Citric acid-assisted hydrothermal synthesis of α-NaYF4:Yb3+,Tm3+ nanocrystals and their enhanced ultraviolet upconversion emissions

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Tm3+ and Yb3+ ions co-doped α-NaYF4 nanocrystals (NCs) were prepared by a one step hydrothermal method with citric acid as the organic additive. The morphology of the as-prepared materials maintains a spherical shape when the surfactant amount, hydrothermal time, and hydrothermal temperature were varied. Under 980 nm excitation, intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from the α-NaYF4 NCs and their intensities were much higher than those of the β-NaYF4 NCs modified by trisodium citrate. The intensities of the 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than those of the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. It is for the first time, to our knowledge, that such intense UV UC emissions were obtained with α-NaYF4 NCs. The intense UV UC emissions and the relatively long fluorescent lifetimes for the 1I6, 1D2, and 1G4 levels of Tm3+ ions in α-NaYF4 NCs indicate that they could be one of the promising candidates for potential application in short-wavelength solid-state lasers.