Thermal-enhanced near-infrared-II luminescence from Sb3+/Er3+ co-doped Cs3GdCl6 microcrystals
Near-infrared-II (NIR-II, 1000-1700 nm) luminescent materials are pivotal for deep-tissue bioimaging and optical communication, yet their performance is often limited by low quantum yields and thermal quenching. Here, we report a thermal-enhanced NIR-II luminescence in Sb3+/Er3+ co-doped Cs3GdCl6 microcrystals synthesized via a modified Bridgman method. Under ultraviolet excitation, the co-doped microcrystals exhibit intense NIR-II emission centered at 1532 nm corresponding to Er3+: 4I13/2 → 4I15/2 transition, with a maximum relative sensitivity of 1.2% K−1 at 303 K. Notably, the integrated NIR-II emission intensity increases by 2.3-fold from 298 K to 373 K, demonstrating anomalous thermal enhancement. This behavior is attributed to the thermally activated energy transfer from Sb3+ sensitizers to Er3+ activators, as confirmed by temperature-dependent photoluminescence spectra and decay kinetics. The energy transfer efficiency reaches 86% at room temperature and further improves with rising temperature. The microcrystals also show excellent photostability, retaining 95% of initial intensity after 120 min continuous UV irradiation. Furthermore, we demonstrate a proof-of-concept wireless optical communication link using the microcrystals as a NIR-II phosphor, achieving a signal-to-noise ratio of 30 dB at 400 Hz modulation frequency. These findings provide a new strategy for designing thermal-enhanced NIR-II luminescent materials and expand their potential in temperature sensing and optical communication.