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This paper presents a review of current research activities on ZnO nanorods (or nanowires). We begin this paper with a variety of physical and chemical methods that have been used to synthesize ZnO nanorods (or nanowires). There follows a discussion of techniques for fabricating aligned arrays, heterostructures and doping of ZnO nanorods. At the end of this paper, we discuss a wide range of interesting properties such as luminescence, field emission, gas sensing and electron transport, associated with ZnO nanorods, as well as various intriguing applications. We conclude with personal remarks on the outlook for research on ZnO nanorod
ZnO nanorods: synthesis, characterization and applications This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2005 Semicond. Sci. Technol. 20 S22 (http://iopscience.iop.org/0268-1242/20/4/003) Download details: IP Address: 147.46.179.80 The article was downloaded on 16/10/2010 at 04:17 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY Semicond. Sci. Technol. 20 (2005) S22–S34 doi:10.1088/0268-1242/20/4/003 ZnO nanorods: synthesis, characterization and applications Gyu-Chul Yi, Chunrui Wang and Won Il Park National CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea E-mail: gcyi@postech.ac.kr Received 5 October 2004, in final form 21 October 2004 Published 15 March 2005 Online at stacks.iop.org/SST/20/S22 Abstract This paper presents a review of current research activities on ZnO nanorods (or nanowires). We begin this paper with a variety of physical and chemical methods that have been used to synthesize ZnO nanorods (or nanowires). There follows a discussion of techniques for fabricating aligned arrays, heterostructures and doping of ZnO nanorods. At the end of this paper, we discuss a wide range of interesting properties such as luminescence, field emission, gas sensing and electron transport, associated with ZnO nanorods, as well as various intriguing applications. We conclude with personal remarks on the outlook for research on ZnO nanorods. 1. Introduction One-dimensional (1D) semiconductor nanostructures such as rods, wires, belts and tubes have in recent years attracted much attention due to their many unique properties and the possibility that they may be used as building blocks for future electronics and photonics [1–3], as well as for life-science applications [4]. It is generally accepted that 1D nanostructures are useful materials for investigating the dependence of electrical and thermal transport or mechanical properties on dimensionality and size reduction (or quantum confinement) [5]. They are also expected to play an important role as both interconnects and functional units in fabricating electronic, optoelectronic, electrochemical and electromechanical nanodevices [6, 7]. In the past few years, much effort has been devoted to developing various 1D semiconductor nanostructures [8–10]. Vapour–liquid– solid (VLS) [11–13] and vapour–solid (VS) [14] mechanisms for growth of whiskers or fibres at high temperature are well recognized, and have been used to synthesize various group III–V and II–VI compound semiconductor nanostructures [15–17]. 1D semiconductor nanostructures have also been obtained via laser ablation-catalytic growth [18, 19], oxide-assisted growth [20], template-induced growth [21, 22], solution–liquid–solid growth in organic solvents [23, 24] and metal-organic chemical vapour deposition (MOCVD) [25]. Zinc oxide (ZnO) is a direct band-gap (E g = 3.37 eV) semiconductor with a large exciton binding energy (60 meV), exhibiting near UV emission, transparent conductivity and piezoelectricity. Furthermore, ZnO is bio-safe and biocompatible, and may be used for biomedical applications without coating. Intensive research has been focused on fabricating 1D ZnO nanostructures and in correlating their morphologies with their size-related optical and electrical properties [26–30]. Various kinds of ZnO nanostructures have been realized, such as nanodots, nanorods, nanowires, nanobelts, nanotubes, nanobridges and nanonails, nanowalls, nanohelixes, seamless nanorings, mesoporous single-crystal nanowires, and polyhedral cages [31–33] (figure 1). Among the 1D nanostructures, ZnO nanorods and nanowires have been widely studied because of their easy nanomaterials formation and device applications. This paper reviews recent research activities that have focused on ZnO nanorods (or nanowires). The main text of this paper is organized into four sections. The next section (section 2) will introduce several concepts related to the growth of ZnO nanorods (or nanowires). The following section highlights a wide range of unique properties associated with ZnO nanorods (nanowires), as well as their potential applications in various areas. The final section concludes with personal remarks on the outlook for research on ZnO nanorods. 