Achieving Ultrahigh Thermal Stability in Cr3+-Activated Garnet Phosphors through Electron Migration between Thermally Coupled Levels.

Abstract

Recently, Cr3+-activated near-infrared (NIR) phosphors have received much more attention due to their excellent photoluminescence (PL) properties. However, most of them suffer from poor thermal stability which limits further application. Herein, a novel Lu2CaGa4SnO12:Cr3+ phosphor with broadband NIR emission (λem = 750 nm) is synthesized successfully. Despite the good luminescence property, its PL intensity decreases obviously with temperature (I425K = 79%). To improve the thermal stability, a series of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ (x = 0-1.0) solid solutions with tunable thermal quenching performance have been designed. It is found that the fluorescence intensity ratio (FIR) of 4T2 → 4A2 to 2E → 4A2 [I(4T2)/I(2E)] transitions (i.e. electron occupation) decreases monotonously with increasing [Lu3+-Ga3+] co-substitution, resulting from a strengthened crystal field strength and increased energy difference between 4T2 and 2E energy levels. Benefiting from the various thermal population and energy difference Δ', the PL thermal quenching behavior of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ can be adjusted easily, and the corresponding mechanism is explored in detail. Most notably, the emission intensity of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ at 425 K can reach up to 142% compared with that at 300 K, which may be the best for Cr3+-activated NIR phosphors. This work may provide an alternative path for the development of thermally stable broadband NIR phosphors.