Fe3+-sensitized blue-light-excited NIR-II emission and highly sensitive thermometry in Cs2NaLuCl6:Er3+ double perovskites.

Near-infrared-II (NIR-II) luminescent materials have attracted increasing attention owing to their excellent penetration capability and low background interference, making them promising for optical thermometry and spectroscopic sensing. However, the weak intrinsic absorption of Er3+ severely limits its luminescence performance under blue-light excitation. Here, enhanced blue-light-excited NIR-II emission is realized in Lu-based double perovskite Cs2NaLuCl6 through an Fe3+/Er3+ codoping strategy. The introduction of Fe3+ induces a broadband excitation band extending from 250 to 600 nm and boost the characteristic emissions of Er3+ at 1542 and 1559 nm by up to 8.6-fold under 418 nm excitation. Leveraging the temperature-dependent intensity ratio of these two NIR-II bands, a non-contact optical thermometer with a maximum relative sensitivity of 5.15% K-1 is achieved. Moreover, effective identification of water, ethanol, and acetonitrile is demonstrated based on the spectral overlap between the NIR-II emission and molecular vibrational absorptions. This work presents a feasible route toward blue-light-excited NIR-II emitters through Fe3+ broadband sensitization and expands the application prospects of halide luminescent materials.