The kinetics of the radiative and nonradiative processes in nanocrystalline ZnO particles upon photoexcitation

This report presents the results of steady-state and time-resolved luminesc ence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes and at different temperatures. In all cases two emissio n bands are observed. One is an exciton emission band and the second an int ense and broad visible emission band, shifted by approximately 1.5 eV with: respect to the absorption onset. As the size of the particles increases, th e intensity of the visible emission decreases while that of the exciton emi ssion increases. As the temperature decreases, the relative intensity of th e exciton emission increases. In accordance with the results presented in a previous paper, we assume that the visible emission is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the bulk (V-O(..)) of the ZnO particle. The temperature dep endence and size dependence of the ratio of the visible to exciton luminesc ence and the kinetics are explained by a model in which the photogenerated hole is transferred from the valence band to a V-O(.) level in the bulk of the particle in a two-step process. The first step of this process is an ef ficient surface trapping, probably at an O2- Site.