Enhancement of luminescence efficiency of ZnS:Cu nanocrystals by pH control

Abstract

ZnS:Cu nanocrystals are synthesized by solution synthesis technique, and the photoluminescence (PL) intensity of samples are substantially enhanced by pH control with the change of urea concentration. The crystal structure, position of absorption edge, and particle shape are not changed by urea addition. However, the PL intensity is maximum for the optimum urea concentration of 3 M and minimum for 24 M. This is the first observation for the ZnS:Cu nanocrystals synthesized by solution synthesis technique. In order to explain this phenomenon, the X-ray photoelectron spectroscopy (XPS) is carried out. It is shown from Cu2p 3/2 analysis obtained from XPS that at the optimum urea concentration, Cu acts as the normal acceptor, leading to the high PL intensity. With high urea concentration, however, Cu is transformed into CuO, and CuO acts as the nonradiative recombination center, resulting in the substantial decrease of PL intensity.