic AI and a mixture of NH3-N2 (NH3 4 vol ) in the presence of a small amount of Ni(N03)2 as the catalyst at llOOdegC The AlN NWs possess a hexagonal structure with an average diameter of around 15 nm and up to tens of microns long The FE properties of hshyAlN NWs are described in Figure 18 A film sample with an area of 02 cm2 (inset of Fig 18(araquo is first affixed to the surface of an oxygen-free high-conductivity copper electrode using silver paint and the transparent tin oxide film anode is used to observe emission of the materials As shown in 18(b) a rather small field of less than 1 V I-Lm-1 is needed to obtain a current density of 1 rnA cm-2

bull The straight line plot of log(JiE2) vs liE (Fig 18(c)) reveals that the FE from the AlN NWs follows the F-N behavior The good FE properties that is high current density under a small applied field arise from the rather small electron affinity of AIN and small diamshyeters of the ID NW geometry Tang et al [291] synthesized

Stability testing time CdS emitters Synthesis method E to (V 1-U111) Eth (V -Lm-I) and tluctuation Ref

NWs

Aligned Solvothermal 67 NA NA [110] Aligned Solvothennal 78 at 01 fJA cm-2 122 at 1 rnA cm 2 NA [279] Aligned MOCVD 134 NA NA [280] Algned BiI sheet-assisted NA 1057 at 1 rnA cm-2 NA [281]

solution-phase growth

NTs core-sheath NWs and Thennal evaporation 14 NA NA [282] tube-wire nanojunctions

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

40 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

conductivity of the ZnO NRs thereby rendering improved FE properties However it has also been pointed that the FE current is suppressed after O

2 exposure [236] The FE

properties of the as-prepared 10 ZnO nanostructures can also be enhanced by further annealing under Hz atmosphere or by hydrogen plasma treatment [231 256] The enhaneed FE properties are ascribed to the improved electrical conductivity arising from hydrogen doping Moreover the FE current of 10 ZnO nanostructures can be influenced by UV illumination [236] Upon UV illumination the FE current from the ZnO NWs increases immediately and can eventually rises by about two orders of magnitude When the UV is turned off it takes much longer for the emisshysion current to fall to its original value which is owing to the change in the carrier concentration and tunneling width upon ionosorption of the respective gas molecules because photo excited electrons participate in the field-tunneling process The high FE current sensitivity of ZnO NWs to gas exposure and UV illumination can be utilized for new applishycations such as FE-based gas and UV sensors [236]

Tremendous progress has already been made pertaining to the FE properties and applications of ZnO nanostrucshytures in the past few years There have been over 100 papers published on this topic recently and Table 3 summarizes some important results reported on the FE properties of 1D ZnO nanostructures so far along with a brief descripshytion of the corresponding synthetic methods

32~2 ZnS Zinc sulfide (ZnS) has two types of crystal structures hexagshyonal wurtzite ZnS and cubic zinc bIen de ZnS having wide band gaps of372 and 377 eV respectively In comparison with the wurtzite structure the zinc bIen de structure is more stable at room temperature [257 258] ZnS is well known for its photoluminescence and electroluminescence propershyties and have many applications in flat-panel displays elecshytroluminescent devices infrared (rR) windows sensors and photocatalysts [259-261] Generally it is considered that a material with a lower (j) value can produce a higher electron emission current at a specific field Although the (j) value of ZnS (70 eV) is larger than that of Si (36 eV) SiC(40 eV) or ZnO (53 eV) it has been reported that the FE propershyties of 10 ZnS nanosiructures are comparable to or even better than those of many other 10 nanostructures because of their high aspect ratios and unique geometry structures

Fang et al [262] have synthesized ultrafine ZnS nanoshybelts by controlling the evaporation and agglomeration rates during synthesis using commercial ZnS C and S powders as precursors A typical width of the nanobelts is in the range of 5-30 nm and the mean is 10-20 nm The nanobelt thickness is likely to be only several nanometers The FE measurements show that is about 347 V Jlm- 1

and J is 115 rnA cm- 2 at a macroscopic field of 55 V Jlm-1bull

The f3 value is calculated to be over 2000 The excellent FE characteristics are attributed to the specific ultrashyfine nanobelt shapes ZnS nanobelt arrays have also been fabricated in situ on Zn metal foils by solvethermal reacshytion and subsequent heat treatment [263] These nanobelts are 300-500 nm wide up to 4 lID long and about 30 nm thick They are aligned in a dense array and approximately

perpendicular to the substrate surface When the distance between the anode and sample is 300 lm the measured E is 38 V lm-I

Such a low E to for ZnS nanobelt arrays can be assigned to the sharp corners and edges of the nanobelts suitable emitter density well-aligned morphologies and good electrical contact with the substrate

We have recently fabricated quasi-aligned ultrafine ZnS NW arrays on zinc substrates via a simple hydrothermal method [111 264] In a typical synthesis 038 g of thiourea and 024 g of sodium dodecyl sulfate (SOS) are added to a 40 ml solution composed ofethylenediamine (en) and deionshyized water (~O) with the volume ratio of 11 After stirring magnetically for 30 min a transparent mixture solution is obtained and then transferred to a 50 ml Teflon-lined stainshyless-steel autoclave The zinc foil is subsequently immersed in the solution and reacts for 10 h at 180degC Herein the Zn foil serves as both the Zn source and substrate and direct synthesis and assembly of ZnS NWs on an electrishycally conductive Zn substrate are accomplished in one step The SEM and TEM images and corresponding X-ray energy-dispersive spectroscopy (EDS) of the as-prepared product are shown in Figures 16(a)-16(c) revealing that quasi-aligned NW arrays are grown on the Zn foil with an ultrafine diameter of about 5-15 nm and length up to micrometers The HRTEM image (Fig 16(draquo discloses that the ZnS NWs are single crystalline with growth along the [0 0 1] direction Figure 16( e) depicts the plot of the current density (J) as a function of the applied electric field (E) According to the J-E plot E to is about 54 V Jlm-- 1bull

When the applied electric field is increased to 142 V Jlmt a current density of 1 rnA cm-z is obtained The linearity of the F-N curve within the measurement range demonstrated in the inset of Figure 16( e) suggests that electron emission from ZnS NW arrays follows the F-N behavior The ~ value calculated from a slope of the fitted straight line is about 5790 assuming that ifJ value is 70 eV for ZnS

Although the reported FE properties of 1D ZnS nanoshystructures are comparable to or even better than those of many other 10 nanostructures it should be emphasized that studies on the FE properties of 1D ZnS nanostructures have hitherto been rather limited Table 4 summarizes the recent work on the FE properties of ZnS nanobelts [262 263 265] NRs-NWs [264-269] and branched ZnS NT-In NW core-shell heterostructures [270]

323 CdS Cadmium sulfide (CdS) is a direct band-gap II-VI semiconshyductor having a band gap of 24 e V It is considered an excelshylent optoelectronic material because its band gap is in the visible region and has important applications in nonlinear optical devices flat panel displays light-emitting diodes (LEDs) photoelectrochemistry and so on [271 272] In the past few years various 1D CdS nanostructures and arrays such as NWs-NRs [273 274] nanobelts NTs [276] nanocombs [277] and CdS-C nanostructures [278] have been synthesized However there have been few reports on the FE properties of ID CdS nanostructures [279-282] although CdS has a low work function of 42 e V which is favorable for FE in comparison with ZnS (70 eV) and ZnO (53 eV)

41 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 3 ID ZnO nanostructures with various shapes and corresponding FE properties

Stability testing time ZnO emitters Synthesis method E to (V fim ) Eth (V flilll) and fluctuation Ref

lVWs

Random Thermal oxidation of brass alloy 25 NA 120 min lt6 [224]

Aligned Thermal evaporating 62 at 01 flA cm-2 15 at 1 mA cm-2 NA [225J

Aligned Olemical reaction in a dilute 2 at 01 rnA em-2 5atlmAcm-2 NA [226] solution

Aligned Solvothermal route 53 NA NA [227J

Random Thermal evaporation NA 646-1877 at 1 NA [231] mAcm-2

Aligned VLSgrowth 083 at 025 flA NA NA [232] cm-2

NWs grown on C cloth Carbon-thermal vapor transport 07 at 1 mAem 2 NA NA [233]

Aligned Solution-phase growth 71-167 13-275 NA [237]

Nanosheet Thermal oxidation of brass alloy 39 NA NA [238]

Nanoeomb Thermal oxidation of brass alloy 38 NJA NA [238]

Gallium-doped Vapor-phase transport 24 atO1 flAem-2 NA NA [240] nanofiber arrays

lvTs

Aligned Solution-based growth 7 at 01 flAcm- 2 NA 24h lt10 [241]

Aligned template-based CVD 73 at 01 flA cm- 2 NA NA [242]

Nanobelts

Aligned Thermal oxidation of brass alloy 23 NA NA [238]

Random Molten-salt-assisted thermal 13 29 NJA [243] evaporation

Nanonails Thermal evaporation 79 NA NA [244]

Nanopencils Thermal evaporation 72 NA NA [244]

Nanoneedle

Aligned Solution-phase growth 42 72 NA [249]

Aligned Template-free electrochemical 189 NA NA [250] deposition

Nanopins Vapor transport 192 at 01 flA cm-2 NA NA [251]

NCs Thermal evaporation 48 NA NA [253]

Nanoscrews Vapor-phase self-catalyst growth 36 NA 30 min lt 10 [254]

Tetrapod-like nanostructures

Vapor transport NA 115 at 18mAem-2 36 h lt2 [255]

Lin et al [280] have controllably synthesized CdS nanoparticles quasi-aligned NWs and well-aligned NWs on Si substrates via the noncatalytic and template-free metal~organic chemical vapor deposition (MOCVD) process by changing the reaction temperature from 350 to 5500c Cd(S2CNProP2)2 is used as the precursor To invesshytigate the effects of the morphologies of nanostructures on the FE properties the electron-emission properties of these nanostructures are measured with a 100 jJm gap between the anode and cathode Both quasi-aligned and well-aligned NWs exhibit FE characteristics with Eo (at the current density of 01 lA cm-2

) of 89 and 78 V JlIll-1 and Eth (at the current density of 1 rnA cm-2) of 144 and 122 V lID- respectively However no apparent FE is observed from the nanoparticle film sample The highEto of CdS NWs is ascribed to the thick CdS buffer layer between the roots of the NWs and substrate surface and thus the stronger electric fields are needed to push electrons through this buffer layer

We have recently reported the solvothermal fabrication and FE properties of CdS NW arrays on the Cd substrate without using a template and catalyst [110] Figure 17 displays the SEM (a b) TEM (c) and HRTEM (d) images of the synthesized CdS NW arrays on a Cd foil The diamshyeter of the CdS NWs is about 20-40 nm and they coalesce together at the ends The TEM image in Figure 17(c) reveals that the NWs retain the wire-like array geometry even after they have been scraped off from the substrate and have undergone ultrasonic treatment for 30 min suggesting that the as-grown NWs have good alignment The HRTEM and selected area electron diffraction (SAED) results shown in Figure 17( d) confirm that the NWs are single crystalline with growth along the [0 0 1] direction FE measurements are performed in order to study the electron-emission properties of the CdS NW arrays fabricated on Cd foils Figure 17 ( e) is the curve of current density J as a function of elecshytric filed E The F-N plot of In (1E2) vs 1E is shown in

42

~ ~ -8 0 --J 10 006008010012014018018020

- liE (mIV)

05

00

One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

(c) en

Zn S

Zn

IL Ao M ~~ 023 4 507 8 9

Energy (keV)

30-=-===-==--==-c=-----

---S5251~~~-1 (e)

sect -60 bull middot 20 10 middotmiddot middotE sect-7G ~ bullbull

o 15 1 -7$ ~

E (Vlj1m)

Figure 16 (a) SEM image (b) TEM image (e) EDS and (d) HRTEM image of the ultrafine ZnS NW arrays (e) FEJ-E curve and (f) corresponding F-N plot Inset in (a) is the high-magnification SEM image Reprinted with permission from [264] O X Qian et aI J Nanosci Nanotech 93347 (2009) copy 2009 American Scientific Publishers

Figure 17(f) The measured E to is about 134 V Jllll~l at a sample-anode distance of 200 j1m The good linearity in the F-N plot (Fig 17(f)) within the measurement range implies that electron emission from the CdS NW arrays follows the F-N behavior The f3 calculated from the slope of the fitted straight line is about 820 assuming that (Jgt value is 42 e V for CdS In comparison with the FE characteristics of CdS NW arrays reported in Ref [280] the f3 value of our CdS NW arrays is lower resulting in a little higher Eo It may be ascribed to the large screening effect owing to coalescence of the NWs at their ends Table 5 is the summary of the FE properties of 1D CdS nanostructures and their correshysponding synthesis methods reported so far

324 ZnSe Zinc selenide (ZnSe) has a room temperature bulk band gap of 270 eV and large exciton binding energy of 21 meV [283J Recently ZnSe has received much interest and is being

widely investigation for applications to LEDs laser diodes photodetectors and so on [284 285] However investigation of the FE properties of ZnSe is relatively scarce although it has a low work function of 484 [286] Zhao et al [287] studied the FE characteristics of ZnSe NB arrays grown directly on Zn foils at 140degC via a solvothermal process and subsequent heating in N at 300degC for -5 h The zinc foil and Se powder z served as the zinc and Se sources respectively Ethyleneshydiamine (en) is the solvent and NzH4

bull HzO is the reducing agent The FE measurements show that the E to of the ZnSe nanobelt arrays producing a current density of 01 j1A cm~2 is 50 V Jlml The calculated f3 value is of 1382 assuming that (Jgt value is 484 eV for ZnSe Moreover the stability examishynation discloses that the current fluctuation with time at a fIXed voltage (500 V~l kV dc mode) is as low as plusmn8 at least during the first 30 min though high-voltage (2 kV) FE can damage some areas in the sample These results demonstrate that lD ZnSe nanostructures are good candidates for elecshytron emitters and have potential applications in FE devices

43 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 4 FE properties of representative lD ZnS nanostructures

Stability testing time ZnS emitters Synthesis method E

to (V lID-I) (VIID-I) and fluctuation Ref

Nanobelts

Random Thermal evaporation 347 55 at 115 rnA cm-2 NA [262]

Aligned Solvothermal reaction 38 NA NA [263]

Random Thennalevaporation 355 NA 6h lt3 [265]

NRsINWs

Aligned Solvothennal reaction 54 NA NA [264]

Aligned Thermal evaporation 541 at 01 fJA cm-2 NA NA [266]

Aligned Epitaxially grown on ZnP 2

crystals 372 NA NA [267]

Random Vapor-phase deposition NA 117 at 1 rnA cm-2 NA [268]

Random Radio frequency magnetron

sputtering technique 29-63 at 245 fJA

cm-2 NA NA [269]

Branched ZnS NT-In NW core-shell Carbotherrnal CVD technique 543 745 at 11 rnA cm-2 NA [270] heterostructures

33 Group III-V Semiconductors

331 AIM Aluminum nitride (AlN) an important member of the group III-V compound semiconductors with the highest band gap of about 62 e V has excellent thermal conductivity good elecshytrical resistance low dielectric loss high piezoelectric response and ideal thermal expansion (matching that of silicon) AIN nanostructures especially 1D AlN nanostructures have been shown to be very promising field emitters because of their large aspect ratio excellent mechanical and thermal properties and very small ( lt025 e V) or even negative electron affinity [288 289] implying that electrons excited into the conduction band can be freely emitted into vacuum

Since AlN nanostructures are easily hydrolyzed and oxidized in an aqueous solution 1D AlN nanostructures are generally produced by vapor-phase growth mostly utilizing the direct reaction between Al and NH at high temperature Zhao et al [290] have fabricated AlN nanoneedle arrays with an average tip diameter of about 15 rIm on a Si substrate by the vaporshyphase reaction between AI from vaporized AI powders and NH3 at 875degC The emission current density (1) observed from the AlN nanoneedles is as high as 47 rnA cm-2 at the field of

Table 5 FE properties of ID CdS nanostructures

99 V 11m-I and theE to

defined as the field where the emission current density is distinguished from the background noise is about 31 V JUIl-1bull The estimated f3 value is 748 and mueh lower than the theoretical value of a single nanoneedle of 3 X 1()4 It is ascribed to the great field-screening effect Wu et al [48] synthesized AlN NWs by the extended VLS growth in which the reaction occurred between metallic AI and a mixture of NH3-N2 (NH3 4 vol ) in the presence of a small amount of Ni(N03)2 as the catalyst at llOOdegC The AlN NWs possess a hexagonal structure with an average diameter of around 15 nm and up to tens of microns long The FE properties of hshyAlN NWs are described in Figure 18 A film sample with an area of 02 cm2 (inset of Fig 18(araquo is first affixed to the surface of an oxygen-free high-conductivity copper electrode using silver paint and the transparent tin oxide film anode is used to observe emission of the materials As shown in 18(b) a rather small field of less than 1 V I-Lm-1 is needed to obtain a current density of 1 rnA cm-2

bull The straight line plot of log(JiE2) vs liE (Fig 18(c)) reveals that the FE from the AlN NWs follows the F-N behavior The good FE properties that is high current density under a small applied field arise from the rather small electron affinity of AIN and small diamshyeters of the ID NW geometry Tang et al [291] synthesized

Stability testing time CdS emitters Synthesis method E to (V 1-U111) Eth (V -Lm-I) and tluctuation Ref

NWs

Aligned Solvothermal 67 NA NA [110] Aligned Solvothennal 78 at 01 fJA cm-2 122 at 1 rnA cm 2 NA [279] Aligned MOCVD 134 NA NA [280] Algned BiI sheet-assisted NA 1057 at 1 rnA cm-2 NA [281]

solution-phase growth

NTs core-sheath NWs and Thennal evaporation 14 NA NA [282] tube-wire nanojunctions

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

41 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 3 ID ZnO nanostructures with various shapes and corresponding FE properties

Stability testing time ZnO emitters Synthesis method E to (V fim ) Eth (V flilll) and fluctuation Ref

lVWs

Random Thermal oxidation of brass alloy 25 NA 120 min lt6 [224]

Aligned Thermal evaporating 62 at 01 flA cm-2 15 at 1 mA cm-2 NA [225J

Aligned Olemical reaction in a dilute 2 at 01 rnA em-2 5atlmAcm-2 NA [226] solution

Aligned Solvothermal route 53 NA NA [227J

Random Thermal evaporation NA 646-1877 at 1 NA [231] mAcm-2

Aligned VLSgrowth 083 at 025 flA NA NA [232] cm-2

NWs grown on C cloth Carbon-thermal vapor transport 07 at 1 mAem 2 NA NA [233]

Aligned Solution-phase growth 71-167 13-275 NA [237]

Nanosheet Thermal oxidation of brass alloy 39 NA NA [238]

Nanoeomb Thermal oxidation of brass alloy 38 NJA NA [238]

Gallium-doped Vapor-phase transport 24 atO1 flAem-2 NA NA [240] nanofiber arrays

lvTs

Aligned Solution-based growth 7 at 01 flAcm- 2 NA 24h lt10 [241]

Aligned template-based CVD 73 at 01 flA cm- 2 NA NA [242]

Nanobelts

Aligned Thermal oxidation of brass alloy 23 NA NA [238]

Random Molten-salt-assisted thermal 13 29 NJA [243] evaporation

Nanonails Thermal evaporation 79 NA NA [244]

Nanopencils Thermal evaporation 72 NA NA [244]

Nanoneedle

Aligned Solution-phase growth 42 72 NA [249]

Aligned Template-free electrochemical 189 NA NA [250] deposition

Nanopins Vapor transport 192 at 01 flA cm-2 NA NA [251]

NCs Thermal evaporation 48 NA NA [253]

Nanoscrews Vapor-phase self-catalyst growth 36 NA 30 min lt 10 [254]

Tetrapod-like nanostructures

Vapor transport NA 115 at 18mAem-2 36 h lt2 [255]

