Upconversion luminescence in monoclinic BaY2F8:Yb3+, Ln3+ (Ln = Er, Ho, Tm) nanoparticles synthesized by hydrothermal method

Upconverting nanoparticles (UCNPs) are among the most investigated lanthanide-doped materials as they offer unique spectroscopic properties, beneficial in bioimaging, medicine, sensors, analytical applications, anticounterfeiting, and many others. One of the most critical factors leading to intense emission is the choice of the chemical compound forming UCNPs, and the type of lanthanide ions (Ln³⁺) used as dopants. Inorganic fluorides are among the best and the most investigated host compounds for Ln³⁺ ions due to their low phonon energy and intense emission of Ln³⁺. This article presents the spectroscopic properties of UCNPs composed of BaY₂F₈, an excellent choice for Ln³⁺ host material. The known reports demonstrate the properties of BaY₂F₈:Ln³⁺ in their form as single crystals or bulk materials. However, obtaining this inorganic fluoride as nanoparticles may extend the available host compounds for the wide range of UCNPs' applications. Here, a method for synthesizing BaY₂F₈:Ln³⁺ nanoparticles using a hydrothermal approach is proposed. Thanks to the determined conditions for the synthesis, the formed UCNPs were single-phased and not larger than 30 nm. As dopants, Yb³⁺ ions were used for sensitizing products for the near-infrared radiation with a wavelength of 960 nm, and Tm³⁺, Ho³⁺, and Er³⁺ as co-dopants responsible for intense, multicolor emission. This article also presents measurement results of excitation spectra and the dependencies of emission characteristics on laser energy. These findings fully characterize the obtained UCNPs and allow for proposing a mechanism responsible for their emission.