Systematic synthesis and optical property studies of monodisperse NaYF₄: Yb³⁺, Er³⁺ nanocrystals with pure hexagonal phase

Near infrared (NIR)-to-visible upconversion nanoparticles of pure hexagonal phase β-NaYF₄: Yb³⁺, Er³⁺ nanocrystals were successfully synthesized by a high boiling temperature organic phase synthesis strategy in oleic acid (OA), which acted as a ligand. The upconversion photoluminescence intensity of the nanocrystals was modulated by varying the reaction temperature and the concentration of sensitizer Yb³⁺ ions and activator Er³⁺ ions. The prepared nanocrystals were investigated by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and photoluminescence spectroscopy. All the results show that the as-prepared monodispersed upconversion nanocrystals emit high photoluminescence when excited by a 980 nm laser. It was found that the up-conversion emission spectra of β-NaYF₄: Yb³⁺, Er³⁺ presented three peaks at about 655, 540, and 521 nm. The optimized concentrations of the sensitizer Yb³⁺ and the activator Er³⁺ are 20% and 2% mmol, respectively. These highly efficient nanocrystals have exhibited great potential in bioimaging and photoelectronic devices.