Modifying the size and uniformity of upconversion Yb/Er:NaGdF4 nanocrystals through alkaline-earth doping

Abstract

NaGdF₄ is regarded as an ideal upconversion (UC) host material for lanthanide (Ln³⁺) activators because of its unique crystal structure, high Ln³⁺ solubility, low phonon energy and high photochemical stability, and Ln³⁺-doped NaGdF₄ UC nanocrystals (NCs) have been widely investigated as bio-imaging and magnetic resonance imaging agents recently. To realize their practical applications, controlling the size and uniformity of the monodisperse Ln³⁺-doped NaGdF₄ UC NCs is highly desired. Unlike the routine routes by finely adjusting the multiple experimental parameters, herein we provide a facile and straightforward strategy to modify the size and uniformity of NaGdF₄ NCs via alkaline-earth doping for the first time. With the increase of alkaline-earth doping content, the size of NaGdF₄ NCs increases gradually, while the size-uniformity is still retained. We attribute this “focusing” of size distribution to the diffusion controlled growth of NaGdF₄ NCs induced by alkaline-earth doping. Importantly, adopting the Ca²⁺-doped Yb/Er:NaGdF₄ NCs as cores, the complete Ca/Yb/Er:NaGdF₄@NaYF₄ core–shell particles with excellent size-uniformity can be easily achieved. However, when taking the Yb/Er:NaGdF₄ NCs without Ca²⁺ doping as cores, they could not be perfectly covered by NaYF₄ shells, and the obtained products are non-uniform in size. As a result, the UC emission intensity of the complete core–shell NCs increases by about 30 times in comparison with that of the cores, owing to the effective surface passivation of the Ca²⁺-doped cores and therefore protection of Er³⁺ in the cores from the non-radiative decay caused by surface defects, whereas the UC intensity of the incomplete core–shell NCs is enhanced by only 3 times.