Lin et al [280] have controllably synthesized CdS nanoparticles quasi-aligned NWs and well-aligned NWs on Si substrates via the noncatalytic and template-free metal~organic chemical vapor deposition (MOCVD) process by changing the reaction temperature from 350 to 5500c Cd(S2CNProP2)2 is used as the precursor To invesshytigate the effects of the morphologies of nanostructures on the FE properties the electron-emission properties of these nanostructures are measured with a 100 jJm gap between the anode and cathode Both quasi-aligned and well-aligned NWs exhibit FE characteristics with Eo (at the current density of 01 lA cm-2

) of 89 and 78 V JlIll-1 and Eth (at the current density of 1 rnA cm-2) of 144 and 122 V lID- respectively However no apparent FE is observed from the nanoparticle film sample The highEto of CdS NWs is ascribed to the thick CdS buffer layer between the roots of the NWs and substrate surface and thus the stronger electric fields are needed to push electrons through this buffer layer

We have recently reported the solvothermal fabrication and FE properties of CdS NW arrays on the Cd substrate without using a template and catalyst [110] Figure 17 displays the SEM (a b) TEM (c) and HRTEM (d) images of the synthesized CdS NW arrays on a Cd foil The diamshyeter of the CdS NWs is about 20-40 nm and they coalesce together at the ends The TEM image in Figure 17(c) reveals that the NWs retain the wire-like array geometry even after they have been scraped off from the substrate and have undergone ultrasonic treatment for 30 min suggesting that the as-grown NWs have good alignment The HRTEM and selected area electron diffraction (SAED) results shown in Figure 17( d) confirm that the NWs are single crystalline with growth along the [0 0 1] direction FE measurements are performed in order to study the electron-emission properties of the CdS NW arrays fabricated on Cd foils Figure 17 ( e) is the curve of current density J as a function of elecshytric filed E The F-N plot of In (1E2) vs 1E is shown in

42

~ ~ -8 0 --J 10 006008010012014018018020

- liE (mIV)

05

00

One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

(c) en

Zn S

Zn

IL Ao M ~~ 023 4 507 8 9

Energy (keV)

30-=-===-==--==-c=-----

---S5251~~~-1 (e)

sect -60 bull middot 20 10 middotmiddot middotE sect-7G ~ bullbull

o 15 1 -7$ ~

E (Vlj1m)

Figure 16 (a) SEM image (b) TEM image (e) EDS and (d) HRTEM image of the ultrafine ZnS NW arrays (e) FEJ-E curve and (f) corresponding F-N plot Inset in (a) is the high-magnification SEM image Reprinted with permission from [264] O X Qian et aI J Nanosci Nanotech 93347 (2009) copy 2009 American Scientific Publishers

Figure 17(f) The measured E to is about 134 V Jllll~l at a sample-anode distance of 200 j1m The good linearity in the F-N plot (Fig 17(f)) within the measurement range implies that electron emission from the CdS NW arrays follows the F-N behavior The f3 calculated from the slope of the fitted straight line is about 820 assuming that (Jgt value is 42 e V for CdS In comparison with the FE characteristics of CdS NW arrays reported in Ref [280] the f3 value of our CdS NW arrays is lower resulting in a little higher Eo It may be ascribed to the large screening effect owing to coalescence of the NWs at their ends Table 5 is the summary of the FE properties of 1D CdS nanostructures and their correshysponding synthesis methods reported so far

324 ZnSe Zinc selenide (ZnSe) has a room temperature bulk band gap of 270 eV and large exciton binding energy of 21 meV [283J Recently ZnSe has received much interest and is being

widely investigation for applications to LEDs laser diodes photodetectors and so on [284 285] However investigation of the FE properties of ZnSe is relatively scarce although it has a low work function of 484 [286] Zhao et al [287] studied the FE characteristics of ZnSe NB arrays grown directly on Zn foils at 140degC via a solvothermal process and subsequent heating in N at 300degC for -5 h The zinc foil and Se powder z served as the zinc and Se sources respectively Ethyleneshydiamine (en) is the solvent and NzH4

bull HzO is the reducing agent The FE measurements show that the E to of the ZnSe nanobelt arrays producing a current density of 01 j1A cm~2 is 50 V Jlml The calculated f3 value is of 1382 assuming that (Jgt value is 484 eV for ZnSe Moreover the stability examishynation discloses that the current fluctuation with time at a fIXed voltage (500 V~l kV dc mode) is as low as plusmn8 at least during the first 30 min though high-voltage (2 kV) FE can damage some areas in the sample These results demonstrate that lD ZnSe nanostructures are good candidates for elecshytron emitters and have potential applications in FE devices

43 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 4 FE properties of representative lD ZnS nanostructures

Stability testing time ZnS emitters Synthesis method E

to (V lID-I) (VIID-I) and fluctuation Ref

Nanobelts

Random Thermal evaporation 347 55 at 115 rnA cm-2 NA [262]

Aligned Solvothermal reaction 38 NA NA [263]

Random Thennalevaporation 355 NA 6h lt3 [265]

NRsINWs

Aligned Solvothennal reaction 54 NA NA [264]

Aligned Thermal evaporation 541 at 01 fJA cm-2 NA NA [266]

Aligned Epitaxially grown on ZnP 2

crystals 372 NA NA [267]

Random Vapor-phase deposition NA 117 at 1 rnA cm-2 NA [268]

Random Radio frequency magnetron

sputtering technique 29-63 at 245 fJA

cm-2 NA NA [269]

Branched ZnS NT-In NW core-shell Carbotherrnal CVD technique 543 745 at 11 rnA cm-2 NA [270] heterostructures

33 Group III-V Semiconductors

331 AIM Aluminum nitride (AlN) an important member of the group III-V compound semiconductors with the highest band gap of about 62 e V has excellent thermal conductivity good elecshytrical resistance low dielectric loss high piezoelectric response and ideal thermal expansion (matching that of silicon) AIN nanostructures especially 1D AlN nanostructures have been shown to be very promising field emitters because of their large aspect ratio excellent mechanical and thermal properties and very small ( lt025 e V) or even negative electron affinity [288 289] implying that electrons excited into the conduction band can be freely emitted into vacuum

Since AlN nanostructures are easily hydrolyzed and oxidized in an aqueous solution 1D AlN nanostructures are generally produced by vapor-phase growth mostly utilizing the direct reaction between Al and NH at high temperature Zhao et al [290] have fabricated AlN nanoneedle arrays with an average tip diameter of about 15 rIm on a Si substrate by the vaporshyphase reaction between AI from vaporized AI powders and NH3 at 875degC The emission current density (1) observed from the AlN nanoneedles is as high as 47 rnA cm-2 at the field of

Table 5 FE properties of ID CdS nanostructures

99 V 11m-I and theE to

defined as the field where the emission current density is distinguished from the background noise is about 31 V JUIl-1bull The estimated f3 value is 748 and mueh lower than the theoretical value of a single nanoneedle of 3 X 1()4 It is ascribed to the great field-screening effect Wu et al [48] synthesized AlN NWs by the extended VLS growth in which the reaction occurred between metallic AI and a mixture of NH3-N2 (NH3 4 vol ) in the presence of a small amount of Ni(N03)2 as the catalyst at llOOdegC The AlN NWs possess a hexagonal structure with an average diameter of around 15 nm and up to tens of microns long The FE properties of hshyAlN NWs are described in Figure 18 A film sample with an area of 02 cm2 (inset of Fig 18(araquo is first affixed to the surface of an oxygen-free high-conductivity copper electrode using silver paint and the transparent tin oxide film anode is used to observe emission of the materials As shown in 18(b) a rather small field of less than 1 V I-Lm-1 is needed to obtain a current density of 1 rnA cm-2

bull The straight line plot of log(JiE2) vs liE (Fig 18(c)) reveals that the FE from the AlN NWs follows the F-N behavior The good FE properties that is high current density under a small applied field arise from the rather small electron affinity of AIN and small diamshyeters of the ID NW geometry Tang et al [291] synthesized

Stability testing time CdS emitters Synthesis method E to (V 1-U111) Eth (V -Lm-I) and tluctuation Ref

NWs

Aligned Solvothermal 67 NA NA [110] Aligned Solvothennal 78 at 01 fJA cm-2 122 at 1 rnA cm 2 NA [279] Aligned MOCVD 134 NA NA [280] Algned BiI sheet-assisted NA 1057 at 1 rnA cm-2 NA [281]

solution-phase growth

NTs core-sheath NWs and Thennal evaporation 14 NA NA [282] tube-wire nanojunctions

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

42

~ ~ -8 0 --J 10 006008010012014018018020

- liE (mIV)

05

00

One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

(c) en

Zn S

Zn

IL Ao M ~~ 023 4 507 8 9

Energy (keV)

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---S5251~~~-1 (e)

sect -60 bull middot 20 10 middotmiddot middotE sect-7G ~ bullbull

o 15 1 -7$ ~

E (Vlj1m)

Figure 16 (a) SEM image (b) TEM image (e) EDS and (d) HRTEM image of the ultrafine ZnS NW arrays (e) FEJ-E curve and (f) corresponding F-N plot Inset in (a) is the high-magnification SEM image Reprinted with permission from [264] O X Qian et aI J Nanosci Nanotech 93347 (2009) copy 2009 American Scientific Publishers

Figure 17(f) The measured E to is about 134 V Jllll~l at a sample-anode distance of 200 j1m The good linearity in the F-N plot (Fig 17(f)) within the measurement range implies that electron emission from the CdS NW arrays follows the F-N behavior The f3 calculated from the slope of the fitted straight line is about 820 assuming that (Jgt value is 42 e V for CdS In comparison with the FE characteristics of CdS NW arrays reported in Ref [280] the f3 value of our CdS NW arrays is lower resulting in a little higher Eo It may be ascribed to the large screening effect owing to coalescence of the NWs at their ends Table 5 is the summary of the FE properties of 1D CdS nanostructures and their correshysponding synthesis methods reported so far

324 ZnSe Zinc selenide (ZnSe) has a room temperature bulk band gap of 270 eV and large exciton binding energy of 21 meV [283J Recently ZnSe has received much interest and is being

widely investigation for applications to LEDs laser diodes photodetectors and so on [284 285] However investigation of the FE properties of ZnSe is relatively scarce although it has a low work function of 484 [286] Zhao et al [287] studied the FE characteristics of ZnSe NB arrays grown directly on Zn foils at 140degC via a solvothermal process and subsequent heating in N at 300degC for -5 h The zinc foil and Se powder z served as the zinc and Se sources respectively Ethyleneshydiamine (en) is the solvent and NzH4

bull HzO is the reducing agent The FE measurements show that the E to of the ZnSe nanobelt arrays producing a current density of 01 j1A cm~2 is 50 V Jlml The calculated f3 value is of 1382 assuming that (Jgt value is 484 eV for ZnSe Moreover the stability examishynation discloses that the current fluctuation with time at a fIXed voltage (500 V~l kV dc mode) is as low as plusmn8 at least during the first 30 min though high-voltage (2 kV) FE can damage some areas in the sample These results demonstrate that lD ZnSe nanostructures are good candidates for elecshytron emitters and have potential applications in FE devices

43 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 4 FE properties of representative lD ZnS nanostructures

Stability testing time ZnS emitters Synthesis method E

to (V lID-I) (VIID-I) and fluctuation Ref

Nanobelts

Random Thermal evaporation 347 55 at 115 rnA cm-2 NA [262]

Aligned Solvothermal reaction 38 NA NA [263]

Random Thennalevaporation 355 NA 6h lt3 [265]

NRsINWs

Aligned Solvothennal reaction 54 NA NA [264]

Aligned Thermal evaporation 541 at 01 fJA cm-2 NA NA [266]

Aligned Epitaxially grown on ZnP 2

crystals 372 NA NA [267]

Random Vapor-phase deposition NA 117 at 1 rnA cm-2 NA [268]

Random Radio frequency magnetron

sputtering technique 29-63 at 245 fJA

cm-2 NA NA [269]

Branched ZnS NT-In NW core-shell Carbotherrnal CVD technique 543 745 at 11 rnA cm-2 NA [270] heterostructures

33 Group III-V Semiconductors

331 AIM Aluminum nitride (AlN) an important member of the group III-V compound semiconductors with the highest band gap of about 62 e V has excellent thermal conductivity good elecshytrical resistance low dielectric loss high piezoelectric response and ideal thermal expansion (matching that of silicon) AIN nanostructures especially 1D AlN nanostructures have been shown to be very promising field emitters because of their large aspect ratio excellent mechanical and thermal properties and very small ( lt025 e V) or even negative electron affinity [288 289] implying that electrons excited into the conduction band can be freely emitted into vacuum

Since AlN nanostructures are easily hydrolyzed and oxidized in an aqueous solution 1D AlN nanostructures are generally produced by vapor-phase growth mostly utilizing the direct reaction between Al and NH at high temperature Zhao et al [290] have fabricated AlN nanoneedle arrays with an average tip diameter of about 15 rIm on a Si substrate by the vaporshyphase reaction between AI from vaporized AI powders and NH3 at 875degC The emission current density (1) observed from the AlN nanoneedles is as high as 47 rnA cm-2 at the field of

Table 5 FE properties of ID CdS nanostructures

99 V 11m-I and theE to

defined as the field where the emission current density is distinguished from the background noise is about 31 V JUIl-1bull The estimated f3 value is 748 and mueh lower than the theoretical value of a single nanoneedle of 3 X 1()4 It is ascribed to the great field-screening effect Wu et al [48] synthesized AlN NWs by the extended VLS growth in which the reaction occurred between metallic AI and a mixture of NH3-N2 (NH3 4 vol ) in the presence of a small amount of Ni(N03)2 as the catalyst at llOOdegC The AlN NWs possess a hexagonal structure with an average diameter of around 15 nm and up to tens of microns long The FE properties of hshyAlN NWs are described in Figure 18 A film sample with an area of 02 cm2 (inset of Fig 18(araquo is first affixed to the surface of an oxygen-free high-conductivity copper electrode using silver paint and the transparent tin oxide film anode is used to observe emission of the materials As shown in 18(b) a rather small field of less than 1 V I-Lm-1 is needed to obtain a current density of 1 rnA cm-2

bull The straight line plot of log(JiE2) vs liE (Fig 18(c)) reveals that the FE from the AlN NWs follows the F-N behavior The good FE properties that is high current density under a small applied field arise from the rather small electron affinity of AIN and small diamshyeters of the ID NW geometry Tang et al [291] synthesized

Stability testing time CdS emitters Synthesis method E to (V 1-U111) Eth (V -Lm-I) and tluctuation Ref

NWs

Aligned Solvothermal 67 NA NA [110] Aligned Solvothennal 78 at 01 fJA cm-2 122 at 1 rnA cm 2 NA [279] Aligned MOCVD 134 NA NA [280] Algned BiI sheet-assisted NA 1057 at 1 rnA cm-2 NA [281]

solution-phase growth

NTs core-sheath NWs and Thennal evaporation 14 NA NA [282] tube-wire nanojunctions

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

43 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 4 FE properties of representative lD ZnS nanostructures

Stability testing time ZnS emitters Synthesis method E

to (V lID-I) (VIID-I) and fluctuation Ref

Nanobelts

Random Thermal evaporation 347 55 at 115 rnA cm-2 NA [262]

Aligned Solvothermal reaction 38 NA NA [263]

Random Thennalevaporation 355 NA 6h lt3 [265]

NRsINWs

Aligned Solvothennal reaction 54 NA NA [264]

Aligned Thermal evaporation 541 at 01 fJA cm-2 NA NA [266]

Aligned Epitaxially grown on ZnP 2

crystals 372 NA NA [267]

Random Vapor-phase deposition NA 117 at 1 rnA cm-2 NA [268]

Random Radio frequency magnetron

sputtering technique 29-63 at 245 fJA

cm-2 NA NA [269]

Branched ZnS NT-In NW core-shell Carbotherrnal CVD technique 543 745 at 11 rnA cm-2 NA [270] heterostructures

33 Group III-V Semiconductors

331 AIM Aluminum nitride (AlN) an important member of the group III-V compound semiconductors with the highest band gap of about 62 e V has excellent thermal conductivity good elecshytrical resistance low dielectric loss high piezoelectric response and ideal thermal expansion (matching that of silicon) AIN nanostructures especially 1D AlN nanostructures have been shown to be very promising field emitters because of their large aspect ratio excellent mechanical and thermal properties and very small ( lt025 e V) or even negative electron affinity [288 289] implying that electrons excited into the conduction band can be freely emitted into vacuum

Since AlN nanostructures are easily hydrolyzed and oxidized in an aqueous solution 1D AlN nanostructures are generally produced by vapor-phase growth mostly utilizing the direct reaction between Al and NH at high temperature Zhao et al [290] have fabricated AlN nanoneedle arrays with an average tip diameter of about 15 rIm on a Si substrate by the vaporshyphase reaction between AI from vaporized AI powders and NH3 at 875degC The emission current density (1) observed from the AlN nanoneedles is as high as 47 rnA cm-2 at the field of

Table 5 FE properties of ID CdS nanostructures

99 V 11m-I and theE to

defined as the field where the emission current density is distinguished from the background noise is about 31 V JUIl-1bull The estimated f3 value is 748 and mueh lower than the theoretical value of a single nanoneedle of 3 X 1()4 It is ascribed to the great field-screening effect Wu et al [48] synthesized AlN NWs by the extended VLS growth in which the reaction occurred between metallic AI and a mixture of NH3-N2 (NH3 4 vol ) in the presence of a small amount of Ni(N03)2 as the catalyst at llOOdegC The AlN NWs possess a hexagonal structure with an average diameter of around 15 nm and up to tens of microns long The FE properties of hshyAlN NWs are described in Figure 18 A film sample with an area of 02 cm2 (inset of Fig 18(araquo is first affixed to the surface of an oxygen-free high-conductivity copper electrode using silver paint and the transparent tin oxide film anode is used to observe emission of the materials As shown in 18(b) a rather small field of less than 1 V I-Lm-1 is needed to obtain a current density of 1 rnA cm-2

bull The straight line plot of log(JiE2) vs liE (Fig 18(c)) reveals that the FE from the AlN NWs follows the F-N behavior The good FE properties that is high current density under a small applied field arise from the rather small electron affinity of AIN and small diamshyeters of the ID NW geometry Tang et al [291] synthesized

Stability testing time CdS emitters Synthesis method E to (V 1-U111) Eth (V -Lm-I) and tluctuation Ref

NWs

Aligned Solvothermal 67 NA NA [110] Aligned Solvothennal 78 at 01 fJA cm-2 122 at 1 rnA cm 2 NA [279] Aligned MOCVD 134 NA NA [280] Algned BiI sheet-assisted NA 1057 at 1 rnA cm-2 NA [281]

solution-phase growth

NTs core-sheath NWs and Thennal evaporation 14 NA NA [282] tube-wire nanojunctions

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

44 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

10

08

M

M 02

0middotdeg1 I

lIE (11mV)E(VIt=

Figure 17 (a) Low magnification of SEM images (b) high magnification of SEM images (c) TEM and (d) HRTEM images of CdS NW arrays (e and f) FE J-E curve and the corresponding F-N plot Insets in (d) are the corresponding ED pattern and HRTEM image Reprinted with permission from [1101 G X Qian et aI 1 Appl Phys 104014312 (2008) copy 2008 American Institute of Physics

AlN nanotip arrays by a CVD method at 700degC using a mixture of Al and FcZ0 3 nanoparticles under NHJ It is very interesting that the product shows an Eiffel-tower-shaped nanostructure morphology as shown in Figures 19( a) and 19(b) Independent of their size AlN nanotips are always composed of two parts a submicrometer-sized base and a sharp tip The base is a polygshyonal pyramid a few hundred nanometers in size and the sharp tip is several to tens of nanometers in diameter and -10 Jll11 in length The FEJ-E curve (Fig 19(c)) shows that theE

to andEtb

of the A IN nanotip arrays are 47 and 106 V JlITl-l respectively The low Ero and Elh originate from the single-crystalline strucshyture nanosize ofthe emitter and relatively low density of about 106-107 tips cm-2

bull The stability measurement reveals that the fluctuation in the FE current at a density of 10 rnA cm-2 for 4 h is as low as 074 suggesting that the nanotips have a high FE stability The submicrometer-sized base of the emitters is considered to be responsible for the good FE stability It has a large connecting area and can quickly transfer the heat from the tip to the Si substrate without subjecting the nanotips to overheating and destruction

