Studies On Electrostatically Self-Assembled Nanocomposite Thin Films

The self-assembly strategy will play an important role in the development of nanotechnology and nanostructured materials, and the electrostatic self-assembly monolayer technique is a novel approach to the build-up of nanocomposite functional thin films and thin-film-based devices. In the present dissertation, nanocomposite thin films and extended molecular devices were prepared via the electrostatic self-assembly monolayer technique. The structures and the properties of the thin films were studied by UV-vis spectroscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, laser Raman spectroscopy, atomic force microscopy, fiber optic experimental setups and so on. A novel fiber optic humidity sensor based on the self-assembled polyelectrolyte multilayer thin films was presented, and other thin film devices were also described. On basis of ionic exchanges and electrostatic adsorption of charged groups, multilayer thin films of polymer/polymer, polymer/organic molecules, polymer/gold nanoparticles and polymer/inorganic nanoparticles were self-assembled onto versatile substrates including silicon, glass and optic fibers. Polyelectrolyte PDDA-polymeric dye PS-119 system exhibited a favorite self-assembling feature. The interior structures of the assembled PDDA/PS-119 films are not clearly laminar, and the films can serve the function of single-layer coatings. The monolayer thickness of the PDDA/PS-119 films is at the nanometer scale and dependent on the concentrations of the two polyelectrolytes. Self-assembled PDDA/PS-119 films are sensitive to the humidity of air. A 0.85um- or 1.3um-LED-lighted fiber optic humidity sensor based on a film of 40¹00nm in thickness self-assembled on the end face of the fiber can work in the range of 1¹00%RH with a response time less than 1s. This novel humidity sensor can be used for monitoring human breathing-conditions. Organic molecules including Congo Red, a derivative of copper phthalocyanine and a fullerol were self-assembled with polymers into multilayer thin films, and organic molecules with more than 4 charged groups exhibited satisfactory features for multilayer self-assembling. Gold colloids with different particle sizes were synthesized using hydrogen tetrachloroaurate as the precursor and ' Na3-citrate/NaBH4 as thereducing agents. Two-dimensional arrangements of gold particles and layered composites of gold with polymer PDDA were prepared. Red shift of the plasmon absorption band was observed after gold particles were assembled into multilayer thin films, and the resulted gold films exhibited a strong surface enhanced Raman scattering (SERS) effect. Both the red shift of the plasmon absorption band and the SERS effect result from the interparticle interactions of gold nanoparticles within the films. The electrical conductance of the gold/polymer composite films was tunable, and a low resistivity of the order of 10^-4cm was yield. The conducting mechanism was not determined, perhaps the tunnel effect and the foreign-substance-electron-transfer should be considered. Colloidal Si02 particles were self-assembled into multilayer thin films with PDDA. The resulted films were densely packed uniform amorphous films with substantial overlap between adjacent Si02 layers, and can serve the function as antireflection coatings. Colloidal Ti02 particles of 3⁴nm in diameter were prepared using tetra-H-butyl titauate as precursor, and composite multilayer films of PSS/TiO2 were electrostatically self-assembled. The TiO2 colloidal particles are brookite nanocrystals, and the PSS/Ti02 films are polycrystalline films with relatively high hardness and can serve the function as reflection-enhancing coatings. X/4-multilayer high reflection coatings were also prepared by assembling SiO2 particles, Ti02 particles and polyelectrolytes into thin films.