Tuning crystal field symmetry of hexagonal NaY0.92Yb0.05Er0.03F4 by Ti4+ codoping for high-performance upconversion

Abstract

378nm, 408nm and 521nm upconversion emissions of Er3+ ions were obviously enhanced by Ti4+ codoped with Yb3+/Er3+ in hexagonal NaYF4, and the corresponding upconversion luminescence lifetimes were also prolonged, especially for 378nm and 408nm emissions. X-ray powder diffraction, field emission scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscopy and upconversion emission spectra were employed to explore the relationships of the structure and properties. From these characterizations we made a novel discovery that Na+ could be induced to occupy Y3+ sites for establishing valence balance of the system if Ti4+ ions were codoped with appropriate concentration. As a result the crystal field asymmetry of NaY0.92Yb0.05Er0.03F4 was enhanced and then its upconversion properties were improved because the hypersensitive electron transition of Yb3+/Er3+ ions was promoted greatly. At the same time, the crystal sizes of the codoped NaYF4 became smaller because the crystal growth was prevented by more negative charges gathering at the crystal surface. This study provides an exploration of the relationship among impurity doping, structural changes and upconversion performance, which may be useful for design and synthesis of high-performance upconversion codoping materials.