Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb3+/Er3+ co-doped Gd6MoO12 and Yb3+/Er3+/Li+ tri-doped Gd6MoO12 phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb3+ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb3+ concentration quenching effects, the critical distance between Yb3+ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er3+(4F9/2, 2H11/2, 4S3/2) → Er3+(4I15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li+ ions mixing, the upconversion emission enhanced largely, and the optimum Li+ concentration was obtained while fixed the Yb3+ and Er3+ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er3+ after Li+ ions doping into the system. This result indicates that Li+ is a promising candidate for improving luminescence in some case.