The luminescence of nanocrystalline ZnO particles: the mechanism of the ultraviolet and visible emission

Results of steady-state luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes are presented. In all cases two emission bands are observed. One is an exciton emission band in the UV and the second an intense and broad emission band in the visible, 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. Tn accordance with previous results, a model is presented in which the visible emission is assigned to the radiative recombination of an electron from a level close to the conduction band edge and a deeply trapped hole in the bulk (Vo) of the ZnO particle. The size dependence of the intensity 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 Vb 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. (C) 2000 Published by Elsevier Science B.V, All rights reserved.