Size-tunable β-NaYF4:Yb/Er up-converting nanoparticles with a strong green emission synthesized by thermal decomposition

Size-tunable β-NaYF4: Yb/Er (50–400 nm) up-converting nanoparticles (UCNPs) with a strong green emission were successfully prepared by a facile thermal decomposition method in a short period of time. The effects of the reaction time, the oleic acid (OA) content, and the doped Gd3+ concentration on the crystal structure, the size evolution mechanism and the luminescence mechanism of the β-NaYF4: Yb/Er nanoparticles were discussed in detail. Characterization was completed using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution (HR-TEM), scanning electron microscopy (SEM), and photoluminescence (PL). The results reveal that the phase transition was completed at 15 min and the morphology of the NaYF4: Yb/Er UCNPs changed from spherical to large hexagonal disks (200–400 nm) in non-doped Gd3+. An appropriate decrease in the volume ratio of oleic acid (OA) to 1-octadecene (ODE) promoted NaF production, resulting in the formation of the hexagonal phase crystals and the subsequent increase of luminous intensity by 30–40 times. More importantly, the size of the β-NaYF4: Yb/Er decreased with Gd3+ doping, and the UCNPs ultimately transformed into small hexagonal prisms (50 nm) with high monodispersity when the doping concentration of Gd3+ was 60 mmoL. The PL spectra confirmed that the luminous intensity of the UCNPs with doped Gd3+ was much higher than that of the undoped UCNPs. The results show that this route for UCNP fabrication with high luminous efficiency and a small size could be extended to other UCNP systems for multifunctional biomedical applications.