Phase transition and upconversion luminescent properties of NaYF₄: Yb, Er@SiO₂ at high temperature

Hexagonal phase NaYF₄: Yb, Er upconversion nanocrystal with the size of around 27 nm were fabricated via solid-liquid thermal decomposition method. NaYF₄: Yb, Er@SiO₂ core-shell nanoparticles were prepared, with the thickness of shell layer about 8 nm. NaYF₄: Yb, Er@SiO₂ were annealed at different temperatures for 3 h respectively. The hexagonal phase NaYF₄ transited to amorphous phase instead of other phase as the annealing temperature increased to 600 ℃. When it further increased to 700 ℃ or 800 ℃, the NaYF₄ reacted with SiO₂ shell and formed a new phase, NaYSiO₄. The samples were excited by the 980 nm infrared laser and showed strong visible upconversion emission. The original NaYF₄: Yb, Er@SiO₂ were taken as comparison samples, the luminescence intensity of samples increased successively with the increase of annealed temperature. This is not consistent with the higher luminescence intensity of fluoride with low phonon energy. The possible reason is that some lattice defects and organic impurities were removed from the samples annealed at high temperature.