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

Abstract

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.