A comparison of morphology, structure and optical properties of ultrasmall, small and core–shell up-converting NaYF4/NaGdF4 nanocrystals co-doped with Tm3+ and Yb3+ ions

A comparison of morphology, structure and optical properties of ultrasmall, small and core–shell up-converting NaYF4/NaGdF4 nanocrystals co-doped with Tm3+ and Yb3+ ions