Inorganic semiconductor nanostructures and their field-emission applications

FEATURE ARTICLE www.rsc.org/materials Journal of Materials Chemistry Inorganic semiconductor nanostructures and their field-emission applications Xiaosheng Fang,* Yoshio Bando, Ujjal K. Gautam, Changhui Ye and Dmitri Golberg Received 21st August 2007, Accepted 17th October 2007 First published as an Advance Article on the web 13th November 2007 DOI: 10.1039/b712874f 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 for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO 3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area. 1. Introduction Nanomaterials and nanostructures are not only at the forefront of the hottest fundamental materials research nowadays, but they are also gradually intruding into our daily life. 1 10 There s plenty of room at the bottom, the principles of physics, as far as I can see, do not speak against the possibility of manoeuvring things atom by atom, put the atoms down where the chemist says, and so you make the substance, this famous statement of legendary the Richard Feynman Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. E-mail: xshfang@yahoo.cn; Fang.Xiaosheng@nims.go.jp; Fax: (+81) 29-851-6280 made in 1959 with immense foresight has been realized in less than half a century by consistent efforts and significant contributions from the scientific community across the globe. 11,12 Structures with at least one dimension between 1 and 100 nm are called nanostructures. They have steadily attracted growing interest due to their fascinating properties, as well as novel characteristics originating from their morphologies. 13 21 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 research is put on 1D nanostructures at the expense of 0D and 2D ones, perhaps due to the intriguing possibility of using them in a majority of short-term future applications. Xiaosheng Fang completed his PhD thesis from the Institute of Solid State Physics, Chinese Academy of Sciences in 2006 under the supervision of Prof. Lide Zhang. After then, he joined Prof. Yoshio Bando s group as a JSPS postdoctoral fellow. He has authored and co-authored over 40 refereed journal publications, an English monograph, and four book chapters. His current research topic is the controlled Xiaosheng Fang fabrication and novel properties of 1D nanostructures. He is a member of the Editorial Advisory Board of several international journals. Yoshio Bando received his PhD degree in physics from Osaka University in 1975 and joined the National Institute for Research in Inorganic Materials (at present NIMS) in the same year. Currently, he is a Director of the International Center for Young Scientists and a Fellow within the National Institute for Materials Science (NIMS). His current research concentrates on preparation, electron microscopy, and Yoshio Bando property analysis of various inorganic nanostructures. He has authored and co-authored more than 400 original papers and more than 150 patents. He received the Tsukuba Prize in 2005. This journal is ß The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 509 522 509

Table 1 Fundamental physical properties of some important inorganic semiconductors Crystal structures Band gap/ev Exciton binding energy/mev Work function/ev ZnO Hexagonal wurtzite 3.37 60 5.3 ZnS Cubic zinc blende/hexagonal wurtzite 3.72/3.77 39 7.0 Si Face-centered (diamond-like) cubic 1.12 15 3.6 WO 3 The crystal structure of tungsten 2.5 3.5 5.7 trioxide is temperature dependent AlN Hexagonal wurtzite 6.2 75 3.7 SiC b-sic (similar to diamond and zinc blende or aphalerite) 2.30 27 4.0 is the general structure for 1D SiC nanostructures CdS Hexagonal wurtzite 2.42 29.4 4.2 There is a large number of new opportunities that could be realized by down-sizing currently existing structures to the nanometre scale (,100 nm), or by making new types of nanostructures. The most successful examples are seen in microelectronics, where smaller has always meant a greater performance ever since the invention of transistors: e.g. higher density of integration, faster response, lower cost, and less power consumption. 13 Inorganic semiconductor nanostructures are ideal systems for exploring phenomena at the nanoscale and studying the dependence of functional properties on size and dimensionality. The building blocks made of inorganic nanoscale semiconductors with tailored geometries are also expected to play key roles in future nanodevices. 22 34 Table 1 shows the fundamental physical properties of some important inorganic semiconductors, including zinc oxide (ZnO), zinc sulfide (ZnS), silicon (Si), tungsten oxide (WO 3 ), aluminium nitride (AlN), silicon carbide (SiC) and cadmium sulfide (CdS). Field-emission (FE), is one of the main features of nanomaterials and nanostructures, and is of great commercial interest in displays and other electronic devices. In the case of a nanostructure, it has the advantage of faster device turn-on time, compactness and sustainability, as compared to conventional bulky technologies. Recent progress in the synthesis and assembly of nanostructures has resulted in a considerable increase in the current density and lowering of the turn-on voltage for a variety of nanomaterials. Field-emission, also known as Fowler Nordheim tunneling, is a form of quantum tunneling in which electrons pass from an emitting material (which is negatively biased) to the anode through a barrier (vacuum) in the presence of a high electric field. This phenomenon is highly dependent both on the properties of the material and the shape of the particular cathode, and the materials with higher aspect ratios and sharp edges produce