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ZnS Nanostructure Arrays: A Developing Material Star

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Xiaosheng Fang , * Limin Wu , * and Linfeng Hu

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Semiconductor nanostructure arrays are of great scientifi c and technical interest because of the strong non-linear and electro-optic effects that occur due to carrier confi nement in three dimensions. The use of such nanostruc-ture arrays with tailored geometry, array density, and length-diameter-ratio as building blocks are expected to play a crucial role in future nanoscale devices. With the unique properties of a direct wide-bandgap semiconductor, such as the presence of polar surfaces, excellent transport properties, good thermal stability, and high electronic mobility, ZnS nanostructure arrays has been a developing material star. The research on ZnS nanostructure arrays has seen remarkable progress over the last fi ve years due to the unique properties and important potential applications of nanostructure arrays, which are sum-marized here. Firstly, a survey of various methods to the synthesis of ZnS nanostructure arrays will be introduced. Next recent efforts on exploiting the unique properties and applications of ZnS nanostructure arrays are dis-cussed. Potential future directions of this research fi eld are also highlighted.

1. Introduction

Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties and potential applications. [ 1 ] Since the discovery of carbon nanotubes in 1991, [ 2 ] one-dimensional (1D) inorganic semiconductor nanostructures with various shapes and mor-phologies, such as nanotubes, nanowires, nanorods, nanobelts/nanoribbons, nanocables, core/shell, and heterostructures, not only have been attracting a great deal of research interest in recent years due to their unique properties and potential to revo-lutionize broad areas of nanotechnology, [ 3–10 ] but also they have been emerging as one of the most powerful and diverse classes of functional nanomaterials that are having a key impact on science and technology. [ 11–15 ] The estimated publications on nanowire-related topics in 2010 only will exceed the number of 6000. [ 16 ]

Recently, semiconductor nanostructure arrays have shown to be of great scientifi c and technical interest since they have the strong non-linear and electro-optic effects that occur due to car-rier confi nement in three dimensions. [ 17–20 ] Considerable efforts

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2011, 23, 585–598

Prof. X. S. Fang , Prof. L. M. Wu , Dr. L. F. Hu Department of Materials ScienceFudan UniversityShanghai 200433, P. R. China E-mail: xshfang@fudan.edu.cn or xshfang@yahoo.cn ; lmw@fudan.edu.cn

DOI: 10.1002/adma.201003624

have been made with respect to the growth of various semiconductor nanostructure arrays via chemical or physical routes in a controlled way. To date, well-aligned nano-structure arrays have been obtained for the group IV (Si and Ge), groups III-V (GaP, GaAs, and InP), and groups II-VI (ZnO, ZnS, CdS, ZnSe) even ternary compound semiconductors. [ 21 ] High quality nanos-tructure arrays grown on demanded sub-strates in a controlled fashion will not only be desirable to the industry application, but also little or no post-growth manipu-lation or assembly is needed to build useful blocks. It is well known that aligned nanostructures with ideal geometry, array density and length-diameter-ratio can sig-nifi cantly enhance some unique properties and thus optimize their potential applica-tions. For example, Atwater and co-workers demonstrated that Si nanowire arrays have

advantageous optical properties for photovoltaic applications, including reasonable absorption of sunlight despite low areal packing fractions, extended near-infrared absorption compared with planar-sheet absorbers and effective optical concentra-tion over a wide range of incidence angles, and fi nally achieved enhanced absorption and carrier collection in Si nanowire arrays for photovoltaic applications. [ 22 ] Wang and Song have suc-cessfully converted nanoscale mechanical energy into electrical energy by means of piezoelectric ZnO nanowire arrays. [ 23 ]

Zinc sulfi de (ZnS) is one of the fi rst semiconductors discov-ered and it has traditionally shown remarkable fundamental properties versatility, including a direct wide-bandgap semi-conductor, the presence of polar surfaces, excellent transport properties, good thermal stability and high electronic mobility, etc. [ 24–27 ] These unique physical properties not only result in rich morphologies of ZnS formed at the nanoscale, but make ZnS be one of the most important semiconductors in the electronics industry with a wide range of applications including electrolumi-nescence, nonlinear optical devices, light-emitting diodes (LEDs, when doped), fl at panel displays, infrared windows, UV-light sensors, lasers, fi eld-emitters, and biological applications. [ 1 , 28–32 ]

The unique one-dimensionality inherent to e.g nanowires has already solved some of the long-standing technical prob-lems that have plagued the thin fi lm community, such as 1D nanostructure growth provides a natural mechanism for relaxing the lattice strain at the interface and enables disloca-tion-free growth on lattice mismatched substrates. [ 16 , 33 ] Growth substrate choosing is one of the most key factors for the growth of 1D semiconductor nanostructure arrays. Although 1D ZnS nanostructures with various morphologies and shapes have

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Xiaosheng Fang obtained his PhD degree from Institute of Solid State Physics, Chinese Academy of Sciences in 2006. He joined in National Institute for Materials Science (NIMS), Japan as a JSPS postdoctoral fellow and then International Center for Young Scientists (ICYS)-International Center for Materials Nanoarchitectonics (MANA) researcher. Currently,

he is professor at the Department of Materials Science, Fudan University, China. His current research topic is the controlled fabrication, novel properties and optoelectronic applications of semiconductor nanostructures, with a special focus on ZnS nanostructures-based optoelectronic devices.

Limin Wu received his PhD degree from Zhejiang University in 1991. He worked as lecturer then associate pro-fessor from 1991 to 1994. He worked as visiting professor at Pennsylvania State University and Eastern Michigan University from 1994 to 1999. He joined Fudan University in 1999, where he currently is “Changjiang Scholar”

Professor. His current research interests include syn-thesis, assembly and optoelectronic properties of organic-inorganic nanoparticles, semiconductor nanostructures, hollow inorganic particles, development of functional coatings.

Linfeng Hu, was born in Anhui, China, in 1982. He received his B. S degree and M. S. degree in Materials Science and Engineering from Huazhong University of Science and Technology and Tsinghua University, respectively. In 2010, he got PhD in Materials Science and Engineering from NIMS, Japan. Now he is a lecturer at the Department of Material

Science, Fudan University. His current research is inter-ested in the self-assembly of semiconductor nanostruc-tures into functional nano-fi lms/nanoarrays and their applications in optoelectronic devices.

been successfully synthesized via a variety of techniques, [ 34–46 ] the studies on ZnS nanostructure arrays were just initiated in last several years. These new progress on ZnS nanostructure arrays provide a great opportunity for using such materials to exploiting their novel properties and optimizing their poten-tial applications. There have already been issued several decent reviews which mainly focus on 1D ZnS nanomaterials and nanostructures, [ 24 , 25 , 47–50 ] no review focused on ZnS nanostruc-ture arrays. Since this is a rapidly expanding fi eld in which some unique properties have been discovered as well as some poten-tial applications have been achieved, we intended to review in this Progress Report a selection of recent work related to ZnS nanostructure arrays. We begin with a detailed introduction of various methods to the synthesis of ZnS nanostructure arrays, and then discuss the recent efforts and great developments on exploiting the unique properties and potential applications of ZnS nanostructure arrays. Finally, the challenges and opportu-nities that researchers face in this fi eld will be highlighted, in particular we provide some perspectives on using ZnS nano-structure arrays in current energy and environment research.

