Fabrication And Formation Mechanism Of Gold Nanocomposite Films

Surface patterning is of great importance for applications in many areas including microelectronics, information-storage devices, nano- and microfluidic devices, and biosensors. Self-assembly provides a bottom-up, lithography-free and cost-effective approach to construct well-ordered patterns and structures. Self-assembly is not only the practical approach for creating a variety of nanostructures, but also shows great superiority in building hierarchical structures with order on different length scales. While the early works focus on molecular self-assembly in solution on the base of non-covalent interactions, self-assembly of non-molecular systems at interfaces have attracted wide interest in recent years. In this thesis, sterically stabilized gold nanoparticles and polymer nanocomposites were used as building blocks to prepare ordered honeycomb macroporous films at air/water interface and at solid surfaces based on solvent evaporation. Highly ordered radial spoke patterns at macroscopic length scales were also obtained in the composite films. The outline and contents of this doctoral dissertation are as follows:In Chapter 1, the properties and synthesis methods of gold nanoparticles were introduced firstly. Then the formation mechanism of coffee rings and surface patterning based on solvent evaporation were discussed. Finally, the research status and the recent development of fabrication of honeycomb films by breath figure method, including the discovery of breath figure method, building blocks, impact factors and post processing techniques, were reviewed. The purpose and significance of this thesis are also demonstrated.In Chapter 2, dodecanethiol-capped gold nanoparticles and DODMAC-stabilized gold nanoparticles were synthesized at room temperature using an established two-phase method. Ordered honeycomb films of gold nanoparticles were fabricated using the breath figure method and were characterized by TEM, SEM and AFM. A two-step procedure is proposed for the fabrication of these macroporous structures, by forming a surfactant monolayer on water, and drop-casting a gold nanoparticle dispersion in chloroform onto the surfactant monolayer. Ordered honeycomb structures with different pore arrays (perforated monolayer films, hexagonal networks and alveoli-like porous films) were obtained. The change in morphology is concentration dependent, and deformed structures with elliptic honeycomb networks are also observed. These films have potential as substrates for surface-enhanced Raman spectroscopy.In Chapter 3, highly ordered polystyrene/nanoparticle macroporous films have been prepared at air/water interface and at solid surfaces using the breath figure method. The microphase separation and interfacial adsorption of AuNPs induce the formation of ordered honeycomb patterns. The self-organized macroporous honeycomb films show the combined optical properties of the gold nanoparticles and an ordered inverse opal structures. Highly ordered iridescent honeycomb films with absorbance in both the visible and near-infrared wave bands have potential as new photonic band gap materials. The influence of nanoparticle concentration and solvent were investigated. At high nanoparticle concentration, the combination of breath figure formation and nanoparticle phase separation leads to hierarchical structures with spherical aggregates under a honeycomb monolayer. Organic ligand-stabilized CdSe/CdS quantum dots or Fe3O4 nanoparticles may be loaded into the honeycomb structure and the fluorescence and superparamagnetic properties of the Fe3O4 nanoparticles are retained in the film. Freestanding honeycomb films with asymmetric through-pore structures were prepared at air/water interface. The morphological difference is closely related with the diameter of water droplets and the thickness of liquid film, that is, the vertical position of water droplets in a liquid film of casting solution. The through-pore structures contribute to the enhanced mechanical property of the freestanding honeycomb film.In Chapter 4, highly ordered radial spoke patterns at macroscopic length scales were spontaneously formed on the surface of AuNP-polymer nanocomposite thin films through a one-step, evaporation method. The spokes have a uniform width of about 1.3μm (measured by SEM). The distance separating adjacent spokes varies from 12 to 35μm. The AFM measurement shows that the periodic spokes are arrays of microchannels on the film surface. The section analysis of the height image reveals that the microchannels have a relatively uniform depth of about 140 nm, which is much smaller than the film thickness. Real time observations under an optical microscope reveal that the droplet evaporation occurs in two stages. In the first stage, the three-phase contact line is steadily pinned to the glass substrate and the receding of solvent front is slow. In the second stage, the solvent front starts to recede rapidly and spoke patterns are formed. The morphological change of surface patterns with different concentrations of AuNP and PS were investigated. The pattern formation was induced by the combination of a fast solvent front retreat and Marangoni convection. The enrichment of AuNPs at the air/solution interface contributes to the surface tension gradient, inducing Marangoni convection.