higher field-emission currents. The current density (J) produced by a given electric field (E) is described by the Fowler Nordheim equation: anode and w is the work function of the emitting materials. The field-enhancement factor, b, is related to emitter geometry, its crystal structure, and the spatial distribution of emitting centers. b can be expressed as b = h/r, where h is the height and r is the radius of curvature of an emitting center. Thus, materials with elongated geometry and sharp tips or edges can greatly increase an emission current. Fig. 1a is a schematic illustration of the field-emission phenomenon. The emission occurs from the tip of an emitter. The emitter can have different emission currents depending upon the tip geometry (Fig. 1b) such as, (i) round tip, (ii) blunt tip and (iii) conical tip. According to the equations 1 3, the emission current (I) is strongly dependent on the following three factors: 35 (i) the work function of an emitter surface, (ii) the radius of curvature of the emitter apex and (iii) the emission area. As known, for different materials, the emission current is strongly dependent on the work function of an emitter material. It is clear that at a specific field, a lower work function material can produce a higher electron emission current. However, not all low work function materials are ideal for constructing field-emission cathodes and this is because their other properties may not be suitable. For instance, the work function of caesium (Ce) w = 1.8 ev, is one of the lowest, however, stable emission and J =(Ab 2 E 2 /w)exp(2bw 3/2 /be), (1) I = S 6 J, E = V/d, (2) or ln(j/e 2 ) = ln(ab 2 /w) Bw 3/2 /be (3) where A and B are constants, (A = 1.54 6 10 26 AeVV 22, B = 6.83 6 10 3 ev 23/2 V mm 21 ), S is the emitting area, V is the applied potential, I is the emission current, b is the fieldenhancement factor, d is the distance between a sample and the Fig. 1 (a) Illustration of the field-emission phenomenon. The emission occurs from the tip of an emitter. (b) The emitter can have different emission currents depending upon the tip geometry (Fig. 1b) such as, (i) round tip, (ii) blunt tip and (iii) conical tip. 510 J. Mater. Chem., 2008, 18, 509 522 This journal is ß The Royal Society of Chemistry 2008

lifetime obtained from Ce or caesiated (caesium coated) cathodes is a serious concern. For a given material, it has been demonstrated that the fieldemission performance, including emission current etc., can be enhanced through increasing its aspect ratio (length to thickness ratio), assembling it into arrays, or decorating its surface with a lower work function material etc. 36 39 For example, it has been shown that the field-emission performance of ZnO nanowires can be significantly enhanced through decreasing the density of the nanowires, and increasing the aspect ratio (length to thickness ratio) of ZnO nanobelts. 40,41 Since the discovery of carbon nanotubes (CNTs), much attention has been paid to exploring the prospects of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios, high mechanical stability, and high conductivity etc. 42 45 A CNT may have an aspect ratio as high as 1000, which leads to effective field enhancement at the apex of a nanotube. This provides a great opportunity for using such materials to obtain electron emission based devices at rather low applied potential. 35 Therefore, a fair amount of recent literature and reviews has dealt with CNTs and their field-emission applications. 46 48 However, we are not discussing CNTs here. Instead, we provide a comprehensive review on the state-of-the-art research that focuses on several kinds of inorganic semiconductor nanostructures, such as ZnO, ZnS, Si, WO 3, AlN, SiC, and their corresponding field-emission applications in detail, and briefly discuss some other inorganic semiconductors. In the end, we conclude this review with some perspectives/ outlooks and future research directions in this field. 2. The field-emission properties of inorganic semiconductor nanostructures In this section, we highlight recent progress with respect to several kinds of inorganic semiconductor nanostructures, including ZnO, ZnS, Si, WO 3, AlN, and SiC, and their fieldemission applications. now, numerous nano- shapes, e.g. ZnO nanoarrays, 63 65 nanorods, 66 69 nanowires, 70 72 nanobelts or nanoribbons, 73 75 nanotubes, 76 78 nanorings, 79 nanohelices, 79 nanobows, 80 nanotips, 81 nanoflowers, 82 nanosheets, 83 85 nanonails or nanopencils, 86 and nanosprings, 79 nanoplatelets, 87 nanoporous structures, 88 nanowalls, 89 nanobridges, 90 and hierarchical nanostructures 91 etc. have been synthesized via a variety of techniques. Some potential applications based on various ZnO nanostructures, e.g. nanowire nanogenerators, have recently been developed. 92 In this segment, we will first give an example which relates to aligned ultralong ZnO nanobelts and their enhanced fieldemission properties, and then report the systematic investigations on field-emission applications which were utilised based on ZnO nanostructures. Recently, in an interesting report, aligned ultralong ZnO nanobelts were synthesized by Ren and coworkers 40 by employing molten-salt-assisted thermal evaporation. In this approach, traditionally called molten flux synthesis, Zn metal powder is evaporated into a liquid environment of molten sodium chloride (NaCl) salt. 40 Fig. 2a shows an optical photograph of the as-grown ZnO nanobelts on an Au substrate, demonstrating that the technique can be effectively used to grow nanobelts whose length can be up to several millimetres. The scanning electron microscopy (SEM) images (Fig. 2b) indicate that the product exhibits beltlike structures which have a width of up to 6 mm. Fig. 2c shows a typical transmission electron microscopy (TEM) image of a single belt. Its transparency to the electron beam (one can see a copper TEM grid beneath the nanostructure) clearly reflects much smaller thickness of the belts as compared to their widths. The ripple like contrast variation in the image suggests the presence of strain in these structures. The corresponding diffraction spots in the rectangular SAED pattern, shown in the inset, can be 2.1 ZnO Zinc oxide (ZnO), a wide-band-gap semiconductor (E g = 3.37 ev at room temperature), has a large exciton binding energy of 60 mev which ensures efficient excitonic ultraviolet (UV) emission at room temperature. 