Optical and electrical properties of ZnO nanowires grown on aluminium foil by non-catalytic thermal evaporation

Well-crystallized ZnO nanowires were grown in large quantity on aluminium foil, by a non-catalytic thermal evaporation method using metallic zinc powder in the presence of oxygen at low temperature. Detailed structural and optical characterizations confirmed that the as-grown nanowires were highly crystalline, possessed a wurtzite hexagonal phase, had grown along the c-axis direction and exhibited excellent optical properties. The electrical characteristics of an individual nanowire were observed in air and vacuum by fabricating field-effect transistor (FET) devices. The transistors turned on typically between -5 and 0 V in ambient air. However, a large threshold voltage (V_th) shift, ∼5 V, towards negative gate bias was observed in high vacuum. The shift of V_th is believed to be related to the charge transfer from the ZnO nanowire surface to the physically adsorbed OH or oxygen. Moreover, the fabricated FETs show a high conductivity ON/OFF ratio of about ∼10 ² with ultraviolet (UV) light and hence provide an effective way to use these devices in nanoscale UV detectors and optoelectronic switches.