Oleylamine-Mediated Synthesis of Small NaYbF4 Nanoparticles with Tunable Size

Hexagonal phase NaYbF 4 has recently been reorganized as a more efficient host material than NaYF 4 for constructing multiphoton upconversion nanoparticles. However, the synthesis and size control of NaYbF 4 nanoparticles have not been completely fulfilled. This study presents a controlled synthesis of small NaYbF 4 nanoparticles as well as a mechanistic investigation of the nanocrystal growth process. The NaYbF 4 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 NaYbF 4 nanoparticles by promoting conversion of the cubic phase intermediates into the hexagonal phase products. Uniform NaYbF 4 nanoparticles with tunable size (from 7 to 70 nm) were readily prepared by controlling a single variable of solvent composition. After coating with NaLuF 4 shells of 10 nm in thickness, the small NaYbF 4:Er (2%) nanoparticles exhibited an over 700-fold enhancement in red upconversion emission. By introducing Yb 3+ ions into the shells, the oleate-capped NaYbF 4:Er (2%)@NaLuF 4: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.