2. Synthesis of ZnS Nanostructure Arrays

During the past decade, many methodologies have been devel-oped to synthesize inorganic semiconductor nanostructures. They are commonly placed into two categories, namely, the bottom-up approach in which the functional structures are assembled from individual atoms and molecules, while the top-down approach relies on dimensional reduction by a com-bination of lithography, etching, and deposition to form func-tional devices and their integrated systems. [ 11 , 16 ] Although both approaches have been exceedingly successful for making inor-ganic semiconductor nanostructures with different advantages and disadvantages, currently most of inorganic semiconductor nanostructures-based nanodevices are fabricated via the facile combination of both strategies. If based on reaction media that were used during the preparation of inorganic semicon-ductor nanostructures, they can also be categorized into two major approaches: solution and gas phase based process. [ 51–53 ] Signifi cant effort has been put into the synthesis of ZnS nano-structure arrays because of their potential benefi ts in showing anisotropic and unique properties, carrier transport, carriers scattering and recombination, thermal stability, index of refrac-tion, and thermal and electrical conductivity. Up to now, tem-plate-assisted growth, epitaxial growth, homoepitaxial growth, heteroepitaxial growth, conversion from other nanostruc-tures, thermal evaporation process, evaporation-condensation approach, plasma-assisted metalorganic chemical vapor depo-sition (MOCVD), vapor-liquid-solid (VLS) and vapor-solid (VS) processes, and hydrothermal/solvothermal reaction and subse-quent heat-treatment process have been successfully developed to synthesize ZnS nanostructure arrays.

2.1. Template-Assisted Growth

A template-based process holds the advantages and promise of large-scale and cost-effective fabrication of highly ordered

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nanostructure arrays with well-defi ned orientation, composition, microstructure, length/diameter ratio, etc. [ 54 ] The use of various templates with controlled morphologies to direct the formation of 1D nanostructure arrays has been successfully developed. For example, different types of templates for the fabrication of 1D nanostructure arrays were adopted including porous anodic alumina membranes (AAM), self-assembly nanosphere lithog-raphy, soft photolithography, polymer, and mesoporous silica SBA-15. On the other side, these “template” methods have been used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale.

In the early 90s, using ordered channel AAM became one of the facile routes to fabricate 1D inorganic semiconductor nano-structure arrays. AAM assisted growth entails synthesis of the desired material within the pores of a nanoporous membrane with controlled pore diameters. In 2004, Lee and co-workers successfully fabricated aligned ZnS nanowire arrays using gold particle-fi lled AAM template assisted growth. [ 55 ] AAM was produced by electrochemical oxidization of aluminum in the presence of an acidic medium, the diameter of the pores in this work could be adjusted in the range 5–200 nm and the pore density can be 10 11 cm − 1 . High-density and uniform-sized gold particle arrays were prepared using the AAM template and alternating-current (AC) electrochemical deposition, and subse-quently these gold particles were used as catalysts to synthesize ZnS nanowire arrays within a horizontal quartz tube via VLS growth mechanism at a high temperature condition. The as-grown ZnS nanowires had a wurtzite (WZ) single-crystal struc-ture and were aligned perpendicularly to the AAM template. [ 55 ] Figure 1 shows a typical scanning electron microscopy (SEM) image of the as-synthesized ZnS nanowire arrays (30 min growth time), revealing each nanostructure has a nanoparticle on its tip end and almost all the nanowires are aligned pre-dominantly perpendicular to the surface of the AAM with a high density. A high-magnifi cation SEM image of a single ZnS nanowire grown for 10 min and a lattice-resolved high-resolution

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 585–598

Figure 1 . Typical SEM image of ZnS nanowire arrays synthesized via using gold particle-fi lled AAM template assisted growth. The inset images are a high-magnifi cation SEM image of a single ZnS nanowire grown for 10 min and a lattice-resolved HRTEM image of a single ZnS nanowire, respectively. Reproduced with permission from [55]. Copyright 2004, American Institute of Physics.

transmission electron microscopy (HRTEM) image of a single ZnS nanowire are shown in the inset of Figure 1 . The length of the ZnS nanowires is estimated to be 24 ± 2 μ m.

In 2006 scientists from the Chinese Academy of Sciences reported the synthesis of highly ordered ZnS nanowire arrays via direct-current (DC) electrodeposition into the nanopores of porous AAM for the fi rst time. [ 56 ] The AAM with channel dia-meters of about 40 nm was fabricated with the method previously proposed by Masuda and co-workers. [ 57 ] Before electrochemical deposition, a layer of Au fi lm was sputtered onto the surface of the AAM to serve as the working electrode, and ZnS nanowires were subsequently deposited into the nanopores of the AAM from the electrolytes contain ZnCl 2 (7.5 g L − 1 ) and elemental S (6.1 g L − 1 ) in dimethylsulfoxide (DMSO) liquid at an effective cur-rent density (the authors defi ned the effective current density = measured current/whole area of nanopores) of 6.62 mA cm − 2 at a temperature between 120 and 130 ° C. By the observations of SEM, TEM, HRTEM, and selected area electron diffraction (SAED) pattern, high-yield and ordered ZnS nanowire arrays with hexagonal-wurtzite phase have been successfully fabri-cated into the nanopores of AAM by the DC electrodeposition method. The growth mechanism was assumed to be that metal cations are fi rstly reduced and then react with elemental S to form ZnS nanowire arrays in the nanopores of AAM. [ 56 ]

In 2007, Zhang and co-workers reported a similar fabri-cation route for the growth of ZnS nanowire arrays into a 40-nm diameter AAM. [ 58 ] Figure 2a presents a representative cross-sectional SEM image of the ZnS nanowires deposited in 40-nm diameter AAM after partial removal of the alumina pore matrix, revealing that large-area, uniform, and highly ordered ZnS nanowires were successfully fabricated. An X-ray energy-dispersive spectroscopy (EDS) spectrum (the inset in Figure 2a ) acquired from the deposited ZnS nanowires confi rm that the nanowires consist of Zn and S with a stoichiometric ZnS com-position. A typical TEM image and SAED patterns of the indi-vidual ZnS nanowires are shown in Figure 2b . The SAED patterns recorded indicate that the wurtzite structures of ZnS nanowires have a single-crystal characteristic and they do not change along the wire length except for a slight variation in the intensity of the diffraction spots. These two independent exper-imental results demonstrate that the ordered channel AAM

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Figure 2 . (a) A representative cross-sectional SEM images of the ZnS nanowires deposited in 40-nm diameter AAM after partial removal of the alumina pore matrix. The inset in (a) is an EDS spectrum acquired from the deposited ZnS nanowires confi rms that the nanowires consist of Zn and S with a stoichiometric ZnS composition. (b) A typical TEM image and SAED patterns of the individual ZnS nanowires. Reproduced with permission from [58]. Copyright 2007, Elsevier B.V.

