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Abstract: 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. [Copyright &y& Elsevier]

Abstract: The upconversion luminescent properties of YF3:Yb3+(20%)/Tm3+(1%) nanobundles with different sizes (240–500nm in length) were studied under 980-nm excitation. Ultraviolet (1I6 → 3F4/3H6 and 1D2 → 3H6), blue (1D2 → 3F4 and 1G4 → 3H6), red (1D2 → 3H4, 1G4 → 3F4, and 3F3 → 3H6), and near infrared (3H4 → 3H6) emissions were observed. The results indicated that the relative intensity of the ultraviolet to the blue as well as the blue to the near infrared increased with decreasing the size of nanobundles. Especially, the position of the dominant red emission peak varied with the size of nanobundles. As the length of nanobundles increased to 500nm, unusual 3F3 → 3H6 transition was observed, which was theoretically explained considering the decrease of the nonradiative transition rate of 3F3 → 3H4. [Copyright &y& Elsevier]