0268-1242/05/040022+13$30.00 © 2005 IOP Publishing Ltd Printed in the UK S22 ZnO nanorods: synthesis, characterization and applications Figure 1. A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders [31]. 2. Growth of ZnO nanorods (or nanowires) 2.1. Growth of ZnO nanorods (nanowires) from the vapour phase Vapour phase synthesis is probably the most extensively explored approach to the formation of 1D nanostructures such as whiskers, nanorods and nanowires [34]. Although the exact mechanism for 1D growth in the vapour phase is still not clear, this route has been used by many research groups to fabricate ZnO nanorods (or nanowires). In a typical process, vapour species are first generated by evaporation, chemical reduction and gaseous reaction. These species are subsequently transported and condensed onto the surface of a solid substrate placed in a zone with a temperature lower than that of the source material. With proper control over the supersaturation factor, one can obtain 1D nanostructures in large quantities. For example, Zhang et al [35], Yao et al [36] and Kong et al [37] have synthesized ZnO nanorods (or nanowires) by evaporating ZnO commercial powder. Nanowire-nanoribbon junction arrays of ZnO and hierarchical ZnO nanostructures have also been fabricated by Gao and Wang [38] and Lao et al [39] by simply heating commercial ZnO and SnO 2 (or In 2 O 3 ) mixed powder at elevated temperature, respectively. Although the thermal evaporation method is simple experimentally, its detailed mechanisms might involve the formation of intermediates or precursors due to the use of relatively high temperature. In many cases, decomposition and other types of side reactions also need to be taken into consideration. Gundiah et al reported the synthesis of ZnO nanorods via a carbothermal reduction process, in which Zn vapour might first be generated in situ through the reduction of ZnO by carbon, transported in a flow reactor to the growth zone, and finally oxidized to form ZnO again [40]. In related studies, ZnO nanorods and nanowires were synthesized by oxidizing Zn powder at 500–550 ◦ C [41, 42]. The VLS process was originally developed by Wagner and Ellis to produce micrometre-sized whiskers during the 1960s [11]. A typical VLS process starts with the dissolution of gaseous reactants into nanosized liquid droplets of a catalyst metal, followed by nucleation and growth of single crystalline rods and then wires (figure 2(a)). The VLS process has been widely used for preparation of ZnO nanorods or nanowires. In this process, Au, Cu, Ni and Sn are used as typical metal catalysts [43–47]. 2.2. Growth of ZnO nanorods or nanowires via MOCVD Metal-organic chemical vapour deposition (MOCVD), a widely used semiconductor thin film process, has also been used for ZnO nanorod growth. Although ZnO thin film and quantum dots have been prepared easily using MOCVD [48–52], MOCVD of ZnO nanorods was very recently developed. Yi and coworkers used catalyst-free MOCVD for growing ZnO nanorod and nanoneedle arrays (figure 2(b)) [53, 54]. In this method, no catalyst is employed for ZnO nanorod formation, which leads to preparation of high purity ZnO nanorods and easy fabrications of nanorod quantum structures and heterostructures as mentioned in sections 2.5, 2.6 and 3.1. After this research, related research of ZnO nanorod growth via MOCVD has been reported by other groups [55–61]. The catalyst-free growth mechanism of ZnO nanorods has not been thoroughly investigated but the main reason for anisotropic growth is anisotropic surface energy in ZnO, which depends on the crystal faces of wurtzite ZnO. In addition, high speed laminar gas flow in a certain growth condition S23 G-C Yi et al ( c )( d ) ( a ) VLS process Catalyst free MOCVD( b ) ( e )( f ) Metal catalyst Figure 2. Schematic diagrams illustrating the growth of ZnO nanorods (nanowires) from (a) the VLS process and (b) catalyst-free MOCVD. FE-SEM images of (c) and (e) VLS-grown (reprinted with permission from reference 28, Science 292 1897. Copyright [2001] AAAS) and (d) and ( f ) catalyst-free MOCVD grown ZnO nanorods [53]. may induce turbulent flow between the nanostructures, which results in adsorption of fresh reactant gases only on nanorod tips. Since more surface steps exist on nanorod tips, the nanorod growth rate is higher on nanorod tips than on side walls. This catalyst-free method may be expanded for one- dimensional nanostructure growth of cubic crystal structure materials as well as anisotropic crystal structure materials. The catalyst-free method excludes possible incorporation of catalytic impurities [53], which might occur during the condensation–precipitation process for the metal catalyst- assisted VLS method [11]. Furthermore the catalyst-free MOCVD enables us to grow ZnO nanorods at 400−500 ◦ C, much lower than the typical growth temperature of 900 ◦ C required for catalyst-assisted nanowire growth [43]. The ability to grow high purity ZnO nanorods at a low temperature is expected to greatly increase the versatility and power of these building blocks for nanoscale photonic and electronic device applications. 