Crystallographic control for Cr4+ activators toward efficient NIR-II luminescence

Abstract

Broadband near-infrared (NIR) emitting phosphors have attracted great interest due to their potential applications in non-destructive examination and bioimaging. However, most of the reported broadband NIR phosphors emit in the NIR-I region with a wavelength shorter than 950 nm, while rare-earth activated NIR-II phosphors can hardly meet the requirements because of their sharp emission. Herein, we successfully synthesized the broadband NIR-II phosphor Li₂ZnGeO₄:Cr⁴⁺. By employing the all-tetrahedron-built matrix, all the Cr ions are stabilized in the tetravalent state due to crystallographic control. This phosphor shows wide absorption from the red to near-infrared region. Under 646 nm excitation, it exhibits broadband NIR-II emission peaking at 1218 nm with an FWHM of 220 nm at room temperature. We also demonstrated the potential applications of Li₂ZnGeO₄:Cr⁴⁺ as an NIR-II light source in non-destructive examination and bioimaging. This work provides a new strategy for exploring broad-band NIR-II luminescent materials.