Nearly pure NIR to NIR upconversion luminescence in Tm3+, Yb3+ co-doped ZnO-TiO2 composite phosphor powder

Abstract

Tm3+ and Yb3+ co-doped ZnO-TiO2 composite system were successfully synthesized via powder-solution mixing method and their upconversion (UC) luminescence characteristics were studied as a function of various ZnO/TiO2 mixing ratios and dopant concentrations under 980 nm laser excitation. The XRD patterns showed that the product fired at 1300 °C consisted of four phases, Zn2TiO4, TiO2, RE2Ti2O7, and RE2TiO5 (RE = Tm3+ and/or Yb3+). The NIR emission centered at 795 nm wavelength was detected as the strongest emission intensity which it was in accordance with the 3H4 → 3H6 transition of Tm3+ ion. The NIR emission intensity of the products was varied by changing ZnO-TiO2 mixing ratios, and Tm3+ and Yb3+ concentrations. The simple chemical formula equations, for interpreting the site preference of Tm3+ and Yb3+ ions in the host crystal matrix, were originated by considering the host crystal structure, its crystal arrangement, and the effect of Tm3+ and Yb3+ ions to the changes in lattice parameters of host crystal. The 1ZnO:1TiO2 (in mole) doped with 0.125 mol% Tm3+, 15 mol% Yb3+ fired at 1300 °C for 1 h was the best condition that displayed highest NIR emission intensity. The possible UC mechanism was suggested and discussed in detail.