Design of Cr3+-activated broadband NIR phosphors with tunable and abnormal thermal quenching behavior for NIR pc-LEDs†

Abstract

Cr³⁺-activated broadband near-infrared (NIR) phosphors usually show controllable and excellent photoluminescence (PL) properties, but their poor thermal stability remains a big challenge. Herein, a series of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ garnet phosphors with tunable and abnormal thermal quenching performance have been successfully proposed. It is found that both the crystal field strength and calculated energetic difference between ⁴T₂ and ²E states decrease obviously with increasing [Ca²⁺–Si⁴⁺] co-substitution, resulting in the thermal occupation of the ⁴T₂ state and broadened PL spectra. More importantly, the Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphors show improved PL thermal stability depending on the different thermal population between ⁴T₂ and ²E states, and the mechanism is investigated in detail. The PL intensity of the optimal sample reaches up to 125% and 121% at 425 K and 475 K compared with that at 300 K, respectively, which is much better than those of most Cr³⁺-activated broadband NIR phosphors. A NIR phosphor-converted light-emitting diode (NIR pc-LED) has been fabricated using the as-prepared thermally stable phosphor and its application in bio-imaging and night vision is demonstrated.