Study of photoluminescence and electronic states in nanophase strontium titanate

Nanophase strontium titanate specimens were prepared by a stearic acid sol-gel process. The phase structures, photoluminescence, and electron states were investigated by X-ray diffraction and fluorescence spectroscopy. A visible emission band centered at 500 nm was observed in the nanosized particles under an excitation with energy lower than the band gap at room temperature. The peak intensity of the visible emission band increases with decreasing particle size. This visible emission is attributed to the recombination of the long-lived self-trapped excitons with forceful binding-energy formed in the nanosized SrTiO₃ mediated by the localized levels which are correlated to the intrinsic surface states and defect centers via a strong electron-phonon interaction. The dependence of the visible emission band on excitation wavelength was examined under both room and liquid nitrogen temperatures where there exist a series of levels of the self-trapped excitons within the forbidden gap and the properties of the self-trapped states are closely related to temperature.