zns material

ZnS nanostructures: From

Xiaosheng Fang a,b,, Tianyou ZhLimin Wu a,, Yoshio Bando b, DaDepartment of Materials Science, Fudan University,b International Center for Young Scientists (ICYS), InteMaterials Science (NIMS), Namiki 1-1, Tsukuba, Ibara

a r t i c l e i n f o

Article history:Received 19 March 2010Received in revised form 21 September 2010Accepted 11 October 2010

2010 Published by Elsevier Ltd.

Corresponding authors. Addressess: Department of Materials Science, Fudan University, Handan Road 220, Shanghai200433, China (X.S. Fang, L.M. Wu), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044,Japan (T.Y. Zhai, U.K. Gautam, L. Li). Tel./fax: +86 21 65643795.

E-mail addresses: [email protected], [email protected] (X.S. Fang), [email protected] (T.Y. Zhai), [email protected] (U.K. Gautam), [email protected] (L. Li), [email protected] (L.M. Wu).

Progress in Materials Science 56 (2011) 175287

Contents lists available at ScienceDirect

Progress in Materials Science

journa l homepage : www.e lsev ie r . com/ loca te /pmatsc i0079-6425/$ - see front matter 2010 Published by Elsevier Ltd.tions. This is followed by in-detail discussions on the recent pro-gress in the synthesis, analysis of novel properties and potentialapplications, with the focus on the critical experiments determin-ing the electrical, chemical and physical parameters of the nano-structures, and the interplay between synthetic conditions andnanoscale morphologies. Finally, we highlight the recent achieve-ments regarding the improvement of ZnS novel properties andnding prospective applications, such as eld emitters, eld effecttransistors (FETs), p-type conductors, catalyzators, UV-light sen-sors, chemical sensors (including gas sensors), biosensors, andnanogenerators. Overall this review presents a systematic investi-gation of the synthesis-property-application triangle for thediverse ZnS nanostructures.doi:10.1016/j.pmatsci.2010.10.001synthesis to applications

ai b,, Ujjal K. Gautamb,, Liang Li b,,mitri Golberg b

Handan Road 220, Shanghai 200433, Chinarnational Center for Materials Nanoarchitectonics (MANA), National Institute forki 305-0044, Japan

a b s t r a c t

Zinc sulde (ZnS) is one of the rst semiconductors discovered. Ithas traditionally shown remarkable versatility and promise fornovel fundamental properties and diverse applications. The nano-scale morphologies of ZnS have been proven to be one of the rich-est among all inorganic semiconductors. In this article, we providea comprehensive review of the state-of-the-art research activitiesrelated to ZnS nanostructures. We begin with a historical back-ground of ZnS, description of its structure, chemical and electronicproperties, and its unique advantages in specic potential applica-

Contents

2.3.

5.

176 X.S. Fang et al. / Progress in Materials Science 56 (2011) 1752875.7. Chemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2655.8. Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2675.9. Nanogenerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

6. Conclusions and outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2735.6. Gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2605.5.2. Individual ZnS nanostructures-based UV-light sensors . . . . . . . . . . . . . . . . . . . . . . . . . 2535.5.3. Multiple ZnS nanostructures-based UV-light sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 2555.5.4. Microscale ZnS nanobelts-based UV-light sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2575.5.1. Unique advantages of ZnS nanostructrues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2515.2. Field effect transistors (FETs) and carrier characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2445.3. p-type conductivity in ZnS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2465.4. Catalytic activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2465.5. UV-light sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2515.1.6. ZnS nanostructures with other novel morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

coreshell heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385.1.5. Multiply enhanced field-emission properties of branched ZnS nanotube-In nanowire5.1.4. Enhanced field-emission properties by multi-angular branched ZnS nanostructureswith needle-shaped tips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236quasi-arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2345.1.2. Ultrafine ZnS nanobelts as field emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2325.1.3. Enhanced field-emission properties by crystal orientation-ordered ZnS nanobelt4.7.3. Red light PL emission from Mn/Cd-co-doped-ZnS nanostructures . . . . . . . . . . . . . . . . 230Potential applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2305.1. Field-emission applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

5.1.1. Field emission phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304.7.1. Optical properties of Mn-doped ZnS nanobelts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2284.7.2. Optical properties of Cu-doped ZnS nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2284.7. Optical property tuning by doping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2284.5. Thermoluminescence (TL). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2244.6. Luminescence mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

4.6.1. Origin of the green photoluminescence from ZnS nanobelts [148] . . . . . . . . . . . . . . . . 2254.6.2. Temperature-dependent PL from elemental sulfur species on ZnS nanobelts [345] . . 2264.4. Electrochemiluminescence (ECL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243.3. 2D nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2114. Luminescence properties of ZnS nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

4.1. Photoluminescence (PL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2124.1.1. Visible emission of ZnS nanostructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2124.1.2. UV emission of ZnS nanostructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

4.2. Cathodoluminescence (CL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2154.2.1. Multi-angular branched ZnS nanostructures with needle-shaped tips . . . . . . . . . . . . . 2164.2.2. Single-crystalline ZnS nanobelts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

4.3. Electroluminescence (EL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2193.2.1. Nanowires (NWs), nanorods (NRs), and nanotubes (NTs) . . . . . . . . . . . . . . . . . . . . . . . 1883.2.2. Nanobelts (NBs), nanoribbons (NRs) and nanosheets (NSs). . . . . . . . . . . . . . . . . . . . . . 1913.2.3. Aligned nanowires and nanobelts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1943.2.4. Complex nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1943.1.1. 0D nanocrystals (quantum dots) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1833.1.2. 0D core/shell nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1843.1.3. 0D hollow nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

3.2. 1D nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177Fundamental properties of ZnS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178Synthesis of ZnS nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.1. 0D nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1821.

ical and physical properties of the nanostructures, in regard of synthetic conditions. This will be

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 177followed by the main objective of the review which is the prospects of ZnS diverse structures in var-ious functional devices. The recent progress on the improvement of their properties and nding novelpotential applications, such as the latest achievements in using various ZnS nanostructures as eldemitters, eld effect transistors (FETs), p-type conductors, catalyzators, UV-light and chemical sensors(including gas sensors), biosensors, and nanogenerators will be highlighted.Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

1. Introduction

Nanostructured materials are not only in the forefront of the hottest fundamental materials re-search nowadays, but they are also gradually intruded into our daily life [13]. Theres plenty of roomat the bottom, the principles of physics, as far as I can see, do not speak against the possibility ofmaneuvering things atom by atom, put the atoms down where the chemist says, and so you makethe substance. . ., this famous statement of legendary Richard Feynman made in 1959 with immenseforesight has been realized in less than half a century by consistent efforts and signicant contribu-tions from the scientic community across the globe [4].

Nanostructured materials are a new class of materials, having dimensions in the 1100 nm range,which provide one of the greatest potentials for improving performance and extended capabilities ofproducts in a number of industrial sectors [5]. Nanostructures can be divided into zero-dimensional(0D when they are uniform), one-dimensional (1D when they are elongated), and two-dimensional(2D when they are planar) based on their shapes. The recent emphasis in the nanomaterials researchis put on 1D nanostructures at the expense of 0D and 2D ones, perhaps due to the intriguing possibilityof using them in a majority of short-term future applications. There is a large number of new oppor-tunities that could be realized by down-sizing currently existing structures into the nanometer scale(

2. Fundamental properties of ZnS

ZnS has two commonly available allotropes: one with a ZB structure and another one with a WZstructure. The cubic form is the stable low-temperature phase, while the latter is the high-tempera-ture polymorph which forms at around 1296 K [7]. For the purpose of comparison, Fig. 1 shows threedifferent views of these structures. The differences can be described either in terms of the relativehandedness of the fourth interatomic bond or by their dihedral conformations. Alternatively, ZB con-sists of tetrahedrally coordinated zinc and sulfur atoms stacked in the ABCABC pattern, while in WZ,the same building blocks are stacked in the ABABAB pattern. The lattice parameters of ZB area = b = c = 5.41 , Z = 4 (space group F4-3 m) and that of WZ are a = b = 3.82 , c = 6.26 , Z = 2 (spacegroup = P63mc).

Such minute difference in atomic arrangements leads to large difference in properties in thesematerials [7], e.g. electronic structures and bandgaps. The WZ phase has a higher bandgap of3.77 eV [8] while the ZB structure of 3.72 eV [9]. The band structure of a solid describes ranges of en-ergy that an electron is forbidden or allowed to have and it determines important electronic and

178 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 1. Models showing the difference between wurtzite and zinc blende crystal structures. (a and b) Show handedness of thefourth interatomic bond along the right (R) for wurtzite and along the left (L) for zinc blende. (c and d) The respective eclipsedand staggered dihedral conformations. (e and f) Show atomic arrangement along the close packing axis. Reproduced from Ref.

[7]. Copyright 1992, The American Physical Society.

optical properties of the material. The optical spectra are related to band structure, its dispersion andprobabilities of inter-band optical transitions. Experimentally, it has been shown that the optical prop-erties of the ZnS ZB and WZ phases are distinct. In order to understand such differences, electronicstructures of these phases have been widely investigated [1014]. Fig. 2 depicts typical band disper-sions for the ZnS ZB and the WZ phases obtained using density functional theory (DFT) calculations. Itcan be seen that the conduction-band minima are much more dispersive than the valence band max-ima for both phases. The authors further state that mobility of electrons in these materials is thereforehigher than that of holes [10]. Also, p-electrons forming the topmost valence band states tightly bindto sulfur and make the valence band holes less mobile. Hence, the contribution of the holes to the con-ductivity is expected to be smaller. The valence band comprises of three regions: a lower region con-sists of the s bands of Zn and S, a higher-lying region contains well localized Zn 3d bands, and a topbroader band originating from the Sp states hybridized with Zn 3d states.

Notably, the phase transition temperature for the two allotropes, the bandgap and electronic struc-ture considerably change when the size of the ZnS particles is of the order of nanometers. In an elegantand simple experiment, Qadri and co-workers used 2.7 nm ZnS ZB nanocrystals and heated them atvarious temperatures [15]. X-ray diffraction (XRD) measurement of these sample showed that ZBnanocrystals start converting to the WZ phase at a temperature as low as 400 C (Fig. 3). During

Fig. 2. Band dispersion for WZ and ZB ZnS calculated according to LDA (solid lines) and LDA + U (dotted lines). The Fermi level isset to zero energy Reproduced from Ref. [10]. Copyright 2007, The American Physical Society.

