Correlation between luminescent characteristics and phase composition of ZnS:Cu powder prepared by self-propagating high temperature synthesis

The powder-like ZnS:Cu grown by self-propagating high temperature synthesis from the mixture of Zn, S and CuCl is investigated before and after annealing at 800 °C by photoluminescence (PL) and X-ray diffraction (XRD) techniques. It is found that after synthesis the ZnS:Cu powder consists of a mixture of cubic and hexagonal ZnS phases as well as crystalline CuxZn1−x solid solution. PL spectrum shows a wide PL band which is the superposition of green and blue Cu-related bands as well as self-activated one. It is shown that annealing at 800 °C gives rise to three processes, controlled by the heating time to annealing temperature: (i) phase transformation of ZnS hexagonal phase to cubic one; (ii) oxidation processes resulting in ZnO formation; (iii) the non-monotonic changes of CuxZn1−x phase composition and decrease of its content. These changes are accompanied by the non-monotonic variation of the blue to green Cu-related PL band intensities ratio which correlates with the variation of CuxZn1−x phase composition. The model that explains the changes of ZnS:Cu PL characteristics by indiffusion of Zn and Cu from CuxZn1−x phase is proposed. The anisotropic character of ZnS phase transformation and oxidation process is found.