Electronic Structure Studies of Graphene and Graphene Oxide Interfaces
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Graphene-based research has grown exponentially since the experimental discovery of graphene in 2004 through the mechanical exfoliation of graphite. This synthetic technique realized several fascinating electronic properties of graphene, including the room-temperature half-integer quantum Hall effect and the Dirac fermion behavior of its charge carriers. However, in order for graphene to be technologically feasible, other synthetic techniques had to be discovered that allowed for large-scale production of high quality graphene sheets. Two synthetic routes with such promise are the oxidation and exfoliation of graphite to produce graphene oxide (followed by subsequent reduction) and the chemical vapor deposition of carbonaceous precursors to produce graphene on the surfaces of transition metals such as Cu and Ni. The oxidation of graphite to graphite oxide is a well-researched subject dating back to 1859 when Brodie used potassium chlorate and nitric acid as oxidizing agents. However, the exfoliation of graphite oxide to graphene oxide, a single layer of graphite oxide, was not realized until recently by Ruoff and co-workers. Subsequent reduction of graphene oxide produces reduced graphene oxide, which is structurally similar to graphene. This allowed for a facile, rapid, and scalable solution-phase synthesis of large amounts of reduced graphene oxide sheets. These sheets can be used in a variety of applications including within polymer composites, as transparent conducting electrodes, and within electronics. In order to be feasible for electronic applications, the reduction of graphene oxide is a crucial step, as it restores electrical conductivity to insulating graphene oxide. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is a particularly useful tool to study graphene oxide and its reduction processes because it is an element specific probe that can elucidate electronic structure changes and provide insight into the surface chemistry of graphene oxide. In addition, polarized NEXAFS studies provide information on surface orientation and the extent of substrate alignment when graphene is interfaced with other materials. This technique underlies most of the experimental work that will be presented in this dissertation. Chapter 2 discusses electrophoretic deposition of graphene oxide onto conducting electrodes followed by subsequent chemical reduction by hydrazine. Polarized NEXAFS measurements has been used to probe the surface chemistry, electronic structure, and alignment of the graphene sheets within the electrophoretically deposited films. These measurements indicate that reduction with hydrazine prior to electrophoretic deposition yields smoother and better aligned films as compared to the post-deposition reduction of electrophoretically deposited graphene oxide films. Chapters 3 and 4 discuss the chemical and thermal defunctionalization of graphene oxide, respectively. NEXAFS studies show that vapor-phase phenylhydrazine and hydrazine, when used as reducing agents, result in the highest obtained intensities of the π* resonance relative to the σ* resonance at the C K-edge. In situ NEXAFS heating experiments combined with sheet resistance measurements show a dramatic enhancement of electrical conductance upon heating to 200°C. Interfacial studies of graphene with other materials are of technological importance because several electronic and mechanical applications require graphene to be interfaced with metals, semiconductors, and/or insulators. The chemical vapor deposition of carbon precursors over transition metals such as Cu and Ni provide high-quality graphene crystals interfaced with a metal substrate. NEXAFS spectroscopy is the ideal technique to study the perturbation of graphene's electronic structure and degree of surface hybridization because changes in the π* band can be observed when graphene is interfaced with different materials. Chapter 5 details NEXAFS measurements of graphene grown on metal substrates and presents clear evidence of substrate hybridization between C 2 p z orbitals and d -orbitals on Cu and Ni. A potentially useful material to interface graphene with is Mg for production of light-weight composites with enhanced mechanical properties. In particular, Mg nanoparticles are of interest because of their enhanced thermodynamic stability as compared to the bulk. This is important because the high negative reduction potential and flammability of Mg make it difficult to synthesize. Currently, synthesis of Mg nanoparticles involves the use of expensive Mg precursors. In Chapter 6 we describe two synthetic techniques that involve the electrochemical and solution-phase reduction of inexpensive Grignard reagents to form well-defined Mg nanostructures. Electrochemical reduction of methyl magnesium chloride and ethyl magnesium chloride lead to the formation of phase-pure and crystalline nanowires. Solution-phase reduction of both Grignard reagents leads to the synthesis of hexagonal nanoplatelets.