Size-Dependent Maximization of Upconversion Efficiency of Citrate-Stabilized ß-phase NaYF4:Yb3+,Er3+ Crystals via Annealing

Upconversion materials show great potential in converting infrared light to visible for many optoelectronic and photovoltaic devices. One of the most promising upconverting materials is Yb3+,Er3+- doped ss-NaYF4 center dot In this study, annealing is shown to have a significant impact on the phase, morphology, and upconversion luminescence of ss-NaYF4:Yb3+,Er3+ crystals of varying sizes (300 nm, 700 nm, and 2.3 mu m, respectively) prepared by hydrothermal synthesis stabilized with sodium citrate. Upconversion luminescence is maximized via annealing while maintaining crystal shape and size dispersity up to a temperature dependent on initial size, with NIR-to-visible quantum yields of 2-5%. Further temperature increases result in growth and agglomeration, increasing luminescence, followed by transformation to the alpha-cubic phase resulting in decreases in overall upconversion performance and shifts to dominant red emission. This study establishes the critical link between annealing temperature and maximal upconversion luminescence in ss-NaYF4:Yb3+,Er3+ crystals, while maintaining particle morphology, which can be very important for technological application.