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

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 Lu3-xCaxGa5-xSixO12: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 Lu3-xCaxGa5-xSixO12: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.