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Figure 3 . SEM images of as-synthesized well-aligned ZnS nanowire arrays epitaxially grown on GaAs (111)B substrates using Ag-assisted MOCVD growth. (a) A typical SEM image at a 30 ° tilted view. (b) A side view image, and (c) a top view image. Reproduced with permission from [65]. Copyright 2010, American Chemical Society.

template-assisted growth is a facile route to fabricate highly ordered ZnS nanowire arrays only via choosing proper pre-cursor and controlling deposited parameters. If the synthesis conditions were controlled well – for example by applying metal organic chemical vapor deposition (MOCVD) but without using of metal catalyst – highly ordered wurtzite-phase ZnS nanotube arrays were successfully synthesized in the pores of the AAM by a single-source MOCVD-template method by heating zinc bis(diethyldithiocarbamate) [Zn(S 2 CNEt 2 ) 2 ] powder to 400 ° C in the absence of any metal catalysts. [ 59 ]

SBA-15 is a member of mesoporous silica with 1D hexagonal arrangement of uniform mesopores, and it can be produced in a large quantity with silica precursor and triblock copolymer, and the mesoporous silica-like SBA-15 has been explored as one of the best hard templates for the synthesis of various nanostructures. [ 60–62 ] In 2003, Zhao and co-workers successfully assembled well-patterned crystalline nanowire arrays of binary compound metal sulfi des, CdS, ZnS, and In 2 S 3 , by employing a simple impregnation reaction procedure from two separate precursors (metal and sulfur precursors) using mesoporous SBA-15 as a hard template. [ 63 ] The detailed structural characteri-zations suggested the as-synthesized ZnS nanostructures have ordered regular nanowire arrays with uniform sizes that exactly copy the main channels of MWD-SBA-15. In 2004, Brieler and co-workers reported the synthesis of highly ordered Zn 1 − x Mn x S with different doping levels x (from 0.01 to 0.3) quantum wire arrays by using three different SiO 2 pore systems with pore diameters of 3 nm (MCM-41) and 6 and 9 nm (both SBA-15) as growth templates. The results from X-ray diffraction (XRD) and physisorption measurements as well as TEM investigations presented by the authors demonstrated that the formation of Zn 1− x Mn x S arrays takes place preferentially inside the pore system of the mesoporous host structures. [ 64 ]

2.2. Heteroepitaxial and Homoepitaxial Growth

The term “epitaxial” is applied to a fi lm grown on top of the crystalline substrate in ordered fashion that atomic arrange-ment of the fi lm accepts crystallographic structure of the substrate. The deposited fi lm is denoted as epitaxial fi lm or epitaxial layer. Heteroepitaxy and homoepitaxy are two kinds of epitaxy, in which a crystalline fi lm is grown on a substrate or fi lm of the different material and a crystalline fi lm grows on a crystalline substrate or fi lm of a same material, respectively. In recent years, heteroepitaxial and homoepitaxial growth have successfully been used in nanotechnology, and indeed they have been demonstrated to be one of the best strategies to grow high-quality and highly oriented semiconductor nanostructure arrays. These unique methods have impressively accelerated the potential applications of semiconductor nanostructure-based technologies, in particular on various state-of-the-art photonics, electronics, solar cells etc environmental and energy science.

Recently, Hark and co-workers fabricated single-crystalline, well-aligned ZnS nanowire arrays epitaxially grown on GaAs substrates by MOCVD method using Ag and Au nanoparti-cles as catalysts. [ 65 ] The GaAs substrates included the GaAs (100), (110), (311)A, and (111)B, and the Ag and Au nanopar-ticles were synthesized by solution method or sputter-coating

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method. Diethylzinc and diethylsulfi de were used as precursors to supply Zn and S species and hydrogen gas was used as the carrier gas, and the reaction chamber was heated quickly to 530 ° C and kept at this temperature for 30 min. The authors observed that the ZnS nanowire arrays always align predomi-nantly along the [ 111 ] B direction of substrate regardless of the crystallographic surface of the GaAs substrates (GaAs (100), (110), (311)A, and (111)B substrates were used for comparison) and size of the catalysts used. Figure 3 depicts some typical SEM images of the ZnS nanowire arrays grown on the GaAs (111)B substrates using Ag as catalyst, revealing that almost all the ZnS nanowires are vertically grown on the (111)B substrate

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Figure 4 . Heteroepitaxial growth of ZnS nanowire arrays on various substrates: (a) CdSe. Reproduced with permission from [66]. Copyright 2004, American Chemical Society. (b) CdS. Reproduced with permission from [67]. Copyright 2007, American Institute of Physics. (c) Zn 3 P 2 . Reproduced with permission from [68]. Copyright 2008, American Chemical Society. (d) Si (111) wafers. Reproduced with permission from [69]. Copyright 2010, IOP Publishing Ltd.

and they have regular and hexagonal cross section. The diam-eter is ranged from 60 to 80 nm, while their length can reach up to 5 μ m. By adopting Au catalyst particles of different size, the diameter of ZnS nanowires can be controlled. During the experiments, a structure transformation was observed from pure WZ phase (when the diameter is < 30 nm) to a mixed phase of WZ and zinc blend (ZB) (the diameter is between 30 and 70 nm) and fi nally to pure ZB (the diameter is larger than 70 nm). [ 65 ]

Besides the above example, heteroepitaxial growth of ZnS nanowire arrays on various substrates (such as CdSe, CdS, Zn 3 P 2 , Si and metal Zn foil) has been demonstrated success-fully. [ 66–70 ] Figure 4 displays the SEM images of the as-grown ZnS nanowire arrays synthesized on the CdSe, CdS, Zn 3 P 2 and Si substrates by various methods, revealing all of the nanowires are orientationally aligned on the top of the corresponding substrates. These examples not only unveil that ZnS nanowire arrays not only can be obtained by a direct evaporation–condensation approach or a two-step thermal evaporation process, but also that the growth mechanism can be catalyst-free VS or metal-catalyzed VLS growth processes. The orienta-tion dependence of ZnS nanowire arrays on various substrates demonstrates the importance of the crystallographic relation-ship and crystalline substrates on the epitaxial growth process of 1D nanostructures. For example, due to a good orientation relationship of [010] Zn3P2 //[1–210] ZnS and (101) Zn3P2 //(0002) ZnS ,

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Mater. 2011, 23, 585–598

highly ordered ZnS nanowire arrays were successfully heteroepitaxially grown on Zn 3 P 2 crystals via a simple thermal evapora-tion method. [ 68 ] During the heteroepitaxial growth of single-crystalline ZnS nanowire arrays on CdS nanoribbon substrates by the Au-catalyzed VLS growth process, the authors discovered that the crystallographic structures of the surface of CdS nanoribbons play a dominant role in the epitaxial growth of ZnS nanowire arrays along preferential orientations although Au seeding particles are necessary for nanowire growth. [ 67 ]