2.3. Synthesis of ZnO nanorods via a chemical route Hydrothermal synthesis provides another commonly used methodology for generating ZnO nanorods or nanowires [62–65]. Other chemical routes such as reverse micelle, sol– gel, aqueous solution and biomineralization methods were used to synthesize ZnO nanorods or nanowires [66–68]. O’Brien and coworkers reported a new synthesis of ZnO nanorods by thermal decomposition of zinc acetate in organic solvent in the presence of oleic acid which produces relatively monodisperse ZnO nanorods with typical diameters of 2 nm and lengths of 40–50 nm [69]. 2.4. Fabrication of ZnO nanorod or nanowire arrays Vertically aligned nanorod arrays with uniform thickness and length distributions have attracted considerable interest because they are highly appropriate for further fabrications of vertical nanodevice arrays. Aligned growth of ZnO nanorods S24 ZnO nanorods: synthesis, characterization and applications (figures 2(c) and (e)) has been successfully achieved on a solid substrate via a VLS process with the use of metal catalysts such as gold [28, 43, 70–76]. Other techniques that do not use any catalyst, such as template-assisted growth [77] and electrical field alignment [78], have also been employed for the growth of aligned ZnO nanorods. Recently, Yi and coworkers [53, 54] have developed a technique for growing vertically aligned ZnO nanorods (figures 2(d) and ( f )) at a low temperature using catalyst-free MOCVD, which leads to fabrication of vertical Schottky nanorod device arrays (see section 3.4). 2.5. Growth of ZnO nanorod heterostructures 1D nanorod heterostructures are potentially ideal functional components for nanoscale electronics and optoelectronics. The ability to fabricate nanoscale heterostructures opens up many new device applications as already proven in micrometre-scale electronics and photonics [79–81]. Recently three groups—Lieber et al [80, 81], Yang et al [82] and Samuelson et al [83–85]—reported the synthesis of nanowire superlattice structures exploiting the general concept of metal- catalysed nanowire growth. In this process, they have grown superlattice nanowires by modulating reactants during VLS growth. The essence of this concept was shown with the growth of a silicon nanowire/carbon nanotube structure [86]. In principle, this approach can be successfully implemented if a metal catalyst is suitable for two different material syntheses. Gudiksen et al [80] prepared the nanowire heterostructures of compositionally modulated superlattices of GaP/GaAs and modulation doped p–n junction nanowires employing laser- assisted catalytic growth or chemical vapour deposition. Wu et al prepared Si/SiGe nanowire superlattices employing a hybrid pulsed laser ablation/chemical vapour deposition (PLA-CVD) process [82]. InAs/InP nanowire superlattices with periods ranging from 100 to just several nanometres were also fabricated by Bj ¨ ork et al using similar metal- catalysed nanowire growth techniques in an ultrahigh vacuum chamber designed for chemical beam epitaxy techniques [83–85]. Moreover, Bj ¨ ork et al also investigated electrical transport properties and observed resonant tunnelling and single electron tunnelling behaviours from InAs/InP nanowire heterostructures where InAs island segments were sandwiched between InP barriers. As an alternative approach, Yi and coworkers have employed a catalyst-free growth technique to minimize the formation of a mixed interfacial layer, i.e., by utilizing direct adsorption of atoms on the top surface of nanorods which demonstrated metal/semiconductor nanorod heterostructures [87, 88]. Metal/semiconductor nanorod heterostructures can be used for metal contacts of many electronic nanorod devices as mentioned in section 3.4.Yiand coworkers also reported that thickness-controlled magnetic metal/semiconductor nanorod heterostructures exhibit the crossover from ferromagnetism to superparamagnetism by decreasing Ni layer thickness [88]. Similarly, numerous nanoscale heterostructures can then be formed by direct epitaxial growth using techniques already developed for growth of thin film heterostructures. As for nanorod heterostructures along the radial direction, termed coaxial nanorod heterostructures, Kim and coworkers reported the growth of amorphous Al 2 O 3 layers on ZnO nanorods by atomic layer deposition [89, 90]. Goldberger et al and An et al also demonstrated fabrications of GaN/ZnO coaxial nanorod heterostructures via an ‘epitaxial casting’ approach [91] and metal-organic vapour phase epitaxy (MOVPE) [92], respectively. These coaxial nanorod heterostructures offer opportunities for fabrication of high performance devices including field-effect transistors and HEMTs [81]. Moreover, amorphous Al 2 O 3 nanotubes and single crystalline GaN nanotubes were fabricated by etching the core ZnO nanorods. These hollow nanotubes may have an important rolein nanocapillary electrophoresis and nanofluidic biochemical sensor applications [93]. 