Enhanced vacuum-photoconductivity of chemically synthesized ZnO nanostructures

Abstract

We report on the enhanced ultraviolet (UV) photoconductivity of zinc oxide (ZnO) nanostructures in vacuum. Nanoparticles and nanorods of ZnO were fabricated using a simple cost-effective solid state grinding method. Morphology of the nanostructures was studied using transmission electron microscopy, while the optical properties were investigated using UV–visible absorption and photoluminescence spectroscopy. The emission spectra of the nanostructures revealed the existence of various native defect states of ZnO and also indicated the presence of surface adsorbed water molecules. In the photoconductivity measurements, although the ZnO nanoparticles exhibited lower photoconductivity in comparison to the nanorods, a similar trend of photoresponse was observed for both the cases. An initial decrease in the photoconductivity followed by a large enhancement was observed in vacuum compared to that in ambient condition. Such unusually increased photoconductivity has been correlated to the desorption of physisorbed water molecules from nanostructure surfaces under vacuum. This desorption is responsible for the rise in dark current and an initial decrease in photoconductivity. Continual UV irradiation in vacuum leads to the desorption of chemisorbed water molecules from the defect sites of the nanostructures, resulting in the occurrence of high photoconductivity.