Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4:Ln3+ (Ln = Eu and Yb/Tm) microcrystals

Cubic and hexagonal NaYF4:Ln3+ (Ln = Eu and Yb/Tm) microcrystals were separately synthesized by an EDTA-assisted solvothermal method. Under 393-nm excitation, the emission spectra of NaYF4:Eu3+ microcrystals show the characteristic Eu3+ emissions. In the excitation spectra of the 610-nm emission, the 7F0 → 5D3 is dominant for the cubic microcrystals, while the 7F0 → 5L6 is dominant for the hexagonal sample. In addition, unusually strong ultraviolet emissions (1I6 → 3H6, 1I6 → 3F4, and 1D2 → 3H6) were observed in the hexagonal NaYF4:Yb3+/Tm3+ microcrystals under 980-nm excitation. In comparison with a cubic sample having the same chemical compositions, the hexagonal microcrystals had a markedly enhanced ability of emitting ultraviolet up-conversion luminescence.