2.6. Growth of ZnO nanorod quantum structures Nanorod quantum structures composed of heterostructures of ultrathin layers in a single nanorod would enable novel physical properties such as quantum confinement to be exploited, such as the continuous tuning of spectralwavelength by varying the well thickness. While there are a number of well-established techniques including molecular beam epitaxy (MBE) and MOVPE for generating quantum well or superlattice thin films, there are only a few papers reporting nanorod heterostructures that exhibit atomically abrupt interfaces [94, 95]. Even though the catalyst- assisted VLS growth process was developed for the synthesis of compositionally modulated nanowire heterostructures, relatively broad hetero-interfaces caused by re-alloying of alternating reactants in the metal catalyst during the condensation–precipitation process lead to difficulties in the fabrication of nanowire quantum structures with an ultrathin quantum well layer [80]. Yi and coworkers fabricated ZnO/Zn 0.8 Mg 0.2 O nanorod quantum structures using catalyst-free MOVPE [94]. Figure 3 shows transmission electron microscopy (TEM) images and photoluminescence (PL) spectra of the nanorod multiple quantum well (MQW) samples with 1.1 and 2.5 nm wells, respectively. Both images exhibit bright and dark layers in the MQW, corresponding to the Zn 0.8 Mg 0.2 OandZnOlayers, respectively. Z-contrast images of the nanorod MQWs are shown in figures 3(b)–(e). Since lighter elements scatter less, ZnO layers are brighter than Zn 0.8 Mg 0.2 O layers in a Z-contrast image. With precise thickness control down to the monolayer level, these heterostructures show the clear signature of quantum confinement, an increasing blue shift with decreasing layer thickness. As shown in figure 3( f ), PL spectra of the series of ZnO/Zn 0.8 Mg 0.2 O MQW nanorod arrays exhibit new peaks with emission energies dependent on well widths, as indicated by arrows. Results from other samples show that the blue shift decreases with increasing well width and is almost negligible at a well width of 110 ˚ A. The systematic increase in PL emission energy by reducing well width is consistent with the quantum confinement effect as expected from theoretical calculation in ten periods of one- dimensional square potential wells. Furthermore, optical properties of individual ZnO/ ZnMgO nanorod single quantum well structures (SQWs) were investigated using scanning near-field optical microscopy (SNOM) [95]. Yatsui et al studied spatially- and spectrally- resolved photoluminescence imaging of individual nanorod S25 G-C Yi et al ( a )( b ) ( f ) ( c )( d )( e ) Figure 3. TEM images and PL spectra of ZnO/Zn 0.8 Mg 0.2 O MQW nanorods. (a) Low magnification TEM image of the sample with 2.5 nm wells. (b)–(e) Z-contrast images of the 2.5 nm well-width sample with increasing magnification. ( f ) 10 K PL spectra of ZnO /Zn 0.8 Mg 0.2 O heterostructure nanorods and ZnO/Zn 0.8 Mg 0.2 O MQW nanorod arrays with band diagrams shown in inset [94]. SQWs [95] for nanophotonic device applications, such as switching devices [96]. From the SNOM PL spectra, they observed band-filling in the ground state and the resultant first excited state of holes in ZnO/ZnMgO nanorod SQW (figure 4). This SNOM study clearly confirms the observation of the quantum confinement effect in nanorod quantum well structures. Even though the axial nanorod quantum structures exhibited a blue shift in the band edge PL peak due to a quantum confinement effect, no quantum confinement effect in the radial direction was observed for nanorod diameters thicker than 20 nm because effective masses of electrons and holes in ZnO are heavy and exciton Bohr radius is as small as 1.25 nm [97]. In order to observe quantum confinement along the radial direction of ZnO nanorods, ultrafine ZnO nanorods with diameters less than 10 nm must be prepared. Very recently, quantum confinement effects along the radial direction have been observed in ultrafine 1D ZnO nanostructures by Wang et al [98] and Park et al [99]. Wang et al have employed a thin Sn film as a catalyst and synthesized the ultrafine ZnO nanobelts showing an average mean diameter of 6 nm with a standard deviation of ±1.5 nm [98]. PL measurement showed a 14 nm blue shift in the emission peak, which presumably results from quantum confinement arising from the reduced width of the nanobelts. Park et al have also fabricated ultrafine ZnO nanorods