Dual-mode optical thermometry of Li2ZnGe3O8:Er3+/Yb3+ upconversion phosphors via fluorescence intensity ratio

Li₂ZnGe₃O₈:Er³⁺ and Li₂ZnGe₃O₈:Er³⁺/Yb³⁺ phosphors were successfully synthesized via a high-temperature solid-state reaction method, and the upconversion luminescence properties and mechanisms of both Er³⁺-doped and Er³⁺/Yb³⁺ co-doped systems under 980 and 1550 nm excitation were systematically investigated. Additionally, the optical temperature sensing performance of the Li₂ZnGe₃O₈:Er³⁺/Yb³⁺ phosphor was explored in detail. To enhance temperature sensitivity and self-calibration features, a dual-mode temperature sensing method was designed and implemented based on fluorescence intensity ratio from thermally coupled energy levels (TCELs) and non-thermally coupled energy levels (NTCELs). Experimental results show that in the TCELs mode, the maximum absolute sensitivity under 980 nm excitation is 0.003197 K^–1 (462 K), and under 1550 nm excitation is 0.0028 K^–1 (303 K); while in the NTCELs mode, the maximum absolute sensitivity under 980 nm excitation is 0.003059 K^–1 (529 K), and under 1550 nm excitation is 0.0013 K^–1 (303 K). These findings indicate that Li₂ZnGe₃O₈:Er³⁺/Yb³⁺ phosphors exhibit excellent dual-mode optical temperature sensing potential under dual-excitation conditions.