Modulation of Sharp-Line Red Emissions to Efficient Broadband Near-Infrared Luminescence in Cr3+-Activated Spinel Phosphors via Defect Engineering

Spectral tuning toward longer emission wavelengths, while maintaining high quantum efficiency (QE) and thermal stability, remains a formidable challenge for near-infrared (NIR) luminescent materials. Herein, a Mg-deficiency strategy is proposed to achieve the redshift and broadening of Cr³⁺ emission in MgAl₂O₄ spinel without sacrificing QE and thermal stability. The emission spectrum shifts from sharp lines around 700 nm for stochiometric MgAl₂O₄:Cr³⁺ to an ultra-broadband centered at 860 nm for Mg-deficient Mg_0.9Al₂O_3.9:Cr³⁺, with a profound increase in full width at half maximum (FWHM) from ≈85 to 303 nm. Meanwhile, the Mg_0.9Al₂O_3.9:0.05Cr³⁺ phosphor exhibits an internal QE of 87% and can maintain 80% of initial emission intensity at 150 °C. Moreover, tunable emission bands peaking from 685 to 908 nm are achieved for Mg_0.9Al₂O_3.9:xCr³⁺ by varying the Cr³⁺ concentration. The Cr³⁺ broadband emission can be attributed to the formation of Al_Mg anti-site defects, while the overall lattice contraction caused by the Mg-deficiency contributes to the maintenance of high QE and low thermal quenching. Finally, a NIR phosphor-converted light-emitting diode (pc-LED) is fabricated and its application in nondestructive testing is demonstrated. This study initiates a new way to improve the spectral performance of NIR phosphors while preserving high QE and thermal stability.