Synthesis and stop band tuning application of colloidal photonic crystal.

摘要

A photonic crystal is a regularly structured material with periodic dielectric constant variation at the length scale of visible light wavelength and near-IR region. Photonic crystals have been the focus of many researchers over the last two decades because of their wide applications ranging from optical communications to chemical and biological sensors.; The initial intense focus on photonic crystals was for applications in telecommunications where a complete photonic band gap (PBG) is required. However, this focus has been defused because of the inherent difficulties in fabricating a photonic crystal with a complete photonic band gap, and in the case of colloidal crystals, synthesizing structures without defects. Emphasis within the photonic crystal community has shifted to explore the value of the stop band (rejection wavelength), and specifically, the opportunities present when the stop band is combined with a photonic crystal that can exhibit a dynamic tunability in its observed optical properties.; The objectives of this dissertation are to fabricate colloidal photonic crystals with desired thermomechanical properties and to develop new methods to tune the stop band of the PBG composite. Physically robust water-free colloidal photonic crystal composites were fabricated using electrostatically stabilized monodisperse colloidal particles followed by two encapsulation steps. The long-range order of the crystalline array was essentially preserved during the encapsulation process. The Tg and storage modulus of the PBG composite were extended to lower values by using a different second step monomer.; A new approach has been developed to tune the stop band of photonic crystals by modulating the electric field applied on the PBG composite film. A 25 nm total rejection wavelength variation from its initial nonbiased state was demonstrated with a PBG film (80 mum in thickness) when subjected to a field strength of 25 V/mum. The thickness strain and the, stop band shift were significantly enhanced by modifying the permittivity of the solid film.; Static and dynamic stop band tuning characteristics of elastomer encapsulated PBG composites were investigated through a dynamic spectroscopic apparatus. This apparatus, which met essentially all of the goals set during the design stage, was constructed in-house. The results of the piezoelectric stop band tuning using this apparatus indicated that deformation of the photonic crystals significantly influenced their reflection spectra. The mechanochromic mechanism was investigated using Kossel diffraction technique, with the result indicating that the stop band attenuating and broadening may be attributed primarily to the disorder induced during deformation. The characteristics of static and dynamic mechanochromic tuning of PBG composites can be controlled by the thermomechanical properties of the encapsulating polymers.; In addition, the electrical percolation of melt-mixed, conductive HDPE-SWNT and HDPE-MWNT composites was studied and compared with HDPE-CB composites. The introduction of SWNT or NMWNT into HDPE decreased its resistivity by up to 13 orders of magnitude. The percolation behavior agreed very well with the percolation theory and the threshold was ca. 1.5 wt% (0.96 vol%) for HDPE-SWNT. The morphology, thermostability and crystallinity of the composites were also investigated to understand the structure-property relationship.