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

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.