Thermal Stability Improvement of Cr³⁺-Activated Broadband Near-Infrared Phosphors via State Population Optimization

Technological progress has accelerated the researches into broadband near-infrared (NIR) luminescent materials as next-generation intelligent NIR light sources; however, poor thermal stability restricts the applications of NIR phosphors. Herein, new insights into Cr3+-activated NIR phosphors with improved thermal stability are provided. The photoluminescence intensity originating from the 4T2 -> 4A2 broadband emission of CaLu2Al4SiO12:Cr3+ (CLAS:Cr) with optimal electron occupation (T-4(2)/E-2) increases up to 118% at 475 K compared with that at room temperature, which is the best for Cr3+-activated broadband NIR phosphor. Accordingly, new ideas for the design of thermally stable phosphors are proposed by optimizing the thermal population between T-4(2) and E-2 states and electron migration from E-2 to T-4(2) state. Finally, multifunctional applications of CLAS:Cr in NIR fluorescence intensity ratio thermometry, X-ray detection, NIR phosphor-converted light-emitting diodes, and bio-imaging are demonstrated.