Isolated Mn2+-activated near-infrared phosphors under nephelauxetic effects and strong crystal field
Broadband near-infrared (NIR) phosphors with longer wavelengths are critically needed for deep-tissue biomedical imaging and other emerging applications. However, the detailed mechanism of Mn2+-activated NIR emission remains elusive. Guided by the nephelauxetic effect theory, sulfide phosphors with strong covalent bonding are promising for achieving long-wavelength Mn2+ luminescence. This work reports a series of M(Ga, In)2S4:Mn2+ (M = Ca, Sr, Ba) phosphors featuring strong crystal field environments, and for the first time the luminescence behaviors of Mn2+-doped MIn2S4 (M = Sr, Ba). Preferential site occupancy leads to significant differences between MGa2S4:Mn2+ (M = Ca, Sr) and MIn2S4:Mn2+ (M = Sr, Ba) despite the same crystal structure. Severe polyhedral distortion enhances ultrawideband deep-red to NIR luminescence (600–850 nm, FWHM ≈ 123–257 nm), far superior to similar materials. Fabricated pc-LED devices demonstrate excellent vascular imaging and plant illumination capabilities. This study provides new insights into the NIR luminescence of isolated Mn2+.