Citric acid-assisted phase controlled synthesis of NaYF4:Yb3+,Tm3+ crystals and their intense ultraviolet upconversion emissions

The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980 nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.