Highly intense upconversion luminescence in Yb/Er:NaGdF4@NaYF4 core–shell nanocrystals with complete shell enclosure of the core
The purpose of the present work is to demonstrate that the hexagonal Yb/Er:NaGdF4 core size has a great impact on the completeness of the NaYF4 shell covering the core. With the increase of core size, the morphology of core–shell nanocrystals evolves from nanosphere to hexagonal nanoprism and finally to hexagonal nanoplate and, impressively, compared to that on the lateral faces of cores, the shell thickness on the top/bottom faces of cores becomes thinner. The shell growth mechanism is proposed to be the separate nucleation of the shell precursors followed by their ripening-mediated deposition on the cores. Based on this mechanism, controlling the thickness and completeness of the shell on the core can be easily realized by adopting appropriate size cores, which results in the intensification of upconversion luminescence of core–shell nanocrystals 10000 times as high as that of the core-only ones. Generally, these new findings should be beneficial for designing and fabricating complex core–shell architecture and the understanding of structure-related properties of core–shell nanocrystals.