Theoretical design and experimental realization of Fe3+-doped dual-band near-infrared garnet phosphors†

Abstract

Cr³⁺-free near-infrared (NIR) phosphors based on Fe³⁺ have garnered extensive attention due to their environmentally friendly and tunable optical properties. However, the reported luminescence predominantly originates from the Fe³⁺ ion in tetrahedral coordination, with wavelengths in the range of 670–830 nm. Phosphors with luminescence from octahedrally coordinated Fe³⁺, which are expected to shift to longer wavelengths over 900 nm, are limited due to the challenges such as quenching mechanisms. Garnets with the formula A₃B₂C₃O₁₂ are excellent hosts for phosphors due to their rigid structures and tunable luminescence properties. Theoretical analysis, supported by first-principles calculations, indicates that Fe³⁺ can occupy both tetrahedral (Fe(T)) and octahedral (Fe(O)) sites, potentially producing dual-band emission in garnet crystals with large octahedral host ions, such as Sr₃ (Sc/Lu/Y)₂Ge₃O₁₂ crystals. This has guided us in the experimental realization of dual-band NIR luminescence, peaking at 720–730 nm (Fe(T)) and 980–990 nm (Fe(O)), in these materials. Consistent with our optical transition analysis, the luminescence intensities of Fe(T) and Fe(O) show different temperature dependencies. Fe(T) exhibits weaker temperature dependence, while Fe(O) experiences severe temperature quenching via the ²T₂ intermediate energy level. The dual-band NIR phosphors Sr₃ (Sc/Lu/Y)₂Ge₃O₁₂:Fe³⁺ show potential applications in luminescence intensity ratio-based and luminescence decay time-based thermometers, with a significant maximum relative sensitivity of 1.24% K⁻¹ at 155 K. The materials designed here provide a foundation for related application explorations, and the strategy developed can be applied to the exploration and development of Fe³⁺-activated advanced optical materials.