49 Besides, the non-central symmetry in wurtzite, combined with its large electromechanical coupling, results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in actuators, piezoelectric sensors and nanogenerators. 50 52 ZnO is also bio-safe and biocompatible, and it can be directly used for biomedical applications without coating. As a consequence, ZnO is recognized as a promising photonic material in the blue UV region and as an ideal piezoelectric sensor and nanogenerator. 53 58 The synthesis of ZnO nanomaterials and nanostructures has been of growing interest owing to their promising applications in nanoscale technology and devices. 58 62 The morphology of ZnO has been proven to be the richest one among inorganic semiconductors, and its electrical and optical properties depend sensitively on both the morphology and size. Up to Fig. 2 (a) A side-view optical camera photograph of as-grown ZnO nanobelts on Au, showing the material alignment over a length of 3.3 mm, (b) typical SEM image showing the beltlike structures; the nanobelts have a width of up to 6 mm, (c) TEM image and SAED pattern (inset) of a single nanobelt, d) HRTEM image of a nanobelt showing its high crystallinity. Reproduced from Adv. Mater., 2006, 18, 3275. 40 Copyright E 2006, Wiley-VCH. This journal is ß The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 509 522 511

straight line in Fig. 3b, the as-grown ZnO nanobelts have a very high field-enhancement factor of 1.4 6 10 4. It is believed that the enhanced field-emission of the aligned ultralong ZnO nanobelts is the result of the extremely high aspect ratio of the obtained emitter geometry. 40 It can be stated that within last few years, tremendous progress has already been made in field-emission applications of ZnO nanostructures. For instance, we realised that there have been almost 100 research papers published which are related with ZnO field-emission characteristics. Here, we tabulate the representative results on field-emission properties of ZnO nanostructures reported so far, along with a brief description of the corresponding synthetic methods and fieldemission performances (Table 2). 2.2 ZnS Fig. 3 Field-emission characteristics of the aligned ZnO nanobelts. a) J F curve of the nanobelts and b) the corresponding Fowler Nordheim plot. Reproduced from Adv. Mater., 2006, 18, 3275. 40 Copyright E 2006, Wiley-VCH. indexed to a hexagonal wurtzite cell (lattice parameters a = 3.25 Å and c = 5.21 Å, JCPDS No. 36-1451). The HRTEM image of this belt is shown in Fig. 2d. The clear lattice fringes imply perfect crystallinity and defect-free nature of the nanobelts. 40 The field-emission performances of the aligned ultralong ZnO nanobelts are shown in Fig. 3. Fig. 3a is a typical fieldemission current density applied field (J F) curve. The turnon electric field is extrapolated as y1.3 V mm 21 at a current density of 10 ma cm 22. Fig. 3b shows the Fowler Nordheim plot for the nanobelts, which fits well to the linear relationship given by eqn (3). As obtained from the slope of the fitted Zinc sulfide (ZnS), 33 was one of the first semiconductors discovered and is probably one of the most important materials in the electronics industry with a wide range of applications including electroluminescence, 101 nonlinear optical devices, 102 LEDs (when doped), 103 flat panel displays, infrared windows, sensors, lasers, 104 110 and biological applications etc. 111 115 ZnS has two kinds of structures, namely, zinc blende and wurtzite, which both have wide bandgaps of 3.72 ev and 3.77 ev, respectively. 33 The zinc blende crystal structure is thermodynamically more stable than wurtzite up to 1020 uc. In a recent report, Wang et al. 116 demonstrated that ZnS nanobelts with a thickness of y10 nm surprisingly take on an ultrastable wurtzite structure, even at high pressure. The stabilization mechanism for the metastable phase of wurtzite-zns is possiblly based on size and morphology tuning. 116 ZnS, in its bulk form, is typically found to have a zinc blende crystal structure at room temperature. Bulk ZnS can undergo a phase transformation from a cubic zinc blende structure to a hexagonal crystal structure (known as wurtzite structure) at elevated temperatures. 3 Notably, wurtzite ZnS is much more desirable due to its optical properties than a zinc blende phase. 117 Another important characteristic of ZnS is its polar surfaces. The most common polar surface is the basal plane, where the oppositely charged ions produce positively charged Zn (0001) and negatively charged S (000 1) polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis, as well as a divergence in surface energy. 