NIR‐II Luminescence in Cr4+ Activated CaYGaO4 toward Non‐Invasive Temperature Sensing and Composition Detection

Abstract Near‐infrared (NIR) II region (1000–1700 nm) lighting sources possess special potential for temperature sensing and quantitative composition analysis. However, it is a challenge to tune the luminescence toward such long wavelength NIR‐II emission. Herein, Cr 4+ ions occupation is designed and verified in olivine‐type CaYGaO 4 as [CrO 4 ] tetrahedron by site occupancy confinement. Under 620 nm red light excitation, the phosphors display broadband NIR‐II emission centered at 1330 nm with a full width at half maximum (FWHM) of 233 nm, resulting from spin‐allowed 3 T 2 to 3 A 2 transition of Cr 4+ ions in T d symmetry. The optical thermometry application is demonstrated by decoding the spectral shift and decay times varying with temperature, and the maximum relative sensitivities are 5.82% and 0.78% K −1 , respectively. Furthermore, a flexible film containing CaYGaO 4 :Cr 4+ is fabricated to implement the non‐invasive detections of fillet temperature and NH 3 concentration in ammonia. This work represents a step toward developing next‐generation NIR‐II luminescence materials and widens non‐invasive detection applications.