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 179Fig. 3. XRD patterns of the ZnS nanocrystals annealed at various temperatures taken with Cu Ka radiation. Peaks labeled withZB and WZ correspond to the zinc blende and wurtzite structures, respectively. Reproduced from Ref. [15]. Copyright 1999,

The American Physical Society.

annealing, the particle size, cell volume, lattice parameters also changed considerably, as depicted inTable 1.

Akiyama and co-workers using empirical calculations have demonstrated that when the sizes ofthe ZnS nanostructures reduce to just a few nanometers, the high temperature WZ structure becomesstabilized [16]. They calculated cohesive energies for nanowires with ZB and WZ (hexagonal, H) struc-tures and demonstrated that the stability of a crystal structure depends on the nanowire diameter.Fig. 4 shows the plot of energy differences between 6H and ZB structures DE6HZB, 4H and ZB struc-tures DE4HZB, and WZ and ZB structures DEWZZB of ZnS NWs as functions of nanowire diameters.

As seen in the plot, the energy differences converge into those of the bulk phase as diameter in-creases, indicating the appearance of bulk features at a large diameter. The W structure was foundfavorable for diameters less than 4 nm. On the other hand, the ZB structure, which is the most stablestructure in the bulk, is favorable for diameters above 24 nm. The authors explained this behavior onthe basis of two- and three-coordinated surface atoms on nanowire facets. Different from the fourfoldcoordination in the bulk solid, the Zn and S atoms at the side surfaces of WZ-ZnS nanowires are allthreefold coordinated with one unsaturated bond. In the case of the ZB-ZnS nanowires, in additionto the threefold coordinated atom, there are also certain twofold coordinated atoms located at theedges of the nanowires facets. This makes the surface energy of ZB-ZnS nanowires larger than that

Table 1Calculated values of the unit-cell parameters, specic volumes, and particle sizes of the ZnS particles obtained at various annealingtemperatures. Reproduced from Ref. [15]. Copyright 1999, The American Physical Society.

Annealingtemperature

Phase Percentof phase

Lattice parameters()

Specic volume(3)

Particle size()

23 C Zinc blende 100 a = 5.42 0.01 38.8 + 0.3 21c = 5.28 0.02

300 C Zinc blende 100 a = 5.42 0.01 38.7 0.2 29c = 5.27 0.02

350 C Zinc blende 100 a = 5.41z:0.01 3S.7 0.3 32c = 5.29 0.02

400 C Zinc blende 72 a = 5.404 0.012 39.5 0.3 74400 C Wuitzite 28 a = 3.S2 39.5 74

c = 6.26500 C Zinc blende 72 c = 5.41 39.6 232500 C Wmtzite 28 a = 3.82 39.6 243

c = 6.26

180 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 4. Plot of energy differences betweenWZ and ZB structuresDEWZZB (triangles), 6H and ZB structuresDE6HZB (squares), 4Hand ZB structures DE4HZB (circles), and for a ZnS nanowire as a function of nanowire diameter. Reproduced from Ref. [16].

Copyright 2007, The Japan Society of Applied Physics.

of WZ-ZnS nanowires, which results in the lower stability. Therefore, when the size of the nanostruc-ture is small so that a large number of constituting atoms reside on the surface, it prefers to be in theWZ phase with a fewer number of dangling bonds.

Different theoretical models have been developed in order to understand the electronic propertiesof ZnS nanostructures [1719,8]. Wang and co-workers calculated the energy bandgaps of WZ-ZnSnanowires using DFT [17]. They showed that the ZnS nanowires have wider bandgaps than that of bulkZnS crystal. The bandgap decreases with increasing diameter. More recently, the geometric, energetic,

Fig. 5. Formation energies of ZnS nanowires (solid circles), single-walled nanotubes (up-triangles), double-walled nanotubes(squares) and triple-walled nanotubes (down-triangle) as a function of tube diameter. The dashed violet lines represent theformation energies of ZnS nanosheets containing one to four atomic layers, respectively. Reproduced from Ref. [19]. Copyright2008, Institute of Physics.

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 181Fig. 6. Band structures of a WZ-ZnS (a) nanowire; (b) double-walled nanotube with faceted morphology; (c) (9, 0) single-wallednanotube; (d) bulk crystal. The energies at the Fermi levels are set to zero. Reproduced from Ref. [18]. Copyright 2008,

American Chemical Society.

and electronic properties of WZ-ZnS nanowires with hexagonal cross sections have been studied byusing rst-principles calculations [18,19,8]. Theoretical investigations were also carried out to studythe properties of different ZnS nanostructures, such as nanowires, nanotubes and nanosheets. In addi-tion, the evolution of energy of nanowires and nanotubes as a function of diameter and wall thickness

Fig. 7. Projection of PDOS of a ZnS nanowire onto the surface atoms (S0 , Zn0) and bulk atoms (S, Zn). Red and blue lines representS and Zn, respectively. The energies at the Fermi levels are set to zero. The arrows indicate the positions of the peak of the 3pstates of S. Reproduced from Ref. [18]. Copyright 2008, American Chemical Society.

182 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287was calculated.Fig. 5 shows the evolution of formation energy as a function of nanostructure diameter. The forma-

tion energy of the nanowires decreases drastically with increasing diameter. Interestingly, it was alsofound that the energy is proportional to the inverse of diameter. Furthermore, both nanowires andnanotubes with faceted morphologies were found to be energetically more favorable than nanotubeswith round cross sections. The formation energy of the thick-walled nanotubes is very close to that ofnanosheets of the same thickness.

Fig. 6 shows the plot of the electronic band structures of the nanowires, nanotubes and bulk WZ-ZnS [18]. The nanostructures have wider bandgaps as compared to a bulk crystal due to quantum sizeeffect. The highest valence band (HVB) and the lowest conduction band (LCB) of the ZnS nanostruc-tures have relatively high dispersions along the growth direction. Fig. 7 depicts the partial electrondensity of states (PDOS) of a ZnS nanowire. Notably, 3p states of both the bulk and the surface S atomscontribute to the HVB, whereas the LCB arises mainly from the Zn(4s) states of the bulk.

3. Synthesis of ZnS nanostructures

Nanostructures have attracted steadily growing interest due to their fashinating properties, as wellpropertymicrostructure correlation [6,20]. They can be divided into three kinds, namely 0D, 1D, and2D nanostructures based on their shapes.

3.1. 0D nanostructures

In the past decades, signicant progress has been made in the eld of 0D nanomaterials and nano-structures. A rich variety of methods have been developed for fabricating 0D nanostructures withwell-controlled dimensions [2125]. In this section, we highlight some progress in the synthesis

and characterization of 0D ZnS nanostructures, such as nanocrystals (quantum dots), core/shell nano-crystals (NCs), and hollow nanocrystals.

3.1.1. 0D nanocrystals (quantum dots)Initially, Brus and co-workers successfully synthesized ZnS nanocrystals with high colloidal stabil-

ity in aqueous and methanolic media using Na2S and Zn(ClO4)2 as precursors. However the size tun-ability and monodispersity of these nanocrystals were limited [26]. Bawendi and co-workersdeveloped a ground-breaking nonohydrolytic method to prepare nearly monodisperse CdSe nanocrys-tals by injecting a solution containing dimethyl cadmium (Cd(CH3)2) and trioctylphosphine selenide(TOPSe) into hot trioctylphosphine oxide (TOPO) [27]. Since then this nonohydrolytic chemical routehas been the most widely adopted method for synthesizing high-quality inorganic nanocrystalsincluding ZnS nanocrystals [28]. Hyeon and co-workers fabricated cubic ZnS nanocrystals with varioussizes and shapes under a thermal reaction of ZnCl2 and S in oleylamine in the presence of TOPO. A

2

[29]. Copyright 2003, American Chemical Society. (b) TEM image of hexagonal ZnS nanocrystals (

low-magnication transmission electron microscopy (TEM) image of ZnS nanocrystals (Fig. 8a) showsuniform nanoparticles with a size of 11 nm. High-resolution TEM (HRTEM) and XRD studies revealhighly crystalline nature of cubic ZnS nanocrystals [29]. In contrast, Zhao and co-workers synthesizedhexagonal ZnS nanocrystals at temperatures as low as 150 C using ZnCl2 and S as precursors in apolyol medium [30]. As demonstrated in Fig. 8b, these ZnS nanocrystals are quite uniform in bothshape and size. An average size is 4.2 nm with a standard deviation of 0.6 nm. The inset of Fig. 8bshows an HRTEM image of an individual particle where the lattice pattern illustrates the well-struc-tured phase.

Peng and co-workers developed another synthetic approach to grow ZnS nanocrystals using green-er chemicals at elevated temperatures. This was achieved by introduction of new terms: activation ofprecursors and identical injection/growth temperatures. By using ZnO or Zn-fatty-acid precursorsrather than highly reactive alkyl zinc precursors, the environmentally more benign synthesis was pos-sible [31]. Recently, a simple synthetic method using single-source molecular precursor was reportedby Li and co-workers (Fig. 8c) [32]. The injection of zinc ethylxanthate (Zn(exan)2)/trioctylphosphine(TOP) solution into a hot hexadecylamine (HDA) + TOP or octylamine (OA) mixed solution yields well-dened ZnS nanocrystals. The authors demonstrated that in the HDA + OA system, diameter- and as-pect-ratio-tunable hexagonal ZnS nanorods were attained in the temperature range of 150250 C,while in the HAD + TOP system, a shape change from rod to spherical particle and a phase transitionfrom hexagonal to cubic simultaneously occurred with an increase of TOP content in the solution. Thisapproach can be successfully applied for the synthesis of ZnxCd1xS nanocrystals by thermolyzing amixture of Cd(exan)2 and Zn(exan)2 precursors (Fig. 8d) [33].

