Rapid Synthesis of Sub‐10 nm Hexagonal NaYF(4)‐Based Upconverting Nanoparticles using Therminol(®) 66

We report a simple one‐pot method for the rapid preparation of sub‐10 nm pure hexagonal (β‐phase) NaYF(4)‐based upconverting nanoparticles (UCNPs). Using Therminol(®) 66 as a co‐solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd(3+) and Yb(3+) sensitizer ions and Er(3+) activator ions). The size and phase‐purity of the as‐synthesized core and core–shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X‐ray diffraction, and small‐angle X‐ray scattering studies. In‐depth photophysical evaluation of the UCNPs was pursued by using steady‐state and time‐resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface‐related luminescence quenching effects.