Upconverting SrF₂:Er³⁺ Nanoparticles for Optical Temperature Sensors

The upconversion (UC) phenomenon is one of the most frequently investigated in lanthanide ions (Ln3+)-doped phosphors. Usually, to obtain UC luminescence, two types of dopants are used: one is a sensitizer, and the other one is an activator, e.g., Yb3+/Er3+. Exceptional UC luminescence can also be presented by systems doped with one type of Ln3+ ions, which are characterized by self-sensitization properties, for instance, Er3+ ions. Herein, the structural and spectroscopic properties of SrF2 nanoparticles (NPs) doped with Er3+ ions in concentration from 2.1 to 29.0% were investigated. The SrF2:Er3+ samples, synthesized using the hydrothermal method, were characterized by the crystalline structure and sizes of their formed NPs below 18 nm. The UC emission under 975 and 1532 nm excitations was observed, wherein the most intense luminescence was registered for SrF2:11.7%Er3+ NPs under 975 nm excitation and for SrF2:4.7%Er3+ NPs under 1532 nm excitation. The UC mechanisms were proposed on the basis of the measured dependencies of integral luminescence intensities on the laser power densities and luminescence lifetimes. The thermometric properties of ratiometric luminescent thermometers based on luminescence intensity ratios (LIRs) from 2H11/2 and 4S3/2 thermally coupled levels in SrF2:Er3+ were evaluated in the 273–373 K temperature range. The interesting temperature dependencies of the UC emission from 4I9/2 and 4I11/2 states were registered; these have not been previously reported.