Synthesis of highly fluorescent LaF3:Ln3+/LaF3 core/shell nanocrystals by a surfactant-free aqueous solution route

A surfactant-free aqueous solution route has been established for the synthesis of LaF3:Ln3+/LaF3 core/shell nanocrystals (Ln=Ce, Tb, Nd) heated at 75 °C at ambient pressure. All the as-prepared nanocrystals with spherical shape have an average size around 20 nm, and consist of well crystallized hexagonal phases. The X-ray photoelectron spectra was used to confirm that the LaF3 shells have coated the LaF3:Ce3+, Tb3+ cores. Compared with that of the original cores under the same conditions, the emission intensity of the LaF3:Ce3+, Tb3+/LaF3 and LaF3:Nd3+/LaF3 core/shell nanocrystals increased significantly of 120% and 60%, respectively. The quantum yield of the LaF3:Ce3+, Tb3+/LaF3 core/shell nanocrystals reached about 27% in aqueous solution. These results indicate that a significant reduction of the quenching from the surface of the core nanocrystals can be obtained by the synthesis of the core/shell structures, and this method can provide more desirable lanthanide-doped nanocrystals for potential biological applications.