Strategies for broadening the emission spectra of Cr3+-doped near-infrared emitting phosphors

Cr³⁺-activated near-infrared (NIR) phosphors have significant application potential in food detection and analysis of biological fluids and tissues components owing to the large spectrum overlap between Cr³⁺ emission and the absorption bands of water and organic groups. To realize real-time, convenient and multi-component detection, a wideband NIR phosphor with a large bandwidth (FWHM > 200 nm) is urgently required to effectively cover the absorption band of the target analyte. This review simply offers a retrospect of the researches on existing Cr³⁺-doped wideband phosphors and summarizes the key strategies, including crystal field regulation, lattice site engineering, Cr³⁺–Cr³⁺ pairs, Cr³⁺/Cr⁴⁺ double fluorescence centers, energy transfer process and the new emission centers resulting from the lattice distortion caused by Cr³⁺ doping, for broadening the Cr³⁺ bandwidth. The feature of the Cr³⁺ emission is also discussed using the Tanabe–Sugano energy level diagram and configuration coordinate model. In addition, the existing problems and future prospects of Cr³⁺-activated wideband emission phosphors are elucidated, providing a reference to broaden the bandwidth of Cr³⁺ for the development of efficient and stable wideband near-infrared phosphors.