The Synthesis,Surface Modification And Self-asssembly Of NaYF₄:Yb,Er Nanospheres

NaYF₄ is one of the most effective substrates for low lattice energy and transparency in visualble region. Though it has a lot of advantages such as narrow emission peak, long fluorescence lifetime, low toxicity and so on, the low upconversion efficiency and the difficulty to adjust to the luminescence process of the upconversion materials is still the most serious limitation for their applications. Resently, combining upconversion materials and photonic crystals which makes the luminous efficiency and luminous process able to be adjusted has become a focus of study. Currently, most of researches make use of the properties of photonic crystals to controll the light by filling the UCNPs into the cracks of photonic crystals templates or coating the UCNPs on the surface of photonic crystals. Because the photonic crystals has different photonic density of states on the different positions, the function of light controlling is different. In order to be better to controll the light of UCNPs, in this paper, we firstly designed to synthsize uniform NaYF₄:Yb, Er nanospheres. Then the self-assembly of the as-prepared uniform NaYF₄.Yb, Er nanospheres had been systematically researched to obtain the three-dimensional photonic crystals of UCNPs. Due to the UCNPs prepared by thermal decomposition method have less defects and high luminous intensity and prepared by hydrothermal method have mild reaction conditions, various of morphologies and sizes and good dispersion, thermal decomposition method and hydrothermal method were chosen as the research methods in this paper.Firstly, The spherical NaYF₄:Yb, Er nanomaterials were prepared by thermal decomposition in precence of sulphur benzoic acid. However, the size distribution of spheres were broad and dispersion is bad after changing the conditions including the amount of ligand, reaction time and the amount of solvent.Secondly, the NaYF₄:Yb, Er nanospheres with uniform size and good dispersion were synthesized by hydrothermal method in the presence of ethylenediamine tetraacetic acid tetrasodium salt. We found that the spheres formed by particles aggregation and the crystalline phase transformation from cubic to hexagonal was rapid. The reaction time played an important role on the uniformity of nanospheres. According to change the concentration of Re?NO₃?₃, anionic species of rare earth salt, pH value and sodium salts we obtained spheres with diferent sizes. We had prepared 0-NaYF₄:Yb, Er spheres with sizes of 200 nm,220 nm,270 nm,350 nm respectively. The spheres had uniform sizes and high Zeta potential values beyond 20 mV. The self-assembly of NaYF₄:Yb, Er spheres had been studied by a series of methods, such as solvent evaporation, dip-coating and centrifugal self-assembly. We obtainded the upconversion materials with green light by centrifugal self-assembly and large self-assembled membrane materials arranged orderly by the dip-coating method.To obtain the NaYF₄:Yb, Er spheres with smooth surface and high luminous efficiency, NaYF₄:Yb, Er@SiO₂ core-shell structure was prepared by stober method. The thickness of silica coating was controlled by the ratio of NH₃H₂O and tetraethoxysilane. We found that with the amount of TEOS ranging, the thicknesses changed from 10 nm to 35 nm. The NaYF₄:Yb, Er@SiO₂ with thickness of 10 nm had higher fluorescence efficiency than before. The self-assembly of NaYF₄:Yb, Er@SiO₂ materials had been studied by solvent evaporation and dip-coating. We found that the spheres with thinner SiO₂ coating contacted each other and had obvious effects of assembly, which provide the feasibility for the preparation of high strength films. The spheres with thicker SiO₂ coating went against to the assembly.