Upconversion enhancement and temperature sensing studies in Li ions incorporated GdPO:Tm/Yb phosphor.

The GdPO:Tm/Yb phosphor codoped with various concentrations of Li  ions were synthesized for upconversion emission and optical thermometry studies. Excitation using 980 nm laser diode results in three upconversion (UC) emission bands with centre wavelengths of 478, 648 and 692 nm. These bands are originated from G→H, G→F, and F→H transitions of the Tm ion, respectively. Li ions modified the local crystal symmetry around dopant ions, resulting enhanced UC emissions. The lifetime of the G level of Tm ion was studied using a 980 nm laser excitation. The temperature sensing performances of GdPO:Tm/Yb and GdPO:Tm/Yb/Li based on fluorescence intensity ratio (FIR) technique were evaluated in the temperature range 301-713 K under 980 nm excitation. Non-thermally coupled levels F (692 nm) and G (478, 648 nm) were utilized for FIR estimation. A maximum absolute sensitivity of 6.28 × 10 K at 653 K and 18.71 × 10 K at 713 K were observed for Li  undoped and codoped phosphors respectively. The result indicates that codoping of Li  ions improved the UC emission as well as optical thermometry. Moreover, CIE colour coordinates and anti-counterfeiting application were also exhibited.