Self-assembly of nanoparticles in nanometer-thin polymer films

Using "soft" materials such as block copolymers to organize self-assembly of nanoparticles into specific, ordered geometries is a promising route to nanostructure fabrication. In the case of block copolymers, which are the materials used in this thesis, the usual attractiveness for creation of nanostructures stems from the nanometer scale structure of their ordered matrices composed of phase-separated microdomains. The characteristic length scale of ordered block copolymer matrices typically lies below the resolution of conventional lithography techniques. Another advantage of block copolymers as a templating tool is that the geometry and periodicity of their structure can be tuned by changing their molecular parameters such as overall length of the block copolymer chain or the relative size of the blocks. This thesis investigates the self-organization of thiol-passivated gold nanoparticles in polymer films with thickness comparable to the particle diameter. Various nanostructures were observed to form, and their fundamental origins were investigated. In most cases, the polymers comprising the films were attached by one end to an underlying surface, in a "polymer brush" motif.; Chapter 1 gives an introduction of the work done by other groups on the self-assembly of nanometer components on substrates, especially on using block copolymers as manipulating tools for nanostructure fabrication. Chapter 2 describes experimental methods and sample characterization, such as nanoparticle synthesis, preparation of polymer/nanoparticle composite films and sample characterization with various techniques, including TEM, AFM, SEM, FTIR, and XPS. In Chapter 3, organization of nanoparticles deposited on top of a polymer brush is investigated experimentally. A physical understanding of how the constraint of end-attachment of the polymer chains affects the organization of the nanoparticles and determines the morphology of nanoparticle aggregation (elongated stripes, extensive circular aggregates) is developed. Chapter 4 discusses a method to prepare polymer brushes with a nonuniform structure and the organization of nanoparticles on such structures. It is found that deposition of nanoparticles on nonuniform brushes can cause the particles to arrange into circular structures at both nanometer and micrometer scales. Formation mechanisms of the two types of ring structures are considered. In Chapter 5 methods are developed to "enhance" a pre-made mesh structure by deposition of additional gold. The effects of UV-ozone exposure, elevated temperatures, and irradiation with electron beams (e-beams) are also investigated. These approaches were chosen for their expected ability to degrade the organic stabilizing surfactants on the nanoparticles, as well as the polymer support films, thus perhaps enabling nanoparticles to fuse together and convert a nanostructure consisting of individual particles into a continuum while preserving the overall structural geometry. Preliminary investigation of the changes in optical properties of thus modified nanoparticle assemblies is also presented.