NIR-to-NIR and NIR-to-Vis up-conversion of SrF₂:Ho³⁺nanoparticles under 1156 nm excitation

Recently, the up-converting (UC) materials, containing lanthanide ions (Ln³⁺)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 around 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 SrF₂NPs 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.