We report metalorganic vapor-phase epitaxial growth and structural and photoluminescent characteristics of ZnO nanorods. The nanorods were grown on Al 2 O 3 (00•1) substrates at 400°C without employing any metal catalysts usually needed in other methods. Electron microscopy revealed that nanorods with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates. The mean diameter of the nanorods is as narrow as 25 nm. In addition, x-ray diffraction measurements clearly show that ZnO nanorods were grown epitaxially with homogeneous in-plane alignment as well as a c-axis orientation. More importantly, from photoluminescence spectra of the nanorods strong and narrow excitonic emission and extremely weak deep level emission were observed, indicating that the nanorods are of high optical quality.

Silicon is a promising candidate for electrodes in lithium ion batteries due to its large theoretical energy density. Poor capacity retention, caused by pulverization of Si during cycling, frustrates its practical application. We have developed a nanostructured form of silicon, consisting of arrays of sealed, tubular geometries that is capable of accommodating large volume changes associated with lithiation in battery applications. Such electrodes exhibit high initial Coulombic efficiencies (i.e., >85%) and stable capacity-retention (>80% after 50 cycles), due to an unusual, underlying mechanics that is dominated by free surfaces. This physics is manifested by a strongly anisotropic expansion in which 400% volumetric increases are accomplished with only relatively small (<35%) changes in the axial dimension. These experimental results and associated theoretical mechanics models demonstrate the extent to which nanoscale engineering of electrode geometry can be used to advantage in the design of rechargeable batteries with highly reversible capacity and long-term cycle stability.

method reported previously [20]. In a typical procedure to synthesize the MWCNT/CdS core±shell nanowires, 14 mg purified MWCNTs were added into dry tetrahydrofuran (THF), which contained 0.1± 0.4 mmol S (99.999 %) powder and an identical stoichiometric amount of anhydrous CdCl 2 . After ultrasonical dispersion for 30 min, excess KBH 4 was slowly added to the flask under vigorous stirring at room temperature. The S was reduced to S 2± and the suspension gradually turned light yellow. After the mixture was stirred for 12 h, a dark yellow precipitate was produced. The dark yellow powder was filtered and washed thoroughly with dry THF, deionized water, and absolute ethanol several times to remove the impurities.The Electroluminescence in n-ZnO Nanorod Arrays Vertically Grown on p-GaN** By Won Il Park and Gyu-Chul Yi* ZnO is a promising material for short-wavelength photonic device applications due to its characteristic direct and wide bandgap with a large exciton binding energy of 60 meV.[1] Recently, both ZnO epitaxial films and single crystalline nanorods have shown excellent optical characteristics.[2±5] Further research on ZnMgO alloys has enabled the control of the bandgap energy in ZnO-based materials and the fabrication of ZnO/ZnMgO nanorod quantum structures.[6] Despite significant progress on ZnO films and nanostructures, the difficulty of p-type doping in ZnO has impeded the fabrication of ZnO p±n homojunction devices. As an alternative approach to homojunction, an n-ZnO/p-GaN heterojunction has been suggested as a strong candidate for device applications, [7,8] since these materials have a similar fundamental bandgap energy (~3.4 eV), the same wurtzite crystal structure, and a low lattice constant misfit of 1.9 %. In general, however, p±n heterojunction devices show a lower efficiency than homojunction devices, [9] because an energy barrier formed at the junction interface decreases carrier injection efficiency for heterojunction devices with a large band offset. [10] This problem may be solved by increasing the carrier injection efficiency by making nanosized junctions, since, as previously reported, the carrier injection rate significantly increases for nanocontacts in Schottky diodes. [11,12] Here, we report on the fabrication of n-ZnO/p-GaN nanorod electroluminescent (EL) devices and their EL characteristics. A schematic of n-ZnO/p-GaN nanorod heterojunction arrays is shown in Figure 1a. For EL device fabrication, vertically well-aligned n-type ZnO nanorod arrays were epitaxially grown on p-GaN(0001) substrates employing catalyst-free metal-organic vapor phase epitaxy (MOVPE). The growth parameters of ZnO nanorods on GaN(0001) substrates were similar to those on Al 2 O 3 (0001) substrates.[13] The synthesis of ZnO nanorods resulted in a preferential growth direction along the c-axis of ZnO normal to the substrate surface. As shown in Figure 1b, field-emission scanning electron micros-COMMUNICATIONS

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.

