Improve the Optical Properties of Yb/Er: NaGdF$_{\mathrm{4}}$ Nanocrystals

Ln$^{\mathrm{3+}}$-doped NaGdF$_{\mathrm{4}}$ UC nanocrystals (NCs) have been widely investigated as bio-imaging and magnetic resonance imaging agents recently. We provide a facile and straightforward strategy to modify the size and uniformity of NaGdF$_{\mathrm{4}}$ NCs \textit{via} alkaline-earth doping for the first time. By increasing of alkaline-earth doping content, the size of NaGdF$_{\mathrm{4}}$ NCs increases gradually, while the size-uniformity is still retained. Importantly, adopting the Ca$^{\mathrm{2+}}$-doped Yb/Er: NaGdF$_{\mathrm{4}}$ NCs as cores, the complete Ca/Yb/Er: NaGdF$_{\mathrm{4}}$@NaYF$_{\mathrm{4}}$ core-shell particles with excellent size-uniformity can be easily achieved. As a result, UC emission intensity of the complete core-shell NCs increases for about 30 times in comparison with that of the cores, owing to the effective surfaces passivation of the Ca$^{\mathrm{2+}}$-doped cores and therefore protection of Er$^{\mathrm{3+}}$ in the cores from the non-radiative decay. To further enhance the upconversion efficiency, we are combining the core-shell NCs and gold NCs into nano-structured optical fiber that provide an unique and novel platform for studying NC UC effects.