Lithium induced emission enhancement of Cr3+-activated fluoride with high thermal stability, toward night vision and information encryption applications

Near-infrared (NIR) luminescent materials are promising NIR light sources for multifunctional applications; providing a strategy to tailor the emission and enhance the performance of NIR phosphors is crucial. Herein, a green HF-free synthetic strategy is realized to tune the NIR emission advantageously by substituting on the adjacent octahedral, completely different from breaking the d–d parity forbidden transitions of Cr3+ through conventional lattice distortion. Broadband NIR phosphor K2Na1−xLixAlF6:Cr3+ with excellent thermal quenching resistance is prepared using the hydrothermal method. Controllable emission tuning from 726 to 765 nm and broadening full width at half-maximum from 105 to 130 nm, is achieved by substituting Li+ for Na+, resulting from weakening the crystal field and increasing electron–phonon coupling. Lithium ions play a critical role in suppressing nonradiative relaxation of the emission centers, thereby significantly enhancing the emission intensity of K2NaMF6:Cr3+ (M = Al, Ga, In) phosphors. The as-fabricated prototype NIR phosphor-converted light-emitting diode device, exhibits significant promise in the fields of night vision and information encryption. This study provides an effective design strategy for developing broadband NIR luminescent materials with high efficiency and good thermal stability.