AIN nanostructures could also be synthesized by replacing Al with AlCl

3 as the precursor to reduce the

synthesis temperature Song et al [292] fabricated hexagonal AlN NR and nanoneedle arrays on a silicon wafer by CVD at 750degC AICI powders and NH3 gas served as the aluminum and nitrogen sources respecshytively By increasing the NH3 flow rate an evolution from NRs to nanoneedles was observed The FE measurements show that AlN nanoneedle arrays have a much lower Eta of 31 V Ilm-l compared toE

to of153 V Ilm-lforNRarrays The

f1 values of the AlN NRs and nanoneedles are calculated to be about 191 and 1413 respectively The larger 3 value for the AIN nanoneedles is owing to their sharper tips which can generate a higher local electric field enhanceshyment at the sharpening tips of AIN nanoneedles Liu et al [293] synthesized aligned AIN NCs on Ni catalyst-coated Si wafers via the reactions between AlCl3 vapor and NH gas at around 700degC As shown in Figure 20(a) high densishyties of AIN NCs with lengths of several micrometers grow perpendicularly to form the quasi-arrays The TEM image of a typical AIN NC scraped from the wafer indicates that its diameter decreases along the growth direction with the tip size of 60 nm and length up to 25 11m (inset in Fig 20(araquo The FEJ-E curve and corresponding F-N plot

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

45 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

-104-----------r1=24 o O7mm e OSmm

-10620

~-108_ 16 -shyIN Ol ~-110~ 12

E-08

04

00 06 07 08 09 10 11 12 08 09 10 11 12 13 14 15 16

E (VJ~lm) 11E (JlmN)

Figure 18 (a) A typical spatial distribution of emission sites recorded from the h-AlN NWs (b and c) the J-E curves and the corresponding F-N plots ofthe AlN NWs with sample-anode distance d of 07 mm (0) and 08 mm (e) The inset in (a) shows the size of the sample Reprinted with permission from [48] Q Wu et aI J Mater Chern 132024 (2003) copy 2003 The Royal Society of Chemistry

are shown in Figure 20(b) It is noted that the E to of the AIN NCs is 12 V 11m-I The high E is considered to be caused by the high field-screening ~fIect and the silicon oxide film on the Si wafer formed in the synthesis process owing to the oxygen impurity The two-sectional feature of the F-N plot (Inset in Fig 20(b)) may result from the space charge effect [294]

Zhao et al [295] investigated the effects of thermal annealing on the FE properties of AlN NRs in a different way The experimental results suggested that FE properties of AlN NRs could be improved by post annealing in NH3 for 1 h at 875degC after the synthesis The threshold FE at a current density of 1 rnA cm-2 decreases from 252 V IIm-1

for as-grown AlN NRs to 212 V IIm-1 for NH3-annealed

samples It is suggested that the NH3 thermal-annealing process can improve the FE properties of AlN NRs owing to low nitrogen vacancy density and work function since the defects (the nitrogen vacancy) can act as the trap center for the electrons

To date various ID AlN nanostructures such as NWsshyNRs [48296297] NTs [298299] nanotips [291-301] NCs [302] nanoneedles [292 303] nanobelts [304 305] and hierarchical nanoarchitecture [306] have been synthesized and their excellent FE properties have been disclosed A summary can be found in Table 6

10 (c)

10

10lt 10-5 = 10)f c ~ a 10 8 u

Hr t ~~ ~ l bull

4 bull 0 I 2 3 410lIV (XU)4yI)

10 200 400 600 800 1000 1200

Voltage (V)

Figure 19 SEM images of Eiffel-tower-shaped AlN nanotip arrays (a) top view (b) side view and (c) corresponding FE I-Vcure The inset in (c) is the F-N plot Reprinted with permission from [291] Y B Tang et al Appl Phys Lett 86233104 (2005) copy 2005 American Institute of Physics

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

46 One-Dimensional Semiconductor Nanostructures Fabrication and Field-EmiSiSiion Properties

Figure 20 SEM images (a) and the measured FE l-E curve (b) of the quasi-aligned AlN NCs fabricated on a Ni-coated silicon wafer under 700degC The inset in (a) is a typical TEM image of AlN nanotip The insct in (b) is the corresponding F-N plots Reprinted with permission from [293] C Uu et aIl Am Chem Soc 1271318 (2005) copy 2005 Amerishycan Chemical Society

332 GaN As a Group III-V group semiconductor gallium nitride (GaN) with a wide direct band gap of34 eV at room tempershyature has attracted much attention in various fields such as LEDs sensor and electronic devices [307-309] GaN is also a promising cold cathode FE material owing to its low work function of 41 eV and low electron affinity (27-33 eV) high carrier mobility as well as excellent chemical and mechanical stability [310-312] 1D GaN nanostructures are expected to possess improved FE performance owing to the combination of good intrinsic properties of GaN with high f3 value of ID nanomaterials

Terada et a1 [313] fabricated GaN vertical nanotips by RIE method on a presynthesized GaN layer formed via the metal organic vapor-phase epitaxy route The density

1010of nanotips is approximately 2 X cm-2 bull The radius

of curvature of the sharpest top is less than 10 nm The height of the nanotips is approximately 200 nm The FE

Table 6 FE properties of 1D AlN nanostructures reported iu the literature

AlN emitters Synthesis method (V ILm-1)

I-V characteristics reveal that the microscopic current I at an anode voltage of 900 V is OJ nA (392 nA cm-Z) The measured maximum electron current density is 51 rnA cm-2

bull

The f3 value estimated from F-N plot is 300 assuming that the electron affinity of GaN is 33 e V The FE stability measurements suggest that the current fluctuation is less than 55 at 1200 V for 3600 s suggesting good FE stability of GaN nanotips Kim et al [314] have reported the prepashyration of GaN NR arrays grown by hydride vapor-phase epitaxy (HYPE) growth In this process both the shape and the diameter and density of the NRs can be controlled by adjusting the growth temperature The FE measureshyments disclose their excellent FE behavior with a low E to of 05 V Ilm-1 and J could reach 54 pA cm-2 at the applied field of206 V JLm-1 The emission stability of GaN NRs was assessed by measuring the current fluctuation with time at a fixed voltage During 20 h of continuous operation using a current density of about 98 jLA cm-2 the current fluctuashytion was as low as 7 Liu et al [315] produced P-doped GaN NWs by simply evaporating the powders of Ga20 3

GaNlnP with a molar ratio of 20101 at 1423 K for 30 min The J-E curve reveals that the E to is about 51 V JLm-l which is lower than those of reported un doped GaN nanobelts and NWs of61 or 12 V JUl11 [41 316] The lower Etc is owing to the large amount of nanoneedle-like facets caused by p doping ensuring that the GaN NWs possess a high field enhancement factor Recently needle-like bicrysshytalline GaN NWs with triangular prism morphology and uniform diameter of about 100 nm have been synthesized by using a simple thermal evaporation process The FE measurements show that the needle-like bicrystalline GaN NWs have a lower E to of 75 V Ilm-1 [317] It is believed that both the bicrystal twin defect and the needle-like morphology are responsible for the superior FE perforshymances of bicrystalline GaN NWs

Stability testing time Eb (V Jllll-l) and fluctuation Ref

NWs Extended EVLS lt10 at 1 mAcm-2 NA NA [48]

Nanoneedle Vapor deposition 31(defined as theJis NA NA [290] distinguished from the background noise)

Eiffel-tower-shaped nanotips CVD 47 106 4h lt074 [291]

Nanoneedle CVD 31 NA NA [292]

NCs CVD 12 NA NA [293]

NRs CVD NA 252 (as-grown) NA [295] 22 (NH-treat)

NRs Nitrogen arc 88 192 at 103 rnA crn-2 1 h lt2 [296] discharge

NRs with milltitipped VS 38 7 at 1 rnAcm~2 NA [297] surfaces

Nanotips CVD 6 NA 10 h lt 10 [300]

Nanotips CVD 108-122 136-152 at 1 rnA crn-2 NA [301]

Flower-like Si-doped CVD 18 46 5 h lt5 [303] nanoneedles

Hierarchical CVD 245-376 367-517 2h lt7 [306] nanoarchitecture

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

47 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Recently GaN heterostructures such alt triangular GaN-BN core-shell nanocables have also been synthesized by thermal CVD utilizing the reaction of GaP-B20 3-NH3 at high temperature They consist of single-crystalline wurtzite GaN NWs (average diameter = 60 nm) and 3 nm thick crystalline h-BN outerlayers [318] The measured E to is 4 V lffil- 1 at the eurrent density of 01 ~ cm-2 and the emission current is up to 01 rnA cm~2 at an applied voltage of 34 V Ilm-1 Table 7 summarizes the recently reported important FE properties of ID GaN nanostructures and their corresponding fabrication methods

333 InN Indium nitride (InN) which has the smallest effective electron mass of all the group III nitrides possesses some promising transport and electronic properties such as high mobility high saturation velocity and a large drift velocity at room temperature [324 325] These properties bode well for novel applications in the electronics industry such as LEDs laser diodes high-efficiency solar cells and highshyfrequency field-effect transistors [326] Among the group III nitrides ID GaN and AlN nanostructures have been more extensively studied but reports on the fabrication and propcrties of InN are still limited owing to its low thermal decomposition temperature and high equilibrium vapor pressure of nitrogen making the synthesis of high-quality ID InN nanostructures very difficult [327] So far there have been only a few reports on the FE properties of ID InN nanostructures

Ji et a1 [328] reported the synthesis of wurtzite InN nanofingers by the ion beam-assisted filtered cathodic vacuum arc technique The InN nanofingers exhibit polyshycrystalline and columnar structures with diameters of 20 nm and lengths of 250 nm oriented normal to the surface The E to is measured to be 97 and 112 V lm-1 under 50 and 100 Ilm cathode-anode distances respectively The correshysponding f3 values are about 156 and 77 The emission stability performed at a macroscopic field of 14 V jlID~1 for

1000 min shows that the emission current only degrades slightly indicating high FE stability Well-aligned and unidirectional single-crystalline InN nanotips have been produced by an etching process conducted on an MBE grown InN(O 0 0 2)-AlN(0 0 0 2)-Si(111) sample [329] The InN nanotips exhibit an E

to of 090 plusmn 034 V Ilm~l at

a current density of 1 JJA cm~2 The low E to is attributed to the surface layer in the InN surface making a downward band bending near the surface region that helps to lower the electron-tunneling barrier However the stability can only be maintained for 180 s at 10 V lm The short-term stability may be attributed to the intrinsic materials quality of the InN nanotips emitters being easy to damage and the fabrication process Assuming the work fuuction of the InN nanotips to be 41 eV the f3 value reaches 69000 which is believed to be a consequence of geometrical enhancement and the inherently high carrier concentration in the degenshyerate InN semiconductor

34 Other Typical Metal-Oxide Semiconductors

341 CuO Copper (II) oxide (CuD) is a narrow band-gap (Eg 12 e V) p-type semiconductor [330] and recognized as an indusshytrially important material for a variety of practical applishycations including magnetic storage media solar energy transformation electronics batteries sensors and catalysis and so on It has been shown that ID CuO nanostructures are good field emitters with a low Eto and high FE stability In addition their FE properties easily vary with heating or illumination by visible light

The thermal oxidation technique has been widely used to grow ID CuD nanostructures from and on copper substrates [331-334] It was first reported by Jiang et a1 [335] who synthesized CuD NWs on Cu mesh in 2002 Zhu et al [336] synthesized vertical-oriented CuD NWs by directly heating bulk copper plates at 390-430degC in air for 1-3 days

Table 7 Important FE properties of ID GaN nanostructures

Stability testing time GaN emitters Synthesis method EIO (V Iml) Eh (V )lm-1

) and fluctuation Ref

RandomNWs CVD 12 NA NA [41) Aligned nanotips RIE 900 Vat 392 nA em-2 NA 3600 s lt55 [313] NRarrays HVPE 05 206 at 54 pA cm-2 20h lt7 [314) Needle-like P-doped NWs Thermal evaporation 51 NA NA [315] Random NBs Ammoniating Ga20 3 thin films 61 at 01 pAcm-2 NA 6h lt6 [316) Needle-like bicrystalline

NWs Thermal evaporation 75 NA NA [317]

GaN-BN core-shell nanoeables CVD 14 at 01 pA cm-2 NA NA [318)

Aligned needle-like NWs Thermal evaporation 21 (at 1 pA em-2) 451 at 1 rnA cm-2 NA [319) Patterned NWs PLD 906 NA NA [320]

AlignedNRs Radio-frequency plasma-

enhanced MBE 125 at 01 pAcm-2 NA NA [321]

RandomNWs CVD 85 at 01 pA cm-2 NA 6h lt87 (322) Random lWs Vapor-phase epitaxy 396 NA NA [323]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

48 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Two typical NWs with different sizes and densities can be fabricated by controlling the growth temperature and time as shown in Figure 21 Type I (Fig 21(a)) and n (Fig 21(b)) NWsare produced by heating the Cu plates in air to 390degC for 3 days and to 430degC for 1 day respectively Type I NWs have an average length of 30 plusmn 3 Jlm and a diameter of 60 plusmn 15 nm at the tips whereas type II NWs have an average length of 4 plusmn 07 Jlmand a diameter of 90 plusmn 10 nm The densities of type I and II are about lOB and 4 X 107 cm-2

respectively The FE examination indicates that type I NWs have a low E

to of 33 V Ilm-I which is lower than that of type

II NWs being 45 V Jlml (Fig 21(c)) Figures 21(d) and 2I(e) are the fluorescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V respectively The dotted rectangles represent the boundshyaries of the samples Based on the FE experimental data and finite element calculation of the screening effect the work function of the CuO NW films is estimated to be about 25-28 eY Hsieh et aL [337] synthesized three types of CuO nanostructures including NRs nanofibers and nanoparshyticles by a Cu self-catalytic growth process at 400 500 and 600degC respectively Among the three nanostructures nanoshyfibers have the largest aspect ratio and sharpest tips The FE tests show that the J-E curves strongly depend on the

geometric morphologies of the CuO nanostructures All the CuD samples possess fairly good FE properties during 10 cycles and have similar E of 6-7 V flm- whereas the to nanofibers NRs and nanoparticles have different maximum FE currents of 9002 and 8 X 10-4 mA cm-2 at 11 V Ilm-l respectively The FE properties of CuO nanofiber arrays have also been investigated [338] The materials were fabricated by thermal oxidation of a plated copper nanoshycrystalline layer The FE J-E curve shows that E

to is about

05 V Ilm-I and the obtained maximum current density is up to 25 rnA cm-2 at an applied field of 27 V Jlill-1bull These results indicate that the CuO nanofiber arrays can be a promising candidate as FE emitters owing to their high aspect ratios and sharp tips

Chenet al [339] reported the FEbehavior ofCuO nanobelt arrays prepared by immersing a copper foil into an aqueous solution of NaOH and (NH4)2SP8 These nanobelts have widths of 20-150 nm and lengths up to 10 11m The temperashyture dependence of the FE characteristics has been studied from 300 to 750 K by heating the substrate Figure 22(a) shows typical plots of the emission current vs applied fields (I-E) from the CuO nanobelt films at 300400500600 and 700 K respectively The (at a current density of 10 JtA cm-2) decreases with increasing temperature from 300

a s shyApplied field Wpm

Figure 21 (a and b) SEM images of CuO NWs of type I and type II (c) FEJ-E curves of CuO NW films (type I (a) and type II (braquo) (d and e) Fluoshyrescence FE images of CuO NWs of type I (a) and type II (b) under an applied voltage of 1000 V The dotted rectangles represent the boundaries of the samples The inset of (a) shows a side view of the NWs of type I The inset in (c) depicts the corresponding F-N plot Reprinted with permission from [336] Y W Zhu et aI Nanotechrwlogy 16 88 (2005) copy 2005 lOP Publishing

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

49 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

to 750 K At room temperature Etu is -11 V (lm-I but Eth

decreases to 6 V (lm-I at 700 K The current variations with increasing temperature at fixed fields are also measured and the result obtained at 10 V j1m-1 is shown in Figure 22(b) About three orders of magnitude increase in the current is recorded over a temperature range of 450 K Moreover the current increases nearly exponentially with temperature It must be noted thattypically for tungsten at a field of 4 x 103

V Jlm-l increasing the temperature from room temperashyture to 1000 K results in about 45 increase in the emisshysion current [2 340] Thus it is believed that the dramatic increase in the current with temperature is attributed to the semiconducting nature of CuO CuO is a narrow band-gap (-12 e V) semiconductor At a higher temperature phonon excitation of the electrons ill increase the carrier density in the conduction band by exciting the interband states in the CuO nanobelts as defects are usually inevitable in these nanomaterials which may have small activation enershygies less than 05 e V Hence electrons can be excited to

(a)

o Q ~

18

(b)

bull

bull E=10MVm

2QO 300 400 500 600 100 800

Temperature (K)

Figure 22 (a) Plots of emission current vs applied field measured at different temperatures (b) The current variation with increasing temperature at a fixed applied field of 10 MV m- I The inset in (a) shows the corresponding In(J)-E2 plots Inset (b) shows the corresponding In(IP1T plot Reprinted with pennission from [339J J Chen et al Appl Phys Lett 83746 (2003) copy 2003 American Institute of Physics

the conduction band by heating and the carrier number in the conduction band increases Chen et al [341J have also investigated the influence ofvisible light illumination on FE from the CuO nanobelts At a low FE current level the FE current generally increases under pulsed irradiation At an emission current of 11 (lA a 19 increase in the emission current is recorded The photoinduced current increase is reduced with increasing emission current and is undetectshyable when the emission current is higher than 15 flA The authors believe that the pulsed irradiation-induced increase in the emission current can be attributed to the excitation of carriers into the conduction band The decrease in the emission current caused by continuous light illumination may result from photoinduced oxygen desorption

The FE properties of CuO nanostructures can be further improved by post-Oz CF4 plasma treatment [342] H2 plasma treatment [343] light illumination [341] and laser irradiation [344] It is believed that both O2 and CF

4 plasma treatments can sharp the tips by reducing their sizes Furthermore Oz plasma removed the amorphous layer on the surface of the as-grown NWs whereas CF

4 plasma

induced fluorinated carbon on the surface of NWs and reduced the work function by about 05 e V All these factors contribute to the large enhancement of the FE perforshymance after the plasma treatment Additionally Zhu et aL [344] also found that the FE current of CuO NWs could be enhanced effectively by laser irradiation The improved FE properties were attributed to the laser-induced electron transition to the excited states and surface oxygen desorpshytion similar to the discussion about the FE improvement of CuO nanobelt arrays via light illumination [341] Table 8 gives a summary of the important FE properties of ID CuO nanostructures reported recently and their corresponding fabrication methods

342 Sn02

Tin (IV) oxide (SnO ) having the rutile structure is anzn-type semiconductor with a large band gap (E = 362 e V at 300 K) and widely used in gas sensors dye-based solar cells transistors electrode materials catalysis and electroshychromic devices On account of favorable inherent propershyties including thermal stability oxidation resistance as well as large aspect ratio lD Sn02 nanostructures are potential FE materials So far ID SnO nanostructures including nariowhiskers [346 347] beak-lIke NRs [348] NWs [349 350] nanobelts [351 352] nanograss [353] and hierarchical nanostructures [354J have been synthesized and their good FE characteristics have been identified as shown in Table 9

He et at [348] synthesized beak-like SnO NRs by a VLS z approach using Au as a catalyst A comparative study on the FE characteristics between Sn0

