Solvothermal synthesis, cubic structure and multicolor upconversion emission of ultrasmall monodisperse lanthanide-doped BaYF5 nanocrystals

Lanthanide (Ln3+) doped BaYF5 (Ln=Yb3+, Er3+, Tm3+) nanocrystals (NCs) with a mean size of approximately 10 nm are synthesized by a solvothermal method using oleic acid as a stabilizing agent at 210 °C. The size of BaYF5 NCs can be controlled by simply tuning the reaction parameters such as reaction temperature, reaction time and the molar ratio of F−/Y3+. The detailed structure investigation reveals that the as-synthesized BaYF5 NCs are in the cubic structure with space group Fm3¯m instead of the reported tetragonal structure. Ln3+ cations occupy crystal lattice positions with lower point symmetry, which may lead to high upconversion efficiency under the excitation of a 980 nm diode laser. By adjusting the dopant concentrations of Yb3+, Er3+ and Tm3+, intense near-infrared, blue, yellow and white upconversion emissions are readily realized, respectively. The desirable property of the ultrasmall monodisperse NCs makes it the promising material for the applications in miniaturized solid-state light sources, multicolor three-dimensional display devices and fluorescent labels for biomedicine imaging.