Unique Spectral Broadening Induced by Exchange Coupling Between Cr³⁺ Ions in LiAl₅O₈:Cr³⁺ Phosphors for Versatile Optical Applications

Abstract

Abstract Broadband near‐infrared (NIR) phosphors activated by Cr 3+ ions are critical for developing NIR phosphor‐converted light‐emitting diodes (pc‐LEDs). The temperature‐dependent behaviors of the narrowband 2 E → 4 A 2 and broadband 4 T 2 → 4 A 2 transitions of Cr 3+ ions differ, which makes Cr 3+ ‐activated phosphors highly promising for ratio optical thermometers. How to obtain Cr 3+ ‐activated phosphors with both high luminescence efficiency and thermal stability is a challenge. Herein, a strategy of adjusting the doping levels is adopted to precisely control the exchange coupling between Cr 3+ ions in LiAl 5 O 8 : Cr 3+ phosphors. As Cr 3+ ions doping concentration increases, the NIR emission shifts from narrowband to broadband. Under heavy doping concentrations, the sample exhibits an unusual broadband emission in a strong crystal field, covering 645 to 1300 nm, with a peak emission wavelength of 950 nm and a FWHM of 295 nm. Simultaneously, a high internal quantum efficiency ( IQE = 85%) and excellent thermal stability ( I 423K / I 293K = 86%) are achieved, highlighting the broad application potential of the developed NIR pc‐LEDs. Interestingly, under low‐concentration doping, the sample exhibits dual emission characteristics, and the temperature dependence of its two emission peaks shows opposite trends. Utilizing fluorescence intensity ratio (FIR) technology, the sample achieves a relative sensitivity of 0.30% K⁻¹, highlighting its promising potential for temperature sensing applications.