Fast synthesis of sub-10 nm β-NaYF4:Yb3+, Er3+@NaYF4 core-shell upconversion nanocrystals mediated by oleate ligands

Ultrasmall (∼5.3 nm) and monodisperse β-NaYF4:Yb3+, Er3+ nanocrystals were successfully synthesized by introducing oleate anions (OA−) as ligands, based on which the highly bright sub-10 nm β-NaYF4:Yb3+, Er3+@NaYF4 core-shell nanocrystals were obtained. [Display omitted] •Fast synthesize ultra-small β-NaYF4:Yb3+, Er3+ UCNCs with 0 min reaction or down to 5.3 nm.•Highly bright sub-10 nm β-NaYF4:Yb3+, Er3+ core-shell NCs were obtained.•OA− ligands cannot only promote the α-to-β phase transition, but also inhibit the crystal growth.•OA− ligands manipulated phase transition and size evaluation is also applicable to other UCNCs. We developed a facile approach for the fast synthesis of ultrasmall and monodisperse β-NaYF4:Yb3+, Er3+ nanocrystals down to 5.3 nm with the assistant of oleate ligands, based on which the highly bright sub-10 nm β-NaYF4:Yb3+, Er3+@NaYF4 core-shell nanocrystals with three order of magnitude increased upconversion emission were successfully obtained by a seed-mediated growth method. The reaction time and temperature dependent experimental results implied that the introduciton of extra oleate ligands can both promote the α-to-β phase transition and impeded the crystal growth of NaYF4, which is due to the oleate ligand induced orderly arrangement of Y3+ and capping effect, respectively. It is expected that the oleate ligands mediated phase transition and size evaluation, demonstrated here in NaYF4:Yb3+, Er3+ NCs, is also applicable to other lanthanide-doped upconversion nanocrystals.