Persistent photoconductivity in the ZnO MW device. (a), (b) Trapping and photoconduction mechanism in the ZnO MW. The top panel in (a) shows the schematic energy band diagram of the ZnO MW in the dark. CB and VB are the conduction and valence bands, respectively. The bottom panel shows oxygen molecules adsorbed on the ZnO MW surface that capture free electrons, resulting in a low-conductivity depletion layer near the surface. The top panel in (b) shows the schematic energy band diagram of the ZnO MW under UV illumination. Photogenerated holes migrate to the surface and recombine with the trapped electrons, releasing the absorbed oxygen molecules. The unpaired electrons in the MW contribute to the photoconductivity. (c) Time-resolved photocurrent rise and decay of the ZnO MW device in air and vacuum.