3.1.2. 0D core/shell nanocrystalsBesides the development of synthesis techniques to prepare semiconductor NCs with narrow size

distribution, an intense experimental work has been devoted to the molecular modication towardthe improvement of the luminescence efciency and colloidal stability of the NCs. Post-synthesis inor-ganic surface modication not only passivated the nanocrystals but also improved quantum efcien-cies (QE) [23]. Thus this may be applicable to a core/shell system, where the bandgap of the core lieswithin the bandgap of the shell material and the photogenerated electrons and holes are mainly con-ned inside the core material. ZnS usually acts as a shell material in such core/shell system.Weller andco-workers reported the fabrication of CdSe/ZnS core/shell nanoparticles with high reaction yields andphotoluminescence (PL) QE of 5060% [34]. Firstly, monodispersed CdSe nanocrystals were preparedin a three-component HDATOPOTOP mixture. The PL QE of CdSe nanocrystals was only in the rangeof 1025% and had a tendency to decrease with increasing particle size. Secondly, ZnS shell was grownon the surface of CdSe nanocrystals. The amount of Zn:S stock solution necessary to obtain the desiredZnS shell thickness was calculated from the ratio between the core and shell volumes using bulk lat-tice parameters of CdSe and ZnS. Fig. 9a shows a HRTEM image of CdSe/ZnS nanocrystals (1.6 mono-layers of ZnS) synthesized from 4.0 nm large CdSe cores. Fig. 9b shows a set of absorption and PLspectra of CdSe/ZnS nanocrystals with different thicknesses of the shells. A maximum of the PL QE(66%) was observed from a 1.6 monolayer (MLs) thick ZnS shell and was reproducibly 50% or morefor a wide range of the shell thicknesses and core sizes [34].

Combining the advantages of two or more shell materials, Mews and co-workers fabricated CdSe/CdS/Zn0.5Cd0.5S/ZnS multishell nanocrystals with high QE of 7085% [35]. This CdSe-NC with a sand-wich shell-structure consists of 1 MLs CdS followed by 3.5 MLs Zn0.5Cd0.5 MLs and nally 2 MLs ZnS asthe outermost shell, as shown in Fig. 9d. The Zn0.5Cd0.5S is used as a buffer layer because the bandgapas well as the lattice parameters can in principle be adjusted by the composition of alloyed materials.The authors developed a successive ion layer adhesion and reaction (SILAR) technique, and could grad-ually change the shell composition from CdS to ZnS in the radial direction. Due to the stepwise adjust-ment of the lattice parameter in the radial direction, the resulting multi-shell NCs showed a high-crystallinity and were almost perfectly spherical, as was detected by XRD and TEM (Fig. 9c). Also, be-cause of the radial increase of the respective valence- and conduction-band offsets, the NCs were wellelectronically passivated. This led to a high orescence QE of 7085% for the amine terminated mul-tishell particles in organic solvents and a QE of 50% for mercapto propionic acid (MPA)-covered par-

184 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287ticles in water [35].

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 1853.1.3. 0D hollow nanocrystalsHollow nanospheres (HNSs) have attracted much attention due to their unique properties includ-

ing low density, high specic surface area and good permeation; and potential applications in cataly-sis, nanoelectronics, nano-optics, drug delivery systems and lightweight structural materials [3640].General methods for preparing ZnS HNSs use polystyrene (PS) microspheres [41,42] and SiO2 spheres[43] as sacricial templates, but the sizes of the obtained hollow spheres are larger than 100 nm. Xuand co-workers demonstrated a facile template-free route for mass production of ZnS HNSs withnano- and submicro-sizes, employing thiocarbamide, (CH3COO)2Zn2H2O and water as the raw mate-rials [44]. ZnS HNSs were obtained via hydrothermal reaction at 140 C for 5 h. XRD analysis showedthat these hollow spheres had been of hexagonal phase structure. A TEM image of the ZnS spherestreated with dilute HCl solution to thin thickness of the shell is shown in Fig. 10a. These hollowspheres are homogeneous and the size ranges from 200 to 300 nm, most of them falling in the rangeof 240260 nm. The mean thickness of the shells is about 20 nm. In contrast, Qi and co-workers devel-oped another synthesis of hollow ZnS nanospheres with diameters smaller than 100 nm using thioac-

Fig. 9. (a) HRTEM image of CdSe/ZnS core/shell nanoparticles (CdSe covered with 1.6 monolayers of ZnS). (b) RT absorption andemission spectra of CdSe nanocrystals before and after deposition of ZnS shells of different thicknesses (in monolayers, ML).Reproduced from Ref. [34]. Copyright 2001, American Chemical Society. (c and d) HRTEM image and structural model of aCdSe/CdS/Zn0.5Cd0.5S/ZnS multishell nanocrystal. Reproduced from Ref. [35]. Copyright 2005, American Chemical Society.

186 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287etamide as the sulfur source in aqueous solutions of the triblock copolymer P123 at RT [45]. Fig. 10bpresents a typical TEM image of hollow ZnS nanospheres, which shows that the hollow spheres arerather uniform with diameters ranging from 50 to 70 nm and a shell thickness of 15 nm. HRTEM

Fig. 10. (a) TEM image of hollow ZnS spheres. Reproduced from Ref. [44]. Copyright 2005, Institute of Physics. (b) TEM imageof hollow ZnS nanospheres in the presence of P123. Reproduced from Ref. [45]. Copyright 2003, American Chemical Society.(c and d) TEM image and HRTEM image of Fe3O4/ZnS core/shell hollow spheres. (e) Schematic diagram of the formation ofFe3O4/ZnS HNSs. Reproduced from Ref. [46]. Copyright 2009, American Chemical Society.

and selected-area electron diffraction (SAED) results indicate that these ZnS nanospheres are cubic,and the shells consist of ZnS nanocrystals.

Core/shell and hollow nanospheres have also attracted tremendous interest in recent research.Very recently, Wu and co-workers have fabricated superparamagnetic uorescent Fe3O4/ZnS HNSswith diameters of

3.2. 1D nanostructures

1D nanostructures have stimulated an increasing interest due to their importance in basic scienticresearch and potential technological applications [61]. It is generally accepted that 1D nanostructuresare ideal systems for exploring a large number of novel phenomena at the nanoscale and investigatingthe size and dimensionality dependence of functional properties. They are also expected to playimportant roles as both interconnects and the key units in fabricating electronic, optoelectronic, elec-trochemical, and electromechanical devices with nanoscale dimensions [5,62,63]. Several kinds of 1Dnanostructures have been reported in the literature. Fig. 11 shows the morphologies of ZnS-related 1Dnanostructures, such as nanowires, nanobelts, nanotubes, nanocombs, nanoawls, which have beensynthesized in our laboratory. In this section, we present some typical recently developed processesfor the synthesis of 1D ZnS-related nanostructures and their structural characterizations.

3.2.1. Nanowires (NWs), nanorods (NRs), and nanotubes (NTs)Nanowires, nanorods, and nanotubes are the most important and popular 1D nanostructures. Typ-

ically, nanowires are relatively long, exible, and have a circular cross-section. Nanorods are shorter(and therefore stiffer) and though circular cross-sections are prevalent, can often have hexagonalcross-sections. When hexagonal cross-sections are present, the surfaces of the nanorods are well-fac-eted [64]. Both nanowires and nanorods have solid structures, whereas nanotubes have hollowstructures.

3.2.1.1. NWs/NRs growth in vapor. Vapor-phase synthesis is probably the most extensively explored ap-proach to form ZnS NWs/NRs. There are several processing parameters such as temperature, pressure,carrier gas, substrate and evaporation time period that can be controlled and need to be properly se-lected before or during the vapor-phase synthesis. Several techniques can be assigned to vapor-phasemethods, such as thermal evaporation, chemical vapor deposition (CVD), molecular beam epitaxy

188 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 11. Several typical morphologies of ZnS-related 1D nanostructures fabricated in our laboratory.

(MBE), laser ablation and metalorganic CVD (MOCVD), which are utilized for the growth of ZnS NWs/NRs, as shown in Table 3. For example, Wang and co-workers reported the fabrication of single-crys-talline ZnS nanowires by thermal evaporation of ZnS powders with the presence of Au lms as a cat-alyst at 900 C [65]. Scanning electron microscopy (SEM) image (Fig. 12a) reveals that thesesynthesized products consist of a large number of nanowires with typical lengths of several tens ofmicrometers, and diameters of 3060 nm. TEM observation shows that most of the ZnS nanowiresterminate with an Au nanoparticle at one end. HRTEM image and SAED pattern indicate that thesenanowires grow along the [1 1 0] direction [65]. Lieber and co-workers demonstrated the rst useof single-source molecular precursors for the growth of ZnS nanowires at a low temperature [66].By using well-dened molecular reactants, the authors were able to prepare single-crystalline ZnS

Table 3Synthetic routes for ZnS nanowires/nanorods.

Nanostructure Synthesis method T (C) Ref.

Nanowires Thermal evaporation method 900 [65]MOCVD process [66]Liquid crystal template by c-irradiation RT [68]Solvothermal route 180 [69]One-step wet-chemical approach 140 [70]VLS process 1200 [71]Thermal evaporation 400500 [72]Molecular beam epitaxy (MBE) technique 430 [73]Solvothermal route 180 [74]Mild-solution chemistry approach 180 [75]Thermal evaporation method 1250 [76]Electrochemical-template method RT [77]Thermal evaporation process 1300 [78]Vapor phase deposition method 900 [79]Thermal evaporation of zinc powder and sulfur powder 580, 90 [80]Electrochemical-template method 120 [81]One-step CVD method 1200 [82]Hydrothermal synthesis route 180 [83]Electrodeposition-template method 120130 [84]Hydrogen-assisted thermal evaporation method 1100 [85]Thermal physical evaporation 1000 [86]Carbothermal CVD process 500 [87]Organic assistant VLS method 1000 [88]VLS method 1100 [89]Thermal evaporation method 700 [90]Two-step thermal evaporation process 1050 [91]Pulsed laser vaporization 950 [92,93]Hydrogen-assisted thermal evaporation 1100 [94]Intermittent laser ablation-catalytic growth process 950 [95]Chemical vapor transport and condensation 900950 [96]Direct reaction of Zn and S powders 750 [97]Solid-state reaction RT [98]Thermal evaporation method 1000 [99]VLS process [100]

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 189Thermal evaporation 1050 [101]

Nanorods Aqua-solution hydrothermal process 95 [102]Micro-irradiation-assisted growth RT [103]Solvothermal approach 135250 [104]Suldation of ZnO nanorod arrays 600 [105]Radio frequency magnetron sputtering technique 40 to 0 [106]Solvothermal decomposition 150200 [107]Solution method in the presence of block copolymer 105 [108]Plasma-assisted MOCVD process 650 [109]Thermolysing Zn(exan)2 with OA and TOP as precursor solvents 150250 [32]Thermal evaporation 1100 [110]Thermal evaporation 970 [111]Hydrothermal synthesis 180 [112]

nanowires with controlled diameters and a high yield. Their synthetic approach follows the nanoclus-ter-catalysted VLS growth process, where Zn(S2CNEt2)2 molecular precursor serves as a source for Znand S reactants (Fig. 12b). In this synthetic method, the single-source precursor undergoes thermaldecomposition in the growth region, forms ZnSAu liquid solutions with the Au nanocluster cata-

Fig. 12. (a) SEM image of ZnS nanowires fabricated by thermal evaporation of ZnS powders under controlled conditions withthe presence of Au lms as a catalyst. Reproduced from Ref. [65]. Copyright 2002, Elsevier. (b) Nanowire growth from asingle-source molecular precursor via a gold nanocluster-catalysted vaporliquidsolid mechanism. (c and d) HRTEM images ofZnS nanowires. Scale bar is 10 nm. Reproduced from Ref. [66]. Copyright 2003, American Chemical Society.

