Valence state control of Cr⁴⁺-activated Li₂SrGeO₄ for NIR-II light source to distinguish deuterium and non-deuterium reagents

Cr-activated phosphor-converted light sources have attracted widespread attention in various applications based on near-infrared (NIR) spectroscopy. Unfortunately, Cr ions usually face the issue of multivalence and typically exist in the valence of +3, which gives the emission mainly at NIR-I (700-1000 nm) and greatly limits its potential application. In this study, we propose the control strategies of site occupancy confinement and sintering atmosphere to synthesize Cr4+-activated Li2SrGeO4 (LSGO:Cr4+) phosphors. Interestingly, the as-optimized LSGO:Cr4+ achieves a NIR broadband emission peak at 1170 nm with an FWHM of 208 nm upon excitation at 680 nm and exhibits excellent thermal stability and high efficiency with the external quantum efficiency of 30.0%. Furthermore, the as-fabricated LSGO:Cr NIR phosphor-converted light emitting diode (NIR pc-LED) device shows a high power of up to 69.1 mW at 550 mA. Furthermore, as NIR-II (1000-1700 nm) light source, the phosphor successfully distinguishes deuterium and non-deuterium reagents. For example, the quantitative detection of H2O in heavy water (D2O) is demonstrated by NIR spectroscopy with a limit of detection (LOD) of 0.47 vol%. Our findings will open up a new avenue to develop novel Cr4+-activated NIR-II phosphors and NIR-II light sources.