Upconversion luminescence properties of Yb3+, Gd3+, and Tm3+ co-doped NaYF4 microcrystals synthesized by the hydrothermal method.

Yb3+/Gd3+/Tm3+ co-doped prism-like NaYF4 microcrystals were synthesized through a facile ethylene diamine triacetic acid (EDTA)-assisted hydrothermal method. The microcrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), upconversion (UC) emission spectra, and decay curve. After annealing in an argon atmosphere, the microcrystals emitted bright blue and intense ultraviolet (UV) light under a 980-nm continuous wave diode laser excitation. Besides the UC emissions of the Tm3+ ions, 6D(J), 6I(J) and 6P(J) --> 8S7/2 UC emissions of Gd3+ were observed. During the emission process of Gd3+ ions, both Yb3+ and Tm3+ ions served as sensitizers. In the complex excitation and frequency UC processes, excited state absorption of Gd3+ ions, cross relaxation energy transfer (CRET) between two excited Gd3+ ions, and energy transfer (ET) between Gd3+ and Yb3+ or Tm3+ ions played key roles for the UC emissions of Gd3+ ions.