Homoepitaxial growth displays several unique advantages in comparison to het-eroepitaxial growth, such as avoiding a lat-tice and thermal expansion mismatch and a lower-energy barrier existing at the interface between the grown materials and substrate, which is the reason that it has been attracting a great deal of interest. Recently, Fei, Zhang, and co-workers have successfully achieved large-scale homoepitaxial growth of ZnS nanowire arrays by utilizing ZnS column arrays as a transition layer. [ 71 ] The growth of single-crystal ZnS column arrays were pre-pared in a tube furnace by using a sintered ZnS wafer shaped from high-purity ZnS powders under a pressure of 2 MPa and pure ZnS powers under high-temperature con-ditions. About 2 nm Au fi lm was coated on the as-prepared ZnS column arrays through ion sputtering. After the heat treatment of

the ZnS columns coated with the Au fi lm, large-scale homoepi-taxial growth of ZnS nanowire arrays was fi nally realized via a VLS growth process. Figure 5 depicts some representa-tive SEM images and a typical XRD pattern of the as-grown ZnS nanowires arrays on single-crystal ZnS column arrays, revealing that most of the ZnS nanowires with a preferred growth orientation of [001] are perpendicular to the substrate surface and have a catalyst tip at each tips. The authors further demonstrated that the density of ZnS nanowire arrays is well controlled via a Au–catalyzed VLS growth process ranging from 0.33 to 3.04 wire/ μ m 2 when the density of Au nanoparticles is altered through annealing ZnS columns coated with Au fi lms at different temperatures. [ 71 ]

2.3. Hydrothermal/Solvothermal Synthesis

In the 21st century, hydrothermal and solvothermal (using non-water as solvents) technology is not just confi ned to the crystal growth or leaching of metals, but it is going to take a very broad shape covering several interdisciplinary branches of sci-ence. It links all the important technologies like geotechnology, biotechnology, nanotechnology, and advanced materials tech-nology. The hydrothermal and solvothermal method has been demonstrated to be one of the most important technologies for producing 1D nanostructures at low temperature. [ 72 ] In 2006,

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Figure 5 . Homoepitaxial growth of ZnS nanowire arrays by utilizing ZnS column arrays as a transition layer synthesized on the ZnS wafer. (a) Low-magnifi cation SEM image of the ZnS nanowire arrays on the surface of every ZnS column. (b) The corresponding XRD pattern. (c) and (d) High-magnifi cation SEM images of ZnS nanowire arrays vertically aligned on the ZnS column surface. Reproduced with permission from [71]. Copyright 2009, American Chem-ical Society.

Cai and co-workers reported the synthesis of large-scale and well-aligned ZnS nanobelt arrays via a simple hydrothermal method and subsequent heat treatment at 250 ° C ( Figure 6 a ). The nanobelts have a typical width ranging from 300 to 500 nm and a thickness of 30 nm, and the length is up to 4 μ m. [ 73 ] Sub-sequently, Shi and Huo reported a mild one-step solvothermal method for growing uniform well-aligned 1D ZnS nanostruc-ture arrays at lower temperatures. [ 74–76 ] Single-crystalline ZnS nanowires, nanotubes, or nanoribbons can be selectively synthe-sized by controlling the reaction temperature and sulfur concen-tration. Figure 6b and 6c show typical SEM images of the highly oriented ZnS nanowire arrays and quasi-aligned ZnS nanowire arrays, respectively. The diameter of the ZnS nanowires can be controlled from several nanometers to several hundred nanometers with a typical length up to several micrometers by altering proper experimental parameters. The controlled size and large surface area of these 1D ZnS nanostructures result in promising properties and potential applications.

2.4. Conversion from Other Nanostructures

The synthesis of one nanostructure through conversing from other nanostructures is one kind of template-assisted syn-thesis where the original nanostructures are used as template, and the shape and size of the synthesized nanostructures are closely related with the original nanostructures. This novel method was widely used for the synthesis of carbide and nitride nanostructures from C nanotubes through vapor phase con-version and transport processes since 1990s, [ 77–80 ] and then it

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was enriched to synthesize various inorganic semiconductor nanostructures and arrays conversed from carbon and non-carbon nanostructures. [ 81–84 ] For example, Yang and co-workers reported an “epitaxial casting” approach for the synthesis of single-crystal GaN nanotube arrays with inner diameters of 30–200 nm and wall thicknesses of 5–50 nm by removing hexagonal ZnO nanowire array templates (the diameters of ZnO nanowires are 30–200 nm) by thermal reduction and evaporation. [ 81 ] Crystalline Si with tubular nanostructures were synthesized by using ZB ZnS nanowires as removable templates. [ 82 ] Li and co-workers developed a simple template-based heat-treatment method to achieve the conversion of metal nanowire arrays into arrays of metal–metal oxide core/shell nanowires and single-crystalline metal oxide nanotubes. [ 15 ] This templating process should be applicable to many other semiconductor systems.

Up to now, the synthesis of various ZnS nanostructure arrays, including nanowire, nanorod and nanotube arrays, have been suc-cessfully fabricated by the conversion from ZnO or ZnO–ZnS nanostructure arrays. [ 85–87 ] For example, large-scale ZnS nanotube arrays were successfully prepared through sulfura-om ZnO column arrays based on a hydro-

tion conversion fr

thermal method at a low temperature of 130 ° C for 48 h. The conversion ratio from ZnO to ZnS can be conveniently con-trolled by reaction time, and cable-like ZnS–ZnO structures were obtained from ZnO column arrays with less sulfuration reaction time (12–15 h). [ 85 ] Similarly, using well-aligned ZnO nanowire arrays synthesized on the zinc foil substrate via direct oxidation of zinc foil with ammonium persulfate oxidant in the alkali solution as an ideal sacrifi cial template, ZnO–ZnS nano-cable arrays with ZnO as the inner core and ZnS as the outer shell were synthesized via a thioglycolic acid-assisted solution route fi rstly. The well-aligned ZnS nanotube arrays with per-fect hexagonal structures were fi nally fabricated by removing the ZnO core with the KOH treatment. [ 86 ] By heating reaction of pre-synthesized aligned ZnO nanorod arrays under an H 2 S and Ar gas mixture (H 2 S/Ar: 1:20) in an evacuable quartz tube oven at atmospheric pressure at 600 ° C for 2 h, vertically aligned ZnS nanorod arrays with crystallographic orientation along the c -axis could be formed easily. Figure 7 a displays a typical SEM image of the synthesized ZnS nanorod arrays after sulfi dation, and although a considerable surface roughness was formed, no break up in the alignment of the nanorods occurred and the original shape and size were maintained. [ 87 ] Recently, Wang and co-workers demonstrated a one-step thermal evaporation method to the synthesis of axial heterostructure ZnO–ZnS arrays in which ZnO nanowires were grown on top of ZnS nanowires via epitaxial growth, and fi nally pure ZnS nanowire arrays were obtained by etching ZnO–ZnS NW arrays with 1 M KOH solution for 120 min. [ 88 ] Figure 7b and 7c show the corre-sponding top-view and side-view SEM images, respectively. It is

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Figure 6 . Hydrothermal/solvothermal synthesis of ZnS nanostructure arrays on metal zinc substrate. (a) Well-aligned and oriented wurtzite ZnS nanobelt arrays are fabricated by a simple solvothermal reaction and subsequent heat treatment at 250 ° C. The inset images are an enlarged belt near top part and a cross-sectional image, respectively. Reproduced with permission from [73]. Copyright 2006, American Institute of Physics. (b) Highly oriented ZnS nanowire arrays can be selectively grown on Zn foil by simply modulating reaction temperature and sulfur concentration at 100 ° C. Reproduced with permission from [74]. Copyright 2009, Amer-ican Chemical Society. (c) Quasi-aligned ZnS nanowire arrays synthesized via a mild one-step solvothermal method at 180 ° C. Reproduced with permission from [75]. Copyright 2009, American Chemical Society.

easy to see that highly ordered and vertical ZnS nanowire arrays with a slight damaged surface during the selective etching process have been successfully synthesized.

