The many mysteries of graphene oxide
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Graphene, the first two-dimensional crystal ever found, is a material that has attracted fervent and sustained interest from condensed matter researchers from around the world. It has a unique and unprecedented band structure in a bulk material: the bands near the Fermi level are linear, leading to massless charge carriers that propagate at the speed of light. However, graphene does not possess a band gap, and as such, it cannot be used to process information in any electronic device that uses digital logic. Graphene is oxidized when several different basic functional groups like hydroxyls, carboxyls, and epoxides bond to the hexagonal carbon basal plane to make graphene oxide (GO). The result is a nonstoichiometric and highly disordered system that, according to the results shown in this thesis, consists of zones of densely-packed functional groups interspersed between zones of relatively small functional group concentration. This has been confirmed by DFT calculations presented here, which is the first time that a successful simulation of the GO density of states has been compared to X-ray data. Contrary to many assumptions in the literature, many of the features in the density of states of GO are due not to carbon sites bonded to functional groups, but are due to nearby non-functionalized carbon sites. The band gap of graphene oxide is principally controlled by oxidation level. Reduction, followed by heating, will regenerate the near-Fermi states and close the band gap significantly as has been seen by others. However, heating non-reduced graphene oxide can also result in a much-reduced band gap, which occurs because intercalated water can react with the heated GO sample to remove functional groups by creation and eventual expulsion of carbon dioxide. The band gap of GO is further complicated by stacking effects if it is multilayered, because residual pi-conjugated states in neighboring planes interact. The two major types of stacking in graphite are AA-stacking and AB-stacking. AA-stacking interactions cause the pi * resonance to broaden and push states to lower energy, which means that AA-stacking determines the width of the gap in highly oxidized samples. However, direct oxidation of graphene is not the only way that one alter the electronic structure of GO. Other results presented here also show that non-covalent functionalization of graphene oxide by amorphous solid water is a powerful, reversible way to dramatically change the GO electronic structure.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentPhysics and Engineering Physics
CommitteeXiao, Chijin; Bradley, Michael; Tse, John; Kelly, Timothy; Banerjee, Sarbajit
Copyright DateDecember 2013
density functional theory