Highly Sensitive Dual-Mode Optical Thermometry of Er³⁺/Yb³⁺ Codoped Lead-Free Double Perovskite Microcrystal

In this Letter, erbium (Er³⁺) and ytterbium (Yb³⁺) codoped perovskite Cs₂Ag_0.6Na_0.4In_0.9Bi_0.1Cl₆ microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er³⁺ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er³⁺ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K^-1 and 8.20 × 10^-2 K^-1. Moreover, Er³⁺ luminescence becomes stronger with the feeding Yb³⁺ increasing under 980 nm laser excitation. The energy transfer of Er³⁺ and Yb³⁺ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er³⁺/Yb³⁺ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.