Enhancing Upconversion Luminescence of LiYF₄:Yb,Er Nanocrystals by Cd²⁺ Doping and Core-Shell Structure

In this work, we report a phenomenon that upconversion emission intensity of Er³⁺ was enhanced by cadmium. Cd²⁺ was taken with 0, 3, 10, 20, 30, and 40 mol % as a new dopant to codope LiY_0.78Yb_0.20Er_0.02F₄, meanwhile the doping effects on the crystal structure, morphology, and upconversion fluorescence emission are investigated by XRD, Rietveld refinement, TEM, and upconversion spectroscopy analysis methods in detail. The green upconversion emission intensity was enhanced, and the maximum emission intensities were enhanced to twice when 10 mol % Cd²⁺ was codoped into tetragonal LiYF₄, while its decay lifetime was reduced. Additionally, we further designed the synthesis of the homogeneous LiYF₄:20%Yb,2%Er,10%Cd@LiYF₄ of which the emission intensity is 11 times higher than that of the Cd²⁺-free nanocrystals, meanwhile an intriguing morphology of core-shell nanocrystals was obtained. This investigation may be useful for the design and improvement of upconversion materials for fulfilling the prerequisite of the practical application.