The upconversion (UC) phenomenon is one of the most frequently investigated in lanthanide ions (Ln 3+ )-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., Yb 3+ /Er 3+ . Exceptional UC luminescence can also be presented by systems doped with one type of Ln 3+ ions, which are characterized by self-sensitization properties, for instance, Er 3+ ions. Herein, the structural and spectroscopic properties of SrF 2 nanoparticles (NPs) doped with Er 3+ ions in concentration from 2.1 to 29.0% were investigated. The SrF 2 :Er 3+ 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 SrF 2 :11.7% Er 3+ NPs under 975 nm excitation and for SrF 2 :4.7%Er 3+ 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 2 H 11/2 and 4 S 3/2 thermally coupled levels in SrF 2 :Er 3+ were evaluated in the 273−373 K temperature range. The interesting temperature dependencies of the UC emission from 4 I 9/2 and 4 I 11/2 states were registered; these have not been previously reported.

Recently, the up-converting (UC) materials, containing lanthanide (Ln3+) ions have attracted considerable attention because of the multitude of their potential applications. The most frequently investigated are UC systems based on the absorption of near-infrared (NIR) radiation by Yb3+ ions at around 975-980 nm and emission of co-dopants, usually Ho3+, Er3+ or Tm3+ ions. UC can be observed also upon excitation with irradiation with a wavelength different than 975-980 nm. The most often studied systems capable of UC without the use of Yb3+ ion are those based on the properties of Er3+ ions, which show luminescence resulting from the excitation at 808 or 1532 nm. However, also other Ln3+ ions are worth attention. Herein, we focus on the investigation of the UC phenomenon in the materials doped with Ho3+ ions, which reveal unique optical properties upon the NIR irradiation. The SrF2 NPs doped with Ho3+ ions in concentrations from 4.9% to 22.5%, were synthesized by using the hydrothermal method. The structural and optical characteristics of the obtained SrF2:Ho3+ NPs are presented. The prepared samples had crystalline structure, were built of NPs of round shapes and their sizes ranged from 16.4 to 82.3 nm. The NPs formed stable colloids in water. Under 1156 nm excitation, SrF2:Ho3+ NPs showed intense UC emission, wherein the brightest luminescence was recorded for the SrF2:10.0%Ho3+ compound. The analysis of the measured lifetime profiles and dependencies of the integral luminescence intensities on the laser energy allowed proposing the mechanism, responsible for the observed UC emission. It is worth mentioning that the described SrF2:Ho3+ samples are one of the first materials for which the UC luminescence induced by 1156 nm excitation was obtained.

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