Highly Sensitive Dual-Mode Optical Thermometry of Er 3+ /Yb 3+ Codoped Lead-Free Double Perovskite Microcrystal.

In this Letter, erbium (Er ³⁺ ) and ytterbium (Yb ³⁺ ) codoped perovskite Cs ₂ Ag _0.6 Na _0.4 In _0.9 Bi _0.1 Cl ₆ 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 Er ³⁺ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er ³⁺ 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, Er ³⁺ luminescence becomes stronger with the feeding Yb ³⁺ increasing under 980 nm laser excitation. The energy transfer of Er ³⁺ and Yb ³⁺ 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 Er ³⁺ /Yb ³⁺ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.