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By integrating bottom-up synthesis with top-down lithographic techniques, various single-crystalline nanowires have been synthesized. They are characterized by different methods such as TEM, SEM, XRD, photoluminescence, photoconductance, scanning surface potential microscopy, and electrical transport measurement. Using ZnO nanowires as an example, they are configured as field effect transistors (FET) and have demonstrated a range of potential applications as UV emitters, photodetectors, as well as chemical sensors. In order to optimize the device performance, surface passivation procedures are conducted and the nanowire FETs exhibit orders of magnitude improvement in the on/off ratio, field effect mobility and sub-threshold swing. To clarify the size effect in nanowires with decreasing diameters but not yet reaching quantum confinement region, ZnO nanowires with diameters around 10 nanometers are characterized and show significant increase in conductivity with diminished gate modulation and reduced mobility. This phenomenon is attributed to the enrichment of surface states owing to the increased surface-to-volume ratio. This enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and explains the reported blue shift. This study shows that surface state play a dominant role in the electrical and optical properties of low dimensional materials. Furthermore, high density vertical zinc oxide nanowire arrays were synthesized using highly ordered channels in anodic alumina membranes via chemical vapor deposition assisted by electrochemical deposition methods. A negative photoconductivity was first observed as a result of electron trapping in the alumina template. In contrast, positive photoconductivity was observed after using a thermally-annealed anodic alumina membrane as the nanowire growth template. These studies render a pathway for constructing high density nanoscale electronic and optoelectronic circuits. |