Upconversion luminescence properties of Yb ³⁺, Gd ³⁺, and Tm ³⁺ Co-Doped NaYF ₄ microcrystals synthesized by the hydrothermal method

Yb ³⁺/Gd ³⁺/Tm ³⁺ co-doped prism-like NaYF ₄ 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 Tm ³⁺ ions, ⁶D _J, ⁶I _J and ⁶P _J → ⁸S _7/2 UC emissions of Gd ³⁺ were observed. During the emission process of Gd ³⁺ ions, both Yb ³⁺ and Tm ³⁺ ions served as sensitizers. In the complex excitation and frequency UC processes, excited state absorption of Gd ³⁺ ions, cross relaxation energy transfer (CRET) between two excited Gd ³⁺ ions, and energy transfer (ET) between Gd ³⁺ and Yb ³⁺ or Tm ³⁺ ions played key roles for the UC emissions of Gd ³⁺ ions.