Size-Dependent Maximization of Upconversion Efficiency of Citrate-Stabilized β-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 Yb(3+),Er(3+)- doped β-NaYF4. In this study, annealing is shown to have a significant impact on the phase, morphology, and upconversion luminescence of β-NaYF4:Yb(3+),Er(3+) crystals of varying sizes (300 nm, 700 nm, and 2.3 μm, 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 α-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 β-NaYF4:Yb(3+),Er(3+) crystals, while maintaining particle morphology, which can be very important for technological application.