Energy-looping strategies in Cs2NaYF6 double perovskites: Enabling ultra-low temperature self-calibrating thermometry and fluorescent anti-counterfeiting

Abstract

Remote temperature sensing based on luminescence intensity ratio (LIR) remains a significant challenge in extreme environments, particularly at ultra-low temperatures. Herein, multimodal dynamically tunable fluorescence is achieved in Cs₂NaYF₆ double perovskites through single or co-doping with Eu³⁺ and Tb³⁺. Dipole–dipole interaction is confirmed to mediate energy transfer between the Tb³⁺ and Eu³⁺, establishing enhanced energy-looping among ⁵D₄ (Tb³⁺) and ⁵D₁ (Eu³⁺) levels under 377 nm irradiation. The ⁵D₄ → ⁷F₅ transition of Tb³⁺ gives rise to four well-resolved emission, arising from the Stark splitting of the ⁵D₄ level under O_h symmetry. Energy-looping facilitates sustained back energy transfer from the ⁵D₁ level of Eu³⁺ to Stark sublevels, thereby amplifying the temperature-dependent variation of the LIR for four-peak emission. This results in a high relative sensitivity of 1.07%/K at 80 K and a low temperature uncertainty of 0.61 K. Meanwhile, real-time feedback is enabled by employing multimodal LIR self-calibration based on the non-thermally coupled levels of Tb³⁺ and Eu³⁺. Furthermore, by dual-wavelength switching excitation, photon barcodes and encrypted QR codes are successfully developed. Overall, this study establishes an effective LIR-based strategy for high-precision remote thermometry in ultra-low temperature environments, and opens avenues for the development of advanced fluorescent anti-counterfeiting.