Oleylamine-Mediated Synthesis of Small NaYbF4 Nanoparticles with Tunable Size

Hexagonal phase NaYbF4 has recently been reorganized as a more efficient host material than NaYF4 for constructing multiphoton upconversion nanoparticles. However, the synthesis and size control of NaYbF4 nanoparticles have not been completely fulfilled. This study presents a controlled synthesis of small NaYbF4 nanoparticles as well as a mechanistic investigation of the nanocrystal growth process. The NaYbF4 nanoparticles were synthesized in a ternary solvent mixture composed of oleylamine, oleic acid, and 1-octadecene by an injection technique. The oleylamine molecule as a surface capping ligand is found to play critical roles in controlling the growth of NaYbF4 nanoparticles by promoting conversion of the cubic phase intermediates into the hexagonal phase products. Uniform NaYbF4 nanoparticles with tunable size (from 7 to 70 nm) were readily prepared by controlling a single variable of solvent composition. After coating with NaLuF4 shells of 10 nm in thickness, the small NaYbF4:Er (2%) nanoparticles exhibited an over 700-fold enhancement in red upconversion emission. By introducing Yb3+ ions into the shells, the oleate-capped NaYbF4:Er (2%)@NaLuF4:Yb (25%) upconversion nanoparticles showed an inverse thermal quenching above room temperature. The findings described here are expected to provide a general strategy for facile control of particle size and optical property in other nanomaterials systems.