118 Table 2 Representative results on ZnO nanostructures with different shapes and their corresponding field-emission performances ZnO emitter Synthesis method Turn-on field/v mm 21 factor (b) Field-enhancement Stability: testing time and fluctuation Ref. Nanowires Electrochemical deposition 15.5 and 9.5 1188 and 1334 93 Vapor phase growth 6.0 at 0.1 ma cm 22 847 94 Nanowires grown on C cloth Carbonthermal vapor transport 0.7 at 1 ma cm 22 4.11 6 10 4 41 Nanotubes Hydrothermal reaction 7 at 0.1 ma cm 22 910 24 h,,10% 95 Nanobelts or nanoribbons Molten-salt-assisted thermal evaporation 1.3 1.4 6 10 4 40 Nanoneedles Metal organic chemical vapor deposition 0.85 at 0.1 ma cm 22 8328 96 (MOCVD) Solution-based method 4.2 2350 97 Chemical vapor deposition (CVD) 2.5 2 h,,20% 98 Nanonails and nanopencils Thermal evaporation process 7.9 and 7.2 85 Nanorods Thermal evaporation process 4.1 at 0.1 ma cm 22 99 Nanoscrews Vapor phase growth 3.6 30 min,,10% 100 a We define the turn-on field at a field producing emission current density of 10 ma cm 22. If other values are used, this is mentioned separately. 512 J. Mater. Chem., 2008, 18, 509 522 This journal is ß The Royal Society of Chemistry 2008

Generally, it is established that a material with a lower work function can produce a higher electron emission current at a specific field. The work function for ZnS is only marginally higher than for some other popular field-emission materials, as listed in Table 1. Thus the question is arising: what about the field-emission properties of ZnS nanostructures; is it possible to turn ZnS nanostructures into excellent fieldemitters? We designed the following experiments to answer these questions. 119 Crystal orientation-ordered ZnS nanobelt quasi-arrays were fabricated using a non-catalytic and template-free thermal evaporation process. 119 The nanobelts were not only aligned on the micro/macroscopic scales but also displayed similar crystallographic orientation of their growth axes. Fig. 4a and 4b are SEM images of fabricated ZnS nanobelts. The images reveal that the typical length of the belts is of the order of several hundreds of micrometres; some of them may even be as long as a millimetre. These nanobelts form in bundles. Within a bundle they are quasi-aligned. In many cases even perfectly parallel ensembles are visible. Fig. 4c shows a typical TEM image of an array of the parallel nanobelts. The inset depicts an electron diffraction pattern recorded from the entire bunch in Fig. 4a. The similar orientation of all nanobelts within the bunch along the [001] ZnS direction is obvious. HRTEM images of an individual nanobelt, as in Fig. 4d, display the defect-free (001) lattice plane of wurtzite ZnS with an inter-planar d-spacing of 0.62 nm, suggesting that the growth direction is [001]. 119 We have carried out field-emission measurements on these crystal orientation-ordered ZnS nanobelt quasi-arrays and found that they have much improved properties as compared to random nanowires: a low turn-on field (y3.55 V mm 21 ) and a high field-enhancement factor (y1850). The emission characteristic plots are shown in Fig. 5a c. As can be seen, the Fig. 4 (a) and (b) low- and high-magnification SEM images of ZnS nanostructures, displaying their quasi-aligned belt-like structures; the nanobelts are nearly parallel to each other, (c) a low-magnification TEM image of a ZnS nanobelt array. The inset is a SAED pattern taken from the entire bunch showing uniform alignment along the [001]-ZnS orientation; (d) a HRTEM image of a nanowire. Reproduced from Chem. Commun., 2007, 3048. 119 Copyright E 2007, Royal Society of Chemistry. Fig. 5 Field-emission properties of quasi-aligned ZnS nanobelt arrays compared with randomly-oriented ZnS nanobelts. (a) J E plots of the two samples. The inset in (a) is a magnified J E curve of random ZnS nanobelts; (b) the corresponding Fowler Nordheim plots; (c) a J T plot of quasi-aligned ZnS nanobelt arrays at an applied electric field of 5 V mm 21. Reproduced from Chem. Commun., 2007, 3048. 119 Copyright E 2007, Royal Society of Chemistry. This journal is ß The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 509 522 513

Table 3 Key performance parameters of ZnS nanostructure field-emitters reported in the literature ZnS emitters Synthesis method Turn-on field/v mm 21 factor (b) Field-enhancement Stability: testing time and fluctuation Ref. Nanobelts or nanoribbons Thermal-evaporation process 3.55 1.85 6 10 3 6h,,3% 119 CVD 3.47 2.01 6 10 3 120 Solvothermal reaction 3.8 1811 121 Nanowires Vapor phase deposition 11.7 at 0.1 ma cm 22 522 122 Nanorods Radio frequency magnetron sputtering technique 2.9 6.3 at 2.452 ma cm 22 420 105 123 emission current density reaches y14.6 ma cm 22 at a macroscopic field of 5.5 V mm 21 for quasi-aligned and wellordered ZnS nanobelt arrays. Importantly, this value is more than 20 times higher than that of randomly distributed ZnS nanobelts which have a current density of 0.68 ma cm 22 at the same macroscopic field. These findings demonstrate that despite the higher work function of ZnS (Table 1), quasialigned ZnS nanobelt arrays can indeed rival the previously reported FE cold cathode materials. 119 The J T plot in Fig. 5c shows variations of emission current density of a sample continuously for 4 hours at an applied electric field of 5 V mm 21. The initial current density and the average current densities of 3.87 and 3.77 ma cm 22, respectively, did not undergo any notable degradation during this period. The emission current fluctuations were found to be as low as 2.58%, proving the high stability of ZnS aligned emitters. 119 More recently, we have also demonstrated that field-emission performances of ZnS nanobelts can be significantly enhanced through increasing their aspect ratios in ultrafine nanobelts. 