There has been considerable interest in the growth of one-dimensional (1D) semiconductor nanostructures including nanowires. [1±3] In addition to nanowires, semiconductor nanoneedles are of particular interest because their tips show a sharp curvature, offering potential applications as probing tips with high spatial resolution in both vertical and horizontal dimensions or field-emission tips due to the increased field-enhancement factor. [4,5] Nevertheless, semiconductor nanoneedles have rarely been studied, while numerous semiconductor nanowires including Si, InP, GaN, GaAs, GaP, and ZnO have been synthesized. [1±3,6] To prepare the nanowires, metal-catalysis-assisted vapor±liquid±solid (VLS) deposition has been widely employed because this method was used for Si microwhisker growth in the 1960s. [7] Since as-grown nanowires prepared by the VLS method yield a metal nanoparticle on their tips, nanoneedles with sharp tips are difficult to prepare using the VLS method. However, we recently developed a metal-catalyst-free growth method for preparing ID nanostructures, [8] which enables the growth of sharp-tipped ZnO nanoneedles.One-dimensional ZnO nanowires and nanorods have been studied recently for optoelectronic nanodevice applications as the material has a wide and direct fundamental bandgap energy of 3.4 eV, a large excitonic binding energy of~60 meV, and high mechanical and thermal stabilities. [9] Both ZnO films and nanostructures have been prepared, typically on sapphire substrates as the substrate shows a good lattice match with ZnO. [10±12] Nevertheless, the use of Si substrates enables the deposition of nanomaterials on large and cheap substrates, offering possible mass production of the nanomaterials. More importantly, the preparation of nanomaterials on Si represents a breakthrough for nanomaterial integration in Si-based electronic devices. Despite the importance of the nanomaterials growth on Si substrates, vertically well-aligned one-dimensional ZnO nanostructure growth on Si has rarely been reported. [6,13] Here, we report metal±organic chemical vapor deposition (MOCVD) of vertically well-aligned ZnO nanoneedles on Si substrates and their structural and optical characteristics.The surface morphology of as-grown ZnO nanoneedles on Si substrates was investigated using scanning electron microscopy (SEM). As shown in Figures 1a±1c, a high density of ZnO nanoneedles is vertically aligned over the entire substrate and they exhibit sharp tips. The diameter and aspect ratio of ZnO nanoneedles depend on growth conditions including growth time. Typically, nanoneedles grown for 1 h exhibit mean lengths of 740 ± 50 nm and mean diameters of 40 ± 5 nm (Figs. 1d and 1e). Normalized standard deviation values (a standard deviation divided by a mean) in nanoneedle diameter and length distributions are as small as 0.16 and 0.06, respectively. These values are comparable to those of ZnO nanorods grown on Al 2 O 3 (0001) substrates and are one or two orders of magnitude smaller than those prepared by the catalyst-as...

We report on the photoluminescent characteristics of ZnO single crystal nanorods grown by catalyst-free metalorganic vapor phase epitaxy. From photoluminescence ͑PL͒ spectra of the nanorods at 10 K, several PL peaks were observed at 3.376, 3.364, 3.360, and 3.359 eV. The PL peak at 3.376 eV is attributed to a free exciton peak while the other peaks are ascribed to neutral donor bound exciton peaks. The observation of the free exciton peak at 10 K indicates that ZnO nanorods prepared by the catalyst-free method are of high optical quality.

Recently, there has been considerable interest in oxide semiconductors for photocatalyst applications. This is due to their high photocatalytic activity and excellent chemical and mechanical stability. [1,2] Many oxide materials including TiO 2and ZnO have been prepared as thin films as well as fine powders. [2] In particular, tremendous attention has been paid to fine powders since high photocatalytic efficiency can be achieved by increasing the effective surface area of the materials. However, these powders have generally been used in a suspended state in water, which limits their practical use, due to problems in their separation and recovery. Such difficulty is more serious with nanometer-scale ultrafine powders. Supporting photocatalytic materials on a rigid substrate can solve this problem. Many thin film deposition techniques have been widely used for the immobilization of photocatalytic materials. [3,4] Among the numerous thin film deposition techniques, metal-organic chemical vapor deposition (MOCVD) has many advantages: excellent crystallinity, mechanical stability, good adhesion of films, and easy mass production. Meanwhile, since photocatalytic activity generally increases with effective surface area, a rough film surface with a high surface-tovolume (S/V) ratio is beneficial. From this point of view, onedimensional (1D) semiconductor nanomaterials with extremely high S/V ratios are good candidates for photocatalytic and photovoltaic applications. [5,6] Nevertheless, photocatalytic applications of oxide nanostructures have rarely been reported. In this communication, we report on the photocatalytic activity of ZnO thin films and nanoneedle arrays grown using MOCVD. We prepared both ZnO thin films and nanoneedle arrays using the same MOCVD growth system, in order to compare photocatalytic characteristics of these materials. The substrates used in this study were Si wafers and glass plates. These materials are suitable for mass production because of their low cost and availability in large sizes. Additionally, MOCVD enables us to deposit the materials on cheap and large substrates. For MOCVD of ZnO thin films and nanoneedle arrays, deposition temperatures were as low as 400 C. [7,8] Low growth temperature is essential for the use of cheap glass substrates. Meanwhile, ZnO nanoneedle arrays were prepared without using any metal catalysts that are generally employed for the one-dimensional growth of semiconductor nanowires. This catalyst-free method excludes possible incorporation of metal impurities into the nanomaterials and the formation of metal nanoparticles on the nanoneedle tips. [7,9] Hence, sharp or flat tips of nanomaterials can be easily made by changing only growth parameters such as deposition pressure and temperature, without any post-treatment. [7,10] As previously reported, both ZnO thin films and nanoneedle arrays exhibited excellent crystallinity and optical properties. [7,9] Photocatalytic activity of ZnO thin films grown by MOCVD was determined by measuring the photoinduced decoloration...

We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ϳ140 nS and a mobility of 75 cm 2 / V s, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 10 4-10 5 , a high transconductance of 1.9 S, and high electron mobility above 1000 cm 2 / V s. The role of the polymer coating in the enhancement of the devices is also discussed.