Citric acid-assisted phase controlled synthesis of NaYF₄:Yb ³⁺,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%Tm ³⁺ 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.