Fig. 1. (a) Cell structure of the KCdF₃ crystal; (b) XRD patterns of 0.07Cr³⁺/0.2Ni²⁺-PG and GC samples at different heat-treatment temperatures, and the inset shows the photograph of the glass samples in daylight; (c), (d) XRD patterns of xCr³⁺/0.2Ni²⁺-GCs (x = 0, 0.03, 0.05, 0.07, 0.09) and 0.07Cr³⁺/yNi²⁺-GCs (y = 0, 0.03, 0.05, 0.1, 0.2, 0.3), heat-treated at 570°C for 2 h, respectively.

Fig. 2. (a) Bright-field TEM image of the 0.07Cr³⁺/0.2Ni²⁺-GC570 sample, and the inset shows the size distribution plot of the KCdF₃ nanocrystals; (b) HRTEM image; (c)–(h) Dark-field HADDF-TEM images and element mapping of K, Cd, F, Cr, and Ni, respectively.

Fig. 3. (a) Transmission spectra of the blank- and 0.07Cr³⁺/0.2Ni²⁺-GC570 samples; (b) PLE (monitored at 784 nm) and PL (excited by 450 nm) spectra of the 0.07Cr³⁺-GC570 sample; (c) PL spectra of the 0.2Ni²⁺-GC570 and 0.07Cr³⁺/0.2Ni²⁺-GC570 samples excited by 450 nm; (d) PEL spectra of the 0.2Ni²⁺-GC570 and 0.07Cr³⁺/0.2Ni²⁺-GC570 samples monitored at 1584 nm.

Fig. 4. PL spectra of the (a) xCr³⁺/0.2Ni²⁺-GC570 (x = 0.03, 0.05, 0.07, and 0.09) and (b) 0.07Cr³⁺/yNi²⁺-GC570 (y = 0, 0.03, 0.05, 0.1, 0.2, and 0.3) samples under 450 nm excitation.

Fig. 5. Decay curves of the (a) 0.07Cr³⁺/0.2Ni²⁺-PG and GC570 as well as (b) 0.07Cr³⁺/yNi²⁺-GC570 samples under 450 nm excitation when monitoring the emission at 784 nm.

Fig. 6. Schematic diagram for the energy transfer from Cr³⁺ to Ni²⁺ in the glass ceramics.

Fig. 7. (a) Driven current-dependent electroluminescence spectra of the as-fabricated NIR LED; (b)–(d) Applications of the as-fabricated NIR LED light source, and photographs taken with a normal camera in daylight (left) and a NIR camera under illumination of the prepared NIR LED light (right), respectively. A 1000 nm long-pass filter was used when capturing images in (c) and (d).