The effect of dopant concentration and pump power on sensitivities of optical thermometry in LiLa9Si6O26:Yb3+-Er3+/Ho3+ phosphors based on upconversion luminescence

In this work, to develop new upconversion (UC) luminescent materials for optical thermometer, a series of LiLa9Si6O26:Yb3+-Er3+/Ho3+ (LLS:Yb3+-Er3+/Ho3+) phosphors were prepared by solid-state reaction method. The phase purity was examined through XRD technique. The concentration- and power-dependent UC luminescence was evaluated, in which the UC mechanism and cross-relaxations were involved. The temperature-dependent luminescence indicates that the as-prepared samples could be promising in temperature-sensing applications. For Yb3+-Er3+ co-doped LLS, the thermally-coupled levels of 2H11/2 and 4S3/2 were used to check the sensing sensitivities. It was found that the 521 and 548 nm emissions of Er3+ follow the Boltzmann distribution. The sensitivities depend on not only Er3+-doping concentration but also pump power. The detailed reason was interpreted by cross-relaxations and thermal effect. For Yb3+-Ho3+ co-doped LLS, the fluorescent intensity ratio (FIR) for the red and green emissions of Ho3+ was employed. It was also found that the relationship between the FIR and temperature follows a linear fitting. The obtained absolute sensitivity is decreased with increasing Ho3+ concentration and excitation power, and the corresponding reason was discussed.