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.