2.1 Materials
Calcium fluoride (CaF2) crystals were doped with trivalent ytterbium ions (Yb³⁺). During optimization of the synthesis procedure the value of Yb3+ was set at 2%. Finally, the concentration series with varying Yb3+ dopant concentration was synthesized. CaF₂ was selected as a host material owing to its exceptionally high optical transparency over a broad spectral range, spanning from the ultraviolet to the infrared as well as due to low maximum phonon energies, which significantly suppress non-radiative relaxation processes and enhance radiative efficiency, thereby favoring anti-Stokes luminescence.
2.2 Reagents
The materials were synthesized using commercially available reagents without any further purification. Ytterbium oxide (99.9%), Calcium nitrate hydrate (99.997%), Pluronic F-127, Sodium tetrafluoroborate (98%), sodium hydroxide (98%), sodium fluoride (99.99%) were purchased from Sigma-Aldrich (Merck). Calcium chloride anhydrous (reagent grade) was purchased from AKTYN, nitric acid (65%) hydrochloric acid (35%-38% analytical grade) were purchased from Chempur, sodium citrate hydrate was purchased from Biomus.
2.3 Synthesis methods
Yb³⁺- doped CaF₂ nano- and microcrystals were synthesized using three different approaches.
Hydrothermal synthesis from solution in a laboratory oven or in microwave reactor
A stoichiometric amount (0.01 mmol) of Yb2O3 was used to prepare ytterbium nitrate Yb(NO3)3 precursor. Yb2O3 was mixed with nitric acid and deionized water and heated at 110°C for 2 hours until complete dissolution and formation of Yb(NO₃)₃. After nitrate formation, the remaining solvents were evaporated. Calcium nitrate, Ca(NO3)2 (0.98 mmol), was dissolved in 8 mL of deionized water and subsequently added to as-prepared ytterbium nitrates solution. The resulting mixture was ultrasonicated for 10 min to ensure homogeneity. Subsequently,
an aqueous solution (10 mL) of NaBF4 (4 mmol) was added under continuous stirring. At this stage, depending on a specific batch, an appropriate amount of surfactant was added to control particle growth and morphology. The synthesis was performed without the surfactant or with 0.1 mg, 1 mg, 0.5 g or 1 g Pluronic F-127 or with 1g sodium citrate. For series of crystals synthesized in different pH values total amount of nitrates was 0.75 mmol. The amounts of the other reagents were reduced proportionally. The resulting solution was stirred for 30 min, during which the pH value was adjusted to the desired level. Then, the mixture was transferred to the Teflon 25 mL reaction vessel and subjected either to conventional hydrothermal treatment in a laboratory oven or to microwave-assisted hydrothermal treatment. In both cases, the reaction was carried out at 180 °C for a specified duration of 3 h 30 min. or 12 h. After completion of the reaction, the autoclave was allowed to cool naturally to room temperature. The resulting particles were collected by centrifugation and thoroughly washed with deionized water and ethanol. Finaly, the particles were dispersed in deionized water for further characterization.
Precipitation method
In the precipitation method, stoichiometric amount of YbCl3 and CaCl2 (total 1 mmol) were dissolved in 7 cm3 deionized water. In parallel, NaF (2 mmol) was dissolved in 7 cm3
of deionized water. The two solutions were then combined and transferred into a Teflon reaction vessel. The reaction mixture was maintained in a laboratory oven at an elevated temperature
of either 80 °C or 180 °C for 12 h. Afterwards, the mixture was naturally cooled down to room temperature, and the particles were collected by centrifugation and washed with deionized water and ethanol. Finally, the purified particles were dispersed in deionized water for further characterization.
2.4 Crystal phase purity verification
The crystal phase purity of all synthesized particles was examined by powder X-ray diffraction (PXRD) using Bragg–Brentano geometry PROTO AXRD diffractometer equipped with Cu Kα₁ radiation (30 kV, 20 mA). The recorded diffraction patterns were compared with the reference diffraction data for the cubic CaF₂ crystal structure to confirm phase purity.
2.5 Morphology characterization
The morphology, size and elemental analysis of the synthesized crystals were investigated by transmission electron microscopy (TEM) using a Thermo Fisher Scientific Talos F200i microscope operated at an accelerating voltage of 200 kV. Particle size distributions were determined from TEM images using ImageJ software by statistical analysis of a representative number of particles. Elemental analysis of the particles was carried out by the energy-dispersive X-ray spectroscopy (EDS), in Scanning Transmission Electron Microscopy (STEM) mode, with High-Angle Annular Dark-Field (HAADF) imaging.
EDS elemental maps were acquired to evaluate the spatial distribution of Ca2+, F-, and Yb3+ ions within the CaF₂:Yb³⁺ crystals. Maps were processed with pre-filtering with Gaussian blur (sigma 3.0).
2.6 Anti-Stokes luminescence measurements
For anti-Stokes luminescence measurements, a droplet of the CaF₂:Yb³⁺ microcrystal suspension was deposited onto a glass coverslip and allowed to dry. An adhesive spacer was placed around the sample to maintain a fixed separation, and the crystals were subsequently sealed with a microscope glass plate. All measurements were performed on individual microcrystals at room temperature using an optical microscope (Nikon Ti2, with 60x, NA=0.6 objective). The samples were excited using a laser diode with Distributed Feedback (DFB) stabilization (AeroDiode), and the resulting anti-Stokes luminescence was measured using a spectrometer (Andor Kymera 328i with grating and iDus camera). Each spectrum accumulated 2x with an exposure time equal . The luminescence intensity was recorded as a function of the excitation laser diode power.
2.7 Raman spectroscopy
For Raman spectroscopy measurements, a droplet of a diluted suspension of CaF₂:Yb3+ microcrystals was deposited onto a glass coverslip and allowed to dry. An adhesive spacer was placed around the deposited microcrystals to prevent mechanical contact and ensure a defined sample thickness. The sample was then sealed with a microscope glass slide and mounted on a temperature-controlled microscope holder. Raman measurements were performed on individual CaF₂ microcrystals using an optical microscope (Nikon Ti2, with 60x, NA=0.6 objective). Each microcrystal was illuminated using the laser (Cobolt) with constant output power equal to , and the resulting Raman scattering was recorded as a function of temperature using a spectrometer (Andor Kymera 328i with grating and iDus camera). Each spectrum was accumulated 2x with an exposure time equal or , depending on a sample. These measurements enabled the evaluation of temperature-dependent changes in the Raman spectra.
