Ca2+/Si4+ Modification of the (Gd,Lu)AG Garnet for Enhanced Broadband Cr3+ Luminescence of High Thermal Stability

Near-infrared (NIR)-emitting phosphors with high quantum efficiency and thermal stability are crucial to NIR pc-LEDs. Garnet-structured (GdLuCa)(Al_4-zSiCr_z)O₁₂ (z = 0.01-0.2) and (Gd_2-xLuCa_x)(Al_4.95-xSi_xCr_0.05)O₁₂ (x = 0.2-1.0) new phosphors with promising NIR luminescence under blue light excitation were designed and fabricated by a solid-state reaction in this work. It was analyzed that the Ca²⁺, Cr³⁺, and Si⁴⁺ ions would replace Gd³⁺ in [GdO₈], Al1 in [Al1O₆], and Al2 in [Al2O₄], respectively, and the optimal Cr³⁺ content is z = 0.05, above which concentration quenching would occur via an electric dipole-dipole interaction. Increasing Ca²⁺/Si⁴⁺ substitution up to x = 1.0 led to luminescence enhancement by a factor of up to 1.85 and internal/external quantum efficiency (%) increment from ∼25.9/10.7 to 63.4/27.5, and all of the phosphors showed excellent thermal stability (I_{423 K}/I_{298 K} ≥ 87.6%). The luminescence properties of Cr³⁺ were discussed in detail through systematic investigation of the effects of Cr³⁺ and Ca²⁺/Si⁴⁺ contents on the crystal structure, local coordination, and crystal field. With the NIR pc-LED device integrated from the optimal phosphor (x = 1.0) and a blue LED chip, electroluminescence manifested potential applications in night vision and medical diagnosis.