Browsing by Author "Smith, Christopher"

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    Charge Transport Processes in Molecular Junctions
    (2016-10) Smith, Christopher
    Molecular electronics (ME) has evolved into a rich area of exploration that combines the fields of chemistry, materials, electronic engineering and computational modeling to explore the physics behind electronic conduction at the molecular level. Through studying charge transport properties of single molecules and nanoscale molecular materials the field has gained the potential to bring about new avenues for the miniaturization of electrical components where quantum phenomena are utilized to achieve solid state molecular device functionality. Molecular junctions are platforms that enable these studies and consist of a single molecule or a small group of molecules directly connected to electrodes. The work presented in this thesis has built upon the current understanding of the mechanisms of charge transport in ordered junctions using self-assembled monolayer (SAM) molecular thin films. Donor and acceptor compounds were synthesized and incorporated into SAMs grown on metal substrates then the transport properties were measured with conducting probe atomic force microscopy (CP-AFM). In addition to experimentally measured current-voltage (I-V) curves, the transport properties were addressed computationally and modeled theoretically. The key objectives of this project were to 1) investigate the impact of molecular structure on hole and electron charge transport, 2) understand the nature of the charge carriers and their structure-transport properties through long (<4 nm) conjugated molecular wires, and 3) quantitatively extract interfacial properties characteristic to macroscopic junctions, such as energy level alignment and molecule-contact electronic coupling from experimental I-V curves. Here, we lay ground work for creating a more complete picture of charge transport in macroscopically ordered molecular junctions of controlled architecture, length and charge carrier. The polaronic nature of hopping transport has been predicted in long, conjugated molecular wires. Using quantum-based calculations, we modeled ‘p-type’ polaron transport through oligophenylenethiophene (OPTI) wires and assigned transport activation energies to specific modes of nuclear motion. We also show control over ‘n-type’, LUMO-mediated transport in short (~2 nm) redox-active perylenediimide (PDI) SAMs bound to contacts through isocyano linkers. By changing the contact work function (Φ) and temperature, we were able to verify thermally-assisted LUMO transport. Transition voltage spectroscopy and the single level model was employed to fit the experimental I-V curves and extract the electronic coupling (Γ) and the EF-LUMO offset (εl). It was found that εl does not change with Φ (LUMO pinning), while Γ changes with both Φ and temperature. Further, the PDI SAMs could be reversibly chemically gated to modulate the transport. These results help advance our understanding of transport behavior in semiconducting molecular thin films, and open opportunities to engineer improved electronic functionality into molecular devices.
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    An Evaluation of Perfusion in Human Body Thermal Modeling through the Integration of a Porous Media Model for Tissue
    (2023-02) Smith, Christopher
    Historically there has been one primary method for modeling the thermal condition of the human body. This method, referencing the bioheat equation, has been and is used widely across the medical device and human comfort industries. The present work leverages an alternate method to biological tissue modeling by using a porous media approach. In doing so, it provides a more physiologically and anatomically representative alternative to human body thermal modeling to contrast the computationally efficient, but low fidelity bioheat method. The present work shows the feasibility of using this porous media approach for high fidelity tissue modeling, allowing both researchers and designers to have an alternative modeling method to leverage – ensuring that they can choose a method best fit for their need.
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    Studies of Molecules and Molecular Complexes Using Microwave Spectroscopy
    (2019-09) Smith, Christopher
    The work presented in this thesis covers a variety of small molecules and complexes, which can roughly be grouped into two categories: molecules with different conformations and molecules/complexes potentially relevant to atmosphere aerosol nucleation. Chapter 1 presents work on the syn- and anti- conformers of thioacetic acid, where we discovered that the methyl group internal rotation barrier differed between the two forms by a nearly a factor of five. With the collaboration of Yirong Mo (Western Michigan University) and Huaiyu Zhang (Hebei Normal University), we aimed to explain this unusually large difference. In regard to the second subject, atmospheric aerosols have become of great interest in recent years. Aerosols are ubiquitous in the atmosphere and can affect the quality of life in several ways, such as negatively affecting human health, decreasing visibility and influencing climate. While the mechanism(s) by which particles nucleate and grow in the atmosphere is not yet fully understood, homogenous nucleation is one possible route for particle formation. Homogeneous nucleation is a process by which individual gas molecules aggregate together to first form complexes, then a stable cluster, and finally an aerosol particle through continued spontaneous growth. Sulfuric acid has long been known to be a key component in particle formation. It is formed in the atmosphere by the oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3), followed by hydrolysis. Sulfuric acid is highly hygroscopic, however, binary models involving sulfuric acid and water do not accurately predict new particle formation rates. Therefore, other constituents must be involved and scientists have started to incorporate common atmospherically relevant organic species, such as amines, carboxylic acids, and oxidative products of hydrocarbons into their models. However, while much has been learned about aerosols and their formation over the past few decades, a full understanding of the nucleation pathways, new particle formation rates, and aerosol composition at various stages, is still unclear. Recent theoretical research incorporating carboxylic acids in the early stages of nucleation, which showed that formic acid (HCOOH) catalyzes the hydration of SO3, converting SO3 to sulfuric acid, caught our attention. Furthermore, calculations showed that the activation barrier of this formic acid catalyzed reaction was not only lowered, but essentially zero. Finally, the authors proposed that this alternative pathway for generating atmospheric sulfuric acid could be competitive with current proposed mechanisms. When our group set out to investigate related complexes such H2SO4 – HCOOH and SO3 – HCOOH, a new project transpired, as an entirely new molecule was discovered, FSA (formic sulfuric anhydride). While this new class of molecules, carboxylic sulfuric anhydrides, is relatively unknown in the chemical literature, they could have great importance in the mechanisms for formation of atmospheric aerosols. Chapter 2 presents work on s-cis- and s-trans¬-acrylic sulfuric anhydride (s-cis-AcrSA and s-trans-AcrSA), which are formed from trans- and cis-acrylic acid, respectively, and provides experimental evidence that a variety of carboxylic acids can react with SO3 to generate their corresponding carboxylic sulfuric anhydrides Chapter 3 illustrates our work on the acetic sulfuric anhydride – water complex. Our aim was to hydrate a carboxylic sulfuric anhydride in order to understand its interaction with water, a first step in understanding anhydride hydrolysis, which would result in the generation of the sulfuric acid – acetic acid complex. Chapter 4 contains a study of propiolic sulfuric anhydride (PSA) and reviews all the carboxylic sulfuric anhydrides characterized by our lab to date, both experimentally and theoretically. Comparisons among their structures and energetics are made and detailed statistical thermodynamic calculations are carried out to estimate their equilibrium constants and concentrations over a range of atmospherically relevant temperatures. Comparisons between their concentrations and the concentrations of other atmospherically relevant species are also highlighted.