Synthesis Of Uniform Core-shell Structured Nanocrystals And Research Upconversion Fluorescent

In the last decades, considerable attention has been attract to the rare earth ion doped upconversion nanocrystals for the wide potential application in the field of biolabel, biomedical imaging, photodynamic therapy and so on, due to the good optical stability, avoiding the autofluorescence, and increased penetration depth. By far, more work is deserved to synthesize nanocrystals with controllable shape and high upconversion efficiency. Therefore, research contents of this thesis include synthesizing the rare earth ion doped NaYF₄ nanocrystals with monodispersition, small uniform size, and regular sharp and improving the upconversion efficiency of the nanocrystals by coating them with a layer of different fluorides.Cubic NaGdF₄:Yb20%,Ho2% and NaYF₄:Yb 20%, Er,Tm2% have been Prepared by the hydrothermal and oleicthermal approach. Based on this word the fluoride nanocrystal with series of core-shell structure NaGdF₄:Yb20%,Er2% /NaY_xGd_1-xF₄(x = 0,0.5,0.8,1)and NaYF₄:Yb20%,Er,Tm2%/NaY_xYb_1-xF₄(x = 0,0.3,0.6,1) have been obtained. The upconverted luminescence was recorded. The results indicated that the upconversion efficiencies of the core-shell nanocrystals were improved, compared with those from the corresponding naked core nanocrystals. In addition, the upconversion efficiency increases with the concentrations of the Gd³⁺ or Yb³⁺ ions in the shell structure.Afacile, economical and environment-friendly solvothermal method has been developed. Hexagonal NaYF₄ nanocrystals with monodispersition, small uniform size, different sharp have been prepared by modulating the reaction time and doped concentration of surfactant oleic acid. The TEM images showed that, during the same reaction, the growth rate towards each direction tend to be equal and slowed down with the increasing of the ratio of surfactant.The NaYF₄:Yb20%,Er2% /NaY_xYb_1-x F(4x = 0,1) core-shell nanocrystals with monodispersition and uniform core-shell structure have been prepared by the solvothermal method mentioned above. Upconversion emissions and time-solved spectra were record and the result shows that the branch ratio have been changed and the lifetimes of the IR energy levels lengthened by 1.5 times, compared with the naked nanocrystal. It means that, after cladding, the products shows the suppressed nonradiation relaxations and higher upconversion efficiency. Furthermore, we have observed the influence of the sensitizers in the shell on the activators in the core, which further improved the upconversion efficiency of the nanocrystals.