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

Tm3+ and Yb3+ ions co-doped alpha-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 alpha-NaYF4 NCs and their intensities were much higher than those of the beta-NaYF4 NCs modified by trisodium citrate. The intensities of the 5-photon UC emissions from the I-1(6) level of Tm3+ ions were much stronger than those of the 4-photon UC emissions from the D-1(2) level and the 3-photon UC emissions from the (1)G(4) level. It is for the first time, to our knowledge, that such intense UV UC emissions were obtained with alpha-NaYF4 NCs. The intense UV UC emissions and the relatively long fluorescent lifetimes for the I-1(6), D-1(2), and (1)G(4) levels of Tm3+ ions in alpha-NaYF4 NCs indicate that they could be one of the promising candidates for potential application in short-wavelength solid-state lasers.