with very thin diameters below 10 nm employing catalyst-free MOCVD [99]. The high-resolution TEM image shows as-grown ultrafine ZnO nanorods with diameters as small as 8 nm. Ultrafine ZnO nanorods exhibited a blue-shifted PL peak due to the quantum confinement effect along the radial direction in ZnO nanorods (figure 5). While a dominant PL peak for ZnO nanorods with a diameter of 35 nm was observed at 3.285 eV, the same position as that of bulk ZnO, ultrafine ZnO nanorods showed a systematic blue shift in their PL peak position by decreasing their diameter. Moreover, Yi and coworkers have also synthesized the ZnO/Zn 1−x Mg x O coaxial nanorod quantum structures by subsequent depositions of a Zn 1−x Mg x O shell layer on core ZnO nanorods. ZnO/Zn 0.8 Mg 0.2 O coaxial nanorod quantum structures exhibited significantly increased PL intensity and greatly reduced thermal quenching. These quantum building blocks create well-defined potential profiles along the radial direction in the nanorod heterostructures, useful for nanoscale high electron mobility transistors and light-emitting devices. 2.7. Alloying and doping of ZnO nanorods ZnO band gap energy can be tuned via divalent substitution on the cation site to produce heterostructures. The fundamental bandgap energy of ZnO-based alloys can increase from 3.4 to ∼4.0 eV and decrease to ∼3.0 eV by doping with Mg and Cd, respectively [51, 94]. Recently, S-doped ZnO nanowires have been synthesized via a simple physical evaporation approach by Geng et al [100] and via chemical vapour deposition by Bae et al [101]. Wan et al also reported the growth of Cd-doped ZnO nanowires by evaporating metal zinc and cadmium at 900 ◦ C [102]. As for ZnO-based magnetic semiconductors, Chang et al reported the synthesis of diluted magnetic semiconductor Zn 1−x Mn x O nanowires via vapour phase growth [103] and Ip et al demonstrated Mn, Co-doped ZnO nanorods via molecular beam epitaxy [104]. Conductivity of ZnO can be controlled by doping although as-grown ZnO is n-type normally. For higher n-type doping concentration, Ga was used as an n-type dopant, and Ga-doped ZnO nanorods were prepared using pulsed laser deposition [105]. Making p-type ZnO is more difficult, presumably due to high background n-type carrier concentration and self-compensation caused by easily formed donor defects [106, 107]. As far as we know, p-type conduction in ZnO nanorods or nanowires has not been reported yet, although several papers on p-type ZnO epitaxial thin films have been published [108]. S26 ZnO nanorods: synthesis, characterization and applications ( a ) ( b ) ( c )( d ) I QW -1 I QW -2 I QW -1 I QW -2 I ZnMgO Excitation power (W/cm 2 ) Photon energy (eV) 3.4 3.5 3.6 5 meV F 2 F 1 Position (nm) PL intensity (arb. units) PL intensity (arb. units) PL intensity (arb. units) 0.1 0.01 110 3 2 1 0 0 50 100 150 55nm Near field Far field 10.8 12 W/cm 2 9.6 4.8 1.2 Figure 4. (a) Monochromatic PL image of ZnO/ZnMgO nanorod SQWs obtained at a photon energy of 3.483 eV. (b) Cross-sectional PL profile through the spot X.(c) Solid curves show the near-field PL spectra of ZnO /ZnMgO nanorod SQWs at various excitation densities ranging from 1.2 to 12 W cm −2 . Dashed curves (F 1 and F 2 ) show the far-field PL spectra. All spectra were obtained at 15 K. (d) Excitation power dependence of PL intensity at 3.483 eV (open circles) and at 3.508 eV (closed circles) [95]. ( b )( a ) Figure 5. (a) High resolution TEM images of ultrafine ZnO nanorods with a mean diameter of 8 nm and (b) room temperature PL spectra of ZnO nanorods with a different mean diameter (D) of 8, 9, 12 and 35 nm [99]. 3. Properties and applications of ZnO nanorods ZnO semiconductor nanowires and nanorods are attractive components for nanometre scale electronic and photonic device applications because of their unique chemical and physical properties. For example, recently, a wide variety of nanodevices including ultraviolet photodetectors [26, 78, 109–111], sensors [102, 112, 113], field effect transistors [27], intramolecular p–n junction diodes [114], Schottky diodes [115] and light emitting device arrays [116] have been fabricated utilizing ZnO nanorods (nanowires). 