Controlled synthesis of ultrasmall hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals with enhanced upconversion luminescence

Ultrasmall upconversion (UC) nanoparticles with intense luminescence are quite desirable for in vitro and in vivo bioimaging. Herein, we report the successful synthesis of sub-10 nm hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals via a high boiling solvent process. The sizes and phases of the crystals can be finely tailored by tuning the amounts of the ligands used (octadecylamine and sodium oleate) and the temperature. The intensity of the UC emission of the nanocrystals increases as the the relative content of Yb3+ ions increases from 20% to 98%, owing to improved near-infrared absorption and energy-transfer from Yb3+ to Tm3+. The experimental results indicate that hexagonal NaYbF4 is the best host for ultrasmall Tm3+-doped UC nanoparticles and shows great potential in biomedical applications.