2 NRs with uniform diamshy

eters and these beak-like NRs suggests that beak-like SnOz NRs have a lower Eta of 58 V JUll-1 and higher p value of 1400 while NRs with a uniform size without the beak-like structure show a pvalue of494 and a higher E of64 V 11m-ito This suggests that the beak-like SnOz NRs have better FE performance than NRs with a uniform diameter owing to the curved tip shapes Patterned Al-doped NWs [350] have been synthesized on stainless-steel mesh substrates by thermal evaporation of a mixture of Al and SnO powders

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

50 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 8 Important FE properties of 1D CuO nanostructures reported recently in the literatures

Stability testing time CuO emitters Synthesis method Eta (V 11m- I) Eth (V 11m-I) and fluctuation Ref

AlignedNWs Thermal oxidation 33-45 NA NA [336]

Aligned nanofibers Thermal oxidation 6-7 NA NA [337]

Aligned nanoneedle Thermal oxidation 05 NA NA [338]

Aligned nanobelts Solution reaction at room NA 11 (300 K) 6 (700 30h no obvious decay [339] temperature K) at 10 1A cm-2

AlignedNWs Thermal oxidation 36 (as-grown) 30 (10 min NA NA [342] 02 plasma treatment) 42 (as-grown) 37 (10 min CF4

plasma reatment)

Random Wet chemical process 1254 (as-grown) 36 (15 min NA 1800s lt36 [343] Hz plasma treatment) at 01 1A cm-2

AlignedNWs Conductive heating in air 65 at 01 1A cm-z NA NA [345]

at 950aC for 1 h The Eta (at the current density of 1 JJA cm-Z) andE

th (at the current density of 1 mAcm-Z) ofAl-doped SnOz

NWs are 15 and 33 V flm-l respectively both being lower than those of pure SnOz NWs of 253 and 47 V flm-I Wu et al [351] reported the synthesis of SnOz zig-zag nanobelts with the use of CuO powders The as-prepared nanobelts have better FE properties with a lower Eta (at the current density of 01 )tA cm-Z) of 19 V flm-I andE

th (at the

current density of 1 rnA cm-Z) of 51 V )tm-I in comparison with SnOz NWs prepared without CuO powders which have a Eta of 23 V flm-I and Eth of 65 V flm-I The f3 values of the SnOz zig-zag nanobelts and SnOz NWs are calculated to be about 3178 and 2772 respectively These results demonshystrate that SnOz zig-zag nanobelts with many periodic zigshyzag-like tips possess excellent FE performance owing to the thousands of zig-zag-like tips on the zig-zag nanbelts

Luo et al [347] investigated the FE behaviors of SnOz nanowhiskers post annealed in Nz and 0z atmospheres Annealing in Nz leaves the surface locally terminated with N-O bonds This enhances electron emission through changes of the band structure in the surface thus decreasing

Table 9 Typical FE parameters of 1D SnOz nanostructures

the emISSIOn barrier or lowering the work function Annealing in 0z leaves a more chemically uniform surface and increases the emission barrier This result suggests that a thermal treatment in a different gas ambient is an easy and effective way to introduce different atoms onto the SnOz surface and the annealing environment strongly impacts the FE properties of the SnOz NWs The FE behavior of the 1D SnOz nanostructures can also be enhanced by Hz exposure [349] Exposure of the SnOz NWs in Hz gas results in the reduction in the turn-on voltage for a driving current of 10 nA from 76 to 55 V flm-I and increase of the current from 047 to 21 flA at 10 V flm-I owing to the reduced work function This suggests that post treatment encompassing Hz exposure can be a useful process to improve the FE properties of SnOz or other oxides

343 In2 0 and ITO 3 Indium oxide (Inz0 ) is an important transparent conducshy

3

tive oxide semiconductor with a direct band gap of 36 e V which has been widely used in the microelectronics and

Stability testing time Sn02emitters Synthesis method Eta (V I1m-) Eh (V 11mshy I

) and fluctuation Ref

Random whisker Thermal evaporation 137 at 011lA cm-2 811 at 1 rnA cm-2 NA [346]

Random whisker Thermal evaporation 317 (as-grown) 363 (Oz-anshy 819 (as-grown) 895 NA [347] nealed) 259 (N2-annealed) at 1 1A cm-z

(02-annealed) 704 (Nzshyannealed) at 1 rnA cm-z

Random beak- VLS growth 58 (beak-like) NA NA [348] likeNRs 64 (uniform)

RandomNWs Vapor transport 76 (as-grown) 55 NA NA [349] (~-exposure) at 10 nA

Patterned Thermal evaporation 15 at 1 1A cm-z 33 at 1 rnA cm-z 2 h without detectable [350] Al-doped NWs degradation of FE

current

Zig-zag nanobelts Thermal evaporation 19 at 01 1A cm-z 51 at 1 rnA cm-2 NA [351]

Aligned nanobelts Thermal evaporation 23-45 at 1 -LA cm-2 NA 2h lt25 [352] Nanograss Thermal evaporation 561 at 144 1A cm-z NA NA [3531

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

51 One-DimensIonal Semiconductor Nanostructures Fabrication and Field-Emission Properties

other industry as flat-panel display materials optical and electric devices and gas detectors owing to its high elecshytrical conductance high transparency to visible light and the strong sensitivity to certain poisonous gas molecules [355 356] ID Inp3 and tin-doped indium oxide (ITO) are promising field emitters because of their high aspect ratio relatively low electron affinity high chemical inertness high oxidation resistance and sputtering resistance [357-362]

Li et al [359360] fabricated aligned Inp3 NWs on Aushycoated lnAs substrates under an electric field in a plasma sheath at a low temperature (480degC) Unaligned Inz03 NWs could also be synthesized without using plasma The FE J - E curves indicate that the E to values (at the current density of 1 ~ cm-2

) of the well-aligned and nonaligned NWs are 7 and 107 V lm-I respectively Their correshysponding Eh values (at a current density of 1 rnA cm-2

) are 113 and 177 V 1Ufl-1 respectively These results indicate that well-aligned nanowries have better FE performance owing to the stronger local electric field enhancement at their tips and relatively weak field-screening effect

Wan et al [362] have reported the epitaxial growth of vertically aligned ITO on ITO-yttrium stabilized zirconia

10

substrates by a vapor transport method using the mixture powders of In and SnO as precursors Figures 23(a) and 23(b) show the schematic of the growth process and sideshyview SEM image of the as-synthesized aligned ITO NW arrays grown on the ITO buffer layer-(1 0 O)YSZ substrate The average diameter of the NWs is less than 200 nm and the lengths are up to 25 lm The measured J - E curves and corresponding F-N plots are shown in Figures 23( c) and 23( d) The Eta value of the aligned ITO NW arrays at a current density of 1 ~ cm-2 is about 20 V 1Ufl-1 and the fJ value estimated from the F-N plot is up to about 3850 assuming that the tP value of ITO is 43 eV The relationshyship between the emission current and the vacuum presshysure for the aligned ITO NW arrays has been investigated As shown in Figure 23( e) when the vacuum level is higher than 2 x 10-4 Pa a steady emission current density of about 20 rnA cm-2 can be achieved at an applied field of 60 V lm-1bull The current density decreases with increasing pressure and the current density at a pressure of 1 X 10-1 Pa decreases by about one-third compared to that at 1 X 10-5 Pa However if the pressure returns to the initial value of 1 X 10-5 Pa again recovery in emission current takes place

~ 1t (c) ~_~I -~~ (d

~ 111 ~~ 1 2

e lid 0i 10 ~~

2~ 10( R I ~ bull~ J

~ 10 o 1 2 4 5 6 -4

01 1)2 03 04 Fieh (VIm) l1E(vmIV)

25

1 (e)

g c ~ gt 0

30

~25 (f) E6V bull

~ 20 a

g a 1X1J Pa

15 1i b 1X1U Paci 10 ig 05

a 000 20 40 60 80

Pressure (Pal Time (min)

Figure 23 (a) Schematic view of processes for vertically aligned ITO NW arrays grown on ITO buffer layer-(l 0 O)YSZ substrate (b) A side-view SEM image of the vertically aIigned ITO NW arrays grown on ITO-(l 0 O)YSZ substrate (c and d) FEJ-E curves and the corresponding F-N plots of the ITO NW arrays measured at the vacuum of 1 X 10-5 Pa (e) Dependence of emission eurrent density ofvertically aligned ITO NW arrays with the vacuum pressure in the range from 1 X 10-5 to 1 X lO- t Fa (f) FE current stability of ITO NW arrays at 1 X 10-5 and 1 x 10-1 Pa respectively The inset in (e) shows the spatial distribution of emission of ITO NWs at the 1 x 10-5 Fa Reprinted with permission from [362] Q Wan et aI Appl Phys Lett 89 123102 (2006) copy 2006 American Institute of Physics

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

52 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

indicating that the ITO NWs have high environmental stability owing to the high oxidation resistance The inset image in Figure 23(e) is the spatial distribution of emission from ITO NWs at a pressure of 1 X 1O~5 Pa The emisshysion is very intense and relatively homogenous suggesting good emission uniformity The FE stability of the vertically aligned ITO NW arrays has been investigated by measuring the current fluctuation with time at a fixed electric field of 60 V lm- i

As shown in Figure 23(f) there is no obvious degradation in the current density and the current fluctuashytion is as low as plusmn5 during 2 h of continuous operation at 1 X 10-5 Pa However the current fluctuation becomes larger when the vacuum reduced and it is found that the current fluctuation is plusmn28 at 1 X 10-1 Pa Table 10 summarizes the most important FE properties of 10 Inp3 and ITO nanostructures reported so far

344 Ti02

Titanium oxide (TiOz) is an important wide band gap (30-32 e V) semiconductor with three common crystal structures anatase rutile and brookite It has a variety of practical applications such as photocatalysis self-cleaning coatings sensors smart surface coatings solar cells and bioactive coating (363 364] 1D TiOz nanostructures have a large surface area and its unique 1D morphology provides a direct path for electron transport Thus the ID Ti02 nanostructures can increase the photoconvershysion efficiency of water-splitting reaetions as well as the power-conversion efficiency of solar cells in comparison with porous or particular Ti02 films (365 366] In addishytion owing to their sharp tips high aspect ratio and stable physical and chemical properties 10 Ti02 nanostrucshytures such as NTs [367] NWs [368-370] N-doped TiOz NTs [371] as well as quasi-aligned TiOz NW-carbon NC core-shell nanostructure arrays [372] have been demonshystrated to have good FE properties thereby making the materials suitable for applications in FEOs vacuum elecshytron sources and so on

lD TiOz nanostructures are generally synthesized by solution-based techniques such as hydrothermal or solvoshythermal synthesis [373-375] and sol-gel template method [376 377] However the crystalline properties of the asshyprepared TiOz nanostructures are usually not satisfactory and further thermal treatment is usually needed to crystalshylize into the anatase or rutile structures Very recently we have synthesized the quasi-aligned Ti0

2 NW-carbon NC

core-shell nanostructure arrays and TiOz NW arrays on Ti

foils by a simple thermal oxidation process with acetone as the oxidant [370 372] Figures 24(a) -24(e) are the SEM TEM and HRTEM images and corresponding EDS of the quasi-aligned TiOzNW-carbon NC core-shell nanostructure arrays directly grown on a Ti substrate under acetone vapor at 850degC Acetone is introduced into the chamber middotwith argon at a flow rate of 150 sccm The NW cores are single-crystalshyline rutile Ti0

2 with diameters of 15-20 nm and the conical

carbon shells are amorphous with gradually decreasing thickshynesses from 200-300 nm at the bases to 5-10 nm at the tips The measured FE J-E plot is shown in Figure 24(f) Eta is about 31 V ~lm-I and the current density reaches 27 mA cm-z

at a biased electric field of 125 V lm- I bull The f1 value is estimated to be 5304 based on the F-N plot in the inset of Figure 24(f) A current density fluctuation within 8 at an average current of 105 rnA cm-2 over 4 h is observed suggesting high emission stability If the reaction temperashyture of Ti foil and acetone vapor is decreased to 750-8000C and the flow rate of the carrier Ar gas is changed to SO seem the products are core-shell NWs with a TiOz NW core surrounded by a layer of amorphous carbon with a thickness of about 5-10 nm as shown in Figure 2S(a) After annealing in air at 600-650degC the surface carbon layer is removed completely and pure TiOzNW arrays are produced on the Ti substrate As shown in Figures 25(b) -2S(d) the Ti02 NWs have a single-crystalline rutile structure with diameters of 20-50 nm and lengths up to micrometers The measured FE I-Tcurveis shown in Figure 25(e) TheE

to value is 41 V 11m-I

and a current density of 1 rnA cm-2 can be obtained at an electric field of 115 V 11m-I The good linearity in the F-N plot (Fig 25(e)) within the measurement range impiiel that electron emission from the Ti0

2 NW arrays follows the F-N

behavior This simple thermal oxidation method provides a simple and straightforward route for large-area growth of semiconducting TiOz NWs on electrically conductive metal substrates thereby enabling the use of the product as robust field emitters without the need for post processing Liu et al [371] investigated the FE properties of TiO NTs fabricated byanodization and N-doped TiOzNTs by further annealing under NH3bull TheEto and Eth for N-doped TiOz NTs are about 112 and 244 V 11m-I while pure TiOz NTs have no FE The coexistence of doped nitrogen and concomitant oxygen vacancies in Ti02 NTs are responsible for the FE behavior of N-doped Ti02 NTs

10 TiOzmaterials are chemically stable inexpensive and biocompatible Hence they are promising electron emitshyters in FE devices The reported FE properties of 1D Ti02 nanostructures are summarized in Table 11

Table 10 FE properties of ID Inp and ITO nanostructures reported in the literature

Stability Inp3 and ITO emitters Synthesis method Eto (V liTIe1) Ellgt (V 1llJ11) testing time and fluctuation Ref

Inp3 NWs nano- 192 (nanocolumns) 247 Physicalcolumns nano- (NWs) 334 (nanopyramids) NA NA [357Jevaporationpyramids at 01 lA cm-2

Inducement of an 70 (aligned) 107 (nonaligned) 113 (aligned) 177In20

3 NWs 180 min lt10 [359]electric field at 1 lAcm-2 (nonaligned)

ITONRs Vapor transport 38 at 01 lAcm-2 NA Wh lt6 [361J Aligned ITO lWs Vapor transport 2 at llJA cm-2 NA 2 II lt5 [362]

53 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

2 4 6 8 10

Energy (KeV)

28

24

20

16

12

08

04

a 4 6 8 10

E(VIm)

Figure 24 (a and b) Low- and high-magnification SEM image (c and d) TEM image and HRTEM images and (e) the corresponding EDS of quasi-aligned TiOzNW-carbon NC core-shell nanostrueture arrays (f) The measured FE J-E curve 11le inset in (f) is the corresponding F-N plot Reprinted with permission from [372] K F Huo et al Appl Phys Lett 93 013105 (2008) copy 2008 American Institute of Physics

345 Tungsten Oxide sensors and so forth Tungsten oxide can have different Tungsten oxide is an interesting material because its band crystal structures such as cubic and monoclinic W0

3 tetragshy

gap can be varied from 25 to 35 eV by altering the oxygen onal W029 and monoclinic W1S

0 49bull ID tungsten oxide content It can be accomplished by conducting the synthesis nanostructures such as NWs and NRs are promising elecshyunder reducing or oxidizing conditions The materials have tron emitters owing to their high aspect ratios and excellent wide applications in electrochromic devices information physical and chemical properties Up to date ID tungsten displays smart windows gas humidity and temperature oxide emitters with different morphologies and compo-

Table 11 FE properties of ID Ti02 nanostruetures

Stability testing time 1102 emitters Synthesis method E (V jlm-) Eh (V jlffil) and fluctuation Ref

Aligned NTs Hydrothermal reaction 28 at 1 jiA cm-2 NA NA [367J

Aligned NWs Thermal evaporation 13 at 01 jiA cm-1 23 at 1 mAcm-2 24 h no obvious change [368]

AlignedNWs Thermal evaporation 57 67 NA [369J AlignedNWs Thermal chemical reaction 41 115 at 1 mA cm-2 NA [370] Aligned N-doped NTs Anodization and thermal 112 244 4h lt3 [371]

annealing

Aligned Ti02 NWC NC eore-shell nanostructure

Thermal chemical reaction 31 NA 4h lt8 [372]

54 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

12

10

l 08 ()

~ 06

~ 04 J

02

~~304

[-308 iNb -312

~~31$ ~I008 010 012 014 016 018 020

lIE(~mV)

00-1-----shy

2 4 6 8 10 12 E (VlJlm)

Figure 25 (a) SEM image of Ti02-C core-shell NWs and (b) TiOI

NWs (c) TEM and (d) HRTEM images of TiO NWs and (e) FEJ-E curve of quasi-aligned Ti02 NW arrays The inset in (a) is the TEM image of a Ti02-C core-shell NW The inset of (d) is the image of the corresponding fast Fourier transformation (FFT) Inset (e) is the corresponding F-N plot Reprinted with pennission from [370] K F Huo et al] Nanosci Nallotech 93341 (2009) copy 2009 American Scientific Publishers

sitions have been fabricated and exhihited excellent FE with Etc values of 6241 and 28 V )lm-c 1 at d = 01 02 properties including W1S nanotips NWs or NTs [378- and 03 mm respectively The corresponding f1 values are 0 49 382] W029 NRs and three-dimensional (3D) WOH NW 1095 1532 and 2116 respectively assuming a work funcshynetworks [383-385] and W03 NWs [386 387] and so on tion cP of W1P49 is 57 eV Aglfal et al [382] investigated

1D tungsten oxide nanostructures are generally fabricated the atmospheric pressure FE performance of W1aD49 NRs by direct heating of tungsten substrates in the presence of with diameters of 15-20 nm grown on tungsten thin films oxygen or thermal evaporation of W03 powders The growth exposed to ethane and nitrogen at 700aC at atmospheric mechanism can be explained by the VS growth model Zhou pressure Since the mean free path of an electron in air is et al [379] synthesized W

1S0

49 nanotip arrays via a two-step approximately 2 Jlm the gap between the cathode (WIP49

physical evaporation deposition process on Si(111) The NRs) and gold-coated anode is about 2 Jllll and thus the materials exhibit a low Em and Eth of 20 and 437 V Jlm-1 probability of ionization from collisions of gas particles respectively The emission stability examination shows that with FE electrons is very limited Atmospheric pressure FE degradation in the emission current density is -2 during measurements in air show that E to is about 33 V )lm-I and 135 h of continuous operation at a current density of about stable atmospheric pressure FE and pulsed FE are observed 1200 JlA Huang et a1 [380] synthesized W1P49 NW arrays in air with a current density up to 28 rnA cm-2bull

on ITO-coated glass substrates by thermal evaporation of 1D WOZ and W0 nanostructures also have exhibited 9 3 0

W03

powders without external catalysts The FE measureshy good FE properties Liu et a1 [383] fabricated W029 NRs ments reveal that the as-prepared W

1S0

49 nanotip arrays by thermally oxidizing a tungsten spiral coil in low vacuum

on ITO-coated glass substrates have good FE properties The FE measurements suggest that Eta of the NRs is about

55 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Cppcr-gate electrode

Cathode plate

Lowegate electrode

Figure 26 Schematic of the proposed FED with double-gate structure Reprinted with permission from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

