Investigation for the upconversion luminescence and temperature sensing mechanism based on BiPO4: Yb3+, RE3+ (RE3+ = Ho3+, Er3+ and Tm3+)

Rare earth ions doped phosphors have been commonly applied in temperature sensing field. However, it is urgent to specify temperature sensing mechanism owing to the troubles in selecting the suitable phosphors and ascertaining the temperature measurement range. In this work, the Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ co-doped BiPO₄ phosphors are prepared by a simple co-precipitation synthesis with further calcination. Under 980 nm excitation, the up-conversion emissions from the thermally couple levels (TCLs) of RE³⁺ (RE³⁺ = Ho³⁺, Er³⁺ and Tm³⁺) are analyzed as a function of temperature from 313 K to 573 K. Using the fluorescence intensity ratio (FIR) technique, the temperature sensing mechanism based on BiPO₄ matrix is investigated. In order to obtain the deeper comprehension of the temperature sensing mechanism, we explore both the relationship between the absolute sensitivity S_A and temperature T and the relationship between the S_A and energy gap ΔE in detail. The research results indicate that they would have an important guiding significance for the design and selection of phosphors with high temperature sensitivity in the future.