Tm ³⁺ ‐Sensitized Thermal Enhancement of Visible and NIR‐II Luminescence via Phonon‐Tuning in Cs ₂ NaTmCl ₆ Double Perovskite Nanocrystals for Advanced Ratiometric Thermometry and High‐Security Optical Encryption

ABSTRACT Luminescent materials relying on Yb 3+ or Nd 3+ sensitization often suffer from severe thermal quenching at elevated temperatures, which limits their performance in high‐precision ratiometric thermometry and optical encryption. Herein, we report a phonon‐tuning strategy in lead‐free Cs 2 NaTmCl 6 double perovskite nanocrystals that enables thermal enhancement of Tm 3 + ‐sensitized luminescence across the visible (450–680 nm) and near‐infrared (NIR, 860–1600 nm) regions under 808 nm excitation.Tm 3+ acts as an efficient multi‐path sensitizer, facilitating emissions from various lanthanide activators (Nd 3+ , Ho 3+ , Dy 3+ , Yb 3+ , and Er 3+ ). Notably, Tm 3+ ‐sensitized upconversion emissions of Ho 3+ (∼660 nm) and Nd 3+ (∼600 and 614 nm) exhibit substantial thermal enhancement from 313 to 533 K, with intensity increases of 7.6‐ and 1.6‐fold, respectively, arising from optimized cross‐relaxation pathways. In contrast, Tm 3+ ‐sensitized downshifting NIR emissions from Dy 3+ (∼1320 nm), Yb 3+ (∼980 nm) and Er 3+ (∼1550 nm) are dominated by energy transfer from the Tm 3+ 3 H 4 and 3 H 5 excited states. By exploiting the interplay between activator thermal quenching and Tm 3+ thermal enhancement, highly sensitive NIR‐II ratiometric thermometry is achieved, delivering a maximum relative sensitivity of 1.59% K–1 based on the emission bands of Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Tm 3+ ( 3 H 5 → 3 H 6 ). This phonon‐tuning strategy further provides a broadband temperature‐responsive platform for multimodal optical sensing and information encryption.