Photoluminescence mechanism of self-activated titanate phosphors investigated by x-ray absorption spectroscopy under UV irradiation.

ABSTRACT

The electronic states of self-activated titanate phosphors (Ba2TiSi2O8, Sr2TiSi2O8, and Na2TiSiO5) were investigated by Ti K-edge x-ray absorption spectroscopy under ultraviolet (UV) irradiation in order to unveil the photoluminescence mechanism in the absence of particular atomic color centers. The overall spectral features of all samples were identical, main edges and large pre-edges corresponding to the Ti 4p and Ti 3d states, respectively. The pre-edge peak of Sr2TiSi2O8 shifted toward lower energy by 0.2 eV compared with the other two samples, corresponding to a red shift of the photoluminescence spectrum. In the region between these features, a small hump was observed for each sample, which gradually diminishes as a function of UV power. The hump can be attributed to the Ti 4p state based on the calculated electron density of states determined by multiple scattering theory, and this hump serves as a trap for UV-excited electrons.