2.5. Other Methods

Table 1 lists a comprehensive and up-to-date summary of the synthesis of ZnS nanostructure arrays. Besides the above-discussed template-assisted growth, heteroepitaxial and homoepitaxial growth, hydrothermal/solvothermal synthesis, and conversion from other nanostructures, a simple thermal evaporation process and plasma-assisted MOCVD are also very facile routes to the synthesis of ZnS nanostructure array. For example, oriented ZnS nanobelt arrays and bundles of oriented and single-crystalline ZnS nanowires with [0001] axis have been successfully synthesized by a controlled thermal evaporation of commercial ZnS powders in an induction furnace at 1150 and 1200 ° C, respectively. [ 89 ] Sixfold symmetry heptapodlike and threefold symmetry tetrapodlike ZnS structures composed of assembled ZnS nanowire arrays with the preferred [0001] ori-entations have been fabricated by a simple thermal evaporation process using a mixture of ZnS and SiO as source materials in N 2 at 1300 ° C. [ 90 ] Figure 7d show a high-magnifi cation SEM image of sixfold symmetry heptapod-like ZnS nanostructures, revealing the obvious hexagonal shape and they are formed by the assembly of ZnS nanowire array with the diameters of 50–70 nm. Vertical aligned ZnS nanorod arrays were syn-thesized on c -plane Al 2 O 3 substrates through a catalyst-free plasma-assisted MOCVD method at 650 ° C and a low pressure of 10 Torr, as shown in Figure 7e . Detailed structural characteri-zations show that the obtained ZnS nanorods have a uniform diameter of 60 nm with the length of about 400 nm, and they are single crystals with a hexagonal structure. [ 92 ]

3. Novel Property and Potential Application

3.1. Enhanced Field-Emission Performances

Field emission (FE), as one of the main features of nanostruc-tures, is of great commercial interest in displays and other elec-tronic devices. [ 1 ] In case of highly ordered nanostructure arrays, it not only has an advantage of faster device turn-on time, com-pactness, and sustainability compared to the conventional bulky technologies, but aligned nanostructures with a high packing density can signifi cantly enhance the FE properties of mate-rials. [ 101–103 ] As discussed in two previous reviews, [ 1 , 104 ] the turn-on fi eld, threshold fi eld, fi eld enhancement factor, FE current density, and its stability are the key parameters of one mate-rial FE performances. These properties are strongly dependent on the work function of an emitter surface, the radius of cur-vature of the emitter apex, the emission area, the aspect ratio and arrangement style of the emitter, etc. Analyzing all the FE results on ZnS nanostructures, it has been demonstrated three facile routes have been developed to optimize the FE perform-ances of ZnS nanostructures, namely can be enhanced through (i) increasing its aspect ratio (length to thickness ratio), [ 26 ]

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Figure 7 . Rich methods to the synthesis of ZnS nanostructure arrays. (a) Oriented ZnS nanorod arrays formed at 600 ° C by sulfi dation of aligned ZnO nanorod arrays in an atmosphere of an H 2 S and argon mixture of 1:20 at atmospheric pressure for 2h. Reproduced with permis-sion from [87]. Copyright 2007, American Chemical Society. (b) and (c) Top- and tilted-view SEM images of ZnS nanowire arrays obtained by selectively etching ZnO-ZnS nanowire arrays with 1 M KOH solution for 120 min. Reproduced with permission from [89]. Copyright 2009, American Chemical Society. (d) Sixfold symmetry heptapodlike ZnS nanostructures composed of assembled ZnS nanowire arrays synthesized via a simple thermal evaporation process at 1300 ° C. Reproduced with permission from [90]. Copyright 2007, American Institute of Physics. (e) Highly aligned ZnS nanorods arrays synthesized on c -plane Al 2 O 3 substrates through a catalyst-free plasma-assisted MOCVD method at 650 ° C and a low pressure of 10 Torr. Repro-duced with permission from [91]. Copyright 2005, Elsevier B.V.

(ii) synthesizing a sharp-tip nanostructure, [ 44 ] and (iii) assem-bling it into arrays. [ 98 ]

Figure 8 depicts the crystal orientation-ordered ZnS nanobelt quasi-arrays and their enhanced FE performances. The crystal orientation-ordered ZnS nanobelt quasi-arrays were prepared upon self-made seeding ZnS sheets by a non-catalytic and template-free thermal evaporation process at 1100 ° C. [ 98 ] The observations indicate that the nanobelts have a typical length of several tens-to-hundreds of micrometers and typical belt width of 50–120 nm, respectively. Figure 8a manifests a typical TEM image of the aligned ZnS nanobelts and the corresponding SAED pattern (the inset), revealing that the nanobelts are aligned not only along the length direction but also in crystal-lography orientation, so that all the nanobelts within the bunch grew along the [001] ZnS direction. FE measurements show that these crystal orientation-ordered ZnS nanobelt quasi-arrays are decent fi eld emitters although the work function of ZnS ( ∼ 7.0 eV) is larger than that for some other popular inorganic semi-conductor fi eld-emission materials. [ 1 ] As shown in Figure 8b , the current density from these crystal orientation-ordered

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhwileyonlinelibrary.com

ZnS nanobelt quasi-arrays was found to be ∼ 14.6 mA/cm 2 at a macroscopic fi eld of 5.5 V/ μ m, which is more than 20 times higher than that of randomly-oriented ZnS nanobelt ensembles ( ∼ 0.68 mA/cm 2 ) at the same macroscopic fi eld. This value is com-parable or even better than the most studied nanostructured materials, such as ZnO and C nanotubes. For example, the FE current density produced at ∼ 3 V/ μ m for the aligned ultralong ZnO nanobelts (the nanobelts can grow to several millimeters in length) synthe-sized by a molten-salt-assisted thermal evapo-ration is about 1.2 mA/cm 2 . [ 105 ] The good sta-bility of the emission current within 4 h at an applied electric fi eld of 5 V/ μ m (the emis-sion current fl uctuates slightly but does not exhibit degradation) is also benefi tted from the aligned structures. [ 98 ]

