Optical Temperature Sensing Behavior Through Stark Sublevels Transitions of Green and Red Upconversion Emissions for Er3+-Yb3+-Li+ Codoped TiO2 Phosphors

The Er3+-Yb3+-Li+ codoped TiO2 phosphors have been prepared by sol-gel method. The green and red upconversion emissions were observed under a 976 nm laser diode excitation, which were ascribed to H-2(11/2) -> I-4(15/2), S-4(3/2(I))/S-4(3/2(II)) -> I-4(15/2), and F-4(9/2(I))/F-4(9/2(II)) -> I-4(15/2) transitions of Er3+ Stark sublevels. The fluorescence intensity ratios (FIR), which are corresponding to the transitions of H-2(11/2)/(S-4(3/2(I)) + S-4(3/2(II))) -> I-4(15/2), S-4(3/2(I))/S-4(3/2(II)) -> I-4(15/2), and F-4(9/2(I))/F-4(9/2(II)) -> I-4(15/2), have been studied as a function of temperature in the range of 303 similar to 673 K. The temperature sensitivities have been calculated at the maximum value of 0.0020 K-1, 0.0015 K-1, and 0.0011 K-1 at the temperatures of 427 K, 350 K, and 273 K for the three coupled energy level transitions, respectively. The Er3+-Yb3+-Li+ codoped TiO2 phosphor with different temperature sensitivities by Stark sublevels indicated that it is a promising material for application in optical temperature sensing at a wide range of temperature.