Realizing Ultrabroadband NIR-II Emission and Wide-Range Wavelength Tuning in Cr⁴⁺-activated ABO₂ (A = Li, Na; B = Al, Ga) Phosphors

Abstract

Cr 4+ -activated phosphors are important candidate materials for NIR-II light sources, but providing a suitable lattice coordination environment for Cr 4+ and achieving long wavelength broadband emission remains a challenge. In this work, a series of Cr 4+ -activated ABO 2 (A = Li, Na; B = Al, Ga) phosphors were successfully prepared. Due to the presence of only tetrahedral coordination structures available for Cr 4+ to occupy in the matrix crystal ABO 2 , the valence state and luminescence stability of Cr 4+ are effectively guaranteed. Through the cation substitution design of A-site (Na → Li) and B-site (Ga → Al), the [BO 4 ] tetrahedron is distorted and expanded, which degrades the symmetry of the Cr 4+ coordination crystal field. Consequently, the central wavelength of the Cr 4+ emission peak is tuned from 1280 to 1430 nm, and the fwhm is significantly extended from 257 to 355 nm. Thebroadband NIR-II light sources constructed with LiAlO 2 : 0.03Cr 4+ and NaGaO 2 : 0.03Cr 4+ phosphors verify their important potential applications in nondestructive testing and biological imaging.