Controllable Growth And Properties Of ZnO Nanorod Arrays

Zinc oxide (ZnO) is an attractive semiconducting oxide with excellent chemical and physical properties, such as wide direct band gap (3.37eV), large excitation binding energy (～60meV), tunable band gap formed by alloying with CdO or MgO and biocompatibility. Its resistivity can be tuned in a wide range from 10-4Ω.cm to 1012Ω.cm by doping. These properties make ZnO a promising material for fabricating short-wavelength optoelectronic devices, piezoelectric devices, transparent thin film transistor, etc.In recent years, low-dimensional ZnO nanomaterials, such as nanobelts, nanorods, nanowires, nanotubes, nanotetrapods and and nanocombs etc, have attracted much attention in the development of novel optical, electronic, magnetic, and catalytic materials. Vertically well aligned one dimensional ZnO nanorods has been considered for applications to nanoscale light-emitting diodes, field emitters and solar cells. Several methods have been developed to prepare one-dimensional ZnO nanostructures, such as chemical vapor deposition (CVD), hydrothermal route, sputtering, and thermal evaporation. Among these deposition techniques, hydrothermal method may be the most attractive one owing to its simple equipment, low temperature deposition, less hazardous, perfect control of morphology and low cost for large-scale production. However, as reported by many researchers, a large amount of ZnO precipitates nucleated homogeneously from solution during the growth of ZnO nanorods by hydrothermal route, which will consume ZnO precursors rapidly and causes early termination of the heterogeneous nanorod growth on the substrate. The development of a simple and controllable solution method to deposit long ZnO nanorod arrays is still required for many applications.In this thesis, well aligned ZnO nanorod arrays were synthesized by a simple hydrothermal method on fluorine-doped SnO2 conductive glass (FTO) at relatively low temperature. The morphology, crystal structure and optical property of the ZnO nanorod arrays were characterized. The main research content and results were as follows:1. The growth of ZnO nanorod arrays by hydrothermal method was systematically investigated under different 1,3-diaminopropane (DAP) concentrations, growth temperatures, and growth times. DAP was added to control growth of ZnO nanorods. The morphology and length of the resulted nanorods were strongly affected by DAP concentration. A morphological change from well-defined hexagonally tipped nanorods towards tapered nanoneedles is observed as the DAP concentration is increased. This is accompanied by an overall increase in length of the nanorods.2. Field emission property was measured for ZnO nanorod arrays deposited under different DAP concentrations. The results indicate that as the DAP concentration is increased, ZnO nanorods with sharper tip will result, which show a lower turn-on field. The field enhancement factor y was estimated from the I-V characteristics and the corresponding Fowler-Nordheim (F-N) plots.3. The crystalline structure of the needle-like ZnO nanorods was characterized by a transmission electron microscope (TEM). Selective area electron diffraction (SAED) and high resolution (HRTEM) image indicate that the nanorods are single crystalline and [0001] is the preferred growth direction for ZnO nanorods. A sharp tip with a diameter about 3nm was also observed by the HRTEM image.4. The optical properties of ZnO nanorod arrays were characterized by transmittance spectra, Raman and photoluminescence measurement. The strong UV emission at 382 run (3.25 eV) with a full width at half-maximum (FWHM) of only 12 nm and very weak defect-related emission in the PL spectra indicates that the ZnO nanorods have good crystalline structure with excellent optical properties. The bandgap estimated from the transmittance spectrum is 3.24eV.