Recent Developments on Broadband Near-Infrared Luminescent Materials Activated by 3d Transition Metal Ions

Crystalline and noncrystalline materials activated with 3d transition metal ions (TMIs) featured by rich spectral characteristics are widely used as phosphors as well as laser gain media. In recent years, the broadband NIR LED is regarded as an ideal miniaturized light source for NIR spectrometers, which can be easily integrated in portable electronic devices in food security and biomedicine. The near-infrared (NIR) phosphors activated by TMIs like Cr³⁺ ions are considered as a key stage in the development of highly efficient broadband NIR LEDs. Unlike the parity forbidden f–f transitions of trivalent rare-earth ions, the d–d transitions of 3d TMIs exhibit large bandwidth and tunable emission wavelengths due to the intense coupling with the surrounding crystal field. At present, materials doped by TMIs except Cr³⁺ ions are rarely explored for NIR applications. In this review, the spectral characteristics associated with the d–d transition of 3d TMIs were elaborated, and recent development on TMI activators and phosphor matrix for a broadband NIR photoluminescence was represented. In addition, this review also provided a brief summary for the factors that need to be considered in the design of broadband NIR phosphors. This review is expected to offer useful guidelines for the development of advanced NIR pc-LEDs.