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 luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes and at different temperatures. In all cases two emission bands are observed. One is an exciton emission band and the second an intense and broad visible emission band, shifted by approximately 1.5 eV with:respect to the absorption onset. As the size of the particles increases, the intensity of the visible emission decreases while that of the exciton emission increases. As the temperature decreases, the relative intensity of the 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 dependence and size dependence of the ratio of the visible to exciton luminescence 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 efficient surface trapping, probably at an O2- Site.