190 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287lysts, and then undergoes nucleation followed by nanowire growth when the nanodroplets becomesaturated with reactant. TEM structural analyses show that single-crystalline ZnS nanowires with con-trolled diameters are obtained. A TEM image of the nanocluster catalyst at the end of a crystalline ZnSnanowire (Fig. 12c) shows clearly how the nanocluster catalysts dene the diameter of nanowires.HRTEM image and SAED pattern indicate that these ZnS nanowires are single-crystals growing alongthe [1 0 0] direction [66].

3.2.1.2. NWs/NRs growth in solution. One of the disadvantages of high-temperature approaches tonanowire synthesis is the high cost of fabrication and scaling-up. Recent progress using solution-phase techniques, such as solvothermal, templating and chemical routes, has resulted in the creationof 1D nanostructures in high yields (grams scale) using selective ligand precursors or templates [67].For example, ZnS nanowires were fabricated via a direct templating route in an inverted hexagonalliquid crystal under c-irradiation [68]. The c-irradiation plays an important role in the formation ofZnS nanowires using thiourea or CS2 as the sulfur source. Several close-packed nanowires aggregateto form bundles with diameters of 1030 nm, which duplicate the hexagonal structure of close-packedinverted micelles formed by amphiphiles. Moreover, ZnS nanorods were fabricated by a solvothermalroute through thermolysing Zn(exan)2 using OA as a precursor solvent and HDA as the main ligandstabilizer [32]. Using hydrazine hydrate (N2H4H2O) as a solvent, ZnS nanowire bundles with diame-ters of 1025 nm and lengths about 58 lmwere prepared in high yield via a solvothermal route. Thenanowires, growing along the [0 0 1] direction, were aligned not only in the length direction but alsoin accord with the crystallography rules to form the bundle [69]. Other routes for the synthesis of ZnSNWs/NRs can be found in Table 3.

3.2.1.3. Nanotubes (NTs). As an important member of the inorganic nanotubes family, recently the ZnSnanotubes have become of growing interest. Wang and co-workers rst reported the synthesis of ZnS

nanocables and nanotubes by a chemical reaction using as-synthesized ZnO nanobelts as a template[113]. These structures were composed of ZnS nanocrystallites of 7 nm in size and had a high per-centage of pores. Yin and co-workers fabricated faceted ZnS nanotubes by a high-temperature ther-mal-chemical reaction route, in which commercial ZnS powders were used as precursors and H2Ovapor was carried by Ar gas and introduced into a graphite crucible to form reductive agents of COand H2 [114]. Fig. 13a and b shows the SEM images of the synthesized ZnS nanotubes. All the productsdisplay a tubular structure. From the cross-sectional morphologies of the ZnS nanotubes, it is clearlyseen that the ZnS nanotubes are perfectly straight and have highly faceted hexagonal cross-sectionmorphologies. As indicated in Fig. 13b, the nanotubes with hexagonal cross-sections grow along the[0 0 0 1] direction and are closed by low-index faces. Typically, these nanotubes have a length of sev-eral micrometers, a uniform outer diameter of 100120 nm, and a tube-wall thickness of about 10 nm.Fig. 13c and d depict TEM images of two single ZnS nanotubes with hexagonal-faceted open endsgrowing along the [0 0 1] direction, which suggest that the nanotubes are very pure, with no impurityphase coating or lling. The HRTEM image and corresponding SAED pattern (Fig. 13f) again conrmthat these nanotubes are single-crystals growing along the [0 0 0 1] direction, and are closed bylow-index planes. For the formation process of faceted tubular ZnS, no templates or metal catalystswere utilized. It was shown that the synthesis temperature and reactor gas pressure play an importantrole in the formation of ZnS nanotubes, and detailed growth mechanism is discussed in Ref. [114]. Byusing MOCVD-template methods, wurtzite-phase ZnS nanotube arrays were synthesized in the poresof anodic aluminia membranes (AAM)s by Yao and co-workers [115]. Different from the conventionalthermal evaporation and CVD methods in which a ZnS powder/nanopowder or ZnS-based mixturehave always been adopted as source materials, herein low pressure thermal decomposition of zincbis(diethyldithiocarbamate) (Zn(S2CNEt2)2) has been employed. The well-ordered and polycrystalline

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 191ZnS nanotubes with 140250 nm in outer diameter and several tens of micrometers in length wereformed [115]. Other synthetic routes for ZnS nanotubes can be found in Table 4.

3.2.2. Nanobelts (NBs), nanoribbons (NRs) and nanosheets (NSs)Unlike 1D nanostructures (nanotubes, nanowires and nanorods) discussed above, the nanobelts

(NBs)/nanoribbons (NRBs) and nanosheets (NSs) have a rectangular cross section with well-dened

Fig. 13. (a and b) SEM images of the faceted ZnS nanotubes; (c and d) Two single faceted ZnS nanotubes growing along the[0 0 1] direction; (e) Growth habit for the synthesized ZnS nanotubes; and (f) HRTEM image and SAED pattern of ZnS nanotubes.

Reproduced from Ref. [114]. Copyright 2005, Wiley-VCH.

geometry and high-crystallinity. After discovery by the Zhong Lin Wangs group in 2001, NBs/NRBs/NSs have become one of the most interesting nanoobjects [123].

Lee and co-workers introduced a convenient hydrogen-assisted thermal evaporation method tosynthesize ZnS nanoribbons. The synthetic reaction was carried out in a quartz tube furnace usinghigh-purity Ar premixed with 5% H2 as carrier gas at 1100 C, and sphalerite ZnS as sources [124].These ZnS nanoribbons have a typical length in the range of several tens to several hundreds ofmicrometers, a width ranging from 200 nm to 400 nm, and a thickness from one fteenth to one twen-tieth of the width (Fig. 14a and b). The width and length of the nanoribbons were sensitive to the

Table 4Synthetic routes for ZnS nanotubes.


Nanotubes Conversion of ZnO nanobelts 1350, 90 [113]High-temperature thermal-chemical reaction route 15001700 [114]MOCVD-template method 400 [115]Ultrasonic chemical solution method RT [116]Wet-chemistry method 140 [117]Hydrogel template route RT [118]Thioglycolic acid-assisted solution route 130180 [119]Atomic layer deposition 75 [120]Thermochemical process 1250 [121]Chemical conversion of ZnO columns 400 [122]Solution route by using the hard templates of CNTs 120 [39]

192 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 14. (a and b) SEM images of ZnS nanoribbons fabricated by a hydrogen-assisted thermal evaporation method; (c and d)TEM image, EDS spectrum, and HRTEM image of ZnS nanoribbons. Reproduced from Ref. [124]. Copyright 2003, Wiley-VCH.

reaction time and temperature, while the thickness of the ribbons was not sensitive to the growth con-ditions. X-ray energy-dispersive spectroscopy (EDS) spectrum and XRD pattern show that the sampleis hexagonal-structured ZnS with the lattice constants of a = 3.822 and c = 6.257 . TEM imagesshow that each nanoribbon is long and straight, and has a uniform width and thickness along its entirelength. Dark lines on the ribbons that appeared in TEM images are due to strain resulting from ribbonbending. HRTEM image (Fig. 14d) reveals that the ZnS nanoribbons are structurally uniform and singlecrystalline. The corresponding SAED pattern indicates that the ribbon grows along [1 2 0], and it is en-closed by (0 0 1) and (2 1 0) planes.

The hydrogen-assisted thermal evaporation process can be expressed as follows:

ZnSfcc H2 !1000C Zng H2S 1

Zng H2S !900C ZnShcp H2 2

Table 5Synthetic routes for ZnS nanobelts, nanoribbons, and nanosheets.


Nanobelts CVD process 1100 [125]Vapor-phase transport process 900 [126]Hydrogen-assisted thermal evaporation 1120 [127]Under moist gas conditionsThermal evaporation 1100 [128]Thermal evaporation 1100 [129]Conversion reaction of CdS NBs 800 [130]Rapid CVD process with Au as catalyst and graphite as reductant 1050 [131]Thermal evaporation 1150 [132]VLS process 1050 [133]Thermal evaporation 1150 [134]Solidvapor phase thermal-sublimation technique 1050 [135]Thermal evaporation 900 [136]Diethylenetriamine-assisted solvothermal approach 180 [137]Controlled thermal process 1150 [138]Low-temperature thermochemistry route 640 [139]VLS process 1000 [140]Thermal evaporation 970 [141]VPT process 900950 [142]Thermal evaporation 1200 [143]H2-assisted thermal evaporation approach 9001100 [144]Thermal evaporation 1050 [145]Thermal evaporation 1600 [146]Thermal evaporation 1100 [147]Solvothermal reaction and subsequent heat treatment 160, 250 [148,149]Thermal evaporation 1020 [150]

Nan

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 193Two-stage temperature-controllable thermal evaporation and condensationprocess

1180 [155]

Thermal evaporation 1050 [101]Microwave-assisted solvothermal synthesis 400 [156]Hydrogen-assisted thermal evaporation 1100 [124]

Nanosheeets Solvothermal method 180 [157]Solvothermal method 180 [158]Solution method in the presence of block copolymer RT [108]Thermal evaporation process 1050 [159]Thermal decomposition and reduction 1500 [160]Solvothermal method 120180 [161]Solvothermal reaction 120180 [162]oribbons Au mediated thermal evaporation route 10001100 [151]CVD method 450 [152]Thermal evaporation using thiol-capped Au NPs as catalysts 1000 [153]Thermal evaporation process 1050 [154]

The reaction between the sphalerite ZnS powder and H2 at high temperature forms Zn vapor andH2S gas, which are transported to the lower temperature zone where they react with each other andthen crystallize as wurtzite ZnS crystals. This is due to the temperature gradient along the tube axis,which will provide the external driving force for the new phase crystal growth [124]. Other syntheticrouts for ZnS nanobelts, nanoribbons and nanosheets may be found in Table 5.

