Site-selective occupancy of Cr³⁺ enabling tunable emission from near infrared I to II in fluoride LiInF₄:Cr³⁺

Broadband near-infrared (NIR) phosphors are crucial for fabrication of next-generation smart light sources; however, the development of a broadband NIR II emission phosphor with the full width at half maximum (FWHM) above 200 nm remains challenging. Herein, a Cr3+ doped fluoride phosphor LiInF4:Cr3+ with tunable emission from the NIR I to the NIR II spectral region is achieved using a site-occupancy control strategy. By changing the volume ratio of HF to H2O in the synthesized solution, nearly pure NIR I emission at similar to 818 nm to NIR II emission centered at similar to 1000 nm with a FWHM of 220 nm is realized. The density functional theory (DFT) calculations and experimental analysis reveal that the NIR I and NIR II emissions originated from the Cr3+ ions at In3+ and Li+ sites in this fluoride, which are coordinated with six and seven F- ions, respectively. The relatively low local symmetry environment is the main factor for the NIR II emission of Cr3+. A phosphor converted light-emitting diode with broadband NIR II emission is fabricated by combining the phosphor LiInF4:Cr3+ and a commercial 460 nm blue InGaN chip, showing good potential for non-destructive detection, vein imaging, etc. This work also provides insights into the luminescence tuning of Cr3+ ions