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

Abstract

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 ∼818 nm to NIR II emission centered at ∼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.