12 V In addition E to of the tungsten oxide NRs can be tuned by controlling the W02 content in the product which can be realized by simply adjusting the temperature of the tungsten coil during deposition Zhou et aL (385J have synthesized 3D W03_oNW networks by thermal evapshyoration of W powders in the presence of oxygen The FE properties of 3D nanonetworks have been measured and these networks may be useful in 3D field emitters with a low E of 1385 V 1Jll1-1

bullto Chen et aI [388] have recently developed a double-gated

FED by using W03 NWs as cathode emitters They are fabrishycated on heavily n-doped Si substrate by using a thermal evaporation method Figure 26 shows the schematic structure of the proposed double-gate FED The FED is composed of

three separate plates that is cathode gate and anode plates The cathode plate consists of NW emitters grown on a suitshyable substrate The anode plate is a phosphor screen The gate plate is an insulating plate such as glass ceramic with round apertures of 100 -Lm drilled into it On both sides of the ceramic plate the metallic electrode strips are prepared by e-beam evaporation using a lift-off process They are perpenshydicular to each other and electrically isolated by the ceramic substrate Individual pixel addressing can be achieved by applying proper sequence of positive or negative voltages to the lower gate and upper gate respectively The details of the driving method can be found in Ref [388] Figure 27( a) shows the picture of the prepared 8 X 8 arrays of W03 NWs on a silicon wafer The diameter of each cathode is about 1 mm and the distance between the pixels is 25 mm Figure 27(b) shows a picture of an assembled device The dark spots on the anode correspond to the pixel and the color change in the phosphor is owing to electron bombardment The SEM micrographs of the W03 NWs are shown in Figures 27(c) and 27(d) The prepared W03 NWs are well aligned and the diameter of the NW is about 100-200 run The full emission image is depicted in Figure 28 in which the Arabic numerals and Chinese characters appear by switching the individual spots Each pixel can be accurately addressed and no interfershyence is observed

Since ID tungsten oxide emitters can be easily fabricated and have good properties they are considered as excellent field emitters alternative to CNTs The important FE propshyerties of tungsten oxide nanostructures reported so far are summarized in Table 12

Figure 27 (a) Picture of the prepared 8 x 8 arrays oftungsten oxide NW (b) Picture of an assembled 8 X 8 device (c) SEM picture of one tungsten oxide NW cathode (d) SEM cross-section picture of the prepared tungsten oxideNW Reprinted with permiSSion from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

56 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Figure 28 Arabic numerals and Chinese characters displayed by the double-gated FED Reprinted with permission from [388J J Chen et aI Appl Phys Lett 90 253105 (2007) copy 2007 American Institute ofPhysies

4 CONCLUSION AND OUTLOOK In this chapter we provide an up-to-date review of the recent progress on the fabrication and FE properties of some important inorganic ID semiconductor nanostrucshytures The study of various ID nanostructures as cold cathode field emitter has been constantly gaining interest and momentum among science and engineering communishyties since the late 1990s owing to their wide applications and potential markets in vacuum electron sources and FE devices The number of papers related to the fabrication and FE properties of ID nanostructures is very large and increasing exponentially yearly Needless to say owing to space limitation this article only focuses on some important inorganic ID semiconductor nanostructures and does not cover all the exciting work reported in this area Table 13 is an up-to-date summary on FE characteristics of other several inorganic semiconductor nanostructures such as metal sulfide oxide carbide nitride and even silicide Interested readers can refer to the relevant references

Fabrication of various ID nanostructures is the first step and opens up the investigation of dimension- and shape-dependent properties and potential applications to nanoscale electronic and optoelectronic devices In the past decade much effort has been devoted to fabricating high-quality semiconductor nanostructures mainly based on the bottom-up process by various techniques Although various ID nanostructures such as NRs NWs NBs NTs nanocables Nes nanoflowers nanoneedles nanotips nanonails nanopencils nanoporous structures nan on ails nanocolumns nanopyramids nanosphere assembly nanoshysprings nanohelical and hierarchical nanostructures have been fabricated how to control the size uniformity dimenshysionality growth direction atomic structures chemical composition and dopant distribution in the ID semiconshyductor nanostructures and how to integrate the functional nanostructures into devices with fully controlled interface structures or junction are still challenging issues for mateshyrials scientists and nanotechnologists In this article we discuss some typical bottom-up growth approaches used to fabricate ID semiconductor nanostructures They mainly fall into two main categories vapor phase and solution approaches The VLS growth process is reasonably well understood However further fundamental understanding of the surface physical and chemical processes and the processes occurring at the catalyst-nanostructure interface are essential for optimizing and controlling growth of ID nanostructures The VS process has been shown to be a powerful technique to fabricate 1D semiconductor nanoshystructures especially oxide and sulfide NWs or nanobelts However current understanding of this growth process and intrinsic mechanism is relatively poor Solution-based techshyniques such as hydrothermal or solvothermal growth SLS growth capping agents or surfactants-assisted synthesis and template-directed growth have been demonstrated as promising alternative approaches to produce ID semiconshyductor nanostructures with excellent control over the shape and composition with high reproducibility However the

Table 12 Tungsten oxide 1D nanostructures with various shapes and corresponding FE parameters reported in the literature

Stability testing time and Tungsten oxide emitters Synthesis method (V Jll11-1) Eh (V llm~l) fluctuation Ref

W1S0 49

AlignedNWs Thermal evaporation 41 NA NA [378] Aligned nanotips Thermal evaporation 20 437 135 h lt2 [379] Aligned NWs Thermal evaporation 28 NA NA [380] Aligned NWs Thermal evaporation 26 62 NA [381] Random nanobelts Thermal chemical reaction 33 152 at 28 rnA cm~2 1 h lt2 (382) WOZ9

AlignedNRs Thermal evaporation 12 NA 2h lt15 [383] 2D NW networks Thermal evaporation 074 NA NA [384] 3D WO~ NW network Thermal evaporation 1385 NA NA [385]

~-WO

AlignedNWs Thermal evaporation 48 NA NA [386] N-dopedNWs Thermal evaporation 644 942 NA [387J wormhole~like mesoporous

NWs AAO template-directed

growth 0083 175 20h lt30 [389]

57 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 13 Important FE properties of several other ID inorganic semiconductor nanostruetUTes reported in the literature

Stability testing time

Materials Morphology Synthesis method Eo (V)lm-l) Eth (V )lm- l) and fiuctuation Ref

MoO Nanobelts Thennal evaporation 87 132 2h lt15 [390]

AlignedNWs Thermal evaporation 35 765 1 h lt10 [391J

MoOz AIignedNWs Thermal evaporation 24 56 NA [392]

AlignedNRs 1bennal evaporation 4 at 01)lA cm-z NA NA [393]

Cactus-like nanos- Carbon thermal 126 232 NA [394J truetures reduction

Aligned N-WS Thennal evaporation 62 NA 75 h lt13 [395]

Random GapJ-C Thermal evaporation 773 845 NA [396] nanocables

RandomNWs Thermal oxidation 63 (as-grown) 36 (H2- NA NA [397] plasma treatment) at 01 )lA Cm~2

RandomNWs Thermal oxidation 33 NA NA [398]

AlignedNWs Thermal oxidation 63 10 NA [399]

MnO AlignedNWs Electrodcposition 84 at 1 )lA cm~2 NA NA [400]

l1J2O

S AlignedNWs Thermal oxidation 67 92 at 1 rnA cm~2 10 h lt10 [401]

AlignedNRs Metalorganic CVD 56 NA 180 h no detectable degradation

[402]

Ce02 RandomNWs Hydrothermal

method 58 at 1 )lA cmshy 2 NA 5 h lt3 [403]

Ru02 AlignedNRs Metalorganic CVD 110 55 (at the beginning

of emission) 50 h no detectable

degradation [404]

Aligned NRs Metalorganic CVD 103 49 (at the beginning 14 h no detectable [405] of emission) degradation

AlignedNWs Gas-solid reaction 11 (1 )lA cm-Z) NA 16h lt2 [406]

Dendritic nanosshy Gas-solid reaction 41 NA NA [407] tructures

AlignedNWs Gas-solid reaction 22 8 NA [408J

CuS Nanowalls Gas-solid reaction 85 205 (1 rnA cm~2) NA [409]

NbSz RandomNWs CVD 25 NA NA [410]

MoS2 Nanoflower Thermal reaction 45-55 76-86 2h lt20 [411J Random nanoshy Chemical-vapor 198 27 (01 rnA cm-2) NA [412]

belts transport and pyrolysis

Aligned nanobelts Chemical-vapor 095 36 (1 rnA cm~2) 2h lt313 [413] transport and pyrolysis

TiC RandomNWs Chloride-assisted 71 NA 2h lt10 [414] carbon thermal reduction

TiN Randomr--Ws Chloride-assisted 70 NA 2h lt8 [415] carbon thermal reduction

RandomNWs Vapor-phase deposishy 8 at 01 JlA cm-2 NA 40 h no obvious degshy [416] tion radation

major limitation is that the growth mechanisms are poorly known and most structural director agents are chosen by trial and error The essence of ID nanostructures formashytion is its anisotropic growth Most ID nanostructures are formed under nonequilibrium conditions and controlled by kinetic processes Although it is rather complex a solid understanding on the growth kinetics is essential in order to control the growth process of ID nanostructures Many questions about the formation process and mechanisms

explaining the structures morphologies and dimensions still need to be addressed

FE is one of the important applications of ID nanostrucshytures ID nanostructures have an effectively enhanced elecshytric field at the emission centers owing to their high aspect ratios and sharp tips Hence they can emit electrons at low electric fields boding well for FE applications Many papers are available in the literature describing the large variety of nanostructures having various shapes and corresponding

58 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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368 B Xiang Y Zhang Z Wang X H Luo Y W Zhu H Z Zhang and D P YUJ Phys D Appl Phys 38 1152 (2005)

369 J M Wu H C Shih and W T Wu Chern Phys Lett 413 490 (2005)

370 K F Huo J J Fu X M Zhang G X Qian Y C Xin H W Ni and P K Chul Nanosci Nanotech 93341 (2009)

371 G Liu F Li D W Wang D M Tang C Liu X Ma G Q Lu and H M Cheng Nanotechnology 19 025606 (2008)

372 K F Huo X M Zhang L S Hu X J Sun R S Chen J J Fu and P K Chu Appl Phys Lett 93013105 (2008)

373 A R Armstrong G Armstrong J Canales R Garcia and P G BruceAdv Mater 17862 (2005)

374 M Miyauchi and II Tokudomei Mater Chern 172095 (2007) 375 Q Zhang and L Gao Langmuir 19 967 (2003) 376 S M Liu L M Gan L H Liu W D Zhang and H C Zeng

Chem Mater 141391 (2002) 377 1 J Qiu W D Yu X D Gao and X M Li Nanotechnology 17

4695 (2006) 378 K Huang Q Pan F Yang S Ni and D He Mater Res Bull 43

919 (2008) 379 J Zhou L Gong S Z Deng 1 Chen J C Sha N S Xu

R S Yang and Z L Wang Appl Phys Lett 87223108 (2005) 380 K Huang Q T Pan F Yang S B Ni and D Y He Appl Swf

Sci 253 8923 (2007) 381 Y B Li Y Bando and D GolbergAdv Mater 151294 (2003) 382 A Aglral and J G E (Han) Gardeniers J Phys Chern C 112

15183 (2008) 383 J G Liu Z J Zhang Y Zhao X Su S Liu and E Wang Small

1310 (2005) 384 Y M Zhao Y H Li 1 Ahmad D G McCartney Y Q Zhu and

W B HuAppl Phys Lett 89133116 (2006) 385 J Zhou Y Ding S Z Deng L Gong N S Xu and Z L Wang

Adv Mater 172107 (2005) 386 Y Baek and K Yong J Phys Chem C 1111213 (2007) 387 M T Chang L J Chou Y L Chueh Y C Lee C H Hsieh

C D Chen Y W Lan and L J Chen Small 3 658 (2007) 388 J Chen Y Y Dai J Luo Z L Li S Z Deng 1 C She and

N S XuAppl Phys Lett 90253105 (2007) 389 W H Lai M H Hon L G Teoh Y H Su J Shieh and

C K Chen J Electron Mater 37 1082 (2008) 390 Y B Li Y Bondo D Golberg and K Kurashima AppL Phys

Lett 81 5048 (2002) 391 J Zhou S Z Deng N S Xu J Chen and J C SheApp Phys

Lett 83 2653 (2003) 392 J Zhou N S Xu S Z Deng J Chen 1 C She and Z L Wang

Adv Mater 151835 (2003) 393 J G Liu Z J Zhang C Y Pan Y Zhao X Sun Y Zhou and

D P Yu Mater Lett 583812 (2004) 394 C Cao Z Chen X An and H Zhu I Phys Chern C 112 95 (2008) 395 Y Huang Z Wang Q Wang C Gu c Tang Y Bando and

D GolbergJ Phys Chern C 113 1980 (2009) 396 J Zhan Y Bando J Hu Y Li and D Golberg Chem Mater 16

5158 (2004) 397 F Xu K Yu Q Wang M Shi Q Zhang W Bai Q Li and

Z Zhu 1 Nanosci Nanotech 7 2774 (2007) 398 L c Hsu Y Y Li and C Y Hsiao Nanoscale Res Lett 3 330

(2008) 399 Y L Chueh M W Lai J Q Liang L J Chou and Z L Wang

Adv Funct Mater 16 2243 (2006) 400 M S Wu J T Lee Y Y Wang and C C Wan J Phys Chern

E 42 16331 (2004) 401 B Varghese S C Haur and C T LimJ Phys Chern C 11210008

(2008) 402 R S Chen Y S Huang Y M Liang C S Hsieh D S Tsai and

K K TiongAppl Phys Lett 84 1552 (2004) 403 X Q Fu P Feng C Wang and T H Wang Chin Phys Lett 8

2423 (2007) 404 C S Hsieh D S Tsai R S Chen and Y S Huang Appl Phys

Lett 853860 (2004) 405 c L Cheng Y F Chen R S Chen and Y S Huang Appl Phys

Lett 86 103104 (2005) 406 J Chen S Z Deng N S Xu S H Wang X G Wen S H Yang

C L Yang J N Wang and W K GeAppl Phys Lett 803620 (2002)

65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

407 Y Xiao1 J Chen S Z Deng N S Xu1 and S Yangf Nanosci 412 X C Wu Y R Tao Y M Hu Y Song Z Hu J J Zhu and Nanotech 8 237 (2008) L Dong Nanotechnology 17 201 (2006)

408 O B Wu S Ren S Z Deng J Chen and N S Xu J Vac Sci 413 Y L Zhang X C Wu Y R Tao C J Mao and J J Zhu Chern Technol B 22 1282 (2004) Cornmun 2683 (2008)

409 X Feng Y Li H Liu Y Li S Cui N Wang L Jiang X Liu and 414 K F Huo Y M Hu Y W Ma Y N Lii Z Hu and Y Chen M Yuan Nanotechnology 18 145706 (2007) Nanotechnology 18 145615 (2007)

410 Y Z Jin W K Hsu Y L Chueh L J Chou Y Q Zhu 415 Y M Hu K F Huo Y W Ma Y N Lii J Q Xu Z Eu and K Brigatti H W Kroto and D R M WaltonAngew Chern Int Y Chenf Nanosci Nanotech 72922 (2007) Ed 435670 (2004) 416 B Xiang O X Wang Z Wang X Z Zhang L Q lin J Xu and

411 Y B Li Y Bando and D GolbergAppl Phys Lett 82 1962 (2003) D P YuAppl Phygt Lett 86243103 (2005)

54 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

12

10

l 08 ()

~ 06

~ 04 J

02

~~304

[-308 iNb -312

~~31$ ~I008 010 012 014 016 018 020

lIE(~mV)

00-1-----shy

2 4 6 8 10 12 E (VlJlm)

Figure 25 (a) SEM image of Ti02-C core-shell NWs and (b) TiOI

NWs (c) TEM and (d) HRTEM images of TiO NWs and (e) FEJ-E curve of quasi-aligned Ti02 NW arrays The inset in (a) is the TEM image of a Ti02-C core-shell NW The inset of (d) is the image of the corresponding fast Fourier transformation (FFT) Inset (e) is the corresponding F-N plot Reprinted with pennission from [370] K F Huo et al] Nanosci Nallotech 93341 (2009) copy 2009 American Scientific Publishers

sitions have been fabricated and exhihited excellent FE with Etc values of 6241 and 28 V )lm-c 1 at d = 01 02 properties including W1S nanotips NWs or NTs [378- and 03 mm respectively The corresponding f1 values are 0 49 382] W029 NRs and three-dimensional (3D) WOH NW 1095 1532 and 2116 respectively assuming a work funcshynetworks [383-385] and W03 NWs [386 387] and so on tion cP of W1P49 is 57 eV Aglfal et al [382] investigated

1D tungsten oxide nanostructures are generally fabricated the atmospheric pressure FE performance of W1aD49 NRs by direct heating of tungsten substrates in the presence of with diameters of 15-20 nm grown on tungsten thin films oxygen or thermal evaporation of W03 powders The growth exposed to ethane and nitrogen at 700aC at atmospheric mechanism can be explained by the VS growth model Zhou pressure Since the mean free path of an electron in air is et al [379] synthesized W

1S0

49 nanotip arrays via a two-step approximately 2 Jlm the gap between the cathode (WIP49

physical evaporation deposition process on Si(111) The NRs) and gold-coated anode is about 2 Jllll and thus the materials exhibit a low Em and Eth of 20 and 437 V Jlm-1 probability of ionization from collisions of gas particles respectively The emission stability examination shows that with FE electrons is very limited Atmospheric pressure FE degradation in the emission current density is -2 during measurements in air show that E to is about 33 V )lm-I and 135 h of continuous operation at a current density of about stable atmospheric pressure FE and pulsed FE are observed 1200 JlA Huang et a1 [380] synthesized W1P49 NW arrays in air with a current density up to 28 rnA cm-2bull

on ITO-coated glass substrates by thermal evaporation of 1D WOZ and W0 nanostructures also have exhibited 9 3 0

W03

powders without external catalysts The FE measureshy good FE properties Liu et a1 [383] fabricated W029 NRs ments reveal that the as-prepared W

1S0

49 nanotip arrays by thermally oxidizing a tungsten spiral coil in low vacuum

on ITO-coated glass substrates have good FE properties The FE measurements suggest that Eta of the NRs is about

55 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Cppcr-gate electrode

Cathode plate

Lowegate electrode

Figure 26 Schematic of the proposed FED with double-gate structure Reprinted with permission from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

12 V In addition E to of the tungsten oxide NRs can be tuned by controlling the W02 content in the product which can be realized by simply adjusting the temperature of the tungsten coil during deposition Zhou et aL (385J have synthesized 3D W03_oNW networks by thermal evapshyoration of W powders in the presence of oxygen The FE properties of 3D nanonetworks have been measured and these networks may be useful in 3D field emitters with a low E of 1385 V 1Jll1-1

bullto Chen et aI [388] have recently developed a double-gated

FED by using W03 NWs as cathode emitters They are fabrishycated on heavily n-doped Si substrate by using a thermal evaporation method Figure 26 shows the schematic structure of the proposed double-gate FED The FED is composed of

three separate plates that is cathode gate and anode plates The cathode plate consists of NW emitters grown on a suitshyable substrate The anode plate is a phosphor screen The gate plate is an insulating plate such as glass ceramic with round apertures of 100 -Lm drilled into it On both sides of the ceramic plate the metallic electrode strips are prepared by e-beam evaporation using a lift-off process They are perpenshydicular to each other and electrically isolated by the ceramic substrate Individual pixel addressing can be achieved by applying proper sequence of positive or negative voltages to the lower gate and upper gate respectively The details of the driving method can be found in Ref [388] Figure 27( a) shows the picture of the prepared 8 X 8 arrays of W03 NWs on a silicon wafer The diameter of each cathode is about 1 mm and the distance between the pixels is 25 mm Figure 27(b) shows a picture of an assembled device The dark spots on the anode correspond to the pixel and the color change in the phosphor is owing to electron bombardment The SEM micrographs of the W03 NWs are shown in Figures 27(c) and 27(d) The prepared W03 NWs are well aligned and the diameter of the NW is about 100-200 run The full emission image is depicted in Figure 28 in which the Arabic numerals and Chinese characters appear by switching the individual spots Each pixel can be accurately addressed and no interfershyence is observed

Since ID tungsten oxide emitters can be easily fabricated and have good properties they are considered as excellent field emitters alternative to CNTs The important FE propshyerties of tungsten oxide nanostructures reported so far are summarized in Table 12