3.1. Luminescence PL spectra of ZnO bulk single crystals have been investigated in detail and considerable progress has been achieved in S27 G-C Yi et al Figure 6. PL spectrum of high quality ZnO nanorods measured at 10 K. The dominant near band edge emission consists of four distinct peaks at 3.359, 3.360, 3.364 and 3.376 eV with full width at half maximum (FWHM) values of 1–3 meV [30]. the last few years in explaining the origins of different ZnO luminescence peaks [117–119]. With respect to the investigation of single nanowires, the lateral resolution of the primary laser beam in PL is unusually limited to about 1 µm minimum due to optical limitations. Therefore, most ZnO nanowire PL spectra [36, 45, 120–122] were measured on many randomly oriented or aligned nanowires. Room temperature luminescence of ZnO nanowire arrays ( a )( b ) ( c ) Substrate c axis Figure 7. (a) Emission spectra from nanowire arrays below (line a) and above (line b and inset) the lasing threshold. The pump powers for these spectra are 20, 100 and 150 kW cm −2 , respectively. (b) Integrated emission intensity from nanowires as a function of optical pumping energy intensity. (c) Schematic illustration of a nanowire as a resonance cavity with two naturally faceted hexagonal end faces acting as reflecting mirrors (reprinted with permission from reference 28, Science 292 1897. Copyright [2001] AAAS). in general shows only one very broad peak structure and therefore does not give much insight into detailed impurity- related recombination processes as demonstrated in the low- temperature luminescence of ZnO bulk single crystals [120– 122]. Only a very few low temperature photoluminescence experiments on ZnO nanowires have been reported up to now [30, 123–125]. Yi and coworkers [30] identified the free exciton and three donor-bound exaction PL peaks at 10 K from PL spectra of high purity ZnO nanorod arrays (figure 6). Recently, room temperature UV lasing emission from a directionally grown ZnO nanoarray (figure 7) was demonstrated with a threshold power density below 100 kW cm −2 [28, 126]. Choy et al [127] also reported high UV-lasing efficiency of ZnO nanorod arrays’ (NRA) on Si wafer, similar to that of ZnO NRAs on Al 2 O 3 substrate [28]. Meanwhile, Yu et al [128] have observed random laser action with coherent feedback in ZnO nanorod arrays embedded in ZnO epilayers. Cathodoluminescence (CL), in comparison to PL, is an excellent tool for luminescence mapping and for selective area investigation of nanometre-sized structures including nanowires. Lorenz et al [129] presented CL investigation of selected single ZnO nanowires; the CL spectra corresponded only partially to the PL spectra of ZnO nanowires grown via MOVPE [30]. Although ZnO is a promising material for short wavelength photonic device applications, difficult p-type doping in ZnO has impeded fabrication of ZnO p–n homojunction devices. As an alternative approach to S28 ZnO nanorods: synthesis, characterization and applications Figure 8. Emission current density from ZnO nanowires grown on silicon substrate at 550 ◦ C. Inset reveals that the field emission follows Fowler–Nordheim behaviour [29]. homojunction, Yi and coworkers [116] employed p-GaN rather than p-ZnO since these materials have similar fundamental band gapenergy and crystal structure, and fabricated n-ZnO/p- GaN nanorod electroluminescent (EL) devices. 3.2. Field emission Field-emitting cathodes have attracted much attention because they can be used in flat panel displays and power devices. In particular, much effort is being devoted to the fabrication of arrays of field emitters. Most research in this area focused on the fabrication of Spindt-type metallic cones before the discovery of carbon nanotubes (CNTs) [130]. CNTs have a high aspect ratio a few micrometres in length and a diameter of several nanometres. This large aspect ratio makes it possible to achieve a high electric field at the tips of CNTs for electron emission at moderate applied voltages. Several experiments have shown that the CNTs have the potential to be excellent field emitters [130–135]. Nevertheless, the achievement of vertically well-aligned CNTs arrays for applicable field emission devices has not been facile and degradation of CNT field emitters by residual gases including oxygen is one of the problems to be overcome for high performance field emission displays (FEDs). After the discoveries of 1D ZnO nanostructures, many researchers suggested that these nanostructures have the potential for use as electron emitting sources because they also have large aspect ratios such as CNTs and the degradation of field emission characteristics by residual gas may be reduced for an oxide surface of ZnO. Some of the first experiments on field emission from ZnO nanowires were demonstrated by Lee and coworkers [29]. These experiments used an array of ZnO nanowires. Although the array of ZnO nanowires was not vertically well aligned, the turn-on and the threshold field were 6.0 V µm −1 at current density of 0.1 µAcm −2 and 11.0 V µm −1 at 0.1 mA cm −2 , respectively (figure 8). Although these field emission characteristics were lower than those of CNTs (turn-on field of 1 V µm −1 and field emission current of 1.5 mA at 3 V µm −1 ; current density, J = 90 µAcm −2 [135]) they were good enough to be used as an electron emitter. Soon after the report of Lee et al, several groups reported on electron emission of an array of ZnO nanowires, and nanorods [136–138]. The reported results demonstrated that the field emission