120 To date, there have been only a few reports that deal with field-emission properties of ZnS nanostructures. Table 3 lists the details of all field-emission measurements carried out on them so far. 2.3 Si Silicon (Si) is the second most abundent element on the Earth and its use is vital to many modern industries, such as glass, concrete and light-weight alloys. Above all, Si is the most important semiconductor for development of modern microelectronic technology that lead to one of the greatest industrial success stories of the 20th century. Hence, many recent works have been directed towards the preparation of Si nanowires, nanobelts, nanoribbons and nanotubes, and utilization of such nanostructures in miniaturized electronic devices. 124 127 Such attempts have lead to the discovery of several novel properties of Si nanostructures such as enhanced photothermal effect, 128 excellent field-emission, 129 and photovoltaic applications. 130 Si nanostructures can be synthesized through adopting various templates or catalysts and using lithography. 131 133 However, such methods have had certain disadvantages, e.g., the lithography techniques are expensive and template removal is difficult. In the catalyst mediated processes, catalytic metal particles may become electron and hole traps in Si. This poses a serious contamination problem for Si complementary metal oxide semiconductor (CMOS) processing. 134 We have been able to develop a simple thermal evaporation technique without using templates and metal catalysts to synthesize Si nanowires assembled into micro-sized semispheres. Electron microscopy investigations of a sample revealed that the nanowires within each semisphere ensemble are well-aligned and evenly distributed. We observed that a typical nanowire array density was y4 6 10 9 cm 22. Such alignments are desirable for improved field-emission properties. Our measurements show that the arrays possess a turn-on field of y7.3 V mm 21 and a field enhancement factor of y424. 135 Fig. 6a and 6b display low- and high-magnification SEM images of the aligned Si nanowires. As seen, the product morphology appears as evenly distributed micro-sized semispheres. Each semisphere is composed of Si nanowire arrays. The nanowires have an average diameter of 40 nm. The TEM images further confirm the nanowire alignment (Fig. 6c). HRTEM images verify that these arrays are structurally uniform and composed of single crystalline wires (Fig. 6d). The resolved lattice spacing of y0.31 nm corresponds to the (111) lattice planes, indicating that the Si nanowire grew along the [111] orientation. 135 Fig. 7 shows FE current density, J, as a function of an applied field, E, for a J E plot (Fig. 7a) and a ln(j/e 2 ) (1/E) plot (Fig. 7b) measured at anode cathode distances of 100 and 120 mm. The relatively smooth and consistent curves indicate the stability of emission from the Si nanostructure emitters. Fig. 6 (a) and (b) low- and high-magnification SEM images of Si nanowires assembled into semispheres, (c) TEM image of a Si nanowire bundle and (d) a HRTEM image of a single nanowire. Reproduced from Chem. Commun., 2007, 4093. 135 Copyright E 2007, Royal Society of Chemistry. 514 J. Mater. Chem., 2008, 18, 509 522 This journal is ß The Royal Society of Chemistry 2008

The role of Si nanostructures in semiconductor electronic industry would be crucial and could not be overestimated. Table 4 lists some important field-emission parameters of Si nanostructures reported in the literature. 2.4 WO 3 Fig. 7 Field-emission properties of Si semisphere ensembles at the anode cathode distances of 100 and 120 mm. (a) J E curve and (b) corresponding Fowler Nordheim plots (a). Reproduced from Chem. Commun., 2007, 4093. 135 Copyright E 2007, Royal Society of Chemistry. From Fig. 7a, the turn-on field at a current density of 10 ma cm 22 was found to be y7.3 V mm 21. The F N plot is nearly linear (Fig. 7b) indicating that the field-emission from the nanostructures is controlled by a barrier tunneling quantum-mechanical process. The enhancement factor b in this case was calculated to be y424. 135 Tungsten(VI) oxide (WO 3 ), also known as tungsten trioxide or tungstic anhydride, is an interesting material, because its composition can be tuned with respect to oxygen content by performing the synthesis under reducing or oxidizing conditions. WO 3 is a versatile wide band-gap semiconductor for many valuable applications. To date, WO 3 is used for many purposes in everyday life. It has been one of the most extensively studied materials for electrochromic devices, information displays, sensor devices and smart windows. For example, WO 3 has also been employed in the production of electrochromic windows, or smart windows. These windows are electrically switchable glasses that change light transmission properties with an applied voltage. Basically, this allows a user to tint their windows and change the amount of heat or light passing through it. 146 Much attention has been paid to explore the use of WO 3 nanostructures in field-emission devices due to their large surface areas and high aspect ratios. Very recently, Chen et al 147 developed a field-emission display (FED) with much improved performance by using WO 3 nanowires as a cathode material. The device is based on a double-gate structure, 147 as shown in Fig. 8. The FED composes of three separated plates: cathode, anode, and a gate plate. The cathode plate consists of the nanoemitters grown on a suitable substrate. The anode plate is a phosphor screen. The gate plate is designed as an insulating plate (ceramic plate with a thickness of 200 mm in this study) with round apertures of 100 mm drilled on it. Metallic electrode strips were prepared on both sides of the ceramic plate by e-beam evaporation in such a way that they became perpendicular to each other while electrically