Luminescent properties and phase transition in Er3+-Yb3+-co-doped NaYF4/SiO2 core-shell nanoparticles

We successfully synthesized Er3+-Yb3+ doped cubic phase NaYF4 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. NaYF4/SiO2 nanoparticles were prepared. The thickness of shell layer was about 7 nm. The as-prepared samples were annealed at different temperature. The cubic phase NaYF4 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 NaYF4 reacted with SiO2 shell and formed a new phase, NaYSiO4. Under the excitation by a 976 nm laser, the Er3+-Yb3+ doped NaYF4, NaYF4/SiO2, 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 Er3+-doped upconversion phosphors will be useful for light converting in solar cell applications in the future.