published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

This thesis describes the transport properties observed in thermally treated graphite oxide (GO), which holds promise as an economical route to obtaining graphene. Graphene is a material consisting of a single atomic plane of carbon atoms and was first isolated as recently as 2004. Several isolation techniques have been investigated, including mechanical exfoliation, chemical vapor deposition, and the reduction (by various methods) of chemically synthesized graphite oxide. Two fundamental questions are pursued in this work. The first is concerned with the maximum electrical conductivity that can be achieved in atomically thin reduced graphite oxide samples (rGO). As produced, GO is insulating and of little use electronically. By heating and exposure to reducing atmospheres, however, the conductivity can be increased. Through the lithographic definition and fabrication of four-point contact structures atop microscopic samples of GO, the resistance of the sample can be monitored in situ as the reduction process takes place. It was discovered that the resistance of few-layer GO could be decreased by an order of magnitude when heated to 200 °C and subsequently cooled back to room temperature in forming gas. Final resistivities were on the order of 0.5 Ω-cm. An ambipolar field effect was observed in the thermally treated samples, with resistance decreasing by up to 16 % under a substrate bias of ±20 V. Mobilites were inferred to be on the order of 0.1 cm²/V-s. It was also found that the presence of forming gas during reduction decreased the resistance of the GO samples by roughly one half. The second question that this work begins to answer is concerned with the distance that electrons can travel in such thermally-reduced GO before spin-randomizing scattering. The answer can be elucidated with the aid of magnetoresistance measurements using ferromagnetic contacts to inject a spin-polarized current through the sample. The observation of the magnetoresistive effect with the contacts separated by a certain distance can be taken as evidence of a spin coherence length in the material of at least that distance. Though this experiment has not yet been carried out, progress has been made toward its possibility; specifically in the fabrication and characterization of independently switchable magnetic contacts. By exploiting magnetic shape anisotropy, contact pairs have been fabricated and demonstrated to differ in magnetic coercivity by up to 8 Oe. / Graduation date: 2011

The H₂O critical point defines the parabolic vertex of the p(T) vaporization boundary and, as a geometric consequence, a positive vertical asymptote for first partial derivatives of the equation of state. Convergence of these derivatives, isothermal compressibility and isobaric expansivity, to the critical asymptote effectively controls thermodynamic, electrostatic, and transport properties of fluid H₂O and dependent transport and chemical processes in hydrothermal systems. The equation of state for fluid H₂O developed by Levelt Sengers et a1. (1983a) from modern theories of revised and extended scaling affords accurate prediction of state and thermodynamic properties in the critical region. This formulation has been used together with the virial equation of state proposed by Haar et a1. (1984) and predictive equations for the static dielectric constant (Uematsu and Franck, 1980), thermal conductivity (Sengers et a1., 1984), and dynamic viscosity (Sengers and Kamgar-Parsi, 1984) to present a comprehensive summary of fluid H₂O properties within and near the critical region. Specifically, predictive formulations and computed values for twenty-one properties are presented as a series of equations, three-dimensional P-T surfaces, isothermal and isobaric crosssections, and skeleton tables from 350°-475°C and 200-450 bar. The properties considered are density, isothermal compressibility, isobaric expansivity, Helmholtz and Gibbs free energies, internal energy, enthalpy, entropy, isochoric and isobaric heat capacities, the static dielectric constant, Z, Y, and Q Born functions (Helgeson and Kirkham, 1974a), dynamic and kinematic viscosity, thermal conductivity, thermal diffusivity, the Prandtl number, the isochoric expansivity-compressibility coefficient, and sound velocity. The equations and surfaces are analyzed with particular emphasis on functional form in the near-critical region and resultant extrema that persist well beyond the critical region. Such extrema in isobaric expansivity, isobaric heat capacity, and kinematic viscosity delineate state conditions that define local maxima in fluid and convective heat fluxes in hydrothermal systems; at the critical point, these fluxes are infinite in permeable media. Extrema in the Q and Y Born functions delineate state conditions that define local minima in the standard partial molal volumes and enthalpies of aqueous ions and complexes; at the critical point, these properties are negative infinite. Because these fluxes and thermodynamic properties converge to vertical asymptotes at the critical point, seemingly trivial variations in near-critical state conditions cause large variations in fluid mass and thermal energy transfer rates and in the state of chemical equilibrium.

This work involves the synthesis of two types of electro-optic monomers and their corresponding polymers. The first type of monomers contain the p-oxy-α-cyanocinnamate structure and were synthesized from ω-hydroxyalkoxy-substituted benzaldehydes and methyl cyanoacetate. These ω-hydroxy-α-cyanoester monomers show a high degree of electron delocalization. Copolyesters were synthesized by copolymerization of these monomers with methyl 12-hydroxydodecanoate by the standard two-stage, high-temperature polyesterification procedure. The copolyesters, incorporating dipolar units all pointing in the same direction, are soluble and solution- and melt-processable. Second harmonic generation (SHG) measurements on chloroform solutions of the copolymers showed enhancements of χ² as large as 20 relative to the dipolar monomers. These are the first known readily soluble main chain polymers that exhibit SHG behavior. The second type of monomers were acrylates containing substituted phenyl esters of benzoic acid as mesogenic (pendant) groups. Specifically, the mesogenic group contained an oxy-aryl-carboxy-aryl-carboxy-alkyl structure separated from the acrylate carbon-carbon double bond by a spacer group, which had a carboxyethyl-carboxyhexyl structure. A synthetic route was established by synthesizing a model monomer containing a 2-methylpropyl group as the alkyl group at the end of the mesogenic group. The model monomer was polymerized free radically and the resulting polymer found to possess a smectic liquid crystalline phase that became isotropic at 103° C. With the synthetic route established, an optically active monomer containing a (S)-2-methyl-1-butyl group as the alkyl group at the end of the mesogenic group was synthesized and polymerized. The optically active polymer was already in a smectic liquid crystalline phase at room temperature (≈25° C) and the phase persisted up to 72.6° C. These results indicate that it is possible to design polymers containing thermotropic liquid crystalline phases by fixing low molecular weight liquid crystalline molecules to a polymer main chain.