Figure 27 (a) Picture of the prepared 8 x 8 arrays oftungsten oxide NW (b) Picture of an assembled 8 X 8 device (c) SEM picture of one tungsten oxide NW cathode (d) SEM cross-section picture of the prepared tungsten oxideNW Reprinted with permiSSion from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

56 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Figure 28 Arabic numerals and Chinese characters displayed by the double-gated FED Reprinted with permission from [388J J Chen et aI Appl Phys Lett 90 253105 (2007) copy 2007 American Institute ofPhysies

4 CONCLUSION AND OUTLOOK In this chapter we provide an up-to-date review of the recent progress on the fabrication and FE properties of some important inorganic ID semiconductor nanostrucshytures The study of various ID nanostructures as cold cathode field emitter has been constantly gaining interest and momentum among science and engineering communishyties since the late 1990s owing to their wide applications and potential markets in vacuum electron sources and FE devices The number of papers related to the fabrication and FE properties of ID nanostructures is very large and increasing exponentially yearly Needless to say owing to space limitation this article only focuses on some important inorganic ID semiconductor nanostructures and does not cover all the exciting work reported in this area Table 13 is an up-to-date summary on FE characteristics of other several inorganic semiconductor nanostructures such as metal sulfide oxide carbide nitride and even silicide Interested readers can refer to the relevant references

Fabrication of various ID nanostructures is the first step and opens up the investigation of dimension- and shape-dependent properties and potential applications to nanoscale electronic and optoelectronic devices In the past decade much effort has been devoted to fabricating high-quality semiconductor nanostructures mainly based on the bottom-up process by various techniques Although various ID nanostructures such as NRs NWs NBs NTs nanocables Nes nanoflowers nanoneedles nanotips nanonails nanopencils nanoporous structures nan on ails nanocolumns nanopyramids nanosphere assembly nanoshysprings nanohelical and hierarchical nanostructures have been fabricated how to control the size uniformity dimenshysionality growth direction atomic structures chemical composition and dopant distribution in the ID semiconshyductor nanostructures and how to integrate the functional nanostructures into devices with fully controlled interface structures or junction are still challenging issues for mateshyrials scientists and nanotechnologists In this article we discuss some typical bottom-up growth approaches used to fabricate ID semiconductor nanostructures They mainly fall into two main categories vapor phase and solution approaches The VLS growth process is reasonably well understood However further fundamental understanding of the surface physical and chemical processes and the processes occurring at the catalyst-nanostructure interface are essential for optimizing and controlling growth of ID nanostructures The VS process has been shown to be a powerful technique to fabricate 1D semiconductor nanoshystructures especially oxide and sulfide NWs or nanobelts However current understanding of this growth process and intrinsic mechanism is relatively poor Solution-based techshyniques such as hydrothermal or solvothermal growth SLS growth capping agents or surfactants-assisted synthesis and template-directed growth have been demonstrated as promising alternative approaches to produce ID semiconshyductor nanostructures with excellent control over the shape and composition with high reproducibility However the

Table 12 Tungsten oxide 1D nanostructures with various shapes and corresponding FE parameters reported in the literature

Stability testing time and Tungsten oxide emitters Synthesis method (V Jll11-1) Eh (V llm~l) fluctuation Ref

W1S0 49

AlignedNWs Thermal evaporation 41 NA NA [378] Aligned nanotips Thermal evaporation 20 437 135 h lt2 [379] Aligned NWs Thermal evaporation 28 NA NA [380] Aligned NWs Thermal evaporation 26 62 NA [381] Random nanobelts Thermal chemical reaction 33 152 at 28 rnA cm~2 1 h lt2 (382) WOZ9

AlignedNRs Thermal evaporation 12 NA 2h lt15 [383] 2D NW networks Thermal evaporation 074 NA NA [384] 3D WO~ NW network Thermal evaporation 1385 NA NA [385]

~-WO

AlignedNWs Thermal evaporation 48 NA NA [386] N-dopedNWs Thermal evaporation 644 942 NA [387J wormhole~like mesoporous

NWs AAO template-directed

growth 0083 175 20h lt30 [389]

57 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 13 Important FE properties of several other ID inorganic semiconductor nanostruetUTes reported in the literature

Stability testing time

Materials Morphology Synthesis method Eo (V)lm-l) Eth (V )lm- l) and fiuctuation Ref

MoO Nanobelts Thennal evaporation 87 132 2h lt15 [390]

AlignedNWs Thermal evaporation 35 765 1 h lt10 [391J

MoOz AIignedNWs Thermal evaporation 24 56 NA [392]

AlignedNRs 1bennal evaporation 4 at 01)lA cm-z NA NA [393]

Cactus-like nanos- Carbon thermal 126 232 NA [394J truetures reduction

Aligned N-WS Thennal evaporation 62 NA 75 h lt13 [395]

Random GapJ-C Thermal evaporation 773 845 NA [396] nanocables

RandomNWs Thermal oxidation 63 (as-grown) 36 (H2- NA NA [397] plasma treatment) at 01 )lA Cm~2

RandomNWs Thermal oxidation 33 NA NA [398]

AlignedNWs Thermal oxidation 63 10 NA [399]

MnO AlignedNWs Electrodcposition 84 at 1 )lA cm~2 NA NA [400]

l1J2O

S AlignedNWs Thermal oxidation 67 92 at 1 rnA cm~2 10 h lt10 [401]

AlignedNRs Metalorganic CVD 56 NA 180 h no detectable degradation

[402]

Ce02 RandomNWs Hydrothermal

method 58 at 1 )lA cmshy 2 NA 5 h lt3 [403]

Ru02 AlignedNRs Metalorganic CVD 110 55 (at the beginning

of emission) 50 h no detectable

degradation [404]

Aligned NRs Metalorganic CVD 103 49 (at the beginning 14 h no detectable [405] of emission) degradation

AlignedNWs Gas-solid reaction 11 (1 )lA cm-Z) NA 16h lt2 [406]

Dendritic nanosshy Gas-solid reaction 41 NA NA [407] tructures

AlignedNWs Gas-solid reaction 22 8 NA [408J

CuS Nanowalls Gas-solid reaction 85 205 (1 rnA cm~2) NA [409]

NbSz RandomNWs CVD 25 NA NA [410]

MoS2 Nanoflower Thermal reaction 45-55 76-86 2h lt20 [411J Random nanoshy Chemical-vapor 198 27 (01 rnA cm-2) NA [412]

belts transport and pyrolysis

Aligned nanobelts Chemical-vapor 095 36 (1 rnA cm~2) 2h lt313 [413] transport and pyrolysis

TiC RandomNWs Chloride-assisted 71 NA 2h lt10 [414] carbon thermal reduction

TiN Randomr--Ws Chloride-assisted 70 NA 2h lt8 [415] carbon thermal reduction

RandomNWs Vapor-phase deposishy 8 at 01 JlA cm-2 NA 40 h no obvious degshy [416] tion radation

major limitation is that the growth mechanisms are poorly known and most structural director agents are chosen by trial and error The essence of ID nanostructures formashytion is its anisotropic growth Most ID nanostructures are formed under nonequilibrium conditions and controlled by kinetic processes Although it is rather complex a solid understanding on the growth kinetics is essential in order to control the growth process of ID nanostructures Many questions about the formation process and mechanisms

explaining the structures morphologies and dimensions still need to be addressed

FE is one of the important applications of ID nanostrucshytures ID nanostructures have an effectively enhanced elecshytric field at the emission centers owing to their high aspect ratios and sharp tips Hence they can emit electrons at low electric fields boding well for FE applications Many papers are available in the literature describing the large variety of nanostructures having various shapes and corresponding

58 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

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55 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Cppcr-gate electrode

Cathode plate

Lowegate electrode

Figure 26 Schematic of the proposed FED with double-gate structure Reprinted with permission from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

12 V In addition E to of the tungsten oxide NRs can be tuned by controlling the W02 content in the product which can be realized by simply adjusting the temperature of the tungsten coil during deposition Zhou et aL (385J have synthesized 3D W03_oNW networks by thermal evapshyoration of W powders in the presence of oxygen The FE properties of 3D nanonetworks have been measured and these networks may be useful in 3D field emitters with a low E of 1385 V 1Jll1-1

bullto Chen et aI [388] have recently developed a double-gated

FED by using W03 NWs as cathode emitters They are fabrishycated on heavily n-doped Si substrate by using a thermal evaporation method Figure 26 shows the schematic structure of the proposed double-gate FED The FED is composed of

three separate plates that is cathode gate and anode plates The cathode plate consists of NW emitters grown on a suitshyable substrate The anode plate is a phosphor screen The gate plate is an insulating plate such as glass ceramic with round apertures of 100 -Lm drilled into it On both sides of the ceramic plate the metallic electrode strips are prepared by e-beam evaporation using a lift-off process They are perpenshydicular to each other and electrically isolated by the ceramic substrate Individual pixel addressing can be achieved by applying proper sequence of positive or negative voltages to the lower gate and upper gate respectively The details of the driving method can be found in Ref [388] Figure 27( a) shows the picture of the prepared 8 X 8 arrays of W03 NWs on a silicon wafer The diameter of each cathode is about 1 mm and the distance between the pixels is 25 mm Figure 27(b) shows a picture of an assembled device The dark spots on the anode correspond to the pixel and the color change in the phosphor is owing to electron bombardment The SEM micrographs of the W03 NWs are shown in Figures 27(c) and 27(d) The prepared W03 NWs are well aligned and the diameter of the NW is about 100-200 run The full emission image is depicted in Figure 28 in which the Arabic numerals and Chinese characters appear by switching the individual spots Each pixel can be accurately addressed and no interfershyence is observed

Since ID tungsten oxide emitters can be easily fabricated and have good properties they are considered as excellent field emitters alternative to CNTs The important FE propshyerties of tungsten oxide nanostructures reported so far are summarized in Table 12

Figure 27 (a) Picture of the prepared 8 x 8 arrays oftungsten oxide NW (b) Picture of an assembled 8 X 8 device (c) SEM picture of one tungsten oxide NW cathode (d) SEM cross-section picture of the prepared tungsten oxideNW Reprinted with permiSSion from [388] J Chen et alAppl Phys Lett 90 253105 (2007) copy 2007 American Institute of Physics

56 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Figure 28 Arabic numerals and Chinese characters displayed by the double-gated FED Reprinted with permission from [388J J Chen et aI Appl Phys Lett 90 253105 (2007) copy 2007 American Institute ofPhysies

4 CONCLUSION AND OUTLOOK In this chapter we provide an up-to-date review of the recent progress on the fabrication and FE properties of some important inorganic ID semiconductor nanostrucshytures The study of various ID nanostructures as cold cathode field emitter has been constantly gaining interest and momentum among science and engineering communishyties since the late 1990s owing to their wide applications and potential markets in vacuum electron sources and FE devices The number of papers related to the fabrication and FE properties of ID nanostructures is very large and increasing exponentially yearly Needless to say owing to space limitation this article only focuses on some important inorganic ID semiconductor nanostructures and does not cover all the exciting work reported in this area Table 13 is an up-to-date summary on FE characteristics of other several inorganic semiconductor nanostructures such as metal sulfide oxide carbide nitride and even silicide Interested readers can refer to the relevant references

Fabrication of various ID nanostructures is the first step and opens up the investigation of dimension- and shape-dependent properties and potential applications to nanoscale electronic and optoelectronic devices In the past decade much effort has been devoted to fabricating high-quality semiconductor nanostructures mainly based on the bottom-up process by various techniques Although various ID nanostructures such as NRs NWs NBs NTs nanocables Nes nanoflowers nanoneedles nanotips nanonails nanopencils nanoporous structures nan on ails nanocolumns nanopyramids nanosphere assembly nanoshysprings nanohelical and hierarchical nanostructures have been fabricated how to control the size uniformity dimenshysionality growth direction atomic structures chemical composition and dopant distribution in the ID semiconshyductor nanostructures and how to integrate the functional nanostructures into devices with fully controlled interface structures or junction are still challenging issues for mateshyrials scientists and nanotechnologists In this article we discuss some typical bottom-up growth approaches used to fabricate ID semiconductor nanostructures They mainly fall into two main categories vapor phase and solution approaches The VLS growth process is reasonably well understood However further fundamental understanding of the surface physical and chemical processes and the processes occurring at the catalyst-nanostructure interface are essential for optimizing and controlling growth of ID nanostructures The VS process has been shown to be a powerful technique to fabricate 1D semiconductor nanoshystructures especially oxide and sulfide NWs or nanobelts However current understanding of this growth process and intrinsic mechanism is relatively poor Solution-based techshyniques such as hydrothermal or solvothermal growth SLS growth capping agents or surfactants-assisted synthesis and template-directed growth have been demonstrated as promising alternative approaches to produce ID semiconshyductor nanostructures with excellent control over the shape and composition with high reproducibility However the

Table 12 Tungsten oxide 1D nanostructures with various shapes and corresponding FE parameters reported in the literature

Stability testing time and Tungsten oxide emitters Synthesis method (V Jll11-1) Eh (V llm~l) fluctuation Ref

W1S0 49

AlignedNWs Thermal evaporation 41 NA NA [378] Aligned nanotips Thermal evaporation 20 437 135 h lt2 [379] Aligned NWs Thermal evaporation 28 NA NA [380] Aligned NWs Thermal evaporation 26 62 NA [381] Random nanobelts Thermal chemical reaction 33 152 at 28 rnA cm~2 1 h lt2 (382) WOZ9

AlignedNRs Thermal evaporation 12 NA 2h lt15 [383] 2D NW networks Thermal evaporation 074 NA NA [384] 3D WO~ NW network Thermal evaporation 1385 NA NA [385]

~-WO

AlignedNWs Thermal evaporation 48 NA NA [386] N-dopedNWs Thermal evaporation 644 942 NA [387J wormhole~like mesoporous

NWs AAO template-directed

growth 0083 175 20h lt30 [389]

57 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 13 Important FE properties of several other ID inorganic semiconductor nanostruetUTes reported in the literature

Stability testing time

Materials Morphology Synthesis method Eo (V)lm-l) Eth (V )lm- l) and fiuctuation Ref

MoO Nanobelts Thennal evaporation 87 132 2h lt15 [390]

AlignedNWs Thermal evaporation 35 765 1 h lt10 [391J

MoOz AIignedNWs Thermal evaporation 24 56 NA [392]

AlignedNRs 1bennal evaporation 4 at 01)lA cm-z NA NA [393]

Cactus-like nanos- Carbon thermal 126 232 NA [394J truetures reduction

Aligned N-WS Thennal evaporation 62 NA 75 h lt13 [395]

Random GapJ-C Thermal evaporation 773 845 NA [396] nanocables

RandomNWs Thermal oxidation 63 (as-grown) 36 (H2- NA NA [397] plasma treatment) at 01 )lA Cm~2

RandomNWs Thermal oxidation 33 NA NA [398]

AlignedNWs Thermal oxidation 63 10 NA [399]

MnO AlignedNWs Electrodcposition 84 at 1 )lA cm~2 NA NA [400]

l1J2O

S AlignedNWs Thermal oxidation 67 92 at 1 rnA cm~2 10 h lt10 [401]

AlignedNRs Metalorganic CVD 56 NA 180 h no detectable degradation

[402]

Ce02 RandomNWs Hydrothermal

method 58 at 1 )lA cmshy 2 NA 5 h lt3 [403]

Ru02 AlignedNRs Metalorganic CVD 110 55 (at the beginning

of emission) 50 h no detectable

degradation [404]

Aligned NRs Metalorganic CVD 103 49 (at the beginning 14 h no detectable [405] of emission) degradation

AlignedNWs Gas-solid reaction 11 (1 )lA cm-Z) NA 16h lt2 [406]

Dendritic nanosshy Gas-solid reaction 41 NA NA [407] tructures

AlignedNWs Gas-solid reaction 22 8 NA [408J

CuS Nanowalls Gas-solid reaction 85 205 (1 rnA cm~2) NA [409]

NbSz RandomNWs CVD 25 NA NA [410]

MoS2 Nanoflower Thermal reaction 45-55 76-86 2h lt20 [411J Random nanoshy Chemical-vapor 198 27 (01 rnA cm-2) NA [412]

belts transport and pyrolysis

Aligned nanobelts Chemical-vapor 095 36 (1 rnA cm~2) 2h lt313 [413] transport and pyrolysis

TiC RandomNWs Chloride-assisted 71 NA 2h lt10 [414] carbon thermal reduction

TiN Randomr--Ws Chloride-assisted 70 NA 2h lt8 [415] carbon thermal reduction

RandomNWs Vapor-phase deposishy 8 at 01 JlA cm-2 NA 40 h no obvious degshy [416] tion radation

major limitation is that the growth mechanisms are poorly known and most structural director agents are chosen by trial and error The essence of ID nanostructures formashytion is its anisotropic growth Most ID nanostructures are formed under nonequilibrium conditions and controlled by kinetic processes Although it is rather complex a solid understanding on the growth kinetics is essential in order to control the growth process of ID nanostructures Many questions about the formation process and mechanisms

explaining the structures morphologies and dimensions still need to be addressed

FE is one of the important applications of ID nanostrucshytures ID nanostructures have an effectively enhanced elecshytric field at the emission centers owing to their high aspect ratios and sharp tips Hence they can emit electrons at low electric fields boding well for FE applications Many papers are available in the literature describing the large variety of nanostructures having various shapes and corresponding

58 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

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56 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Figure 28 Arabic numerals and Chinese characters displayed by the double-gated FED Reprinted with permission from [388J J Chen et aI Appl Phys Lett 90 253105 (2007) copy 2007 American Institute ofPhysies

4 CONCLUSION AND OUTLOOK In this chapter we provide an up-to-date review of the recent progress on the fabrication and FE properties of some important inorganic ID semiconductor nanostrucshytures The study of various ID nanostructures as cold cathode field emitter has been constantly gaining interest and momentum among science and engineering communishyties since the late 1990s owing to their wide applications and potential markets in vacuum electron sources and FE devices The number of papers related to the fabrication and FE properties of ID nanostructures is very large and increasing exponentially yearly Needless to say owing to space limitation this article only focuses on some important inorganic ID semiconductor nanostructures and does not cover all the exciting work reported in this area Table 13 is an up-to-date summary on FE characteristics of other several inorganic semiconductor nanostructures such as metal sulfide oxide carbide nitride and even silicide Interested readers can refer to the relevant references

Fabrication of various ID nanostructures is the first step and opens up the investigation of dimension- and shape-dependent properties and potential applications to nanoscale electronic and optoelectronic devices In the past decade much effort has been devoted to fabricating high-quality semiconductor nanostructures mainly based on the bottom-up process by various techniques Although various ID nanostructures such as NRs NWs NBs NTs nanocables Nes nanoflowers nanoneedles nanotips nanonails nanopencils nanoporous structures nan on ails nanocolumns nanopyramids nanosphere assembly nanoshysprings nanohelical and hierarchical nanostructures have been fabricated how to control the size uniformity dimenshysionality growth direction atomic structures chemical composition and dopant distribution in the ID semiconshyductor nanostructures and how to integrate the functional nanostructures into devices with fully controlled interface structures or junction are still challenging issues for mateshyrials scientists and nanotechnologists In this article we discuss some typical bottom-up growth approaches used to fabricate ID semiconductor nanostructures They mainly fall into two main categories vapor phase and solution approaches The VLS growth process is reasonably well understood However further fundamental understanding of the surface physical and chemical processes and the processes occurring at the catalyst-nanostructure interface are essential for optimizing and controlling growth of ID nanostructures The VS process has been shown to be a powerful technique to fabricate 1D semiconductor nanoshystructures especially oxide and sulfide NWs or nanobelts However current understanding of this growth process and intrinsic mechanism is relatively poor Solution-based techshyniques such as hydrothermal or solvothermal growth SLS growth capping agents or surfactants-assisted synthesis and template-directed growth have been demonstrated as promising alternative approaches to produce ID semiconshyductor nanostructures with excellent control over the shape and composition with high reproducibility However the

Table 12 Tungsten oxide 1D nanostructures with various shapes and corresponding FE parameters reported in the literature