3.2.3. Aligned nanowires and nanobeltsGrowth of aligned nanostructures is important for applications in lasers, LEDs and FETs [163,164].

Aligned growth of nanostructures can be achieved with the use of substrates and catalysts or seeds.The orientation-aligned ZnS nanowire bundles have been rstly demonstrated on a Si (1 1 1) substratewith CdSe as a buffer layer after a two-step vapor deposition process [91]. The rst step deposits theCdSe base and the second step forms the ZnS nanowire bundles. During the growth, bundles of ZnSnanowires grow on the top of a CdSe layer, which serves as a buffer between the Si(1 1 1) substrateand the nanowires. All of the ZnS nanowires are orientationally aligned, and they grow uniformlyalong the bundle [64,91].

Aligned ZnS nanorods/nanowires have also been successfully grown on several substrates throughheteroepitaxial and homoepitaxial growth [71,76,109,132,148]. The crystal structure of a substratewas crucial for the orientation of nanowires. Epitaxial relationship between the substrate surfaceand ZnS nanowires determines whether there will be an aligned growth or not and how good thealignment can be. For example, well-aligned ZnS nanorods had been grown on the c-plane Al2O3 sub-strates by plasma-assisted MOCVD without using any metal catalysts [109]. The lattice mismatch be-

194 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287tween the Al2O3 substrate and hexagonal ZnS is only 3%. ZnS nanowire arrays on Zn3P2 crystals werefabricated via a thermal evaporation method [76]. The fabrication of quasi-aligned ZnS nanobelt arrayswas achieved through a non-catalytic and template-free thermal evaporation process [132]. Well-aligned ZnS nanobelt arrays on Zn foils were prepared by a simple solvothermal reaction and subse-quent heat treatment [148]. These well-aligned ZnS nanowire/nanobelt arrays are expected to bepromising candidate for various applications.

3.2.4. Complex nanostructuresComplex nanostructures with modulated compositions, structures and interfaces have recently be-

come of particular interest with respect to potential applications in nanoscale building blocks of futureFig. 15. Illustration of assembling individual 1D ZnS nanostructures into complex nanostructures.

optoelectronic devices and systems [165168]. Fig. 15 illustrates assembling of individual 1D ZnSnanostructures into complex nanostructures, such as longitudinal nanoheterostructures (LONHs),coaxial (core/shell) nanoheterostructures (LONHs), side-by-side nanostructures, alloyed nanostruc-tures, doped nanostructures, tetrapodal nanostructures, bicrystalline nanostructures, ZnS/organic hy-brid nanostructures, and hierarchical nanostructures. In this section, we will highlight some of theimportant synthesis of 1D ZnS-based complex nanostructures.

3.2.4.1. Longitudinal nanoheterostructures (LONHs). Silica nanotube-shelled GaZnS nanowire hetero-structures were prepared through thermal evaporation of ZnS, Ga2O3, and SiO powder source materi-als by desired thermo-chemical reactions inside a vertical induction furnace [169]. Fig. 16 shows thetypical TEM images of the heterostructures in a product. The clear contrast variations within the struc-tures suggest that an individual nanostructure consists of the segments of different materials. More-over, each heterostructure is sheathed over its entire length within a thin tube. EDS analysis revealsthat the light and dark contrasting segments are composed of ZnS and Ga, respectively, whereas the

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 195Fig. 16. Typical TEM images of the silica-sheathed GaZnS nanowire heterojunctions: (a and b) Junction areas at thenanostructure center and at the end of a silica tube, respectively. (c and d) Two GaZnS junction areas formed by the Ga and ZnSnanowire end-to-end contacts within a silica tube. (e) The ends of Ga and ZnS nanowires are not in contact, leaving a narrowpart inside the tube. (f) Several junction areas formed by periodic Ga and ZnS nanowire end-to-end contacts along the tube axis.

Reproduced from Ref. [169]. Copyright 2005, Wiley-VCH.

sheathed tubes are made of SiO2. Most of the GaZnS nanowire heterostructures have diameters of150250 nm (a few of them have a diameter of 80120 nm); the shielding silica tube walls are48 nm thick [169171]. Other ZnS LOHs such as ZnOZnS, ZnScyclohexylamine, or multi-walledcarbon nanotubes (MWCNTs)ZnS LOHs are listed in Table 6.

3.2.4.2. Coaxial (core/shell) nanoheterostructures (CONHs). 3.2.4.2.1. ZnS-core CONHs. Coaxial nanohet-erostructures (CONHs), e.g. core/shell nanostructures, are fundamentally interesting and have signif-icant technological potential. CONHs can be fabricated by coating a material with a conformal layer ofthe second material. For example, BN/coated ZnS nanoarchitectures (or ZnS/BN core/shell nanostruc-tures) were synthesized by heating ZnS twinned-crystal whiskers in the presence of BNO vapors in aN2/NH3 atmosphere in an induction furnace [175]. ZnS twinned-crystal whiskers were preliminaryfabricated through controlled evaporation of ZnS powders at 1200 C under N2 atmosphere. Fig. 17ais an SEM image of the as-synthesized products, showing ZnS nanospine arrays grown on thetwinned-crystal whiskers. The nanospines are aligned on both sides with an angle of 59 (Fig. 17b),forming a shbone-like architecture. A TEM image of the typical structure of ZnS nanospine arraysis depicted in Fig. 17c, suggesting that the nanospines have sharp tips of only several nanometers thickand wide bottom of 100 nm at the joint with the whisker. The SAED pattern indicates that each of

Table 6Representative 1D ZnS longitudinal nanoheterostructures.

Nanostructure Composition Synthesis method T (C) Ref.

Heterostructures GaZnS Thermal evaporation 14001500 [169]ZnOZnS Thermal evaporation 1100 [172]ZnSCyclohexylamine Solvothermal method 100180 [173]MWCNTsZnS Ultrasonic pretreatment and heat treatments 180 [174]

196 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 17. (a and b) SEM images, (c) TEM image and (d) HRTEM iamge of BN-coated ZnS nanoarchitectures, i.e. ZnS/BN core/shell

nanostructures. Reproduced from Ref. [175]. Copyright 2004, Wiley-VCH.

the Znrect r 2

can beHSCH2COOH Zn2 $ ZnHS CH2COOH 4ZnHS S2 $ 2ZnSH2S 5

Furthermore, ZnS-core core/shell nanostructures, such as ZnS/SiC, ZnS/SiO2, ZnS/Si, ZnS/C, and ZnS-shell core/shell nanostructures, such as In/ZnS, Sn/ZnS, Zn3P2/ZnS, CdS/ZnS, TiO2/ZnS, CdSe/ZnS, havealso been synthesized as shown in Table 7.

3.2.4.3. Side-by-side heterostructures. Similar to the concept of creating a uniform sheath around ananowire is the idea of coating 1D nanostructures anisotropically, i.e., only along one or two sidesof the nanowire material, which will result in the formation of biaxial or triaxial nanostructures[67,207]. Hu and co-workers fabricated SiZnS biaxial and ZnSSiZnS triaxial nanowires based on

a cataobtained in line with the reactions:ZnOH2S ! ZnSH2O 3Fig. 18a and b shows a comparison of the TEM images taken from ZnO nanobelts before and after

reacting with H2S. The interface between the ZnS shell and ZnO core is fairly sharp and there appearsto be no intermediate layer. A clear picture of the core/shell structure is given in Fig. 18c, which dis-plays a composite core/shell structure with a broken ZnS surface layer. The corresponding SAED pat-terns (Fig. 18d) and EDS spectra (Fig. 18e and f) indicate that the core is made of hexagonal single-crystalline ZnO, and the shell consists of cubic nanocrystalline ZnS.

The conversion of ZnO nanobelts into ZnO/ZnS core/shell structures occurs in solution. Due to thelimited solubility of ZnO in water, the reaction is essentially a substitution reaction; thus, these core/shell nanostructures still preserve the rectangular cross section. The pores in the structures may beformed due to two factors. First, excess H2O produced in the reaction may be present in the structureand lead to the pores. Second, from the structural viewpoint, cubic ZnO and hexagonal ZnS are incom-patible in nature. Thus, the substitution reaction is unlikely to produce single-crystalline ZnS. The for-mation of nanocrystallites is expected especially when the reaction proceeds at room temperature[175,176].

Using a similar strategy, Xue and co-workers fabricated well-aligned ZnO/ZnS nanocable arrays andthen ZnS nanotube arrays by using ZnO nanorod arrays as templates [119]. First, ZnO nanorod arrayswere grown on a Zn foil substrate through direct oxidation of the foil with ammonium persulfateoxide in the alkali solution. Such solution-based methods greatly facilitate the approach toward scal-ing up ZnO nanoarrays (and ZnO/ZnS nanocable arrays and ZnS nanotube arrays) under relatively mildreaction conditions and at low cost. Second, ZnO/ZnS nanocable arrays with ZnO as the inner core andZnS as the outer shell were synthesized via a thioglycolic acid-assisted solution route. Subsequent re-moval of the ZnO core led to the formation of ZnS nanotube arrays. The evolution from ZnO nanorod toZnO/ZnS nanocable or ZnS nanotube arrays is due to the solubility difference between ZnO and ZnSand to the assistance of thioglycolic acid. When ZnO nanorod arrays are introduced into HSCH2COOHsolution, ZnHS+ complex could be formed between the lone pair electrons of sulfur atom ofHSCH2COOH molecule and the vacant d orbital of the Zn2+ ions on ZnO nanorods, and then ZnS nucle-ates and grows by dissolution of ZnO nanorods. After reaction over a certain time, ZnO/ZnS nanocableson:reactiO nanobelt template, Wang and co-workers received ZnO/ZnS core/shell nanostructures by a di-eaction of H S with the surface layer of ZnO within the presence of water, in line with thenanospines is a single crystal. Fig. 17d is an HRTEM image of the tip of a ZnS nanospine, which clearlyreveals the lattice-resolved image with the interplanar spacing d001 = 0.626 nm, conrming that [0 0 1]orientation is the preferred direction for ZnS nanospines. It also clearly displayed that layered BNsheaths uniformly coat the ZnS nanospines. These BN-coated ZnS, in other words, ZnS/BN core/shellnanoarchitectures have excellent stability and are promising for a wide range of nanotechnologyusages [175].3.2.4.2.2. ZnS-shell COHNs. Using ZnO nanowires/nanorods as templates, ZnO/ZnS core/shell nano-structures have been synthesized by a chemical reaction [113]. Based on the geometrical shape of

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 197lyst-free thermal evaporation of a mixture of SiO and ZnS powders via a two-stage process under

198 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287entire temperature control [208]. In this process, Si nanowires were rst formed from the dispropor-tionation of SiO powders, and ZnS nanowires were then grown on the Si nanowire substrates viathermal evaporation of ZnS powders. This results in the formation of side-to-side SiZnS biaxial and

Fig. 18. TEM image of ZnO nanobelts (a) before- and (b) after reaction with H2S, showing the formation of ZnO/ZnS core/shellnanostructures. (c) A ZnO/ZnS nanocable with a broken ZnS shell, and (d) a corresponding SAED pattern recorded from theregion, showing the presence of a single-crystalline ZnO core and the nanostructured ZnS shell. (e and f) EDS spectra acquiredfrom the regions indicated in (c), which prove the local chemical composition. Reproduced from Ref. [113]. Copyright 2002,Wiley-VCH.

