Achieving high-sensitivity dual-mode optical thermometry via phonon-assisted cross-relaxation in a double-perovskite structured up-conversion phosphor

Abstract

High sensitivity is still a challenge for achieving optical thermometry applications. In this work, we effectively regulated the phonon energy of matrix crystals by substituting the A-site ions in the tellurate double perovskite up-conversion (UC) phosphors of ALaLiTeO₆:5%Yb³⁺,0.2%Tm³⁺ with Ca²⁺, Sr²⁺, and Ba²⁺. On this basis, a dual-mode thermometer based on the fluorescence intensity ratio of FIR₁ = I₆₉₄/I₆₄₉ and FIR₂ = I₆₉₄/I₄₇₅ was constructed by utilizing significant differences in the response to temperature changes between 649 nm/475 nm emissions (from both Yb³⁺ and Tm³⁺) and 694 nm UC emission (from Yb³⁺ → Tm³⁺). As the temperature increases, involving the assistance of matrix crystal phonon energy, the collaborative action of two non-radiative transitions (Tm³⁺) and three cross-relaxation processes (Tm³⁺–Tm³⁺) can degrade the emission intensity at 475 and 649 nm, but enhance the UC emission intensity at 694 nm, thus effectively improving the thermometry sensitivity. Finally, we achieved the maximum thermometry sensitivity values of S_r = 4.69% K⁻¹ and S_a = 10.05% K⁻¹. These results provide new insights into exploring and constructing ultra-sensitive optical temperature sensors.