Stability testing time and Tungsten oxide emitters Synthesis method (V Jll11-1) Eh (V llm~l) fluctuation Ref

W1S0 49

AlignedNWs Thermal evaporation 41 NA NA [378] Aligned nanotips Thermal evaporation 20 437 135 h lt2 [379] Aligned NWs Thermal evaporation 28 NA NA [380] Aligned NWs Thermal evaporation 26 62 NA [381] Random nanobelts Thermal chemical reaction 33 152 at 28 rnA cm~2 1 h lt2 (382) WOZ9

AlignedNRs Thermal evaporation 12 NA 2h lt15 [383] 2D NW networks Thermal evaporation 074 NA NA [384] 3D WO~ NW network Thermal evaporation 1385 NA NA [385]

~-WO

AlignedNWs Thermal evaporation 48 NA NA [386] N-dopedNWs Thermal evaporation 644 942 NA [387J wormhole~like mesoporous

NWs AAO template-directed

growth 0083 175 20h lt30 [389]

57 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 13 Important FE properties of several other ID inorganic semiconductor nanostruetUTes reported in the literature

Stability testing time

Materials Morphology Synthesis method Eo (V)lm-l) Eth (V )lm- l) and fiuctuation Ref

MoO Nanobelts Thennal evaporation 87 132 2h lt15 [390]

AlignedNWs Thermal evaporation 35 765 1 h lt10 [391J

MoOz AIignedNWs Thermal evaporation 24 56 NA [392]

AlignedNRs 1bennal evaporation 4 at 01)lA cm-z NA NA [393]

Cactus-like nanos- Carbon thermal 126 232 NA [394J truetures reduction

Aligned N-WS Thennal evaporation 62 NA 75 h lt13 [395]

Random GapJ-C Thermal evaporation 773 845 NA [396] nanocables

RandomNWs Thermal oxidation 63 (as-grown) 36 (H2- NA NA [397] plasma treatment) at 01 )lA Cm~2

RandomNWs Thermal oxidation 33 NA NA [398]

AlignedNWs Thermal oxidation 63 10 NA [399]

MnO AlignedNWs Electrodcposition 84 at 1 )lA cm~2 NA NA [400]

l1J2O

S AlignedNWs Thermal oxidation 67 92 at 1 rnA cm~2 10 h lt10 [401]

AlignedNRs Metalorganic CVD 56 NA 180 h no detectable degradation

[402]

Ce02 RandomNWs Hydrothermal

method 58 at 1 )lA cmshy 2 NA 5 h lt3 [403]

Ru02 AlignedNRs Metalorganic CVD 110 55 (at the beginning

of emission) 50 h no detectable

degradation [404]

Aligned NRs Metalorganic CVD 103 49 (at the beginning 14 h no detectable [405] of emission) degradation

AlignedNWs Gas-solid reaction 11 (1 )lA cm-Z) NA 16h lt2 [406]

Dendritic nanosshy Gas-solid reaction 41 NA NA [407] tructures

AlignedNWs Gas-solid reaction 22 8 NA [408J

CuS Nanowalls Gas-solid reaction 85 205 (1 rnA cm~2) NA [409]

NbSz RandomNWs CVD 25 NA NA [410]

MoS2 Nanoflower Thermal reaction 45-55 76-86 2h lt20 [411J Random nanoshy Chemical-vapor 198 27 (01 rnA cm-2) NA [412]

belts transport and pyrolysis

Aligned nanobelts Chemical-vapor 095 36 (1 rnA cm~2) 2h lt313 [413] transport and pyrolysis

TiC RandomNWs Chloride-assisted 71 NA 2h lt10 [414] carbon thermal reduction

TiN Randomr--Ws Chloride-assisted 70 NA 2h lt8 [415] carbon thermal reduction

RandomNWs Vapor-phase deposishy 8 at 01 JlA cm-2 NA 40 h no obvious degshy [416] tion radation

major limitation is that the growth mechanisms are poorly known and most structural director agents are chosen by trial and error The essence of ID nanostructures formashytion is its anisotropic growth Most ID nanostructures are formed under nonequilibrium conditions and controlled by kinetic processes Although it is rather complex a solid understanding on the growth kinetics is essential in order to control the growth process of ID nanostructures Many questions about the formation process and mechanisms

explaining the structures morphologies and dimensions still need to be addressed

FE is one of the important applications of ID nanostrucshytures ID nanostructures have an effectively enhanced elecshytric field at the emission centers owing to their high aspect ratios and sharp tips Hence they can emit electrons at low electric fields boding well for FE applications Many papers are available in the literature describing the large variety of nanostructures having various shapes and corresponding

58 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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57 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

Table 13 Important FE properties of several other ID inorganic semiconductor nanostruetUTes reported in the literature

Stability testing time

Materials Morphology Synthesis method Eo (V)lm-l) Eth (V )lm- l) and fiuctuation Ref

MoO Nanobelts Thennal evaporation 87 132 2h lt15 [390]

AlignedNWs Thermal evaporation 35 765 1 h lt10 [391J

MoOz AIignedNWs Thermal evaporation 24 56 NA [392]

AlignedNRs 1bennal evaporation 4 at 01)lA cm-z NA NA [393]

Cactus-like nanos- Carbon thermal 126 232 NA [394J truetures reduction

Aligned N-WS Thennal evaporation 62 NA 75 h lt13 [395]

Random GapJ-C Thermal evaporation 773 845 NA [396] nanocables

RandomNWs Thermal oxidation 63 (as-grown) 36 (H2- NA NA [397] plasma treatment) at 01 )lA Cm~2

RandomNWs Thermal oxidation 33 NA NA [398]

AlignedNWs Thermal oxidation 63 10 NA [399]

MnO AlignedNWs Electrodcposition 84 at 1 )lA cm~2 NA NA [400]

l1J2O

S AlignedNWs Thermal oxidation 67 92 at 1 rnA cm~2 10 h lt10 [401]

AlignedNRs Metalorganic CVD 56 NA 180 h no detectable degradation

[402]

Ce02 RandomNWs Hydrothermal

method 58 at 1 )lA cmshy 2 NA 5 h lt3 [403]

Ru02 AlignedNRs Metalorganic CVD 110 55 (at the beginning

of emission) 50 h no detectable

degradation [404]

Aligned NRs Metalorganic CVD 103 49 (at the beginning 14 h no detectable [405] of emission) degradation

AlignedNWs Gas-solid reaction 11 (1 )lA cm-Z) NA 16h lt2 [406]

Dendritic nanosshy Gas-solid reaction 41 NA NA [407] tructures

AlignedNWs Gas-solid reaction 22 8 NA [408J

CuS Nanowalls Gas-solid reaction 85 205 (1 rnA cm~2) NA [409]

NbSz RandomNWs CVD 25 NA NA [410]

MoS2 Nanoflower Thermal reaction 45-55 76-86 2h lt20 [411J Random nanoshy Chemical-vapor 198 27 (01 rnA cm-2) NA [412]

belts transport and pyrolysis

Aligned nanobelts Chemical-vapor 095 36 (1 rnA cm~2) 2h lt313 [413] transport and pyrolysis

TiC RandomNWs Chloride-assisted 71 NA 2h lt10 [414] carbon thermal reduction

TiN Randomr--Ws Chloride-assisted 70 NA 2h lt8 [415] carbon thermal reduction

RandomNWs Vapor-phase deposishy 8 at 01 JlA cm-2 NA 40 h no obvious degshy [416] tion radation

major limitation is that the growth mechanisms are poorly known and most structural director agents are chosen by trial and error The essence of ID nanostructures formashytion is its anisotropic growth Most ID nanostructures are formed under nonequilibrium conditions and controlled by kinetic processes Although it is rather complex a solid understanding on the growth kinetics is essential in order to control the growth process of ID nanostructures Many questions about the formation process and mechanisms

explaining the structures morphologies and dimensions still need to be addressed

FE is one of the important applications of ID nanostrucshytures ID nanostructures have an effectively enhanced elecshytric field at the emission centers owing to their high aspect ratios and sharp tips Hence they can emit electrons at low electric fields boding well for FE applications Many papers are available in the literature describing the large variety of nanostructures having various shapes and corresponding

58 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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FE properties Various fascinating nanostructures with different InOIphologies provide models to investigate shapeshydependent FE properties and playa very important role in the development of next-generation FE devices Tables 1-13 list the important FE parameters of some important 1D semiconductor nanostructures However there are various emitter parameters that influence FE and more often than not the contributions by individual parameters are difficult to identify and quantify Thus it is rather difficult to unamshybiguously compare their emission properties According to eqns (1) and (2) the effective enhanced field at the emisshysion centers is PE and so it is possible to emit electron from a given nanostructured material if it is long and sharp at an applied field E As described in Section 3 the Pvalues of field emitters or FEAs are highly dependent on the aspect ratios tip shapes alignment of emitters and density of emitters as well as measurement vacuum conditions and sample-anode distances The reported 1D nanostructures have different tip shapes and sizes aspect ratios and densishyties and their FE properties are measured under different vacuum conditions and sample-anode distances Therefore it is difficult to compare and determine which FE perforshymance is better by simply comparing the Eo and E or Pth

values Many inorganic semiconductor 1D nanomaterials such as ZnO and AlN can be prepared in a variety of sizes shapes and aspect ratios and they display excellent FE characteristics Hence in order to properly compare the FE performance and efficiency of different emitters they must be synthesized to have a similar geometry alignment and density and measured under the same vacuum conditions and sample-anode distances This requires more dedicated work than that reported so far The intrinsic work function (4raquo varies from 2 to 7 e V for most of the inorganic semishyconductor materials and it is also an important parameter influencing the FE current density As discussed in Section 3 the work function of 1D semiconductor nanostructures could be varied by changing the surface states of adsorption and defects even gas exposure A lower tP value is better for higher emission efficiency

The long-term stability of a field emitter that is stability of the emission current over a long period of time is another key parameter crucial to FE applications and performance From the emission mechanism standpoint it is clear that the current emission depends strongly on a number of factors including the applied electric field tip geometry emitter density and work function of the emitter materials Variashytions in any of these factors should be reflected by the emisshysion currents The thermal instability is one of the reasons causing emitter failure In some cases emission from some materials appears to drop abruptly when large currents are emitted and it is often associated with sudden melting of the emitters [2] Therefore the emitter stability must also be considered along with the overall enhancement of the FE properties Inorganic semiconducting nanomaterials such as ZnO AlN and W03 having much higher melting points and good thermal stability compared to that of organic materials are more preferable for future FE applications

In order to expedite and expand industrial acceptance of these novel nanostructured emitter materials one of the major challenges is the synthesis of more robust field

emitters In fact a constant topic for vacuum microelecshytronics is how to produce a cathode of high performance because the cathode is the core of a FE device The search for 1D nanostructure with a low work function and good physical and chemical properties suitable for FE applicashytions will continue by rigorous theoretical and experishymental investigation Future work in this direction should continue to focus on techniques to produce addressable 1D inorganic semiconductor nanoemitter arrays in a more controllable method with ideal separation to overcome the field-screening effect and ways to grow them in designable devices configuration on large-area substrates demanded by industrial applications

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NSFC 50902104) Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No CityU 112307 Key Project of Chinese Ministry ofEducation (No 208087) Hubei Province Natural Science Foundation (No 2008CDB015 and 2008CDB033) and Project of Educational Commission of Hubei Province of China (Z200711001 and Q20081105)

GLOSSARY absorbate states Absorbates on semiconductor surface result in electronic states in energy band and impact on the electrical behavior of semiconductor These electronic states are called absorbate states band gap In a semiconductor or insulators the minimum energy necessary for an electron to transfer from the valence band into the conduction band conduction band A vacant or partially occupied energy band in which the electrons can move freely or nearly so electron affinity Electron affinity is defined as the energy required removing an electron from the bottom of conducshytion band to vacuum level In this Case that vacuum level lies below the conduction band minimum the surface has a negative electron affinity Fenni level For metals the highest occupied molecular orbital in the valence band at absolute zero temperature (T = 0 K) In insulators and semiconductors the valence and conduction bands are separated Therefore the Fermi level is located in the band gap Based on Fermi-Dirac distribution function the probability for an electron to occupy Fermi level is one half field emission Process where electrons tunnel through a surface potential barrier of a solid that is a field emitter into vacuum under an intense electric field F-Nplot Curve ofln(Jpound2) vslE ionization The process of adding or removing an elecshytron tofrom an atom thereby creating a charged atom (ie ion) I-V curve Current vs voltage characteristics J-E curve Current density vs electric field characteristics

59 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

surface barrier Increased potential at the surface of a material thermionic emission Process where electrons are emitted from hot filaments via heating to overcome the surface work function of the materials tunneling Quantum mechanical process where a particle can pass through a barrier rather than having to go over the barrier valence band The highest electronic energy band in semishyconduetors or insulators whieh can be filled with electrons work function Potential barrier between the Fermi energy level (E

F) of the materials and vacuum level

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1526 (2005) 362 Q Wan P and T H Wang AppL Phys Lett 89 123102

(2006) 363 K Y Cheung C T Yip A B Djurisic Y H Leung and

W K ChanAdv Funct Mater 17555 (2007) 364 X Chen and S S Mao Chern Rev 172891 (2007) 365 Y Ohsaki N Masaki T Kitamura Y Wada T Okamoto

T Sekino K Niihara and S Yanagida Phys Chern Chern Phys 74157 (2005)

366 G K Mor K Shankar M Paulose O K Varghese and c A Grimes Nano Lett 6215 (2006)

367 M Miyauchi H Tokudome Y Toda T Kamiya and H Hosono Appl Phys Lett 89043114 (2006)

368 B Xiang Y Zhang Z Wang X H Luo Y W Zhu H Z Zhang and D P YUJ Phys D Appl Phys 38 1152 (2005)

369 J M Wu H C Shih and W T Wu Chern Phys Lett 413 490 (2005)

370 K F Huo J J Fu X M Zhang G X Qian Y C Xin H W Ni and P K Chul Nanosci Nanotech 93341 (2009)

371 G Liu F Li D W Wang D M Tang C Liu X Ma G Q Lu and H M Cheng Nanotechnology 19 025606 (2008)

372 K F Huo X M Zhang L S Hu X J Sun R S Chen J J Fu and P K Chu Appl Phys Lett 93013105 (2008)

373 A R Armstrong G Armstrong J Canales R Garcia and P G BruceAdv Mater 17862 (2005)

374 M Miyauchi and II Tokudomei Mater Chern 172095 (2007) 375 Q Zhang and L Gao Langmuir 19 967 (2003) 376 S M Liu L M Gan L H Liu W D Zhang and H C Zeng

Chem Mater 141391 (2002) 377 1 J Qiu W D Yu X D Gao and X M Li Nanotechnology 17

4695 (2006) 378 K Huang Q Pan F Yang S Ni and D He Mater Res Bull 43

919 (2008) 379 J Zhou L Gong S Z Deng 1 Chen J C Sha N S Xu

R S Yang and Z L Wang Appl Phys Lett 87223108 (2005) 380 K Huang Q T Pan F Yang S B Ni and D Y He Appl Swf

Sci 253 8923 (2007) 381 Y B Li Y Bando and D GolbergAdv Mater 151294 (2003) 382 A Aglral and J G E (Han) Gardeniers J Phys Chern C 112

15183 (2008) 383 J G Liu Z J Zhang Y Zhao X Su S Liu and E Wang Small

1310 (2005) 384 Y M Zhao Y H Li 1 Ahmad D G McCartney Y Q Zhu and

W B HuAppl Phys Lett 89133116 (2006) 385 J Zhou Y Ding S Z Deng L Gong N S Xu and Z L Wang

Adv Mater 172107 (2005) 386 Y Baek and K Yong J Phys Chem C 1111213 (2007) 387 M T Chang L J Chou Y L Chueh Y C Lee C H Hsieh

C D Chen Y W Lan and L J Chen Small 3 658 (2007) 388 J Chen Y Y Dai J Luo Z L Li S Z Deng 1 C She and

N S XuAppl Phys Lett 90253105 (2007) 389 W H Lai M H Hon L G Teoh Y H Su J Shieh and

C K Chen J Electron Mater 37 1082 (2008) 390 Y B Li Y Bondo D Golberg and K Kurashima AppL Phys

Lett 81 5048 (2002) 391 J Zhou S Z Deng N S Xu J Chen and J C SheApp Phys

Lett 83 2653 (2003) 392 J Zhou N S Xu S Z Deng J Chen 1 C She and Z L Wang

Adv Mater 151835 (2003) 393 J G Liu Z J Zhang C Y Pan Y Zhao X Sun Y Zhou and

D P Yu Mater Lett 583812 (2004) 394 C Cao Z Chen X An and H Zhu I Phys Chern C 112 95 (2008) 395 Y Huang Z Wang Q Wang C Gu c Tang Y Bando and

D GolbergJ Phys Chern C 113 1980 (2009) 396 J Zhan Y Bando J Hu Y Li and D Golberg Chem Mater 16

5158 (2004) 397 F Xu K Yu Q Wang M Shi Q Zhang W Bai Q Li and

Z Zhu 1 Nanosci Nanotech 7 2774 (2007) 398 L c Hsu Y Y Li and C Y Hsiao Nanoscale Res Lett 3 330

(2008) 399 Y L Chueh M W Lai J Q Liang L J Chou and Z L Wang

Adv Funct Mater 16 2243 (2006) 400 M S Wu J T Lee Y Y Wang and C C Wan J Phys Chern

E 42 16331 (2004) 401 B Varghese S C Haur and C T LimJ Phys Chern C 11210008

(2008) 402 R S Chen Y S Huang Y M Liang C S Hsieh D S Tsai and

K K TiongAppl Phys Lett 84 1552 (2004) 403 X Q Fu P Feng C Wang and T H Wang Chin Phys Lett 8

2423 (2007) 404 C S Hsieh D S Tsai R S Chen and Y S Huang Appl Phys

Lett 853860 (2004) 405 c L Cheng Y F Chen R S Chen and Y S Huang Appl Phys

Lett 86 103104 (2005) 406 J Chen S Z Deng N S Xu S H Wang X G Wen S H Yang

C L Yang J N Wang and W K GeAppl Phys Lett 803620 (2002)

65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

407 Y Xiao1 J Chen S Z Deng N S Xu1 and S Yangf Nanosci 412 X C Wu Y R Tao Y M Hu Y Song Z Hu J J Zhu and Nanotech 8 237 (2008) L Dong Nanotechnology 17 201 (2006)

408 O B Wu S Ren S Z Deng J Chen and N S Xu J Vac Sci 413 Y L Zhang X C Wu Y R Tao C J Mao and J J Zhu Chern Technol B 22 1282 (2004) Cornmun 2683 (2008)

409 X Feng Y Li H Liu Y Li S Cui N Wang L Jiang X Liu and 414 K F Huo Y M Hu Y W Ma Y N Lii Z Hu and Y Chen M Yuan Nanotechnology 18 145706 (2007) Nanotechnology 18 145615 (2007)

410 Y Z Jin W K Hsu Y L Chueh L J Chou Y Q Zhu 415 Y M Hu K F Huo Y W Ma Y N Lii J Q Xu Z Eu and K Brigatti H W Kroto and D R M WaltonAngew Chern Int Y Chenf Nanosci Nanotech 72922 (2007) Ed 435670 (2004) 416 B Xiang O X Wang Z Wang X Z Zhang L Q lin J Xu and

411 Y B Li Y Bando and D GolbergAppl Phys Lett 82 1962 (2003) D P YuAppl Phygt Lett 86243103 (2005)

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64 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

338 Y L Liu L Liao J C Li and C X Pan I Phys Chern C 111 5050 (2007)

339 1 Chen S Z Deng N S Xu W X Zhang X G Wen and S H YangAppl Phys Lett 83746 (2003)