3.2. Room-Temperature UV Lasing

A laser, which is an acronym for Light Ampli-fi cation by Stimulated Emission of Radiation, is a device that converts energy into light with a mechanism for emitting electromagnetic radiation via a process of stimulated emis-sion. Room temperature semiconductor lasers are attractive for the opto-electronic applica-tions, such as optical communications and other lightwave technologies. Early double heterostructure lasers in GaP-GaAs system fabricated by vapor phase epitaxy (VPE) occur only at liquid nitrogen temperature, similarly to homojunction lasers, owing to lattice mis-match. [ 106 ] A continuous wave (CW) operation of semiconductor lasers at room tempera-ture was achieved in the early 1970s. [ 107 ] The

realization of semiconductor lasers emitting short wavelength UV light is of considerable interest for a wide range of applica-tions including optical computing, high-density data storage and material processing, and chemical/biochemical microa-nalysis. [ 108–110 ] The achievements of room temperature green-blue diode laser structures with ZnSe, GaN, SiC and In x Ga 1 −x N as the active layers promoted signifi cantly the development of short-wavelength semiconductor lasers. [ 111–114 ] However, almost all the researches focused on zero-dimensional (0D) and two-dimensional (2D) structures, especially on their thin fi lm and multi-quantum-well (MQW) structures. In 2001, Yang and co-workers demonstrated the fi rst room-temperature ultraviolet lasing in semiconductor ZnO nanowire arrays. [ 109 ] They realized that the ZnO hexagonal nanowires bear striking similarities to the conventional macroscopic laser cavity. This discovery led to a fl urry of researches into photonic properties of 1D nanostructures.

ZnS is a wide bandgap ( ∼ 3.72 eV and ∼ 3.77 eV for cubic ZB and hexagonal WZ ZnS, respectively) compound semicon-ductor that is suitable for a sub-wavelength laser cavity in the

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Table 1. A comprehensive and up-to-date summary on the synthesis of ZnS nanostructure arrays.

Nanostructure Synthesis method Growth temperature ( ° C) Grown substrate Reported property/application Ref.

Nanowire arrays Template-assisted VLS growth process 1100 Within ordered channel AAM Optical properties, PL, and room

temperature UV lasing

55

Template-assisted electrodeposition Between 120 and 130 Within ordered channel AAM —— 56

Template-assisted electrodeposition 120 Within ordered channel AAM —— 58

Template-assisted electrodeposition 120 Within ordered channel AAM —— 92

Template-assisted electrodeposition 120 Within ordered channel AAM Optical properties, PL 93

A simple impregnation reaction proce-

dure using mesoporous silica SBA-15 as

a hard template

150 Within mesoporous SBA-15 —— 63

Heteroepitaxial growth 530 GaAs substrate —— 65

Heteroepitaxial growth 1050 CdSe crystal —— 66

Heteroepitaxial growth 1050 CdS nanoribbon substrate Optical properties, CL, and room

temperature UV lasing

67

Heteroepitaxial growth 1250 Zn 3 P 2 crystal Optical properties, PL and Field-

emission

68

Homoepitaxial growth 1200 ZnS column arrays —— 71

A facile evaporation–condensation

approach

1050 Si wafer Field-emission 69

A simple thermal evaporation route Within 400 to 500 Zn foil Optical properties, PL and Field-

emission

70

One-step, wet-chemical approach 100 Zn foil Optical properties, CL 74

One-step solvothermal method 180 Zn substrate —— 75

One-step solvothermal method 180 Zn substrate Optical properties, PL and Field-

emission

76

A controlled thermal process 1200 —— Optical properties, PL 89

A simple thermal evaporation process 1300 —— Optical properties, PL 90

A simple evaporation process 1050 Highly conductive Cu substrate Field-emission 94

Selective etching of ZnO-ZnS nanowire

arrays

1100 for the synthesis of

ZnO-ZnS and RT for the

subsequent etching

ZnS buffer layers Optical properties, CL and

electricity generation

88

Nanorod arrays Plasma-assisted MOCVD 650 c -plane Al 2 O 3 substrate Optical properties, PL 91

Sulfi dation of ZnO nanorod arrays 600 ZnO thin fi lm Optical properties, PL 87

An aqua-solution hydrothermal process 95 Pulse-plating Zn nanocrystallines —— 95

An aqua-solution hydrothermal process 95 Pulse-plating Zn nanocrystallines Optical properties, PL 96

Nanotube arrays One-step, wet-chemical approach 140 Zn foil Optical properties, CL 74

Sulfuration conversion from ZnO arrays 130 Crystalline quartz substrate Optical properties, PL 85

CVD template method 400 Within ordered channel AAM Optical properties, PL 97

MOCVD template method 400 Within ordered channel AAM Optical properties, PL 59

In Situ chemistry strategy from ZnO

nanorod arrays

130–180 Zn foil —— 86

Nanobelt/nano-

ribbon arrays

A controlled thermal evaporation process 1100 ZnS sheet Field-emission 98

A simple solvothermal reaction and

subsequent heat treatment

250 Zn substrate Field-emission 73

A simple solvothermal reaction and

subsequent heat treatment

250 Zn substrate Field-emission 99

One-step, wet-chemical approach 180 Zn foil Optical properties, CL 74

A controlled thermal process 1150 —— Optical properties, PL 79

Homoepitaxial Growth 1050 Single-crystal ZnS nanoribbon

substrate

Optical properties, PL and CL, and

room temperature UV lasing

100

Horizontal

nanobelt arrays

A simple CVD growth process 1100 A predesigned patterned substrate

(Si covered SiO 2 )

Optical properties, CL and

UV-light sensor

31

A simple CVD growth process 1050 Si wafer UV-light sensor 46

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Figure 8 . (a) TEM image and the corresponding SAED pattern of crystal orientation-ordered ZnS nanobelt quasi-arrays, and (b) its enhanced FE performances. Reproduced with permission from [91]. Copyright 2007, Royal Society of Chemistry.

Figure 9 . Under high-power density optical excitation (266 nm), ZnS nanowire cross arrays synthesized via homoepitaxial growth on micrometer-wide single-crystal ZnS nanoribbon substrates show bandgap lasing emission at room temperature. The pump powers for these spectra are 50, 100, 240, and 350 kW/cm 2 , respectively. Reproduced with per-mission from [100]. Copyright 2006, Wiley-VCH.

