Hydro/solvo-thermal synthesis of surface modified NaYF4 co-doped Yb3+/Er3+ up-conversion nanoparticles

Abstract

Surface modified up-conversion rare earth fluorides have attracted attention in recent years. Owing to their unique optical properties they can be used for biomedical application such as bio-detection, fluorescene imaging and in drug delivery systems. Different synthesis methods which generate nano- and micro-crystals with controllable compositions have been reported. For improved control of size, shape and morphology of the particles surfactants or structure directing agents are used. In this work PEG or PVP capped NaYF4 particles were synthesized using hydro/solvo-thermal synthesis at 200 ºC (3h). Their structural, morphological and luminescence characteristics have been studied based on X-ray powder diffractometry (XRPD), Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), scanning and transmission electron microscopy (SEM/TEM) and photoluminescence measurements. Both polymers proved to be a good structure directing agents enabling generation of the well crystalline polymer coated upconverting particles with efficient emissions in visible spectrum. It was shown that generation of the hexagonal P63/m β- NaYF4:Yb3+/Er3+ phase with the most efficient green emission (CIE 0.31, 0.66) is enhanced when PVP is used during synthesis, while formation of the cubic Fm-3m α-NaYF4:Yb3+/Er3+ phase that has a yellowish spectral output (CIE 0.41, 0.56) was observed in the particles produced in the presence of PEG. Increase of the luminescence intensity was achieved with additional particles annealing in argon atmosphere at 400 ºC (5h).