340 R H Good and E W Muller in Encyclopedia of Physics (S Flugge Ed) Vol XXI Berlin Springer 1956

341 J Chen N Y Huang S Z Deng J C She N S Xu W X Zhang X G Wen and S H Yang AppL Phys Lett 86151107 (2005)

342 Y W Zhu A M Moo T Yu X J Xu X Y Gao Y J Liu C T Lim et aI Phys Lett 419458 (2006)

343 W Y Sung W 1 Kim S M Lee H Y Lee Y II Kim K H Park and S Lee Vacuurn 81 851 (2007)

344 Y W Zhu C H Sow and 1 T L Thong J AppL Phys 102 114302 (2007)

345 S C Yeon W Y Sung W 1 Kim S M Lee H Y Lee and Y H Kim Vac Sci Techno B 24 940 (2006)

346 S H Luo Q Wan W L Liu M Zhang Z F Di S Y Wang Z T Song C L Lin and J Y Dai Nanotechnology 15 1424 (2004)

347 S H Luo P K Chu Z F Di M Zhang W L Liu C L Lin 1 Y Fan and X L WuAppl Phys Lett 88013109 (2006)

348 J H He T H Wu C L Hsin K M Li L J Chen Y L Chueh L J Chou and Z L Wang Srna1l2 116 (2006)

349 H S Jang S O Kang and Y 1 Kim Solid State Commun 140495 (2006)

350 L A Ma Y Ye L Q Hu K L Zheng and T L Guo Physica E 40 3127 (2008)

351 J Wu K Yu L J Li 1 W Xu D J Shang Y E Xu and Z Q Zhu Phys D Appl Phys 41 185302 (2008)

352 Y J Chen O H Li Y X Liang T H Wang Q Zhao and D P YuAppl Phys Lett 855682 (2004)

353 B Wang Y H Yang C X Wang N S Xu and G W Yang J App Phys 98 124303 (2005)

354 Q Wang KYu and F Xu Solid State Cornmun 143260 (2007) 355 1 Hamburg and C G GranqvistJ Appl Phys 60 Rl23 (1986) 356 C Li D H Zhang X L Liu S Han T Tang J Han and

C W ZhouAppl Phys Lett 82 1613 (2003) 35Z S Kar S Chakrabarli and S Chaudhuri Nanotechnology 17 3058

(2006) 358 S Kar and S Chaudhuri Chern Phys Lett 422 424 (2006) 359 S Q Li Y X Liang and Y H Wang Appl Phys Lett 87143104

(2005) 360 S Q Li Y X Liang and T H WangAppl Phys Lett 88 053107

(2006) 361 H S lang D H Kim H R Lee and S Y Lee Mater Lett 59

1526 (2005) 362 Q Wan P and T H Wang AppL Phys Lett 89 123102

(2006) 363 K Y Cheung C T Yip A B Djurisic Y H Leung and

W K ChanAdv Funct Mater 17555 (2007) 364 X Chen and S S Mao Chern Rev 172891 (2007) 365 Y Ohsaki N Masaki T Kitamura Y Wada T Okamoto

T Sekino K Niihara and S Yanagida Phys Chern Chern Phys 74157 (2005)

366 G K Mor K Shankar M Paulose O K Varghese and c A Grimes Nano Lett 6215 (2006)

367 M Miyauchi H Tokudome Y Toda T Kamiya and H Hosono Appl Phys Lett 89043114 (2006)

368 B Xiang Y Zhang Z Wang X H Luo Y W Zhu H Z Zhang and D P YUJ Phys D Appl Phys 38 1152 (2005)

369 J M Wu H C Shih and W T Wu Chern Phys Lett 413 490 (2005)

370 K F Huo J J Fu X M Zhang G X Qian Y C Xin H W Ni and P K Chul Nanosci Nanotech 93341 (2009)

371 G Liu F Li D W Wang D M Tang C Liu X Ma G Q Lu and H M Cheng Nanotechnology 19 025606 (2008)

372 K F Huo X M Zhang L S Hu X J Sun R S Chen J J Fu and P K Chu Appl Phys Lett 93013105 (2008)

373 A R Armstrong G Armstrong J Canales R Garcia and P G BruceAdv Mater 17862 (2005)

374 M Miyauchi and II Tokudomei Mater Chern 172095 (2007) 375 Q Zhang and L Gao Langmuir 19 967 (2003) 376 S M Liu L M Gan L H Liu W D Zhang and H C Zeng

Chem Mater 141391 (2002) 377 1 J Qiu W D Yu X D Gao and X M Li Nanotechnology 17

4695 (2006) 378 K Huang Q Pan F Yang S Ni and D He Mater Res Bull 43

919 (2008) 379 J Zhou L Gong S Z Deng 1 Chen J C Sha N S Xu

R S Yang and Z L Wang Appl Phys Lett 87223108 (2005) 380 K Huang Q T Pan F Yang S B Ni and D Y He Appl Swf

Sci 253 8923 (2007) 381 Y B Li Y Bando and D GolbergAdv Mater 151294 (2003) 382 A Aglral and J G E (Han) Gardeniers J Phys Chern C 112

15183 (2008) 383 J G Liu Z J Zhang Y Zhao X Su S Liu and E Wang Small

1310 (2005) 384 Y M Zhao Y H Li 1 Ahmad D G McCartney Y Q Zhu and

W B HuAppl Phys Lett 89133116 (2006) 385 J Zhou Y Ding S Z Deng L Gong N S Xu and Z L Wang

Adv Mater 172107 (2005) 386 Y Baek and K Yong J Phys Chem C 1111213 (2007) 387 M T Chang L J Chou Y L Chueh Y C Lee C H Hsieh

C D Chen Y W Lan and L J Chen Small 3 658 (2007) 388 J Chen Y Y Dai J Luo Z L Li S Z Deng 1 C She and

N S XuAppl Phys Lett 90253105 (2007) 389 W H Lai M H Hon L G Teoh Y H Su J Shieh and

C K Chen J Electron Mater 37 1082 (2008) 390 Y B Li Y Bondo D Golberg and K Kurashima AppL Phys

Lett 81 5048 (2002) 391 J Zhou S Z Deng N S Xu J Chen and J C SheApp Phys

Lett 83 2653 (2003) 392 J Zhou N S Xu S Z Deng J Chen 1 C She and Z L Wang

Adv Mater 151835 (2003) 393 J G Liu Z J Zhang C Y Pan Y Zhao X Sun Y Zhou and

D P Yu Mater Lett 583812 (2004) 394 C Cao Z Chen X An and H Zhu I Phys Chern C 112 95 (2008) 395 Y Huang Z Wang Q Wang C Gu c Tang Y Bando and

D GolbergJ Phys Chern C 113 1980 (2009) 396 J Zhan Y Bando J Hu Y Li and D Golberg Chem Mater 16

5158 (2004) 397 F Xu K Yu Q Wang M Shi Q Zhang W Bai Q Li and

Z Zhu 1 Nanosci Nanotech 7 2774 (2007) 398 L c Hsu Y Y Li and C Y Hsiao Nanoscale Res Lett 3 330

(2008) 399 Y L Chueh M W Lai J Q Liang L J Chou and Z L Wang

Adv Funct Mater 16 2243 (2006) 400 M S Wu J T Lee Y Y Wang and C C Wan J Phys Chern

E 42 16331 (2004) 401 B Varghese S C Haur and C T LimJ Phys Chern C 11210008

(2008) 402 R S Chen Y S Huang Y M Liang C S Hsieh D S Tsai and

K K TiongAppl Phys Lett 84 1552 (2004) 403 X Q Fu P Feng C Wang and T H Wang Chin Phys Lett 8

2423 (2007) 404 C S Hsieh D S Tsai R S Chen and Y S Huang Appl Phys

Lett 853860 (2004) 405 c L Cheng Y F Chen R S Chen and Y S Huang Appl Phys

Lett 86 103104 (2005) 406 J Chen S Z Deng N S Xu S H Wang X G Wen S H Yang

C L Yang J N Wang and W K GeAppl Phys Lett 803620 (2002)

65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

407 Y Xiao1 J Chen S Z Deng N S Xu1 and S Yangf Nanosci 412 X C Wu Y R Tao Y M Hu Y Song Z Hu J J Zhu and Nanotech 8 237 (2008) L Dong Nanotechnology 17 201 (2006)

408 O B Wu S Ren S Z Deng J Chen and N S Xu J Vac Sci 413 Y L Zhang X C Wu Y R Tao C J Mao and J J Zhu Chern Technol B 22 1282 (2004) Cornmun 2683 (2008)

409 X Feng Y Li H Liu Y Li S Cui N Wang L Jiang X Liu and 414 K F Huo Y M Hu Y W Ma Y N Lii Z Hu and Y Chen M Yuan Nanotechnology 18 145706 (2007) Nanotechnology 18 145615 (2007)

410 Y Z Jin W K Hsu Y L Chueh L J Chou Y Q Zhu 415 Y M Hu K F Huo Y W Ma Y N Lii J Q Xu Z Eu and K Brigatti H W Kroto and D R M WaltonAngew Chern Int Y Chenf Nanosci Nanotech 72922 (2007) Ed 435670 (2004) 416 B Xiang O X Wang Z Wang X Z Zhang L Q lin J Xu and

411 Y B Li Y Bando and D GolbergAppl Phys Lett 82 1962 (2003) D P YuAppl Phygt Lett 86243103 (2005)

64 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

338 Y L Liu L Liao J C Li and C X Pan I Phys Chern C 111 5050 (2007)

339 1 Chen S Z Deng N S Xu W X Zhang X G Wen and S H YangAppl Phys Lett 83746 (2003)

340 R H Good and E W Muller in Encyclopedia of Physics (S Flugge Ed) Vol XXI Berlin Springer 1956

341 J Chen N Y Huang S Z Deng J C She N S Xu W X Zhang X G Wen and S H Yang AppL Phys Lett 86151107 (2005)

342 Y W Zhu A M Moo T Yu X J Xu X Y Gao Y J Liu C T Lim et aI Phys Lett 419458 (2006)

343 W Y Sung W 1 Kim S M Lee H Y Lee Y II Kim K H Park and S Lee Vacuurn 81 851 (2007)

344 Y W Zhu C H Sow and 1 T L Thong J AppL Phys 102 114302 (2007)

345 S C Yeon W Y Sung W 1 Kim S M Lee H Y Lee and Y H Kim Vac Sci Techno B 24 940 (2006)

346 S H Luo Q Wan W L Liu M Zhang Z F Di S Y Wang Z T Song C L Lin and J Y Dai Nanotechnology 15 1424 (2004)

347 S H Luo P K Chu Z F Di M Zhang W L Liu C L Lin 1 Y Fan and X L WuAppl Phys Lett 88013109 (2006)

348 J H He T H Wu C L Hsin K M Li L J Chen Y L Chueh L J Chou and Z L Wang Srna1l2 116 (2006)

349 H S Jang S O Kang and Y 1 Kim Solid State Commun 140495 (2006)

350 L A Ma Y Ye L Q Hu K L Zheng and T L Guo Physica E 40 3127 (2008)

351 J Wu K Yu L J Li 1 W Xu D J Shang Y E Xu and Z Q Zhu Phys D Appl Phys 41 185302 (2008)

352 Y J Chen O H Li Y X Liang T H Wang Q Zhao and D P YuAppl Phys Lett 855682 (2004)

353 B Wang Y H Yang C X Wang N S Xu and G W Yang J App Phys 98 124303 (2005)

354 Q Wang KYu and F Xu Solid State Cornmun 143260 (2007) 355 1 Hamburg and C G GranqvistJ Appl Phys 60 Rl23 (1986) 356 C Li D H Zhang X L Liu S Han T Tang J Han and

C W ZhouAppl Phys Lett 82 1613 (2003) 35Z S Kar S Chakrabarli and S Chaudhuri Nanotechnology 17 3058

(2006) 358 S Kar and S Chaudhuri Chern Phys Lett 422 424 (2006) 359 S Q Li Y X Liang and Y H Wang Appl Phys Lett 87143104

(2005) 360 S Q Li Y X Liang and T H WangAppl Phys Lett 88 053107

(2006) 361 H S lang D H Kim H R Lee and S Y Lee Mater Lett 59

1526 (2005) 362 Q Wan P and T H Wang AppL Phys Lett 89 123102

(2006) 363 K Y Cheung C T Yip A B Djurisic Y H Leung and

W K ChanAdv Funct Mater 17555 (2007) 364 X Chen and S S Mao Chern Rev 172891 (2007) 365 Y Ohsaki N Masaki T Kitamura Y Wada T Okamoto

T Sekino K Niihara and S Yanagida Phys Chern Chern Phys 74157 (2005)

366 G K Mor K Shankar M Paulose O K Varghese and c A Grimes Nano Lett 6215 (2006)

367 M Miyauchi H Tokudome Y Toda T Kamiya and H Hosono Appl Phys Lett 89043114 (2006)

368 B Xiang Y Zhang Z Wang X H Luo Y W Zhu H Z Zhang and D P YUJ Phys D Appl Phys 38 1152 (2005)

369 J M Wu H C Shih and W T Wu Chern Phys Lett 413 490 (2005)

370 K F Huo J J Fu X M Zhang G X Qian Y C Xin H W Ni and P K Chul Nanosci Nanotech 93341 (2009)

371 G Liu F Li D W Wang D M Tang C Liu X Ma G Q Lu and H M Cheng Nanotechnology 19 025606 (2008)

372 K F Huo X M Zhang L S Hu X J Sun R S Chen J J Fu and P K Chu Appl Phys Lett 93013105 (2008)

373 A R Armstrong G Armstrong J Canales R Garcia and P G BruceAdv Mater 17862 (2005)

374 M Miyauchi and II Tokudomei Mater Chern 172095 (2007) 375 Q Zhang and L Gao Langmuir 19 967 (2003) 376 S M Liu L M Gan L H Liu W D Zhang and H C Zeng

Chem Mater 141391 (2002) 377 1 J Qiu W D Yu X D Gao and X M Li Nanotechnology 17

4695 (2006) 378 K Huang Q Pan F Yang S Ni and D He Mater Res Bull 43

919 (2008) 379 J Zhou L Gong S Z Deng 1 Chen J C Sha N S Xu

R S Yang and Z L Wang Appl Phys Lett 87223108 (2005) 380 K Huang Q T Pan F Yang S B Ni and D Y He Appl Swf

Sci 253 8923 (2007) 381 Y B Li Y Bando and D GolbergAdv Mater 151294 (2003) 382 A Aglral and J G E (Han) Gardeniers J Phys Chern C 112

15183 (2008) 383 J G Liu Z J Zhang Y Zhao X Su S Liu and E Wang Small

1310 (2005) 384 Y M Zhao Y H Li 1 Ahmad D G McCartney Y Q Zhu and

W B HuAppl Phys Lett 89133116 (2006) 385 J Zhou Y Ding S Z Deng L Gong N S Xu and Z L Wang

Adv Mater 172107 (2005) 386 Y Baek and K Yong J Phys Chem C 1111213 (2007) 387 M T Chang L J Chou Y L Chueh Y C Lee C H Hsieh

C D Chen Y W Lan and L J Chen Small 3 658 (2007) 388 J Chen Y Y Dai J Luo Z L Li S Z Deng 1 C She and

N S XuAppl Phys Lett 90253105 (2007) 389 W H Lai M H Hon L G Teoh Y H Su J Shieh and

C K Chen J Electron Mater 37 1082 (2008) 390 Y B Li Y Bondo D Golberg and K Kurashima AppL Phys

Lett 81 5048 (2002) 391 J Zhou S Z Deng N S Xu J Chen and J C SheApp Phys

Lett 83 2653 (2003) 392 J Zhou N S Xu S Z Deng J Chen 1 C She and Z L Wang

Adv Mater 151835 (2003) 393 J G Liu Z J Zhang C Y Pan Y Zhao X Sun Y Zhou and

D P Yu Mater Lett 583812 (2004) 394 C Cao Z Chen X An and H Zhu I Phys Chern C 112 95 (2008) 395 Y Huang Z Wang Q Wang C Gu c Tang Y Bando and

D GolbergJ Phys Chern C 113 1980 (2009) 396 J Zhan Y Bando J Hu Y Li and D Golberg Chem Mater 16

5158 (2004) 397 F Xu K Yu Q Wang M Shi Q Zhang W Bai Q Li and

Z Zhu 1 Nanosci Nanotech 7 2774 (2007) 398 L c Hsu Y Y Li and C Y Hsiao Nanoscale Res Lett 3 330

(2008) 399 Y L Chueh M W Lai J Q Liang L J Chou and Z L Wang

Adv Funct Mater 16 2243 (2006) 400 M S Wu J T Lee Y Y Wang and C C Wan J Phys Chern

E 42 16331 (2004) 401 B Varghese S C Haur and C T LimJ Phys Chern C 11210008

(2008) 402 R S Chen Y S Huang Y M Liang C S Hsieh D S Tsai and

K K TiongAppl Phys Lett 84 1552 (2004) 403 X Q Fu P Feng C Wang and T H Wang Chin Phys Lett 8

2423 (2007) 404 C S Hsieh D S Tsai R S Chen and Y S Huang Appl Phys

Lett 853860 (2004) 405 c L Cheng Y F Chen R S Chen and Y S Huang Appl Phys

Lett 86 103104 (2005) 406 J Chen S Z Deng N S Xu S H Wang X G Wen S H Yang

C L Yang J N Wang and W K GeAppl Phys Lett 803620 (2002)

65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

407 Y Xiao1 J Chen S Z Deng N S Xu1 and S Yangf Nanosci 412 X C Wu Y R Tao Y M Hu Y Song Z Hu J J Zhu and Nanotech 8 237 (2008) L Dong Nanotechnology 17 201 (2006)

408 O B Wu S Ren S Z Deng J Chen and N S Xu J Vac Sci 413 Y L Zhang X C Wu Y R Tao C J Mao and J J Zhu Chern Technol B 22 1282 (2004) Cornmun 2683 (2008)

409 X Feng Y Li H Liu Y Li S Cui N Wang L Jiang X Liu and 414 K F Huo Y M Hu Y W Ma Y N Lii Z Hu and Y Chen M Yuan Nanotechnology 18 145706 (2007) Nanotechnology 18 145615 (2007)

410 Y Z Jin W K Hsu Y L Chueh L J Chou Y Q Zhu 415 Y M Hu K F Huo Y W Ma Y N Lii J Q Xu Z Eu and K Brigatti H W Kroto and D R M WaltonAngew Chern Int Y Chenf Nanosci Nanotech 72922 (2007) Ed 435670 (2004) 416 B Xiang O X Wang Z Wang X Z Zhang L Q lin J Xu and

411 Y B Li Y Bando and D GolbergAppl Phys Lett 82 1962 (2003) D P YuAppl Phygt Lett 86243103 (2005)

65 One-Dimensional Semiconductor Nanostructures Fabrication and Field-Emission Properties

407 Y Xiao1 J Chen S Z Deng N S Xu1 and S Yangf Nanosci 412 X C Wu Y R Tao Y M Hu Y Song Z Hu J J Zhu and Nanotech 8 237 (2008) L Dong Nanotechnology 17 201 (2006)

408 O B Wu S Ren S Z Deng J Chen and N S Xu J Vac Sci 413 Y L Zhang X C Wu Y R Tao C J Mao and J J Zhu Chern Technol B 22 1282 (2004) Cornmun 2683 (2008)

409 X Feng Y Li H Liu Y Li S Cui N Wang L Jiang X Liu and 414 K F Huo Y M Hu Y W Ma Y N Lii Z Hu and Y Chen M Yuan Nanotechnology 18 145706 (2007) Nanotechnology 18 145615 (2007)

410 Y Z Jin W K Hsu Y L Chueh L J Chou Y Q Zhu 415 Y M Hu K F Huo Y W Ma Y N Lii J Q Xu Z Eu and K Brigatti H W Kroto and D R M WaltonAngew Chern Int Y Chenf Nanosci Nanotech 72922 (2007) Ed 435670 (2004) 416 B Xiang O X Wang Z Wang X Z Zhang L Q lin J Xu and

411 Y B Li Y Bando and D GolbergAppl Phys Lett 82 1962 (2003) D P YuAppl Phygt Lett 86243103 (2005)