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 c ases two emission bands are observed. One is an exciton emission band in th e UV and the second an intense and broad emission band in the visible, shif ted by approximately 1.5 eV with respect to the absorption onset. As the si ze of the particles increases, the intensity of the visible emission decrea ses, while that of the exciton emission increases. Tn accordance with previ ous 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 co nduction band edge and a deeply trapped hole in the bulk (Vo) of the ZnO pa rticle. The size dependence of the intensity ratio of the visible to excito n luminescence and the kinetics are explained by a model in which the photo generated hole is transferred from the valence band to a Vb level in the bu lk 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.