Preparation And Band Gap Study Of Inverse Opal And FCC Photonic Crystal Of Non-closed-packed TiO₂ Hollow Spheres

Photonic crystal is a new type of optical material with photonic band gap. For a photonic crystal with a complete photonic band gap, the propagation of the electromagnetic wave within the band gap frequency is forbidden in all directions. So, photonic crystals can control the spontaneous emission from atoms. This may bring about important potential in several scientific and technical areas.In this paper, the monodispersed silica microspheres with a very narrow size distribution were prepared on sub-micrometer scale by an improved stober method. The monodispersed micrometer polystyrene spheres were also synthesized by soap-free emulsion polymerization. The results show that the improved stober method and soap-free emulsion polymerization technique are both useful for the preparation of uniform microspheres. The influences of reaction parameters on diameter and size distribution of microspheres were systemically investigated.We investigated the vertical deposition technique mechanisms to fabricate colloid crystals, and then reported on the synthesis conditions of colloidal crystals with high crystalline quality. Using optimizing temperature, solvent and concentration, silica and polystyrene colloidal crystals with high crystalline quality were fabricated quickly by the vertical deposition method. The SEM images show the colloidal crystals are long-range ordered in the plane both parallel and perpendicular to the substrate. Both the theoretical study and experimental analysis of colloid crystals indicate that such colloid crystals have FCC structure, and UV-vis spectrums of colloidal crystals prove that the band gap position is in good accordance with the theoretical value calculated by Bragg equation.Besides applications in the catalysis, adsorption and gas separation, the promising application fields for three-dimensionally ordered macroporous materials are related to photonic crystal. Inverse opal photonic crystal is fabricated by colloidal crystal template technique. Using silica colloidal crystal as template, large-area highly-ordered polystyrene inverse opal was fabricated. High-performance titania inverse opal photonic crystal was derived from a sol-gel procedure using polystyrene colloidal crystals as template and the shrinkage of the inverse opal framework was decreased. The titania X-ray diffraction and electronic diffraction patterns show that the titania inverse opal framework consists of nanocrystallites of anatase. Based on the plane wave expansion method, theoretical calculations show that a complete photonic band gap between the eighth and ninth bands will open in the titania inverse opal photonic crystal and the band gap width between second and third bands could reach 21% around point L.Our presented that a hollow-spherical non-closed-packed structure can present a photonic band gap between the eighth and ninth bands. When the dielectric constant chose as 7.84, the results show that the maximum width of the band gap could reach 9.1% and the critical dielectric contrast for the existence of the band gap is only 5.3, which is lower than that of other FCC structures could be obtained by self-assembling method. With the help of thermal sintering techniques, selective etching techniques and sol-gel process, the FCC photonic crystals of crystalline TiO2 were fabricated using non-close packed silica opals as the templates. We designed a hollow spherical non-closed-packed face-centered cubic uniaxial structure (HNFUS). In HNFUS, each hollow sphere at the lattice sites is only connected to the hollow spheres belonging to the above and below layers by cylindrical tubes, whereas the connectivity between hollow spheres belonging to same layers is suppressed. Band structure calculations show HNFUS possesses a large complete photonic band gap between the low-frequency bands (second and third bands). The results show that the gap size can reach a value of 6.1% at a low dielectric contrast of 7.84 appropriate to the transparent TiO2. The critical value of the dielectric contrast needed to open the complete photonic band gap is as low as 5.8. The influence of structure variations on the band gap, such as hollow spheres, cylindrical tubes and dielectric contrast, is also discussed. The gap size firstly increases with the inner radius of the tubes and then decreases. With the increase of the inner radius of sphere shells, the gap size at first remains essentially invariable and then decreases until it disappears entirely. The midgap frequency is influenced remarkably by the thickness of the spherical shells and tube walls. With the increase of the dielectric contrast, the gap size is enlarged rapidly. We also present a method based on a combination of colloidal self-assembly, thermal sintering, etching, infiltrating technique to achieve the HNFUS crystal.