Impact of Parylene-A Encapsulation on ZnO Nanobridge Sensors and Sensitivity Enhancement via Continuous Ultraviolet Illumination

Abstract

The impact of parylene-A encapsulation and the effect of continuous ultraviolet (UV) exposure on ZnO nanobridge sensor response are investigated. ZnO nanowire (NW) devices are fabricated using a novel method that involves selective growth of ZnO nanobridges between lithographically defined pads of carbonized photoresist (C-PR). We find that a thin coating of parylene-A effectively attenuates the response of NW devices to O2, H2O vapor, and UV illumination. The accessibility of the amine group on parylene-A for chemical functionalization is verified by transforming the amine groups on the surface of the parylene-A coating into aromatic imine groups, followed by UV–Vis absorption. Our results suggest that, in addition to modulating environmental sensitivity and providing protection of the ZnO NWs for liquid- and vapor-phase sensing, the parylene-A encapsulation may also serve as an activation layer for further specific functionalization targeting selective sensing. We also found that the sensitivity and response time of ZnO nanobridge devices to O2 are dramatically improved by continuously exposing the nanobridge devices to UV illumination. Finally, we show that the C-PR directed growth method can also be used to isolate free-standing NW carpet.