Unravelling phase and morphology evolution of NaYbF₄ upconversion nanoparticles via modulating reaction parameters

The applications of upconversion nanoparticles in biodetection, bioimaging, and lighting have greatly fuelled a growing demand for enhancing their brightness. The brightness of upconversion nanoparticles is fundamentally limited by the low doping concentration of either activator or sensitizer ions, in spite of the progress of rational controlling nanoparticles to improve performance. Although NaYbF₄ nanoparticles possessing the largest amount of sensitizers (Yb³⁺) have been proposed as a promising alternative host to achieve bright upconversion luminescence, systemic explorations in synthesizing NaYbF₄ nanoparticles have scarcely been performed. In this work, we report the systematic synthesis and fine control of monodisperse NaYbF₄ nanoparticles through a canonical high-temperature co-precipitation method. The phase and morphology of NaYbF₄ nanoparticles can be rationally tuned, through modulating the amount of NH₄F, amount of NaOH, temperature, and time, respectively. The heating rate has also been found to exert an effect on the phase and morphology evolution of NaYbF₄ nanoparticles for the first time. The size of NaYbF₄ nanoparticles can be modulated from hundreds to several nanometers, in addition to the variation of their phase and morphology. The optical properties of NaYbF₄:Er upconversion nanoparticles obtained under different reaction parameters have been investigated, and moreover, their potential applications in imaging deep tissue and photodynamic therapy have been explored. Unravelling the phase and morphology evolution of NaYbF₄ nanoparticles is expected to hold great potential for boosting the brightness of upconversion nanoparticles.