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

Cubic and hexagonal NaYF4:Ln(3+) (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 F-7(0) -> D-5(3) is dominant for the cubic microcrystals, while the F-7(0) -> L-5(6) is dominant for the hexagonal sample. In addition, unusually strong ultraviolet emissions (I-1(6) -> H-3(6), I-1(6) -> F-3(4), and D-1(2) -> H-3(6)) 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. (C) 2008 Elsevier B.V. All rights reserved.