UV region (below 400 nm). Moreover, the binding energy of the exciton for ZnS is about 39 meV, which is much greater than the thermal energy at room temperature of 26 meV, making ZnS become an ideal candidate for effi cient exci-tonic laser action at room temperature. Based on the aligned ZnS nanowire arrays with perfect WZ-2H structure fabricated using gold particle-fi lled AAM template assisted growth (typ-ical SEM and lattice-resolved HRTEM images were shown in Figure 1 ), the authors observed amplifi ed stimulated emission as well as narrow resonant cavity modes under high-powder density optical excitation using the fourth harmonic (266 nm, 5 ns pulse width) Nd:YAG (yttrium-aluminum-garnet) laser at room temperature in 2004. [ 55 ] Subsequently, they discovered the similar room temperature UV lasing from the homoepitaxial growth of ZnS nanowires and nanoribbons on the surfaces of micrometer-wide single-crystal ZnS nanoribbon substrates. [ 100 ] Figure 9 shows room-temperature PL spectra of the sample under different laser excitation powers and the accumulated intensity of 100 pulses versus input power density, respectively. The lasing threshold was found around 100 kW/cm 2 , which is relatively low in comparison with previously reported values for random lasing ( ∼ 300 kW/cm 2 ) in disordered particles or thin fi lms. [ 115 ] At a low excitation intensity ( < ca. 100 kW/cm 2 ), the spectrum consists of a narrow emission peak at 338.4 nm, and a new emission peak at 339.8 nm appears when the excitation power increases to 240 kW/cm 2 . As the pump power increases further to 350 kW/cm 2 , several narrow peaks emerge (marked by the dashed lines in Figure 9 ). The observed single or multiple sharp peaks represent different lasing modes. [ 100 ] The emission peak narrows with the increase of pump power because of the preferential amplifi cation of frequencies close to the maximum of the gain spectrum, which is similar to the oriented zinc oxide nanowires grown on sapphire substrates. [ 109 ] These results have clearly demonstrated that the highly oriented ZnS nanowire/nanobelt arrays indeed serve as an excellent nanoscopic laser cavity with a room temperature UV lasing.

3.3. Visible-Blind UV-Light Sensors

In recent years, the use of nanostructured materials with tailored geometry as building blocks of functional devices

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including chemical/biological/gas sensors and photodetec-tors has attracted intense attention due to their high surface-to-volume ratio and its rationally designed surface. [ 116–118 ] The electrical integration of chemically synthesized 1D nanostruc-tures has been achieved with lithography, optical integration, which promises high speeds and greater device versatility. [ 119 ] The interest in developing high-performance photodetectors has culminated in the realization of individual nanowire/nano-belt photodetectors with ultrafast recovery speed and ultra-high quantum-effi ciency. [ 120–123 ] Visible-blind UV-light sensors measure the power or intensity of incident UV radiation while it is blind or insensitive to visible light. Such sensors are very likely to be used in our daily life since prolonged exposure to UV radiation is one of the main reasons to cause skin cancer. [ 4 ]

In 2009, we have successfully assembled individual ZnS nanobelts into devices using optical lithography with the assist-ance of a pre-designed mask and electron beam deposition, [ 31 ] as shown in Figure 10 a . The nanobelt has a typical width and thickness of ∼ 400 nm and ∼ 20 nm, respectively, with its uncov-ered part exposed to the incident light. The optoelectronic measurements show the individual ZnS nanobelt-based sensor is extremely sensitive to UV light exposure (320 nm, over 3 orders of magnitude upon the illumination) while the dark current of the device is below the detection limit (10 − 14 A) of the current meter. The photocurrent increase allows us to revers-ibly switch the device between “UV ON” and “UV OFF” states. As can be seen from Figure 10d , the photocurrent is unstable and the intensity needs to be improved. When a better Cr/Au contact designs (e.g., the width of electrodes was altered from

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Figure 10 . ZnS nanostructures based visible-blind UV-light sensors and the corresponding time response of the photoconductor upon 320 nm light illumination measured by a current meter under and without UV light. From (a) and (c) individual ZnS nanobelts based sensors, to (b) and (d) multiple ZnS-nanobelts and (c) and (f) microscale ZnS nanobelts–based sensors (horizontal nanobelt arrays-based sensors). Reproduced with permission from [31,46].

∼ 2 to ∼ 10 μ m) was applied and a larger surface area of individual ZnS nanobelt was exposed to light (the width of the nanobelt was controlled from ∼ 400 nm to ∼ 2 μ m), ∼ 2.5 times increase on photocurrent and a stable photocurrent were achieved. [ 46 ]

To enhance the sensor performances further, we developed two facile and novel methods to fabricate multiple ZnS-nanobelts and microscale ZnS nanobelts–based sensors, in which the hor-izontal nanobelt arrays were applied. [ 31 , 46 ] A pre-designed pat-terned substrate was used to fabricate a sensor made of multiple ZnS nanobelts. The electrodes gaps and lengths between multiple ZnS nanobelts could be tuned while using different masks to fabricate a pre-designed patterned substrate. Figure 10b shows a multiple ZnS nanobelts-based sensor, in which the electrodes gap is about 100 μ m. The interspaces between electrodes are densely fi lled with horizontal ZnS nanobelt arrays, and the nanobelt lengths are enough to join each two neighboring electrodes in one circuit. The corresponding time response of the photoconductor upon 320 nm light illumina-tion was measured by a current meter under and without UV light, and is shown in Figure 10e . The photocurrent inten-sity is enhanced about 20 times compared with an individual ZnS nanobelt-based sensor performance and the photocur-rent stability is also improved signifi cantly. Very recently, we reported on the micrometer-scale ZnS nanobelt-based sensors fabricated by a simple CVD method and subsequent electron-beam deposited using Au microwires as masks. [ 46 ] We designed the electrode lengths at ∼ 1 and 2 mm after considering our measurement device. The sensors are composed of dense and horizontal ZnS nanobelt arrays, like thin-fi lm-like devices. The typical length and width of the belts are several hundred micrometers and several micrometers, respectively. As shown in Figure 10c , most of the nanobelts lie on the substrate pro-viding a good physical contact between ZnS nanobelt arrays and Cr/Au electrodes. The characteristics of this kind of sensor suggest that they are ideal candidates for visible-blind UV-light sensors with ultrafast response speed, low dark current, high ratios of photocurrent immediate decay and photocurrent/dark

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Mater. 2011, 23, 585–598

current, good stability, etc. The above exam-ples justify that the horizontal ZnS nanobelt arrays not only could serve as highly sensi-tive visible-blind UV-light sensors, but also may be valuable to be used as chemical and biological sensors, and switching devices for future nanoscale devices.

3.4. Nanogenerator for Electricity Generation

Electricity generation is the process of cre-ating electricity from other forms of energy. The unique properties of nanostructures associated with their low dimensionality give rise to new opportunities for research on energy conversion, and especially semicon-ductor nanostructure arrays provide a novel route to advance solid-state energy conver-sion and storage devices. [ 124 ] In 2006, Wang and co-workers developed a nanogenerator based on ZnO nanowires converted nano-

l energy into electrical energy by means of

scale mechanicapiezoelectric ZnO nanowire arrays. [ 23 ] This discovery of nano-generator on ZnO nanowire arrays is not by accident, but due to the unique coupling of piezoelectric and semiconducting properties of ZnO and the formation of a Schottky barrier between the metal and ZnO contacts. Although the output power and voltage are still weak to power our homes lights, this nanogenerator is able to power battery-free wireless nano-devices by converting ambient vibrations, hydraulic energy, or mechanical movement to electricity. Similar with ZnO, ZnS exhibits both semiconducting and piezoelectric properties that can form the basis for electromechanically coupled sensors and transducers. In 2009, Wang and co-workers demonstrated ZnS nanowire arrays the potential to be used for piezoelectric energy generation that it is able to convert mechanical energy into electricity when they are defl ected by a conductive AFM tip in contact mode. However, the output signal for ZnS nanowire arrays is lower than that for ZnO nanowire arrays because (1) ZnS has smaller piezoelectric constants and (2) the length of ZnS nanowires here are shorter (the SEM images were shown in Figure 7b and 7c ). [ 88 ]

4. Conclusions and Outlook

Recent development in our understanding of ZnS nanostruc-ture arrays has been reviewed comprehensively. Although the studies of ZnS nanostructure arrays are just initiated a few years ago and the achievements are limited compared with the whole ZnS nanostructures, [ 50 ] the progress is notable and presenting unique properties and potential applications for electronics, optics, photoelectronics, solar cells, etc. reveal that ZnS nano-structure arrays have been a developing material star. Several morphological ZnS nanostructure arrays including nanowire arrays, nanorod arrays, nanotube arrays, nanobelt/nano-ribbon arrays, and horizontal nanobelt arrays have been suc-cessfully synthesized through single and combined methods.

