Synthesis of small-sized hexagonal NaREF4 (RE = Yb, Lu) nanocrystals through accelerating phase transformation

Abstract

Heavy lanthanide element-based NaREF4 nanocrystals have recently been proposed as a promising replacement of classical hexagonal NaYF4 for their efficient luminescence and excellent X-ray attenuation property. However, synthetic control over their particle size remains challenging due to the fast growth and slow nucleation. Herein, a simple and robust approach for the controlled synthesis of small NaYbF4 and NaLuF4 nanocrystals is presented as well as a mechanistic investigation of the nanocrystal growth process. The injection order of NaOH and NH4F plays critical role in controlling the nanocrystal growth by accelerating phase transformation from cubic phase to hexagonal phase. We also verify experimentally that control of the hexagonal-phase nucleation rather than growth process is crucial for controlling the final particle size of NaYbF4 and NaLuF4. After coating an inert shell, we found enhanced downshifting emission of NaYbF4:Er nanocrystals compared with less Yb3+ doping. The results shown here provide a reliable method for preparing small-sized hexagonal NaYbF4 nanocrystals with enhanced downshifting emission, which favors their further applications in bioscience.