Tuning hexagonal NaYbF₄ nanocrystals down to sub-10 nm for enhanced photon upconversion

Enhancing upconversion emission is critical for small-sized lanthanide doped upconversion nanocrystals. A promising way is increasing the doping concentration of excitation energy absorbers, the Yb³⁺ sensitizer. However, it is still a challenge to obtain small-sized hexagonal NaLnF₄ (Ln: lanthanide) upconversion nanocrystals with a high Yb³⁺ concentration due to the fast growth of NaYbF₄ crystals, which hinders their applications particularly in biology. We here demonstrate a highly repeatable and controllable method for tuning the size of hexagonal NaYbF₄ nanocrystals, down to ∼7 nm, without the assistance of additional impurity doping. By monitoring the reaction process, we found that ultrasmall hexagonal NaYbF₄ nanocrystals were formed through an in situ transformation of their cubic counterparts. We observed an enhanced upconversion emission of NaYbF₄:Tm nanocrystals when compared to that of NaYbF₄:Y/Tm nanocrystals with less Yb³⁺ doping. After coating a thin layer of a NaYF₄ shell on NaYbF₄:Tm nanocrystals, a ∼100 times upconversion emission enhancement with over 800 times stronger emission in the ultraviolet and blue ranges was observed. This versatile method, together with the strong upconversion emission of the as-prepared ultrasmall nanocrystals, should facilitate the future applications of upconversion nanocrystals.