Study On Rare-Earth Doping And Self-organizing Of Silica Colloidal Microspheres

Silica colloidal microspheres doped with rare earth (RE) have significantapplications in optics, materials science and biology. Therefore, preparation andinvestigation of RE doped silica colloidal microspheres have very important scientificand applicable value. Fabricating colloidal photonic crystals films by self-organizingmonodisperse silica microspheres is a significant approach to preparethree-dimensional (3D) photonic crystals. In this dissertation, synthesis, RE-dopingand self-organization of silica colloidal microspheres have been studied.First, two methods (the base-catalyzed method and the acid-catalyzed method)have been used to prepare pure silica colloidal microspheres through the hydrolysisand condensation reactions of tetraethyl orthosilicate (TEOS). Microstructureevolvement of silica microspheres during the calcination was also investigated. Themechanism of formation and growth of silica microspheres was given andmicrostructure model was put forward. It is found that four-fold silaxone tingsoligomers first come into being by the hydrolysis and condensation reactions of TEOSafter TEOS is added. Silica nuclei are formed by the condensation of 4-fold ringstructure and the growth of nuclei proceeds through the addition of 4-fold ringstructure due to collision with each other. Synthesized silica colloidal microspheresmainly consist of 4-fold silaxone tings, including incomplete condensed silanols andincomplete hydrolyzed alkoxy groups. The formation of 4-fold ring structure duringthe hydrolysis and condensation reactions of TEOS is propitious to embed REcomplex into silica microspheres during doping.Second, the modified base-catalyzed method and the seed growth method were employed to synthesize RE complex-doped silica hybrid microspheres. The formercan incorporate RE complex into the entire silica microspheres, to obtainmonodisperse silica hybrid microspheres with submicron diameter, while the lattercan produce silica hybrid microspheres surfacely doped with monolayer or multilayerRE complex based on the different demands.The microstructure model of Eu(TTFA)₃ and/or Sm(TTFA)₃-doped silica hybridmicrospheres synthesized by the modified base-catalyzed method was put forward.The results show that the hybrid microspheres are mainly composed of 4-fold silaxonerings, with RE complex embedded into the micropores of silica network. Theexistence of RE complex locally destroys the 4-fold ring structure to form microporeswith larger size. (1) The hybrid microspheres doped with Eu(TTFA)₃ or Sm(TTFA)₃exhibit the characteristic photoluminescence (PL) of the Eu³⁺ or Sm³⁺ ions,respectively. The Eu³⁺ ions in the hybrid microspheres locate in two differentmicropores and thus have two different luminescence centers. In a mixture ofdimethylformamide (DMF) and dimethyl sulfoxide (DMSO), the radiative lifetimes ofthe Eu³⁺ ions embedded into silica hybrid microspheres are about one time longer thanthose in pure RE complex, which indicates that the radiative lifetimes of theluminescence centers are related to not only dielectric boundraries, but also theexistence state of dielectric or interfaces between two media. (2) Silica hybridmicrospheres co-doped with Eu(TTFA)₃ and Sm(TTFA)₃ were first synthesized. In thehybrid microspheres, sensitization effect of Sm³⁺ ions to Eu³⁺ ions was found andmultiple energy transfer processes were achieved, namely the processes from ligandsto Sm³⁺ ions or Eu³⁺ ions and from Sm³⁺ ions to Eu³⁺ ions. Sensitization of Sm³⁺ ionsto Eu³⁺ ions was mostly carried out by the energy transfer process of ⁴G_5/2 (Sm³⁺)→⁵D₀ (Eu³⁺). The effect of doping concentrations of Eu(TTFA)₃ or Sm(TTFA)₃ on thePL intensities of Eu³⁺ ions in the co-doped hybrid microspheres was investigated indetail. Inside the co-doped hybrid microspheres, Eu³⁺ ions still have two differentluminescence centers.RE complex-doped silica hybrid microspheres were prepared for the first time byusing the seed growth method. The results show that the hybrid microspheres have nice monodispersity and smooth surfaces. PL spectrum reveals that the hybridmicrospheres exhibit the characteristic PL of the Eu³⁺ ions. By effectively using theenergy transfer process of RE complex, SiO₂ hybrid microspheres with excellentmonodispersity and PL properties were synthesized by the seed growth, whichovercame the disadvantages of large diameter and polydispersity by theacid-catalyzed method.Finally, the vertical deposition technique was used to prepare 3D colloidalphotonic crystals films. The results reveal the hexagonal arrangement of themicrospheres in the surface of the films, with the ordered arrangement ofmicrospheres in 3D. The transmission spectra show the films self-organized withmicrospheres of different diameters exhibit photonic band gap in visible and nearinfrared wavebands. A new method to determine film thickness, the opticalmicroscopy, was put forward. The method is simple and can't destroy the thin films.Monolayer films of silica microspheres were prepared by the vertical depositiontechnique. The films present polycrystal structure and feeble photonic band gap.