insulated by the ceramic substrate. The details of the driving scheme for the proposed FED structure can be found in ref. 147. Fig. 9a depicts the prepared 8 6 8 arrays of WO 3 nanowires on a silicon wafer. The patterned WO 3 nanowire arrays were prepared upon a heavily n-doped silicon substrate by a Table 4 Key performance parameters of Si nanostructure field-emitters reported in the literature Si emitters Synthesis method Turn-on field/v mm 21 factor (b) Field- enhancement Stability: testing time and fluctuation Ref. Nanowires Microwave plasma enhanced CVD 0.8 455 132 Thermal-evaporation process 7.3 424 135 CVD 7.4 540 136 High-temperature annealing 6.3 7.3 700 and 1000 at low and 137 high fields, respectively CVD template method 14 24 h, y5% 138 CVD 2 139 Vapor liquid solid (VLS) reaction 7.76 0.55 at 10 ma cm 22 y500 140 Nanowires grown VLS reaction 0.7 at 1 ma cm 22 6.1 6 10 4 141 on C cloth Cone arrays Ion-beam sputtering 13 142 C coated Si Plasma etching 1.75 and 2.52 for different 2310 and 6350 for 143 Nanocone arrays substrates at 1 ma cm 22 different substrates Nanotip arrays Si-Based PAAM as a mask 8.5 1100 144 Nanotubes Multistep template replication route 5.1 740 145 This journal is ß The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 509 522 515

Fig. 8 Schematics of a double-gate field-emission display device. Reproduced from Appl. Phys. Lett., 2007, 90, 253105. 147 Copyright E 2007, American Institute of Physics. combination of thermal evaporation and shadow mask technique. 147 The diameter of each cathode is y1 mm and the distance between the pixels is 2.5 mm. Fig. 9b shows a picture of an assembled device. The dark spots on the anode correspond to the pixels. It was also observed that there is a change of color of the phosphor due to the electron bombardment. SEM micrographs of the WO 3 nanowire are shown in Fig. 9c and 9d. The functioning of the device is evident in Fig. 10, where Arabic and Chinese characters appear by switching of individual spots. Each pixel could be accurately addressed without any interference. 147 Table 5 summarizes the most valuable field-emission properties of tungsten oxide nanostructures reported so far. 2.5 AlN Aluminium nitride (AlN), an important semiconductor compound of the III-V group, has a hexagonal crystal structure which is isomorphic with one of the polytypes of ZnS known as wurtzite. It is stable up to very high temperatures in inert atmospheres. For example, considerable surface oxidation occurs in air only above 700 uc. AlN turns out to be a very promising candidate for a field-emitter because of its very small (0.25 ev) electron affinity. Such small electron affinity Fig. 9 Pictures of (a) a prepared 8 6 8WO 3 nanowire array and (b) an assembled 8 6 8 device, (c) and (d) SEM images of one WO 3 nanowire cathode and a cross-section view of the WO 3 nanowires. Reproduced from Appl. Phys. Lett., 2007, 90, 253105. 147 Copyright E 2007, American Institute of Physics. means that an electron can be extracted from the surface with ease when an electric field is applied, which results in a large field-emission current density and excellent field-emission properties. 160 To date, a few methods have been developed to synthesize AlN nanomaterials and their efficient field-emission properties have been investigated (Table 6). 153 163 2.6 SiC Silicon carbide (SiC) is an important ceramic which is manufactured on a large scale for use mainly as an abrasive. SiC also occurs in nature as an extremely rare mineral, Fig. 10 Arabic numerals and Chinese characters created by a double-gated FED. Reproduced from Appl. Phys. Lett., 2007, 90, 253105. 147 Copyright E 2007, American Institute of Physics. 516 J. Mater. Chem., 2008, 18, 509 522 This journal is ß The Royal Society of Chemistry 2008

Table 5 Tungsten oxide nanostructures with various shapes and corresponding key performance FE parameters reported in the literature WO 3 emitter Synthesis method Turn-on field/v mm 21 Field-enhancement factor (b) Stability: testing time and fluctuation Ref. Nanowires Thermal-evaporation process 4.8 148 Thermal-evaporation process 6.44 691 149 Nanowires (W 18 O 49 ) Infrared irradiation to heat W foils 2.6 0.1 150 Nanotips Physical evaporation deposition process 2.0 13.5 h, y2% 151 Nanorods (WO 2.9 ) Thermal oxidization approach 1.2 152 Table 6 Field-emission performances of AlN nanostructures reported in the literature AlN emitters Synthesis method Turn-on field/v mm 21 factor (b) Field-enhancement Stability: testing time and fluctuation Ref. Nanowires Extended VLS growth technique,1 at 1 ma cm 22 8.2 6 10 4 153 Nanoarchitectures CVD 2.5 3.8 2 h,,7% 154 Nanotips CVD 4.7 1175.5 4 h,,0.74% 155 CVD 6 10 h, y5 and 10% 156 CVD 10.8 at 10 ma cm 22 367 and 317 157 Nanoplatelets Vapor phase method 3.2 5.0 1785 5 h,,3% 158 Si-Doped nanoneedles CVD 1.8 3271 5 h,,5% 159 Nanoneedles Vapor deposition method 3.1; the emission current 748 160 density is distinguished from the background noise Nanorods VS process 3.8 950 161 Mobile nitrogen arc discharge 8.8 565 1 h,,2% 162 Nanocones CVD 17.8 1450 and 340 163 moissanite. Due to the rarity of natural moissanite, silicon carbide is typically man-made. SiC exists in at least 70 crystalline forms, alpha SiC (a-sic) being the most commonly encountered polymorph. It can be obtained at temperatures higher than 2000 uc and has a hexagonal crystal structure (similar to wurtzite). The beta modification (b-sic), with a face-centered cubic crystal structure (similar to diamond or zinc blende or sphalerite), is formed at temperatures below 2000 uc and this polymorph structure is most frequent for the synthesized 1D SiC nanostructures. b-sic is an n-type wide band-gap semiconductor with a gap of 2.3 ev. Due to its superior electronic, physical and chemical properties, which are prerequisites in building devices capable of generating stable emission electron current, SiC nanostructures with various morphologies have been thoroughly studied. The studies indeed revealed that SiC is a promising emitting material for application in field-emission technology (Table 7). 