Mechanism of Sensitivity Enhancement of a ZnO Nanofilm Gas Sensor by UV Light Illumination.

Although ultraviolet (UV) light illumination has been widely used to increase the sensitivity of semiconductor gas sensors, its underlying mechanism is still blurred and controversial. In this work, the influence of UV light illumination on the sensitivity of ZnO nanofilm gas sensors is explored experimentally and simulated based on a modified Wolkenstein's model. The influential factors on sensitivity are determined respectively: the surface band bending and Fermi level are measured by Kelvin probe force microscopy, the binding energy and extrinsic surface state are calculated by density functional theory, and the depletion of the whole semiconductor caused by the finite size is illustrated by the transfer characteristics of a field effect transistor. With all these factors taken into consideration, the surface state densities of adsorbed O2 and NO2 molecules in the dark and under UV light illumination are calculated which determine the sensitivity. Good agreement has been obtained between the experiment and simulation results. Accordingly, when NO2 is introduced into the atmosphere, the enhancement of sensitivity is ascribed to the more dramatic increase of surface state density and surface band bending activated by the UV light illumination compared with that in the dark. This finding is critical and would contribute greatly to the development of gas sensors with high sensitivity.