Upconversion of Yb3+/Ho3+ co-doped bismuth oxyfluoride and fluoride microcrystals for high-performance ratiometric luminescence thermometry

Ratiometric luminescence thermometry is awfully applicable for non-contact temperature detection. Lanthanide luminescent materials available for high-performance ratiometric thermometry are still highly desirable until now. Herein, two novel upconversion phosphors, Yb3+/Ho3+ co-doped BiOF and BiF3 microcrystals, were synthesized and investigated systematically in terms of their crystalline structure, room temperature and temperature-dependent upconversion properties. Under 980 nm laser excitation, both microcrystals bring forth the characteristic emissions of Ho3+, with an identical overall luminescent color of green, while the intensity is much higher in BiF3:Yb/Ho microcrystals. When increasing temperature, the upconversion intensity at various wavelengths for both microcrystals is reduced, meanwhile fluorescence intensity ratios of both 545/656 nm and 752/656 nm decrease highly regularly, establishing a ratiometric thermometry therefore. BiF3:Yb/Ho microcrystals were demonstrated to have significantly superior thermometric performance than BiOF:Yb/Ho, and a high relative sensitivity up to 5.41% K-1 as well as a wide temperature sensing range of 298-673 K were obtained.