characteristics of 1D ZnO nanostructures are comparable to those of CNTs. To realize an electron emission source with 1D ZnO nanostructures, researchers have carried out in viewpoint on vertical alignment of ZnO nanoneedle arrays. Highly oriented vertical alignment has been considered as a factor in enhancing field emission properties since electrostatic models in metal cones were investigated by theoretical calculation [139]. Vertically well-aligned ZnO nanowires were fabricated successfully by diverse methods and used to conduct field emission experiments. Li et al showed that field emission characteristics of ZnO nanowires depend on vertical alignment [140]. Nanorod tip morphology is also an important factor because sharper tips increase the effective electric field at the tips [141]. Generally, ZnO 1D nanostructures have shown better field emission characteristics for needle-like structures. Li et al examined the tip surface perturbations of ZnO nanoneedles as an important factor in enhancing field emission characteristics [142]. Xu et al reported that ZnO nanopins also have good field emission characteristics with a turn-on field of 1.9 V µm −1 at a current density of 0.1 µAcm −2 [143]. In particular, ZnO nanoneedle arrays [54] were regarded as structures with great potential for electron emitters due to their extremely sharp tips. Later, ZnO nanoneedle arrays were demonstrated to be one of the most promising candidates for field emitters [144, 145]. A series of experiments on field emission of 1D ZnO nanostructures, such as nanowires, nanorods, nanopins and nanoneedles has attracted considerable interest. Other groups have carried out several experiments on the effect of doping in ZnO 1D nanostructures since several researchers suggested that increases in electrical conductivity with doping enhance field emission characteristics of ZnO 1D nanostructures. Xu et al reported that the enhancement of field emission characteristics of ZnO nanofibre arrays can be achieved by Ga doping, and Jo et al also demonstrated that hydrogen annealing can affect the enhancement of ZnO nanowire field emission properties [137, 138, 146]. Many researchers consider ZnO 1D nanostructures as good field emitters. Research on ZnO 1D nanostructures as field emitters has only recently begun, so their field emission characteristics have not been optimized sufficiently. However, additional experiments on various aspects including electrical conductivity, density control and device structures may yield excellent field emitters based on ZnO 1D nanostructures. 3.3. Gas sensing ZnO, a multifunctional semiconductor metal oxide, is one of the most promising materials for gas sensor applications [147– 149]. ZnO nanostructures have also attracted considerable attention for solid-state gas sensors with great potential for overcoming fundamental limitations due to their ultrahigh surface-to-volume ratio. Wang et al have carefully studied the gas sensing characteristics of ZnO nanowires [102, 112, 113]. They found that the current increased rapidly over three orders of magnitude when Cd-doped ZnO nanowire was exposed to the moist air of 95% relative humidity. In addition, gas sensors based on ZnO nanowires fabricated with a microelectromechanical system exhibited a very high sensitivity to ethanol gas and a fast response time (within 10 s) S29 G-C Yi et al at 300 ◦ C, showing a promising application for ZnO nanowire humidity sensors. Yi and coworkers demonstrated ZnO nanorod sensors for detection of biological molecules [150]. They functionalized ZnO nanorod surfaces with biotin and developed nanosensors for real time detection of biological molecules using surface- modified ZnO nanorods as a conducting channel. For the fabrication of bimolecule nanosensors, single crystalline ZnO nanorods were prepared using catalyst-free metal-organic vapour phase epitaxy. Using an e-beam lithography technique, metal micropatterns were fabricated on a single ZnO nanorod. Conductance of the biotin-modified ZnO nanorod electronic biosensors was drastically increased upon exposure to streptavidin, indicating that ZnO nanorod biosensors are a promising candidate for electrical detection of biological species with high sensitivity. 