The ease of electron loss of fifty alipathic thioethers was studied by electrochemistry, charge transfer and photoelectron spectroscopy. These compounds consisted of mesocyclic thioethers and S-methyl norbornane derivatives. Comparison of charge transfer and photoelectron ionization potential showed a good correlation. Correlation of ionization potential with anodic peak potentials showed the existence of two groups of compounds. Those compounds having an electron rich neighboring group capable of an intramolecular interaction were found to have a good correlation of ionization potential with electrochemical peak potential. For those compounds without this capability, no observable correlation was seen. Photoelectron ionization potentials of thioethers are a function of the alkyl groups attached to the sulfur atom. With substituent constants assigned to alkyl groups from measurements on simple thioethers, the ionization potential of more complicated thioethers can be calculated. Compounds whose experimental value was found to be less than the calculated value were found to fall in the group where an intramolecular neighboring group could facilitate the ease of electron loss by stabilization of the cation radical. Electrochemical peak potentials were seen to be dramatically affected by intramolecular stabilization of the cation radical. Shifts of 600-800 mV were seen for structurally similar compounds whose only difference was the availability of an electron rich neighboring group. Since the electrochemistry of these compounds exhibits irreversible behavior, the shift in peak potential could be due to a change in the formal potential, the heterogeneous rate constant, the rate of a following chemical reaction or a combination of these. Changes in the heterogeneous rate constant or chemical step rate constant alone cannot account for the magnitude of the shift seen. Thus the differences observed must be due to two different processes. In the noninteracting case, E⁰' is a measure of the formation of the cation radical, whereas, in the case of interacting compounds E⁰') is a measure of the formation of an intramolecularly stabilized cation radical where bond formation has occurred. Evidence for the structure of some intramolecular stabilized intermediates is presented. An electron deficient thioether which is a model for biological systems was shown to be capable of phosphorylating adenine nucleotides.

Cytochrome c was immobilized to several supports to study the effects of immobilization on the molecule and to serve as a model for the in vivo system. Immobilization was accomplished by covalent attachment of cytochrome c to the support surface, either Sepharose 6MB or glassy carbon. The effect of the coupling conditions on the covalent attachment reaction was studied with Sepharose 6MB. The reactive groups were monitored colorimetrically and were highly susceptible to hydrolysis. Correction for hydrolysis indicated that the covalent attachment reaction was first order with respect to reacted groups. Coupling conditions most affecting the amount of attached cytochrome c were the initial cytochrome c concentration, temperature, and pH. A detailed study of the resulting immobilized cytochrome c was conducted based on its three characteristic properties: spectra, oxidation reduction potential, and biological activity. The spectral properties demonstrated that no major conformational changes had occurred upon immobilization since the spectra were essentially the same. The redox potentials for most samples of immobilized cytochrome c loaded with different amounts of protein were found to be 20-25 mV lower than native cytochrome c (+ 270 mV). Two samples, the heaviest loaded, were approximately equal to the native protein suggesting that they may be least affected by immobilization. The biological activity measurements provide an indication of the ability of the molecule to function properly. The Michaelis constant (K(m)) for cytochrome oxidase and reductase with immobilized cytochromes c were significantly higher (20-400X) than the K(m) for soluble cytochrome c. The higher K(m)s reflect that about 1% of the immobilized cytochrome c is availble for reaction in agreement with distribution and exclusion studies. Correction of the immobilized cytochrome c K(m)s for available protein results in values similar to the soluble cytochrome c K(m). Immobilization of cytochrome c to glassy carbon was performed by two procedures employing a carbodiimide or 4-vinylpyridine as the coupling reagents. The former resulted in electrodes with higher specific activities and lower protein loadings than the latter. In both cases up to 60% of the immobilized protein was held by adsorption on the surface. Protein coverages were approximately 10⁻⁸ to 10⁻⁹ moles/cm² which corresponds to 100-800 layers.

No description available.

Various metal phthalocyanines have been used as dyes, catalysts, indicators, electrophotographic receptors and more recently as active elements in chemical sensors and photoconductive materials for solar energy conversion applications. Of the MPc's, VOPc, GaPc-Cl and TiOPc have shown promise for solar energy conversion devices. GaPc-Cl has also shown promise as a chemical sensor. Up to this point in time, the focus of Pc research in this group has been in the direction of characterizing the photoelectrochemical properties of these materials. Recently, solid state studies of TiOPc have been done to determine the electronic properties of a tetravalent metal Pc, such as conductivity and photoconductivity. Such solid state measurements are facilitated by the use of interdigitated microelectrode arrays which are coated with thin films of various Pc's.

No description available.