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 199Table 7Representative 1D ZnS-core and ZnS-shell COHs.

Nanostructures Composition Synthesis method T (C) Ref.

ZnS-coreCONHs

ZnS (core)/BN Heating ZnS twinned-crystal whiskers in the present of BNO vapors

1200,600

[175]

ZnS (core)/BN One-step CVD method 1200 [82]ZnS (core)/SiC Two-stage thermal process 1150,

1400[177]

ZnS (core)/ZnO MOCVD process 450 [178]ZnS (core)/SiO2 Thermal evaporation process 900 [179]ZnS (core)/SiO2 Thermal evaporation 1100 [180]ZnS (core)/SiO2 Volume and surface diffusion VLS process 1000 [181,182]ZnS (core)/SiO2 High-temperature VLS process 1350 [183]ZnSSiZnS triaxial nanowires. Fig. 19a shows a scanning TEM (STEM) image of the segment of astraight and long side-to-side biaxial nanowire, which reveals a clear and uniform interface betweenthe Si subnanowire (light contrast) and ZnS subnanowire (dark contrast). The Si, Zn and S elementalmaps (Fig. 19bd) and line-scanning elemental proles (Fig. 19e) from the composite nanowire di-rectly feature a side-to-side geometry of the SiZnS biaxial nanowire, which is different from the char-acteristics of the coreshell nanowire heterostructures [171]. The diameters of Si and ZnSsubnanowires within these biaxial structures are 30 nm and 40 nm, respectively. A HRTEM takenfrom the interface domain between Si-side and ZnS-side indicates that the interface is homogeneousand uniform at the atomic scale. Furthermore, a clear epitaxial relationship, i.e., (1 1 1)Si//(1 1 1)ZnSand (1 1 1)Si//(1 1 1)ZnS, between the Si- and ZnS-sides is seen, and neither mist dislocations(the lattice mismatch of 0.40%) nor strains are observed at the interface under HRTEM studies. Thesandwich-like ZnSSiZnS triaxial nanowire heterostructures, which consist of two separated ZnS

ZnS (core)/SiO2 Thermal evaporation 1200 [184]ZnS (core)/Si Two-step thermal evaporation method 1060,

1260[185]

ZnS (core)/Si Thermal evaporation method 1450 [186]ZnS (core)/C One-step thermal evaporation 780800 [187]ZnS (core)/C Thermal evaporation method 1300 [188]

ZnS-shellCONHs

In/ZnS (shell) Carbon-thermal CVD technique 1300 [189]

Sn/ZnS (shell) Sn-nanorod-templated evaporation process 1150 [190]Zn/ZnS (shell) Thermo-chemical process 1500

1600[191]

Zn/ZnS (shell) Thermal reduction of ZnS 800,1300

[192]

Zn-Cd/ZnS(shell)

Thermal evaporation process 1200 [193]

Zn3P2/ZnS(shell)

Thermo-chemical process 12501350

[194]

Mn-CdS/ZnS(shell)

Two-step solvothermal process 160 [195]

TiO2/ZnS (shell) Wet chemical method RT [196]ZnO/ZnS (shell) Hydrothermal process 160 [197]ZnO/ZnS (shell) Catalyst-free thermal evaporation technique 1100 [198]ZnO/ZnS (shell) Suldation of ZnO nanorod arrays 400 [105]ZnO/ZnS (shell) Catalyst-free thermal vapor transport method 1000 [199]ZnO/ZnS (shell) Thioglycolic acid-assisted solution route 130180 [119]ZnO/ZnS (shell) Low-temperature reaction in Na2S solution 60 [200]ZnO/ZnS (shell) Two-step chemical reaction 200 [201]ZnO/ZnS (shell) Conversion of ZnO nanobelts 1350, 90 [113]CdS/ZnS (shell) MOCVD method 280 [202,203]Mn-ZnS/ZnS(shell)

Solvothermal method 200 [204]

CdSe/ZnS (shell) Two-step solution growth 290, 120 [205]ZnO/ZnS (shell) Low-temperature reaction in Na2S solution 60 [206]

200 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287subnanowires and one central Si subnanowire, were also prepared. Fig. 19f shows a STEM image of asegment of ZnSSiZnS triaxial nanowire, where the external surface and SiZnS interfaces are cleanand uniform, and the central Si subnanowire (light contrast) and two separated ZnS subnanowires(dark parts) have a nearly equal diameter of 2530 nm. The Si elemental mapping (Fig. 19g) showsthat Si is located at the central area along the longitudinal direction of the heterostructure, whilethe Zn and S elemental maps (Fig. 19h and i) display that Zn and S species are both distributed atthe left and right sides along the length and are absent in the center, directly revealing a three-layersandwich-like nanowire structure with well-dened compositional proles. Elemental proles acrossthe ZnSSiZnS nanowire heterostructure were also obtained (indicated by a line, Fig. 19f). The proleof Si, Fig. 19g, has a peak in the center, while the proles of Zn and S both show two peaks at the rightand left sides with a gap in the center, verifying a three-layer sandwiched geometry [171,208].

Recently, two novel semiconducting heterostructures: hetero-crystalline-ZnS/single-crystalline-ZnO biaxial nanobelts and side-to-side single-crystalline ZnS/ZnO biaxial nanobelts (Fig. 20) have beensynthesized via a simple one-step thermal evaporation method using gold as a catalyst [209]. In therst heterostructure a ZnS domain was composed of the hetero-crystalline superlattice (3C-ZnS)N/(2H-ZnS)M[1 1 1][0 0 0 1] with the atomically smooth interface between WZ and ZB ZnS fragments,

Fig. 19. (a) STEM image and (be) Corresponding elemental mappings and line-scanning elemental proles of SiZnS biaxialnanowires. (f) STEM image and (gj) Corresponding elemental mappings and line-scanning elemental proles of ZnSSiZnStriaxial nanowires. Reproduced from Ref. [208]. Copyright 2003, American Chemical Society.

Fig. 20. (ac) Typical TEM images of two novel ZnS/ZnO biaxial nanobelt heterostructures. (df) HRTEM images recorded fromthe ZnO side, the ZnS-side and the interface of hetero-crystalline ZnS/single-crystalline-ZnO biaxial nanobelts. (g and h)Structural models of WZ-ZnS/ZnO and ZB-ZnS/ZnO interfaces marked with I1 and I2 in (f). Reproduced from Ref. [209].Copyright 2008, American Chemical Society.

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 201

where N and M are the numbers of the atomic layers in the ZB and WZ-ZnS sections. Detailed HRTEMresults suggested that N and M had usually varied from 9 to 11 and from 24 to 28, respectively. Thealternating growth of WZ-ZnS/ZnO and ZB-ZnS/ZnO interfaces might reduce the stresses and systemenergy, leading to the formation of hetero-crystalline-ZnS/single-crystalline-ZnO biaxial nanobelts.For example, the length of 26 layers (Ls) on the WZ-ZnS side is slightly larger than the distance of19 Ls on the ZnO side, while the other segment 11 Ls on the ZB ZnS side is slightly smaller than thedistance of 10 Ls on the ZnO side. Importantly, the structures remained stable [209,210]. Otherside-by-side heterostructures can be found in Table 8.

3.2.4.4. Doped 1D nanostructures. To utilize semiconductor nanostructures as building blocks of func-

ExcN-dopsource

202 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287tion atmosphere. These N-doped ZnS nanoribbons have a uniform ribbon-like geometry and lengths of3040 lm, and their morphology remains similar to that of the undoped nanoribbons [217].

Other 1D ZnS-based doped nanostructures, such as N-doped, Cu-doped, Ga-doped, Eu-doped, andAl-doped-ZnS nanostructures have been fabricated as summarized in Table 9.

3.2.4.5. Alloyed 1D nanostructures. Recent advances in ternary semiconductor nanostructures haveshown that their bandgaps and thus their physical properties can be tuned by changing constituentstoichiometries [227]. Lee and co-workers fabricated ZnxCd1xS nanoribbons with variable composi-tions (0 6 x 6 1) by combining laser ablation of CdS with thermal evaporation of ZnS at 950 C[228]. The ZnxCd1xS nanoribbons had thicknesses of 5080 nm, widths of 0.55.0 lm, and lengthsup to several hundreds of micrometers, as shown in Fig. 22a and b. They displayed smooth surfaces,and their morphology varied a little with a substrate temperature in the range of 550800 C. In con-trast, the composition was highly dependent on the substrate temperature. HRTEM images and SAED

Table 8Representative 1D ZnS-based side-by-side heterostructures.