3.4. Electron transport properties As the critical dimension of an individual device becomes smaller and smaller, the electron transport properties of their components become an important issue for study. Results from Harnack et al indicate that the current–voltage (I–V) characteristics of ZnO nanorods are strongly nonlinear and asymmetrical, showing rectifying, diode-like and an asymmetry factor up to 25 at a bias voltage of 3 V [78]. Lee and coworkers reported that the average resistivity of ZnO nanowires in anodic aluminium oxide (AAO) templates was about one order of magnitude higher than that of naked single ZnO nanowires [114]. Prototype devices that have been demonstrated include field effect transistors (FETs), p−n junctions, Schottky diodes and electroluminescent nanodevices based on ZnO nanostructures [27, 115, 116]. FETs, one of the most fundamental and important electronic components, were fabricated by Arnold et al using a ZnO nanobelt [27]. For field-effect transistor fabrications, they deposited the ZnO nanobelts on predefined gold electrode arrays on a 120-nm- thick-SiO 2 gate dielectric/Si (p + ) substrate. The ZnO nanobelt field effect transistor showed a threshold voltage of −15 V, a switching ratio of nearly 100, and apeak conductivity of 1.25× 10 −3 ( cm) −1 (figure 9). The ZnO nanobelt transistor performance is analogous to those of carbon nanotubes deposited on top of Au electrodes or covered by Ti electrodes [151]. Recently, Park et al have fabricated high performance n-channel ZnO nanorod FETs which significantly improved FET characteristics with a high current on/off ratio of 10 5 and a transconductance of 1.8 µS [152]. Furthermore, the electron mobility estimated from transconductance exhibited a maximum value of 1000–1200 cm 2 V −1 s −1 .High performance nanoscale FETs obtained show the feasibility of ZnO nanorods for electronic nanodevice applications. In addition, Yi and coworkers have taken a different approach for ZnO based device fabrications. They made vertically aligned metal/semiconductor (M/SC) nanorod heterostructures simply by evaporating metal onto ZnO nanorod tips [87]. Since metal is selectively deposited on ZnO nanorod top surfaces, the interface between the metal layer and ZnO nanorod was atomically abrupt as determined by TEM. The I–V characteristics of metal/ZnO Figure 9. Source-drain current versus gate bias for a ZnO nanobelt FET in ambient. (Inset) AFM image of ZnO FET across gold electrodes [27]. ( a ) ( b ) Figure 10. Typical I–V characteristic curves of (a)Au/ZnO and (b)Au /Ti/ZnO nanorod heterostructures, indicating Schottky and ohmic behaviour, respectively. Inset shows a TEM image of a Au /ZnO nanorod heterostructure [115]. nanorod heterostructures were measured by placing a Au-coated conducting tip on individual nanorod top surfaces S30 [...].. .ZnO nanorods: synthesis, characterization and applications using current sensing atomic force microscopy (CSAFM) Au /ZnO heterostructure nanorods exhibited rectifying I–V characteristic curves without significant reverse-bias leakage current up to −8 V (figure 10(a)), resulting presumably from the Schottky contact formation due to a well-defined interface between Au and ZnO layers In addition, Au/Ti /ZnO. .. X C, Zhang H Z, Xu J, Zhao Q, Wang R M and Yu D P 2004 Shape controllable synthesis of ZnO nanorod arrays via vapor phase growth Solid State Commun 129 803 ZnO nanorods: synthesis, characterization and applications [76] Zhang Y, Jia H B, Wang R M, Chen C P, Luo X H, Yu D P and Lee C J 2003 Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrate Appl Phys... Y, Xiong Y J, Zhang R and He W 2003 Reverse micelle-assisted route to control diameters of ZnO nanorods by selecting different precursors Chem Lett 32 760 [67] Tian Z R R, Voigt J A, Liu J, Mckenzie B and Mcdermott M J 2003 Biomimetic arrays of oriented helical ZnO nanorods and columns J Am Chem Soc 124 12954 [68] Vayssieres L 2003 Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions... 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Ding Y, Summers C J and Wang Z L 2004 Large-scale synthesis of six-nanometer-wide ZnO nanobelts J Phys Chem B 108 8773 [99] Park W I, An S J, Yi G C and Kim M 2004 Quantum confinement observed in ultrafine ZnO and ZnO/ Zn0.8Mg0.2O coaxial nanorod heterostructures Int Symp Proc Nanomanufacturing 2 668 [100] Geng B Y, Wang G Z, Jiang Z, Xie T, Sun S H, Meng G W and Zhang L D 2003 Synthesis and optical properties... Wang R M, Xu J and Yu D P 2003 Synthesis, optical, and magnetic properties of diluted magnetic semiconductor Zn1−xMnxO nanowires via vapor phase growth Appl Phys Lett 83 4020 [104] Ip K et al 2003 Ferromagnetism in Mn- and Co-implanted ZnO nanorods J Vac Sci Technol B 21 1476 [105] Yan M, Zhang H T, Widjaja E J and Chang R P H 2003 Self-assembly of well-aligned gallium-doped zinc oxide nanorods J Appl . fabrications of vertical nanodevice arrays. Aligned growth of ZnO nanorods S24 ZnO nanorods: synthesis, characterization and applications (figures 2(c) and (e)) has been successfully achieved on a solid substrate. in ZnO nanorods or nanowires has not been reported yet, although several papers on p-type ZnO epitaxial thin films have been published [108]. S26 ZnO nanorods: synthesis, characterization and applications ( a ). SEMICONDUCTOR SCIENCE AND TECHNOLOGY Semicond. Sci. Technol. 20 (2005) S22–S34 doi:10.1088/0268-1242/20/4/003 ZnO nanorods: synthesis, characterization and applications Gyu-Chul Yi, Chunrui Wang and Won Il