Dynamics of surface-excitonic emission in ZnO nanowires

We report on systematic investigations of the dynamics of the surface-excitonic (SX) near-band-edge photoluminescence of ZnO nanowires observed at approximate to 3.365 eV. The temporal evolution of the SX emission of vapor-phase grown ZnO nanowires with diameters of d=40-130 nm is studied as a function of spectral position, temperature, and excitation intensity. A phenomenological rate-equation model is developed and discussed which is able to describe the experimentally observed transients pointing to a biexponential decay. A detailed analysis shows that the dependence of the transients on the spectral position can be explained by the relaxation and decay of excitons that are separated into a strongly localized and another weakly localized fraction. With increasing temperature, the trapped excitons are activated into less localized SX centers at higher lying energies. The distinct SX emission saturation behavior with increasing excitation density observed in time-integrated as well as in time-resolved measurements is clearly related to the limited number of SX near-surface centers.