164 171 3. Conclusion and outlook In this article, we have presented a dense review on recent advances in studies of some important inorganic semiconductor nanostructures and their field-emission properties. The fascinating achievements towards their applications should inspire more research efforts to address the challenges that remain. Table 8 is an up-to-date summary of the important repots on field-emission of several inorganic semiconductor nanostructures. Needless to say, due to space limitations, this article is unable to list all the exciting works reported in this field. The above results show that inorganic semiconducting nanostructures have become a popular research pursuit owing to the richness of their physical and chemical properties and wide range of possible applications. The observed nanostructure morphologies are not only very rich, e.g. nanoparticles, nanorods, nanowires, nanobelts or nanoribbons, nanotubes, nanocables, nanocones, nanoflowers, nanoneedles, nanotips, nanonails, nanopenciles, nanoscrews, nanoarrays, nanorings, nanohelices, nanobows, nanosheets, nanosprings, nanoplatelets, nanoporous structures, nanowalls, nanobridges, hierarchical nanostructures, nanocolumns, nanopyramids and so on, but they also provide shapes that play a very important role for developing next generation fieldemission devices. Table 7 Field-emission performances of SiC nanostructures reported in the literature SiC emitters Synthesis method Turn-on field/v mm 21 Field-enhancement factor (b) Stability: testing time and fluctuation Ref. Nanowires Reacting the aligned C nanotubes with SiO 0.7 1.5 24 h, y3% 164 Catalyst-assisted thermal heating process 3.33 165 Thermal evaporation process 10.1 166 Thermal evaporation process 5 839 100 h, y4% 167 Thermal evaporation process 3.1 3.5 10 4 order 2 h, y15% 168 BN coated SiC nanowires VLS process 6 169 C coated SiC nanowires CVD 4.2 170 SiC/Si heterostructures High temperature carbon implantation into silicon 2.6 at 1 ma cm 22 171 This journal is ß The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 509 522 517

Table 8 Key field-emission performance parameters and work function of several other inorganic semiconductor nanostructures reported in the literature Material Morphology Work Field-enhancement function/ev Synthesis method Turn-on field/v mm 21 factor (b) Stability: testing time and fluctuation Ref. Ge Cone arrays 4 Electron-beam evaporation 15 at a current of 1 ma 560 172 CdS Nanowires 4.2 MOCVD 7.8 at 0.1 ma cm 22 397 173 Solvothermal route 6.7 174 CuS Nanowalls VS process 8.5 175 Cu2S Nanowires Gas solid reaction 11 at 1 ma mm 22 16 h, y2% 176 TaS2 Nanobelts Pyrolysis of TaS3 nanobelts in vacuum 19.8 177 MoS2 Nanoflowers Thermal evaporation process 4.5 5.5 2 h, y20% 178 NbS2 Nanowires CVD 2.5 179 ZnSe Nanoribbons 4.84 Solvothermal route 5 at 0.1 ma cm 22 1382 2 h, y8% 180 SnO 2 Nanorods 4.7 VLS process 5.8 1402.9 181 Nanobelts Thermal evaporation process 2.3 4.5 at 1 ma cm 22 1796 2 h, y2.5% 182 Indium tin oxide (ITO) Nanorods 4.7 Carbon thermal reduction 3.8 at 0.1 ma cm 22 1140 20 h, y6% 183 MoO2 Nanorods Thermal evaporation process 4 at 0.1 ma cm 22 184 MoO3 Nanobelts Thermal evaporation process 8.7 2 h, y15% 185 CuO Nanowires 4.5 CVD 3.0 3.6 186 Nanobelts Aqueous reactions 6 187 Cobalt oxide Nanowalls 4.5 CVD 5 7.5 at 0.1 ma cm 22 302 19; 1118 41 188 In2O3 Nanowires 5 Electric-field-aligned growth 7 and 10.7 at 1 ma cm 22 10 h, y10% 189 Physical evaporation technique 2.47 at 0.1 ma cm 22 1810 190 Nanocolumns 1.92 at 0.1 ma cm 22 3934 Nanopyramids 3.34 at 0.1 ma cm 22 1022 RuO 2 Nanorods 4.87 MOCVD 10.3 1153 14 h no detectable degradation 191 a-fe 2 O 3 Nanoflakes 5.6 Heating an iron-coated AFM tip 2.67 4 2.8 6 10 5 and 1 6 10 5 192 TiO 2 Nanowires 4.5 Thermal evaporation 5.7 3896 and 1650 193 IrO2 Nanorods 4.23 MOCVD 5.6 40 000 8000 192 h no detectable degradation 194 GaN Nanowires 4.1 Catalytic CVD method 8.5 at 0.1 ma cm 22 1170 10 h, y8.7% 195 Thermal evaporation process 7.5 196 P-Doped GaN Nanowires Thermal-evaporation process 5.1 197 GaN/BN Nanocables CVD 1.4 at 0.1 ma cm 22 1400 198 InN Nanopyramids 5.1 MOCVD 19 1 at 5 na cm 22 y230 199 Nanofingers Ion-assisted filtered vacuum arc technique 9.7 1000 min, degraded slightly 200 Nanowires CVD 10.02 201 BN Nanosheets Protruding from Si3N4 nanowires 4.2 10 h, y2% 202 GaAs Nanowires 4.77 Etching GaAs wafer covered with Au film 2.0 at 1 ma cm 22 3500 203 using H plasma TiSi 2 Nanowires 4.6 Vapor phase deposition 8 at 0.1 ma cm 22 400 501 10 40 h no obvious degradation 204 518 J. Mater. Chem., 2008, 18, 509 522 This journal is ß The Royal Society of Chemistry 2008

