ZnO Nanorods
Zinc oxide (ZnO) nanorod, also known as nanowire, has a direct bandgap energy of 3.37 eV, which is similar to that of GaN, and it has an excitation binding energy of 60 meV. More interestingly, the optical bandgap of ZnO nanorod can be tuned by changing the morphology, composition, size etc. Recent years, ZnO nanorods have been intensely used to fabricate nano-scale electronic devices, including field effect transistor, ultraviolet photodetector, Schottky diode, and ultra-bright light-emitting diode (LED). Various methods have been developed to fabricate the single crystalline, wurtzite ZnO nanorods. Among those methods, growing from vapor phase is the most developed approach. In a typical growth process, ZnO vapor is condensed onto a solid substrate. ZnO vapor can be generated by three methods: thermal evaporation, chemical reduction, and Vapor-Liquid-Solid (VLS) method. In the thermal evaporation method, commercial ZnO powder is mixed with SnO2 and evaporated by heating the mixture at elevated temperature. In the chemical reduction method, zinc vapor, generated by the reduction of ZnO, is transferred to the growth zone, followed by reoxidation to ZnO. The VLS process, originally proposed in 1964, is the most commonly used process to synthesize single crystalline ZnO nanorods. In a typical process, catalytic droplets are deposited on the substrate and the gas mixtures, including Zn vapor and a mixture of CO/CO2, react at the catalyst-substrate interface, followed by nucleation and growth. Typical metal catalysts involve gold, copper, nickel, and tin. ZnO nanowires are grown epitaxially on the substrate and assemble into monolayer arrays. Metal-organic chemical vapor deposition (MOCVD) has also been recently developed. No catalyst is involved in this process and the growth temperature is at 400 ~500 °C, i.e. considerably milder conditions compared to the traditional vapor growth method.