Up-conversion fluorescence characteristics and mechanism of Er3+-doped TiO2 thin films

•We propose RF magnetron sputtering to prepare rare-earth doped TiO2 thin films.•We elucidate the unusual up-conversion mechanism of Er3+-doped TiO2 thin film.•We propose that state densities of phonons cause special transition behaviors.

Er3+-doped TiO2 thin films were prepared by RF magnetron sputtering. The effects of Er3+ concentration and annealing conditions upon up-conversion fluorescence were studied. The result shows that 490 nm green emission and 670 nm red emission can be obtained from Er3+-doped TiO2 thin films excited by 980 nm lasers. TiO2 thin films with 1.0 mol% Er3+ manifested the best up-conversion property when the annealing temperature reached 900 °C. Under such conditions, we observed intense 490 nm green emission that has not been observed in powder or sol–gel samples and the mechanism of up-conversion fluorescence were studied. We also confirmed the up-conversion properties are closely related to the formation of Er2Ti2O7 in thin films, which is proved to reduce state densities of phonons and therefore inhibit nonradiative transition in an up-conversion process.