Temperature dependent photoluminescence of anatase and rutile TiO2 single crystals: Polaron and self-trapped exciton formation

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Fran...