Fig. 1. Emission spectra of $({\mathrm{Sr}}_{1-y}{\mathrm{Ba}}_y)({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$ phosphors, synthesized at different conditions and excited with 470 nm. (a) Synthesized at 1300°C, 1350°C, 1400°C, and 1450°C for 4 h with $x=0.01$ for $\mathrm{Sr}({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$; (b) synthesized at 1400°C for 4, 6, 8, 10, and 12 h with $x=0.01$ for $\mathrm{Sr}{({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x)}_{12}{\mathrm{O}}_{19}$; (c) synthesized at 1400°C for 8 h with $x=0.01$ and assisted with variant fluxes for $\mathrm{Sr}({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$; (d) synthesized at 1400°C for 8 h with $x=0.005$, 0.010, 0.015, 0.020, and 0.025 for $\mathrm{Sr}({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$; (e) synthesized at 1400°C for 8 h with $y=0$, 0.05, 0.10, 0.15, and 0. 20 for $({\mathrm{Sr}}_{1-y}{\mathrm{Ba}}_y)({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$; (f) the normalized spectra of (e).

Fig. 2. Excitation spectra of $({\mathrm{Sr}}_{1-y}{\mathrm{Ba}}_y)({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$ phosphors, synthesized at different conditions and obtained by monitoring the emission at 768 nm. (a) Synthesized at 1300°C, 1350°C, 1400°C, and 1450°C for 4 h with $x=0.01$; (b) synthesized at 1400°C for 4, 6, 8, 10, and 12 h with $x=0.01$; (c) synthesized at 1400°C for 8 h with $x=0.01$ and assisted with variant fluxes; (d) synthesized at 1400°C for 8 h with $x=0.005$, 0.010, 0.015, 0.020, and 0.025; (e), (f) $\mathrm{Ba}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ ($\mathrm{BGO}:{\mathrm{Cr}}^{3+}$) and $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ ($\mathrm{SGO}:{\mathrm{Cr}}^{3+}$) synthesized at 1400°C for 8 h (e) by monitoring the emission at 697 and 714 nm and (f) by monitoring the emission at 730 and 768 nm.

Fig. 3. Crystal structures and morphologies of $\mathrm{Sr}({\mathrm{Ga}}_{1-x}{\mathrm{Cr}}_x{)}_{12}{\mathrm{O}}_{19}$ phosphors. (a) XRD patterns of the phosphor synthesized at 1400°C for 8 h with $x=0.005$, 0.010, 0.015, 0.020, and 0.025; (b) XRD patterns of the phosphor synthesized at 1400°C for 8 h with $x=0.01$ and assisted with variant fluxes; (c), (e) SEM pictures of the phosphor synthesized at 1400°C for 8 h with $x=0.01$ and without flux, and the same phosphor by magnified different times for (c) and (e); (d), (f) SEM pictures of the phosphor synthesized at 1400°C for 8 h with $x=0.01$ and with aid of $1\%$${\mathrm{H}}_3{\mathrm{BO}}_3+1\%$${\mathrm{AlF}}_3$ flux, and the same phosphor by magnified different times for (d) and (f).

Fig. 4. Crystal structure and luminescence mechanism of ${\mathrm{SrGa}}_{12}{\mathrm{O}}_{19}:{\mathrm{Cr}}^{3+}$ phosphor. (a) The three-dimensional structure of the ${\mathrm{SrGa}}_{12}{\mathrm{O}}_{19}$ host, (b) the polyhedral coordination of five sites of Ga atoms in the host; (c) the structure refinement of XRD pattern on the site occupation of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{SrGa}}_{12}{\mathrm{O}}_{19}$ host; (d) the Tanabe–Sugano schematic diagram of ${\mathrm{Cr}}^{3+}$ in sixfold coordination.

Fig. 5. (a) The thermal stability and (b) quantum efficiency of $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor luminescence; and (c) the optical and (d) electronic properties of the NIR LED device packaged using the $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor. (a) Emission spectra of $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor under the excitation of 470 nm ranged from 25°C to 225°C. (b) The measurement on absorbance and internal and EQE of $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor, in which the emission spectrum of calibration light source and the emission and reflection spectra of $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor are listed. (c) The emission spectra of the NIR LED device packaged using the $\mathrm{Sr}({\mathrm{Ga}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{12}{\mathrm{O}}_{19}$ phosphor driven under different currents. (d) The output power and energy conversion efficiency of the NIR LED device as a function of current.

Fig. 6. Immunofluorescence detection and activity assay of BV-2 microglia under different light environments (all immunofluorescence staining is scaled to $50\mu \mathrm{m}$). (a)–(d) The immunofluorescence pictures stained with the CD14 and CD16 antibodies for the microglia cultured under different light environments for 24 and 48 h, respectively; (e), (f) the expression of the CD14 and CD16 antibodies for the microglia cultured under different light environments for 24 and 48 h, respectively; (g), (h) the assays on MTT and ATP, respectively, for the microglia cultured under different light environments for 24 h.