Highly sensitive temperature sensors based on the fluorescence intensity ratio of dual-emissive lead-free metal halides.

Given that optical thermometers are widely used due to their unique advantages, this study aims to address critical challenges in existing technologies, such as insufficient sensitivity, limited temperature measurement ranges, and poor signal recognition capabilities. Herein, we develop a thermometer based on the fluorescence intensity ratio (FIR) of Sb-doped Cs₂NaInCl₆ (Cs₂NaInCl₆:Sb). As the temperature increases from 203 to 323 K, the thermally induced transition from triplet to singlet self-trapped excitons (STEs) leads to enhanced 455 nm photoluminescence (PL) from singlet STE recombination. Thus, the FIR monotonically depends on temperature, allowing for temperature sensing with a high absolute sensitivity (S_A) of 0.0575 K^-1 and the maximum relative sensitivity (S_R) of 1.005% K^-1. We demonstrate that spatial temperature distribution can be measured by mapping the PL spectra, even with a transparent medium screening the target. Furthermore, blue emissive Cs₂NaInCl₆:Sb is mixed with yellow emissive Cs₂AgInCl₆:Sb with a thermal quenching feature. The fluorescence color of the mixture dramatically depends on temperature, enabling a user-friendly colorimetric temperature sensing. Therefore, two operational modes are proposed to meet various practical application demands.