Broadband near-infrared luminescence properties of fluoride phosphor K₃AlF₆:Cr³⁺

Near-infrared light can be widely used in non-destructive testing, sensing, and night vision. Recently, phosphor-converted LED (pc-LED) based on near-infrared (NIR) phosphor have attracted considerable interest. In this work, tetragonal structured K₃AlF₆:Cr³⁺ phosphors were synthesized using the co-precipitation method. Cr³⁺ ions occupy the octahedral position, and its Dq/B value (the parameter describing the crystal field strength) is calculated to be 2.08, suggesting a weak crystal field. Upon blue light excitation (430 nm), a broadband NIR emission is observed in the range of 650–900 nm, corresponding to the spin-allow ⁴T₂ → ⁴A₂ transition. Increasing Cr³⁺ concentration, red-shift is observed in its NIR emission with the maximum emission intensity at 5 mol% Cr³⁺. Its activation energy ΔE is calculated to be 0.14 eV. Combining with blue InGaN chips, the fabricated NIR pc-LEDs based on K₃AlF₆:Cr³⁺ phosphor exhibit excellent optoelectronic performance, showing potential applications in biological detection.