Side-by-side heterostructures SiZnS Two-stage thermal evaporation process 1600, 1500 [208]ZnSSiZnSZnOZnS Thermal evaporation 1100 [209]ZnSSi Thermal co-evaporation 1100 [211]ZnOZnS VLS technique 1050 [212,213]ZnOZnS MOCVD method 900, 500 [214]ZnOZnS Thermal evaporation 1050 [215]ept for metal-doped-ZnS nanostructures, non-metal-doped ZnS nanoribbons are also important.ed ZnS nanoribbons were grown via a chemical vapor deposition using ZnS powder as thematerial and high-purity Ar premixed with 5% H2 as the carrier gas. NH3 was added to the reac-tional nanodevices, it is important to synthesize them by having in the end diverse physical properties.This could be realized via appropriate doping. Kim and co-workers synthesized Mn/Fe-doped and co-doped ZnS 1D nanostructures via an Au-assisted chemical vapor transport method [216]. The key inthis synthetic process of co-doping is the use of metal chloride. Typical SEM images of Mn-doped,Fe-doped and Mn/Fe-co-doped nanowires are shown in Fig. 21a, d and g. These nanowires were pro-duced at high density. They uniformly covered the entire substrate. The nanowires are tens of microm-eters long and 70100 nm in diameter. The HRTEM images (Fig. 21b, e and h) of these nanowires showclear lattice fringes, which conrm their single-crystalline nature,The corresponding SAED patternsdemonstrate that these nanowires all grow along the [0 0 1] direction. The EDS spectra indicate thatZn and S are major elements, with about 3 at.% Mn for Mn-doped ZnS nanowires (Fig. 21c), 2 at.%Fe for Fe-doped ZnS nanowires (Fig. 21f), and 1 at.% Mn and 2 at.% Fe for Mn/Fe-co-doped ZnS nano-wires (Fig. 21i), respectively. The authors suggested that the formation of metal-doped ZnS nanowirescould be explained by the following reaction pathway [216]:

ZnOpower MSpowder MCl2 $ Zn1xMxSnanowires Cl2gasM3O4residues in the boatM Mn or Fe 6

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 203patterns conrmed the single-crystal quality and could be indexed to a hexagonal structure. TheseZnxCd1xS nanoribbons possessed sharp, tunable lasing emission within 340390 nm and 485515 nm. Chen and co-workers synthesized ZnxCd1xSe (x = 0, 0.2, 0.3, 0.5, 0.7, and 1) nanowires via

Fig. 21. SEM image, HRTEM image, and corresponding EDS spectra of (ac) Mn-doped, (df) Fe-doped, and (gi) Mn/Fe-co-doped ZnS nanowires grown on Au-coated Si substrates. Reproduced from Ref. [216]. Copyright 2009, American ChemicalSociety.

Table 9Representative 1D ZnS-based doped nanostructures.


Doped 1D nanostructures Mn/FeZnS Au-assisted chemical vapor transport method 950 [216]NZnS NBs CVD method 950 [217]MnZnS NRs Solvothermal approach 200 [218]MnZnS NRs Thermal annealing process 700 [219]CuZnS NRs Solvothermal process 200 [220]CuZnS NBsMnZnS NBs

Vapor-phase transport method 950 [221]

GaZnS NWs Thermal evaporation 1350 [222]Mn, CdZnS NSs CVD method 700 [223]EuZnS NWs Vapor deposition method 1100 [224]Cu, MnZnS NWs Controlled thermal process 1150 [138]Cu, AlZnS NRs Self-assembly method RT [225]MnZnS NBS Thermal evaporation 900 [226]

204 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287a laser-assisted CVD method using sintered mixture of CdSe and ZnSe at 1000 C [229]. The composi-tions of the alloyed nanowires could be adjusted by varying the precursor ratios of the laser ablatedtarget and the CVD deposition temperatures. These ZnxCd1xSe nanowires had diameters of 60150 nm and lengths of several tens of micrometers (Fig. 22e). The structures exhibited strong visiblePL from 712 to 463 nm as a nonlinear function of the compositions and could serve as important inte-grated full-color display elements in nanotechnological applications. Fei and co-workers achievedZnSxSe1x (x = 0.80, 0.71, 0.60, 0.49, 0.35, and 0.21) nanowires via thermal evaporation of the mixturesof ZnS and ZnSe powders at 1100 C.Various compositions could be easily obtained by changing themole ratio of ZnS and ZnSe in the source material [230]. The diameters of the ZnSxSe1x nanowireswith different compositions x were mostly distributed in the range of 100200 nm, and the lengthswere up to several tens of micrometers (Fig. 22f). The HRTEM and SAED studies indicated that thesenanowires were all single crystalline with a hexagonal structure, but they had different growth direc-tions for different compositions, such as [2 1 0] for x = 0.73 and [0 0 1] for x = 0.33, respectively. PLmeasurements demonstrated the tunable bandgap emission of the alloyed ZnSxSe1x nanowiresshifted continuously from 340 nm (pure ZnS) to 463 nm (pure ZnSe).

The process of fabricating alloyed nanostructures using ME powders (M = Zn, Cd; E = S, Se) withhigh melting points (such as 1830 C for ZnS and 1750 C for CdS) involves vapor generation, transportand deposition of target materials, and inevitably requires high temperatures or high-vacuum laser-ablation operations to generate sufcient amounts of vapor for the deposition. Yao and co-workershave developed a low-temperature route for the preparation of single crystalline ternary ZnxCd1xSnanocombs and zigzag nanowires via a one-step MOCVD approach by heating a mixture ofZn(S2CNEt2)2 and Cd(S2CNEt2)2 powders to 420 C [227,233]. Fig. 22c shows typical growth of a

Fig. 22. (a and b) SEM images and EDS spectra of Zn0.86Cd0.14S and Zn0.64Cd0.36S nanobelts. Reproduced from Ref. [228].Copyright 2005, Wiley-VCH. (c) SEM and TEM images of Zn0.54Cd0.46S nanocombs. Reproduced from Ref. [227]. Copyright 2006, Elsevier. (d) SEM image and EDS spectrum of Zn0.78Cd0.22S zigzag nanowires. Reproduced from Ref. [233]. Copyright 2006, Institute of Physics. (e) SEM image of ZnxCd1xSe nanowires. Reproduced from Ref. [229]. Copyright 2006, AmericanChemical Society. (f) SEM image of ZnSxSe1x nanowires. Reproduced from Ref. [230]. Copyright 2007, Wiley-VCH.

comb-like structure, with one side at and the other side with a shorter nanotooth. The diameters ofthe comb ribbons ranged from 500 to 800 nm, and their lengths were in the range of 250 lm. Theseteeth had a length of 300800 nm and a width of 30100 nm. Fig. 22d shows a SEM image of typicalzigzag nanowires. These nanowires have diameters of 200300 nm, widths of 200300 nm andlengths of up to tens of micrometers. TEM results indicate that these nanowires are single crystalline,with the growth axis of [0 0 1], by changing the growth direction from [1 1 3a2/2c2] to [1 1 3a2/2c2](a, c are the respective lattice constants of the a-axis and c-axis). The compositions x of the ZnxCd1xSnanocombs and nanowires are 0.54 and 0.78, respectively [233].

Other ZnS-based alloyed nanostructures, such as Zn1xMnxS NWs and NBs, ZnxCd1xSySe1y NBsCoxZn1xS NWs, and ZnSxSe1x tetrapods have also been achieved as listed in Table 10.

3.2.4.6. Tetrapodal nanostructures. Since Alivisatoss group rst reported the synthesis of tetrapod-shaped CdSe nanocrystals using a complicated thermal decomposition of organometallic precursors[243], several techniques have been developed to prepare tetrapod nanostructures. For example,ZnS tetrapods were synthesized through the direct reaction of Zn vapor with S vapor using a stainlessnetwork as the collecting substrate [244]. The ZnS tetrapods were almost uniform in size and shape(Fig. 23a), and they had four legs with equal lengths of 150 nm protruding from the center. HRTEMobservations show that each branch within this architecture has a triangular cross section connectedto form a tetrapod structure; the branches have the wurtzite phase and the core has the zinc blendephase. These experimental results showed a direct evidence of the zinc blende octahedron nucleusmodel, which could give a clue for explaining the formation of ZnS tetrapod nanostructures [244].Using cubic CdSe nanocrystals as the seeds, Yao and co-workers fabricated size-tunable ZnS tetrapodsvia a one-step seed-epitaxial MOCVD approach [245247]. The ZnS tetrapods had a CdSe nanocrystal

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 205core at the center with four wurtzite ZnS arms growing out from the core along four [0 0 0 1] direc-tions. The diameters of the ZnS tetrapods can be easily tuned by changing the distances betweenthe substrates and precursors. Furthermore, nanocable-aligned ZnS tetrapods were prepared throughthermal evaporation of ZnS and carbon mixed powders by Zhu and co-workers [248]. The ZnS tetra-pods were linked by nanocables (Fig. 23d), and the cables passed through the center of the tetrapods(Fig. 23e). The tetrapods were aligned together with ZnSC nanocables along the [1 0 0] direction. The

Table 10Representative 1D ZnS-based alloyed nanostructures.


Alloyed 1Dnanostructures

ZnxCd1xS NRs Combining laser ablation of CdS with thermalevaporation of ZnS

950 [228]

ZnxCd1x Se NWs CVD method assisted with laser ablation 1000 [229]CoxZn1xS NWs One-step thermal evaporation method 800 [231]ZnxCd1xSNanocombs

MOCVD process 420 [227,232]

ZnxCd1xS zigzagNWs

MOCVD process 400 [233]

ZnxCd1xS NWs Ethylenediamine-assisted solvothermal approach 175 [234]Ternary SiZnSNWs

One-step thermal evaporation method 1150 [235]

Zn1xMnxS NWsand NBs

Vapor phase deposition 900 [236]

Zn1xMnxS NWs Thermal evaporation 1020 [237]Zn1xMnxS NWsand NBs

Ion implanted with Mn 1200 [238]

ZnxMn1xS NRs Solvothermal process 200 [239]ZnSxSe1x NWs Thermal evaporation 1100 [230]ZnSxSe1xTetrapods

Thermal evaporation 800850 [240]

ZnxCd1x Se NWs MOCVD process 550 [241]ZnxCd1xSySe1yNBs

Cothermal evaporation route 1050 [242]

206 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287nanocable-aligned ZnS tetrapods may have potential applications in nanoelectronics and photonics.Other synthetic routes for ZnS tetrapodal nanostructures can be found in Table 11.Fig. 23. (ac) TEM and HRTEM images of ZnS tetrapods. Reproduced from Ref. [244]. Copyright 2006, Wiley-VCH. (df) TEMimages of nanocable-aligned ZnS tetrapods. Reproduced from Ref. [248]. Copyright 2003, American Chemical Society.3.2.4.7. Bicrystalline nanostructures. The periodic twinned structures formed in ZB ZnS with a (1 1 1)facet as the twin plane [251], whereas the bicrystals were observed in WZ-ZnS. Bicrystalline ZnS nano-structures composed of (0 1 2)/(1 0 3), (0 1 0)/(1 0 3) [252], (1 0 1)/(0 1 3) [253], (0 0 1)/(0 1 2) or(0 0 1)/(0 1 3) [254] have been reported. For example, ZnS periodically twinned nanowires (PTNWs)and asymmetrical polytypic nanobelts (APNBs) were synthesized via Au-assisted VLS processes[251]. Fig. 24a shows a TEM image of a ZnS PTNW, revealing sequentially bright/dark contrast stripesthroughout the entire length of the wires. The SAED pattern (inset of Fig. 24b) shows a ZB structurefrom the [1 1 0] zone axis, indicating the existence of a twin defect along the [1 1 1] growth directions.A HRTEM image (Fig. 24b) further reveals periodically alternating twins along the [1 1 1] axis of thewire: atomically sharp twin boundaries appear every 79 ZnS layers. The zigzag angles are 141(70.5 + 70.5), in accordance with the relative rotational angle (1 1 1) twin crystals in face-centeredcubic (fcc) structures. Fig. 24c depicts a TEM image of a ZnS APNB, showing that straight strips parallelto the belt axis are embedded inside the belt. The typical widths of the strips are 3050 nm. An SAEDpattern (Fig. 24d) indicates that the nanobelt contains both WZ and ZB phases. HRTEM image(Fig. 24e) clearly reveals that the strip (as interlayer) is the ZB phase, whereas the left- and right-hand

Table 11Representative 1D ZnS-based tetrapodal nanostructures.


