Structural Confinement for Cr 3+ Activators toward Efficient Near-Infrared Phosphors with Suppressed Concentration Quenching

Trivalent chromium ion-doped near-infrared (NIR) phosphors have been widely studied due to their tunable emission wavelengths and broad applications. High Cr 3+ concentration can improve absorption efficiency but generally results in low emission intensity due to the concentration quenching effect. Herein, we report a series of efficient NIR phosphors with suppressed concentration quenching, Sr 9 M 1– x (PO 4 ) 7: x Cr 3+ (M = Ga, Sc, In, and Lu), showing a broadband NIR emission ranging from 700 to 1100 nm peaking at 850 nm upon the 485 nm light excitation. The emission peak position is almost independent of the type of M ion and the Cr 3+ dopant content, and the type of M ion has little influence on the luminescence thermal quenching, indicating that [MO 6 ] octahedra are rigid enough to keep octahedral volumes and average M 3+ –O 2– distances nearly constant owing to the formation of the framework structure on Cr 3+ substitution. The NIR emission intensities monotonously increase with the Cr 3+ content increasing from 0 to 80% with suppressed concentration quenching, the intensity of Sr 9 Cr(PO 4 ) 7 still maintains 84.23% of Sr 9 Ga 0.2 (PO 4 ) 7:0.8Cr 3+ phosphor, and the thermal quenching behavior is slightly dependent on x; these effects can be attributed to the suppressed energy transfer due to the structural confinement effect. The optimal sample, Sr 9 Ga 0.2 (PO 4 ) 7:0.8Cr 3+, has an internal/external quantum efficiency of 66.3%/29.9%. Finally, we fabricate a NIR phosphor-conversion light-emitting diode and demonstrate its applications in nondestructive examination and medical fields.