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Figure 11 . The schematic illustration of 1D ZnS nanostructure arrays- based solar cell concept. The p-type semiconductors can be Cu 2 O or CuInSe 2 .

Template-assisted growth, heteroepitaxial and homoepitaxial growth, hydrothermal/solvothermal synthesis, conversion from other nanostructures, a simple thermal evaporation process and plasma-assisted MOCVD were highlighted. Each method has advantages and disadvantages for the synthesis of ZnS nano structure arrays. For example, template-assisted growth has the advantages on controlling the shape, size (diameter and length), growth direction, and array density, however, it is nec-essary to remove the template for some applications. The crys-talline of the ZnS nanostructures synthesized by heteroepitaxial and homoepitaxial growth and thermal evaporation process is better than that grown by hydrothermal/solvothermal synthesis, however, a higher growth temperature or a fairly well-controlled environment and sophisticated pieces of equipment with high operating costs are normally adopted.

Thus, there is considerable interest in further study of fab-ricating of ZnS nanostructure arrays: (i) some new fabrication technologies are still developing for the synthesis of ZnS nanostructure arrays. For example, horizontal ZnS nanostruc-ture arrays could be assembled using Langmuir–Blodgett (LB) techniques or microfl uidics and used as the building blocks for various devices. (ii) The good contact between the grown ZnS nanostructure arrays and the substrates will be benefi cial to the direct applications. Heteroepitaxial and homoepitaxial growth provide a successful example. By adopting a metal catalyst or a seed layer, template-assisted growth and hydrothermal/solvo-thermal synthesis are also facile to grow the ZnS nanostructure arrays with a direct interface with the substrates. (iii) One of the key requirements for some applications including fl at panel displays and LEDs of ZnS nanostructure arrays is the cost-effective growth at large scale on a substrate. Wafer-scale high-throughput ordered growth of vertically aligned ZnO nanowire arrays on Si substrate was achieved recently by a combination technology between a hydrothermal chemical method and laser interference patterning. [ 125 ] (iv) The fabrication of devices from nanostructured building block on fl exible substrates has attracted considerable recent attention owing to the prolifera-tion of handheld, portable consumer electronics. [ 126 , 127 ] This kind of device (such as the device made on plastic substrate) not only has many attractive properties including biocompatibility, light weight, shock resistance, bendable, softness, printable and transparency, [ 126 ] but possesses some unique applications, such as piezoelectric nanowire generators. [ 128 ] Template-assisted growth, hydrothermal/solvothermal synthesis and conver-sion from other nanostructures offer some possibilities for the growth of ZnS nanostructure arrays on fl exible substrates at large scale due to most of the fl exible substrates (such as plastics) deform or melt at a relatively low growth temperature region. The excellent intrinsic performances of ZnS nanostruc-tures coupled with fl exible substrates could open up opportuni-ties in many fi elds. (v) 1D nano/heterostructures/superlattices consisting of two or more important functional materials are of prime importance for revealing unique properties and essen-tial for developing potential nanoscale devices, and some new and enhanced performances have been discovered. [ 27 , 129 , 130 ] The fabrication of 1D ZnS nanostructure arrays based heterostruc-tures/superlattices, especially the heterostructures/superlat-tices between ZnS and other important semiconductors (Si, Ge, ZnO, CdS, ZnSe, ZnTe, CdSe etc, and their ternary compounds

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ZnS x O 1 − x , ZnS x Se 1 − x , ZnS x Te 1 − x etc) will be highly valuable for building multi-functional and property-tuning optoelectronic devices, nanogenerators and solar cells. (vi) Surface function-alization, binding and doping with specifi c materials have been proved to be facile and effective routes to optimize the device performances. However, the relative studies on ZnS nano-structure arrays are limited. The work to tune ZnS conductivity, bandgap, surface and optical properties, and so forth through a control in vacancies formation, surface functionalization and binding and hybrid with organic nanostructures will be also an important direction.

The nanocrystalline-based solar cell is now emerging as a competitive technology for practical application due to simple preparation technologies, the possible use of fl exible substrates, savings on energy and source materials, mass production, and low cost. [137–140] Since ZnS has a direct wide-bandgap semi-conductor, the features of easy crystallization and anisotropic growth, an intrinsically n-type semiconductor, good thermal stability, high electronic mobility, and excellent transport prop-erties (reduction of the carriers scattering and recombination), its 1D nanostructure arrays are promising materials for next generation photovoltaic solar cells. For example, as shown in Figure 11 , the device consists of vertically oriented n-type 1D ZnS nanostructure arrays, surrounded by a fi lm constructed from p-type semiconductor nanoparticles with a high solar-light absorption coeffi cient. The p-type semiconductors can be Cu 2 O or CuInSe 2 . Physically, this concept makes full use of the excel-lent electrical properties of the oriented 1D ZnS nanostruc-tures and the highly effi cient absorbing p-type semiconductors. Compared to TiO 2 , ZnS has higher electronic mobility (about two orders of magnitude) that would be favorable for electron transport, with reduced recombination loss when used in solar cells. The advantage of this novel p-n heterojunctions device concept based on 1D ZnS nanostructure arrays is the (i) low cost, (ii) high effi ciency in principle, (iii) avoiding the use of toxic elements, (iv) greatly reduction or no use of rare elements, (v) facile for mass production, and (vi) a low lattice mismatch between ZnS and p-type semiconductors system (e.g. CuInSe 2 ) compared to ZnO.

In summary, the state-of-the-art research activities on ZnS nanostructure arrays have been reviewed comprehensively. The results show that ZnS nanostructure arrays are a rising star in materials science owing to the unique intrinsic characteristics of ZnS, the richness of the physical and chemical properties, and wide range of potential applications. The outlook for the future is that there still is a lot of room for future development in this area. Future research work will be expected to further

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demonstrate that ZnS nanostructure arrays are emerging as one of the most powerful functional nanomaterials that are having a major impact on fundamental science issues at the nanoscale as well as technological applications.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51002032 and 21001028) and the innovative team of Ministry of Education of China (IRT0911). We are grateful to Dr. Meiyong Liao at Sensor Materials Center, National Institute for Materials Science (NIMS), Japan for the valuable discussion on ZnS nanostructure arrays- based solar cell concept.

Received: October 4, 2010Published online: November 22, 2010

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