Upconversion luminescence and magnetic properties of ligand-free monodisperse lanthanide doped BaGdF5 nanocrystals

Lanthanide (Ln=Yb3+, Er3+and Tm3+) doped monodisperse oleate-capped BaGdF5 nanocrystals with a mean diameter of approximately 18 nm were prepared via liquid–solid–solution method. The cell parameter of the as-prepared cubic BaGdF5 nanocrystals is 5.884 Å, which is different from the reported 6.023 Å (JCPDS 24-0098). When excited by a 980 nm laser, these Ln3+ doped nanocrystals exhibit multi-color up-conversion (UC) emissions including blue, yellow and white, by precisely adjusting the dopant concentration of Yb3+, Er3+ and Tm3+. The oleate ligands capped on the surface of the as-synthesized products, which can be conversed from hydrophobic to hydrophilic along with certain extent of weakening of UC intensity, can be moved by the acid treatment process. The measured field dependence of magnetization (M–H curves) of the BaGdF5 nanocrystals shows excellent paramagnetism. At room temperature, the magnetization of BaGd0.798Yb0.2Tm0.002F5 nanocrystals is 0.9165 emu/g and the magnetic mass susceptibility reaches 6.11×10−5 emu g−1 Oe−1 at 15 kOe. Our results indicate that these bi-functional hydrophilic Ln3+ doped BaGdF5 nanocrystals have potential applications in color displays, bioseparation and optical-magnetic dual modal nanoprobes in biomedical imaging.