Abnormal gas pressure sensitivity of the visible emission in ZnO quantum dots prepared by improved sol-gel method: The role of surface polarity

Abstract

ZnO quantum dots (QDs) were prepared with an improved sol-gel approach. In addition to the sensitive response to ambient gas types, the visible emission of ZnO QDs is found to be sensitively dependent on the gas pressure. For example, the fluorescence intensity decreases significantly with decreasing air pressure, and even almost completely quenches at 10 mbar. This is a striking phenomenon which is opposed in the case of ZnO nanowires (NWs), for which the visible fluorescence is enhanced in a vacuum condition. A consistent model is proposed to explain the phenomenon, in which the polar surface induced band bending in ZnO QDs could be significantly compensated by that induced by oxygen adsorption, while such a compensation effect is negligibly small in NWs with long lengths along the c-axis. The central part of the model emphasizes that the wurtzite ZnO QDs may not be simply seen as isotropic spheres, but a polar surface charging effect may form the base of the gas sensitivity.