NIR-to-NIR and NIR-to-Vis up-conversion of SrF:Honanoparticles under 1156 nm excitation.

Recently, the up-converting (UC) materials, containing lanthanide ions (Ln)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 Ybions at around 975-980 nm and emission of co-dopants, usually Ho, Eror Tmions. 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 Ybion are those based on the properties of Erions, which show luminescence resulting from the excitation at 808 or 1532 nm. However, also other Lnions are worth attention. Herein, we focus on the investigation of the UC phenomenon in the materials doped with Hoions, which reveal unique optical properties upon the NIR irradiation. The SrFNPs doped with Hoions in concentrations from 4.9% to 22.5%, were synthesized by using the hydrothermal method. The structural and optical characteristics of the obtained SrF:HoNPs 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, SrF:HoNPs showed intense UC emission, wherein the brightest luminescence was recorded for the SrF:10.0%Hocompound. 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 SrF:Hosamples are one of the first materials for which the UC luminescence induced by 1156 nm excitation was obtained.