Research On Up-conversion Fluorescence Characteristics Of Rare-earth Doped TiO₂ Thin Films

Up-conversion fluorescence is an anti-Stokes process. It is widely used in optical devices such as data communication and storage, as well as solid-state laser and solar energy. Rare-earth elements have special electronic structure, and therefore become a promising candidate for up-conversion fluorescence, and Er3+ is one of them. Besides, TiO2 thin film is widely used as up-conversion host materials because of its lower phonon energy and excellent mechanical property and photovoltaic conversion property. Since Er3+-doped TiO2 thin film is paid attention, Yb3+ is used to improve up-conversion efficiency as sensitization ion. Thus Er3+ and Yb3+ are always researched in up-conversion materials as a system.In this study, Er3+-doped and Er3+/Yb3+-doped TiO2 thin films are prepared by RF magnetron sputtering and the best doping concentration and annealing conditions are confirmed. Up-conversion fluorescence is measured under 980 nm pumping and 490 nm green light and 670 nm red light are obtained. On this base, the factors of up-conversion fluorescence and its mechanism is analyzed. The main conclusions are as follows:(1) Up-conversion fluorescence of Er3+-doped TiO2 thin films is affected by the formation of Er2Ti2O7 crystallites and its relative content. The strongest up-conversion fluorescence was obtained when Er2Ti2O7 crystallites and rutile TiO2 phase reach to an appropriate proportion. The highest up-conversion efficiency could be achieved by doping 1.0 mol% Er3+ into TiO2 thin films, which was subsequently annealed at 900℃ for 150 min. High Er3+ content may cause concentration quenching effect. Compared to green emission, red emission is more sensitive to the change of annealing temperature and doping concentration.(2) 490 nm green light and 670 nm red light can be obtained when Er3+/Yb3+-doped TiO2 thin films are under 980 nm pumping. The doping concentration and annealing conditions will affect formation of ErxYb2-xTi2O7 crystallites which influence up-conversion fluorescence. Up-conversion fluorescence reaches top intensity when 1.0 mol% Er3+/5.0 mol% Yb3+ are co-doped. The intensity of green light increases 9.6 times while the red light increases 4.3 time compared to Er3+-doped TiO2 thin films(3) Both Er2Ti2O7 crystallites and ErxYb2-xTi2O7 crystallites in TiO2 thin film by RF magnetron sputtering could decrease state densities of phonons, and therefore inhibits nonradiative transition that makes red emission much stronger than green emission and appearance of 490 nm green emission.(4) The difference of reflectivity caused by the thickness of thin film will affect up-conversion fluorescence. The light absorption of pump occurred in the surface of thin film. It will be helpful to up-conversion fluorescence if the substrate is roughed. Besides, up-conversion fluorescence will be improved if low phonon energy material is used, such as fluoride.