Luminescent properties and phase transition in Er~(3+)-Yb~(3+) co-doped NaYF_4/SiO_2 core-shell nanoparticles

摘要

We successfully synthesized Er~(3+)-Yb~(3+) doped cubic phase NaYF_4 nanocrystals by a facile solvethermal method. The average size of the as-prepared nanocrystals was about 12 nm based on the observation of scanning electron microscope and transmission electron microscope. NaYF_4/SiO_2 nanoparticles were prepared. The thickness of shell layer was about 7 nm. The as-prepared samples were annealed at different temperature. The cubic phase NaYF_4 did not transit to hexagonal phase, but to amorphous phase as the annealing temperature increased to 700 °C. When temperature further increased to 1100 °C, the amorphous NaYF_4 reacted with SiO_2 shell and formed a new phase, NaYSiO_4. Under the excitation by a 976 nm laser, the Er~(3+)-Yb~(3+) doped NaYF_4, NaYF_4/SiO_2, and NaYSiO4 nanoparticles showed intense visible upconversion emissions. Since the host materials changed from fluoride crystals to amorphous fluoride, then to silicate crystals, the spectral profiles were different. Fluorescent lifetimes obtained from decay curves were applied to analyze the multi-phonon relaxation processes in different hosts. The Er~(3+)-doped upconversion phosphors will be useful for light converting in solar cell applications in the future.