Synthesis of Er(III)/Yb(III)-doped BiF₃ upconversion nanoparticles for use in optical thermometry

The authors describe an ethylene glycol assisted precipitation method for synthesis of Er(III)/Yb(III)-doped BiF₃ nanoparticles (NPs) at room temperature. Under 980-nm light irradiation, the NPs emit upconversion (UC) emission of Er(III) ions as a result of a two-photon absorption process. The temperature-dependent green emissions (peaking at 525 and 545 nm) are used to establish an unambiguous relationship between the ratio of fluorescence intensities and temperature. The NPs have a maximum sensitivity of 6.5 × 10^-3 K^-1 at 619 K and can be applied over the 291-691 K temperature range. The results indicate that these NPs are a promising candidate for optical thermometry. Graphical abstract Schematic of the room-temperature preparation of Er(III)/Yb(III)-doped BiF₃ nanoparticles with strongly temperature-dependent upconversion emission.