Broadband NIR-II phosphors with Cr4+ single activated centers based on special crystal structure for nondestructive analysis

Due to the widespread applications of near-infrared (NIR) light in medical diagnosis, food inspection and biological detection, the need for NIR phosphors with longer wavelengths has become more urgent. Herein, a series of Cr-doped Li2ZnGeO4 (LZGO) phosphors were successfully synthesized. Excited by 470 or 675 nm, the LZGO: Cr phosphor emits a broadband NIR-II spectrum from 1000 to 1600 nm centered at 1230 nm with a full width at half-maximum up to 213 nm. The broadband emission is attributed to the T-3(2)->(3)A(2) spin-allowing transition of Cr4+. The phosphor can be excited by an efficient blue LED chip, and the emitted NIR light can be detected by a cheap In-GaAs detector. The analysis of XPS, EPR, fluorescence lifetime and excitation spectra at different wavelengths show that both Cr3+ and Cr4+ exist in the samples with a content ratio of approximately 3:1, and Cr4+ ions are the sole luminescent center. The reasons for the production of Cr4+ were sum-marized. Furthermore, transmission experiments of several solutions show that LZGO: Cr4+ phosphors have potential applications in physical property identification, quantitative and non-destructive analysis. This work provides a new direction for the development of broadband NIR-II phosphors.