Geometric Nanoconfinement Effects on the Electronic and Mechanical Properties of Self-Assembled Molecular Systems
Ewers, Bradley William
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With the ongoing research and development of nanoscale technologies and materials, it becomes increasingly important to understand how local environment influences molecular and material properties. An important factor in this regard is geometric nanoconfinement, for example, the restriction of molecules to nanostructure surfaces. The bulk or average characteristics of materials and molecules do not appropriately define their behavior in these circumstances, and highly localized measurement techniques developed to specifically identify the influence of confinement on their properties is essential to understanding their characteristics and behavior. In this dissertation, two forms of geometric confinement are considered in the context of different molecular properties. First, the role of radial confinement on the tribological properties of self-assembled monolayers (SAMs) is considered. SAMs are an excellent model lubricant for experimental studies of boundary lubrication, and they have been employed as boundary lubricant additives and surface coatings. The lubricated contacts of technologically relevant surfaces, however, consist of asperity interactions, and the summit curvature of these asperities can impact the critical cohesive forces from which the properties of the SAM are derived. Molecular dynamics simulation was employed to understand the influence of nanoscopic surface curvature, as well as surface coverage density, two factors which together contribute to the cohesive forces of SAMs, on their tribological properties. In particular their dissipative potential and effective surface protection were examined, as well as the influence of these factors on the contact mechanics of functionalized nanoasperity contacts. Another mode of geometric confinement studiedin this work is two-dimensional nanoconfinement of molecules and its influence on the mechanism of charge transport in molecular systems. Effective control of charge transport in molecules is essential for molecular modification of CMOS technologies, and is critical in controlling charge carrier dynamics in dye-sensitized photovoltaics. In this work, the size dependence of the electronic properties of thiol-tethered zinc porphyrin aggregateson the Au(111) surface was investigated. AFM nanolithography was used to confine these molecules within an alkanethiol matrix on the Au(111) surface, forming molecular islands of specific dimensions to investigate the relationship between island size and charge transport, demonstrating a shift from tunneling based charge transport to the more tunable and efficient charge hopping based transport.