Impurity induced controlled growth of a NaGdF4 nanostructure by a core-shell approach

Doping, incorporating appropriate and functional atoms or ions into host lattices of colloid nanoparticles, is an important approach to modifying the shapes and sizes of nanoparticles and endowing them with new and useful properties. Herein, we demonstrate a "core-shell" synthetic strategy to control the shape of core-shell structured NaGdF4 upconversion nanoparticles (UCNPs) by introducing an impurity dopant in the core region. We introduced Cr3+ ions in the core region of NaGdF4 to synthesize "Jasminum sambac flower" shaped non-classic core-shell NaGdF4:Cr3+@NaGdF4:Yb3+,Tm3+ UCNPs. First of all, the effect of doping concentration on the shape of NaGdF4 core was investigated. Then, the core-shell strategy was applied to obtain a shape-controlled hierarchical nanostructure, and the effects of Cr3+ concentration on the phases, shapes and sizes of the products were investigated in detail. TEM images indicate that NaGdF4:Cr3+@NaGdF4:Yb3+,Tm3+ nanocrystals with Cr3+ concentration of 40-50% show highly uniform "Jasminum sambac flower" shaped structures. TEM images and XRD patterns of NaGdF4:Cr3+@NaGdF4: Yb3+,Tm3+ at different reaction times indicate that, initially, the flower shaped products are cubic phase and the size of the flowers is about 50 nm; with an increase of the reaction time, the cubic phase nano crystals discompose to small ones and then transform to a bigger hexagonal phase "Jasminum sambac flower". This "core/shell" doping strategy presents a general route to achieving controlled morphology in lanthanide-doped nanocrystal systems.