Tetrapodalnanostructures

ZnS Direct reaction of Zn vapor with S vapor 1000 [244]ZnS Seed-epitaxial MOCVD deposition 420 [245]Nanocable-aligned ZnS tetrapods Thermal evaporation process 1100 [248]ZnS Thermal evaporation method 800 [249]ZnS Thermal evaporation process 1300 [250]

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 207parts are WZ structures. The formation of ZnS PTNWs and APNBs is based on modulating mass diffu-sion processes in the catalyst droplet and on the nanowire side surfaces, respectively, and more de-tailed information can be found in Ref. [251]. Other 1D ZnS-based bicrystalline nanostructures havealso been synthesized as listed in Table 12.

Fig. 24. (a and b) TEM image, HRTEM image and SAED pattern of ZnS periodically twinned nanowires (PTNWs). (ce) TEMimage, SAED pattern and HRTEM image of ZnS asymmetrical polytypic nanobelts (APNBs). Reproduced from Ref. [251].Copyright 2006, American Chemical Society.

Table 12Representative 1D ZnS-based bicrystalline nanostructures.

Nanostructures Synthesis method T (C) Ref.

Bicrystalline ZnS NRs Thermal evaporation process 1050 [154]ZnS Periodically twinned NWs and polytypic NBs VLS growth 1200 [251]Bicrystalline ZnS MBs Chemical vapor deposition 1300 [252]Dart-shaped tricrystal-ZnS NRs SiO-assisted thermal evaporation process 1100 [253]ZnS bicrystal NRs Thermal evaporation 1150 [254]Bicrystalline ZnS (core/shell) nanocables Physical deposition method 1000 [255]Periodically twinned ZnS NWs Thermal evaporation 1100 [256]ZnS heterocrystal and bicrystal structures Plasma enhanced chemical vapor deposition 700 [257]Bicrystalline ZnS NRs Thermal evaporation 1050 [154]ZnS tricrystals Thermal evaporation 1250 [258]ZnS nanosaws Hydrogen-assisted thermal evaporation 1000 [259]

3.2.4.8. ZnS hierarchical nanostructures.3.2.4.8.1. Homoepitaxial growth. In particular, the hierarchical assembly of nanoscale building blockswith a tunable dimensional and structure complexity is an essential step towards the realization ofmulti-functionality of nanomaterials. Several synthetic approaches have been reported to assemble3D branched and hyper-branched structures using a variety of materials and techniques includingself-assembled dendritic growth of nanowires [260,261], and growth of multi-branched nanowirestructures via sequential seeding of catalyst [262]. For example, Lee and co-workers reported thehomoepitaxial growth of ZnS nanowires and nanoribbons on the surfaces of micrometer-wide sin-gle-crystal ZnS nanoribbon substrates. These homo-branched ZnS nanostructures were formed undera two step thermal evaporation process using ZnS powders as precursors. First, the ZnS nanoribbonbackbones with lengths and widths of tens to hundreds of micrometers, and thickness of 40200 nmwere grown. Second, these ZnS nanoribbon substrates were sputter-coated with a 10 nm thickAu lm for catalytic growth of ZnS nanowires and nanoribbons. For the products synthesized at

208 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Fig. 25. (ad) SEM images of ZnS nanowires and nanoribbons grown on ZnS nanoribbons. (e) Schematic diagram showing thealignments of nanowires grown on different surfaces of the substrates. (f) The scheme of the basic cell of hexagonal structureshowing the crystallographic relations of the lattice planes and directions involved in epitaxial growth. (g) Molecular structuremodel showing the orientation of the cross-array nanowires with respect to the substrate. Reproduced from Ref. [263].

Copyright 2006, Wiley-VCH.

nanoribbon substrate temperatures of 800850 C for 1 h, well-aligned ZnS nanowire arrays with highdensity were grown on both top and side surface of the substrate, as shown in Fig. 25a and b. Thenanowires grown on the side surface were aligned normal to the side surface, while that grown onthe top surface grew dominantly in two directions with an angle of approximately 60 to each otherand to the substrate surface (Fig. 25c). In sharp contrast, for the products fabricated at about 900 C,ZnS nanoribbons perpendicular to the substrate surface were obtained (Fig. 25d). The TEM resultsshow that the nanowires grown on the top surface and side surface grow along the [2 1 0] and[0 0 1] directions, respectively, implying that the growth direction of ZnS nanowires can be controlledby using ZnS planes with different orientations as a substrate for homoepitaxy. The orientation rela-tions between the epitaxial ZnS nanowires and the substrate are shown in Fig. 25eg. On the top sur-face of the substrate, ZnS nanowires grow along the equivalent [2 1 0] and [1 1 0] directions,respectively, forming the cross arrays of nanowires at 60 with respect to each other and to the(2 1 0) surface of the surface. On the side of the substrate, the ZnS nanowires grow along the[0 0 1] direction, or perpendicular to the side surface [263].

3.2.4.8.2. Heteroepitaxial growth. Agarwal and co-workers reported the fabrication of branched nano-wire heterostructures, where the backbones and branches had been assembled with ZnS and CdS,

X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287 209Fig. 26. (a) Schematic illustrating the synthesis of nanowire branched heterostructures. (b and c) SEM and HRTEM images ofZnS nanowire backbones. (d) TEM image of Au catalysts deposited on ZnS nanowires. (e) SEM image of branched structuresshowing multiple branches spread out from each backbone nanowire. (f) SEM image of an isolated branched structure showingAu catalysts at the tips of branches, marked by arrows. Reproduced from Ref. [264]. Copyright 2007, American Chemical

Society.

respectively [264]. Growth of branched and backbones with control over the compositions was en-abled via sequential seeding of gold nanocluster catalyst. The growth method of ZnS (CdS) nanowireswas based on MOCVD process using single molecular precursors, where zinc (cadmium) diethyldithio-carbamate (Zn(S2CNEt2)2), Cd(S2CNEt2)2)) was used to grow ZnS (CdS) nanowires. The strategy to syn-thesize branched nanowire heterostructures is illustrated in Fig. 26a. First, ZnS nanowire backboneswith control over their composition and diameters were fabricated by using Zn(S2CNEt2)2) as precur-sors and monodispersed Au nanoclusters as catalysts via VLS mechanism. The diameters of nanowiresranged approximately from 80 to 120 nm, and the nanowire lengths were about 10 lm (Fig. 26b). TheTEM results indicate that the majority of the ZnS nanowires grow along [1 0 0]. Next, the ZnS nanowirebackbones were seeded with Au colloidal solution followed by sample air-drying for secondary cata-lyst deposition. Fig. 26d shows a TEM image of ZnS nanowires after the Au catalyst deposition, reveal-ing that Au nanoparticles are attached to ZnS nanowires. Finally, the branches were grown by placingthe ZnS nanowires decorated with secondary catalyst in the growth furnace with a ow of CdS precur-sor material. The nanowires branches were formed on the substrate as observed in Fig. 26e. As seen inFig. 26f, the Au nanoparticles were still found at the tips of branches (marked by arrows), and thediameter distribution of the branches were estimated to be similar to the size distribution of theAu catalyst. Further microscopy analysis indicated that the growth of heterostructures branches takesplace epitaxially, thus maintaining over all single-crystalline nature of the entire structure. This kindof new nanowire heterostructures are distinct from axial and radial nanowire heterostructures, pos-sesses large surface area and is expected also be useful as nanoelectronic and photonic building blocksto control the generation and transportation of carriers in three dimensions [264]. Subsequently, manyefforts have been focused on the integration of these building blocks into complex functional architec-tures. Several hierarchical nanostructures, such as ZnO nanorod arrays on ZnS nanobelts [265],ZnSxSe1x nanowire arrays on ZnS nanoribbons [266], ZnS nanowire arrays on CdS nanoribbons[267], ZnS nanowire on ZnO nanobelts [268], have been constructed subsequently, as shown in Table13.

210 X.S. Fang et al. / Progress in Materials Science 56 (2011) 175287Table 13Representative 1D ZnS-based hierarchical nanostructures.


/Hierarchicalnanostructures

ZnS NWsZnS NRs Homoepitaxial growth on ZnS NRs 1200, 1050 [263]

CdS NWsZnS NWs MOCVD process and VLS growth 950, 740 [264]ZnO NRsZnS NBs Two-step vapor method 1150, 650 [265]ZnSxSe1x NWsZnS NRs Metal-catalyzed VLS growth method 1150, 1020 [266]ZnS NWsCdS NRs Metal-catalyzed VLS growth method 880, 1050 [267]ZnS NWsZnO NBs Thermal evaporation method 1100 [268]ZnS nanocantilever structuresZnSNRs

Catalyst-assisted post-annealingtreatment

1050, 600 [269]

ZnS NWsZnS NWs VLS growth process 950 [270]ZnS nanoneedlesZnS NWs Thermal evaporation via VLS

mechanism950 [271]

ZnS tetrapod tree-likeheterostructures

Thermal evaporation method 1200 [272]

ZnS nanosaws Catalyst-free solidvapor depositiontechnique

1000 [

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