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

► Morphology and optical properties of various types of NaYF4:Tm3+/Yb3+ NPs have been investigated. ► Significant elongation of rise and decay times in Tm3+ ions have been found in core-shell structures. ► Temporal slowing down the nucleation processes was crucial for core-shell formation.