The growth kinetics and thermodynamics involved in the synthesis of inorganic semiconductor nanostructures are extremely complex, and presume different mechanisms under different growth conditions. Significant challenges still exist in their syntheses that include, but are not limited to, reliable control in diameter, length, orientation, density, nanostructure network and hierarchical assembly. There should clearly be some new effects to be elucidated, which are different from traditional theory and infinite systems, especially with respect to quantum behaviors. Such phenomena are envisaged to dominate as the size of inorganic semiconductor nanostructures continues to reduce. CNTs have demonstrated remarkable field-emission properties over the past decades. The CNT field-emitters can produce high current densities of tens of ma cm 22 at a low applied electric field of a few V mm 21. Decent progress has been made in regard of making vertically aligned CNTs on a substrate. However, the adhesion of CNTs to the substrates and the stability of CNT field-emitters remain the key issues to be addressed. It is also known that emission currents from CNTs are strongly influenced by the presence of defects in a material. With the development of controlled growth of inorganic semiconductor nanostructures over the last five years, more attention was paid to explore their use at a higher aspect ratio. The data, as shown in Tables 2 8, suggest that inorganic semiconductor nanostructures may not only rival the previously reported CNTs or other excellent FE cold cathode materials, but in some cases may actually surpass them. Some effective routes have been developed to enhance the field-emission performances of inorganic nanostructures. For example, only a 0.35 V mm 21 turn-on field at a 10 ma cm 22 current density was achieved from ultrasharp (y1 nm diameter) and utrahigh density (up to 10 11 cm 22 ) Si nanotip arrays. 205 Very recently, we have developed two alternative methods to dramatically improve the field-emission properties of ZnO nanorods: the first one is to decrease the tip radius of nanorods down to y5 nm; the second one is to decorate the nanorods with small metal particles. 206 Therefore, it is still possible to develop novel effective routes or combine the preexisting methods with new ones to substantially enhance the field-emission performances. The literature documents a large number of nanostructures in various shapes and their field emission properties. Tables 2 8 list the important emission parameters of the most studied nanostructured materials. However, it is rather difficult to unambiguously compare their emission properties. From eqn (1) it can be seen that for an applied field E, the effective enhanced field at the emission centers is be. Thus, it is possible to achieve high field-emission from a given nanostructured material if it is long and sharp. The effective tip sharpness may be different in different nanostructures, for instance round closed CNT tips, blunt ends or conical tips (see Fig. 1). Thus in order to properly compare the efficiency of different emitters, they must be synthesized in a similar geometry. This requires more focused work than reported so far. The intrinsic work function, w, is an important parameter that varies mainly from 1 to 7 for most of the inorganic semiconductor materials, lower being better for higher emission efficiency. On the other hand, the aspect ratio of the nanostructures would be the most appropriate parameter to be considered, since it directly affects the field enhancement factor, b. By far, among all inorganic materials, only ZnO, can be prepared in a variety of sizes, shapes and aspect ratios. This material in fact has displayed excellent field-emission properties. To date, for valuable commercial use of nanostructures in field-emission devices, one of the major challenges has been the structure synthesis. The researchers are expected to focus more on the smart synthesis routes of nanostructures made of suitable inorganic materials with extremely high aspect ratios under their alignment on desirable substrates. The fundamental relationships between the nanostructure emission currents and its density and height has also to be established for different nanomaterials. Perhaps another interesting approach would be to tailor the band-gap of established nanomaterials, through doping for example, to further enhance their emission properties. The stability of a field-emission event, i.e. stability of the emission current over a long period of time, thermal effect etc., is key to many applications and real device performances. From the emission mechanism standpoint, it is clear that the current emission is strongly dependent on a number of factors including applied potential, tip geometry and the cathode material work function etc. Variations in any of these factors should be reflected in the emission currents. Thermal instability is one of the reasons for emitter failures. For instance, the emission of some materials often appears to drop abruptly when large currents are drawn from an emitter, which is associated with its sudden melting. 35 Therefore, one must also consider the emitter stabilities along with overall enhancment of field-emission properties. Inorganic emitters such as metals (e.g. W) and semiconducting materials (e.g. ZnO) have much higher melting points and thermal stability as compared to the organic materials and hence should be preferable for applications in the near future. There is still plenty of room for the development of inorganic semiconductor nanostructures and their field-emission applications. We believe that future work in this direction should continue to focus on generating inorganic semiconductor nanostructures in a more controlled, predictable and simple way, and enhancing their field-emission properties up to the level desirable for many real industrial applications. Acknowledgements The authors acknowledge the financial support from the Japan Society for the Promotion of Science (JSPS), in the form of a fellowship tenable at the National Institute for Materials Science, Tsukuba, Japan (X. S. Fang). The authors also thank Prof. Z. F. Ren, Prof. J. Chen, Prof. N. S. Xu, Ms E. Arisumi, Dr G. Z. 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