Indium and zinc oxide-based nanostructures prepared by thermal evaporation

Abstract

Metal oxides semiconductor nanostructures, such as ZnO and In2O3 nanowires exhibit novel electronic and optical properties owing to their unique one-dimensional nanostructure and possible quantum confinement effects. ZnO and In2O3 are versatile semiconductor oxides with numerous applications ranging from optoelectronics to chemical sensors due to their distinctive optical, electronic and chemical properties. This thesis aims at studying the fabrication technique, growth mechanisms, structural characteristics and physical properties of nanostructured ZnO and In2O3. The fabrication technique is based on a simple thermal evaporation method. Under different conditions, different ZnO nanostructures, such as nanowires, nanobelts, and nanotetrapods, have been synthesized. In particular, the nanotetrapods of ZnO were synthesized by a carbothermal reduction process using a mixture of ZnO powder and graphite at an evaporation temperature of 1100C. Uniform tetrapod-shaped ZnO nanocrystals were formed on the inner wall of the alumina tube located downstream of the carrier gas. Since there is no catalyst used, the growth mechanism mainly follows the vapour-solid (VS) growth process. Cathode luminescence study showed a band-gap emission at 370 nm and a very strong and sharp deep level emission at 580 nm. Field emission device using the ZnO nanotetrapods have been fabricated and characterized. The result showed emission current density as high as 0.9 mA/cm2 under an electric field of 7.5 V/ um. Large amount of In2O3 nanowires were also successfully fabricated from indium particles at 1000C by a thermal evaporation method. Relatively higher quality In2O3 nanowires with average diameters ranging from 100 to 200 nm have been grown at the anodic alumina template compared to the silicon wafer, suggesting that the anodic alumina template can enhance the uniform growth of In2O3 nanowires and confine the growth diameter. The growth of In2O3 is through a vapour-liquid-solid (VLS) mechanism since Au nanoparticles are used as catalyst. A low-temperature thermal evaporation method at 540C was also proposed for the synthesis of In2O3/Sb core-shell cable-like nanostructures by co-evaporating In and Sb, and a unique structure of In2O3 nanowires with rectangular turning and co-axial amorphous Sb shell layer was formed. The amorphous Sb layer was found to undergo a crystallization phase transition under a 200 keV focussed electron beam irradiation. An epitaxial relationship was found to exist between the crystallized Sb and In2O3.