F− substitution-induced high-efficiency broadband near-infrared MgAl2O4:Fe3+ phosphors for night-vision and biomedical imaging

Abstract

Fe³⁺-doped near-infrared (NIR) phosphors are promising for NIR phosphor-converted LEDs (pc-LEDs), but their NIR emission is plagued by low internal quantum efficiency (IQE) and poor thermal stability. To overcome these, herein, a broadband NIR-emitting phosphor MgAl₂O₄:Fe³⁺ has been successfully developed for such applications. It is demonstrated that substituting O²⁻ with anion F⁻ enhances the internal quantum efficiency significantly, from 66.5% to 96.5%. This improvement is attributed to the reduction in oxygen vacancies and the lowered lattice symmetry around Fe³⁺ ions. Simultaneously, due to the dual-site occupancy of Fe³⁺ and the weakened crystal field intensity, the emission peak can be tuned from 740 to 770 nm, and the full width at half maximum is broadened from 98 to 106 nm. The emission intensity of the phosphor at 423 K remains at 82.3% of that at room temperature. The NIR pc-LED fabricated with a 365 nm chip and the MgAl₂O₄:Fe³⁺,F⁻ phosphor achieves a high NIR output power of 5.74 mW at a driving current of 320 mA, and its feasibility as a light source for night vision and blood vessel imaging is verified. Furthermore, this study on anionic F⁻ replacing O²⁻ provides valuable insights for optimizing the performance of Fe³⁺-doped phosphors.