Investigation Of Self-Assembled Silica Photonic Crystals And Their Bandgap Properties

Photonic crystals are materials with regular periodicity of dielectric structures, which can create a range of forbidden frequencies called photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. Photonic crystals have the ability to manipulate, confine, and control light, thus provide the opportunities to shape and mould the flow of light for photonic communication technology and photonic computer and become the frontier of functional materials.Presently, the fabrication of photonic crystals, especially those in visible or near-infrared region, is the key to the development of photonic crystals. And the most effective way to fabricate photonic crystals with visible or near-infrared bandgap is the self-assembly approach, namely, assembly of the colloidal crystals by monodisperse and sub-micrometer colloidal spheres.In this thesis, SiO₂ colloidal spheres were prepared for using as building blocks of photonic crystals, and modified with succinic acid in order to enhance their surface charges and electrostatic repulsive forces, then they were self-assembled into photonic crystals in visible or near-infrared region from aqueous colloidal solution by the vertical deposition method, moreover, the preparation and assembly of SiO₂ colloidal spheres with bimodal size distribution were studied. The synthesized materials, structures and properties of the SiO₂ colloidal crystals and their relationship were analyzed by XRD, FT-IR, SEM, XPS, Zeta potential and UV/Vis/NIR transmission spectra.Firstly, SiO₂ colloidal spheres were synthesized by hydrolysis of tetraethyl orthosilicate (TEOS) in alcohol-water mixed medium using ammonia as catalyst. Effects of concentrations of ammonia and TEOS on the SiO₂ particle size and size deviation were mainly investigated. XRD and FT-IR results respectively show that the prepared SiO₂ colloidal spheres are non-crystalline and there are hydroxyl groups on the surface of SiO₂ colloidal spheres, which are favorable to surface modification with succinic acid. SEM images indicate that the obtained SiO₂ spheres are highly uniform with diameters ranging from 292nm to 473nm and with relative standard deviation less than 5.0%, and when the concentrations of either ammonia or TEOS is increased, the particle size increases and the relative standard deviation decreases.After the SiO₂ colloidal spheres were prepared by St?ber method, they were modified with succinic acid. FT-IR and XPS results indicate that one end of succinic acid is chemically bonded to the SiO₂ spheres through esterification and the other could ionize in water. Zeta potential of the modified SiO₂ spheres in water solution is improved from ?53.72mV to ?67.46mV, and surface charge density of the modified SiO₂ spheres is enhanced from 0.19μC/cm2 to 0.94μC/cm2.Finally, opal photonic crystals with visible and near-infrared photonic bandgap were assembled by vertical deposition method from aqueous colloidal solution using SiO₂ colloidal spheres with diameters of 292nm and 473nm as building blocks. The morphology, structure and defects were investigated by SEM images, and the incomplete bandgap properties were confirmed by UV/Vis/NIR transmission spectra. Effects of modification, colloid concentration, particle size, temperature, pH value, sintering treatment and incident angle on the morphology and photonic bandgap properties of the obtained SiO₂ colloidal crystals were discussed. The results show that: the succinic acid modified SiO₂ spheres can assemble into more ordered structure and the corresponding photonic bandgap is deeper and narrower compared to the unmodified SiO₂ spheres; the obtained SiO₂ colloidal crystals are fcc structure with each sphere neighbouring six others in a layer and (111) planes parallel to the substrate, and have cracks, dot defects and line defects; the number of colloidal crystals layers is related to the colloid concentration, with colloid concentration increasing, the number of the layers correspondingly increases, and under the concentration of 0.3wt%, SiO₂ colloidal crystals with relatively high quality can be obtained; SiO₂ colloidal crystals consist of spheres with diameters of 292nm and 473nm have the bandgap at 604nm and 1047nm respectively, which are near to the calculated values by Braag law, and with the incident angle increasing, a blue shift of the photonic bandgap happens, which indicates that the bandgap is incomplete; temperature and pH value have great influence on the assembly of SiO₂ colloidal crystals, and under 40℃and the pH value of 7.0, the most close-packed structure can be obtained; after sintering treatment, there are less defects in the colloidal crystals and the structures are more closed, correspondingly, photonic bandgap has a blue shift and becomes more deeper and narrower.Additionly, in this thesis, SiO₂ colloidal spheres with bimodal size distribution were prepared by the hydrolysis of TEOS in alcohol-water mixed medium with the ammonia as catalyst when NaCl electrolyte was introduced into the system. A model of nucleation and growth mechanism of SiO₂ colloidal spheres with bimodal size distribution was proposed. And then SiO₂ colloidal crystal which has the zinc blende structure was self-assembled by vertical deposition method in alcoholic solution using SiO₂ colloidal spheres with bimodal size distribution.The innovative points of this thesis are as follows: 1) a new method to enhance surface charge on SiO₂ spheres via modification with succinic acid was presented; 2) the detailed influences on assembly of SiO₂ colloidal crystals from aqueous solution were investigated; 3) the preparation and assembly of SiO₂ colloidal spheres with bimodal size distribution were studied.