Transport Measurements in Intercalated Graphite and Fullerene Compounds.

We have investigated the transport properties of fluorine-intercalated vapor-grown graphite fibers with concentrations spanning the range C_{2.9 }F to C_{6.6}F, corresponding to stage I and stage II samples. The samples were characterized by X-ray diffraction, weight uptake measurements and X-ray Photo-electron Spectroscopy (XPS). Among all Graphite Intercalation Compounds (GICs), fluorine is unique in that intercalation introduces a large disorder into the very ordered pristine graphene planes. Intercalation with an acceptor compound also increases the anisotropy of the conductivity to a value ~10 ^5-10^6. Therefore, dilute samples (up to C_{3.6} F) exhibit weak localization and carrier-carrier interaction effects typical of a two-dimensional disordered metal. As the fluorine concentration increases, the bonding between fluorine and carbon shifts from ionic to covalent, which reduces the carrier concentration, resulting in a metal-insulator transition and strong localization. Magnetotransport at low temperatures (0.5K-30K) has been used to thoroughly investigate the effect of disorder on the electronic properties of both metallic and semiconducting two-dimensional solids. The disorder present in the fluorine GICs was characterized by Transmission Electron Microscopy (TEM) as well as Electron Spin Resonance (ESR). The distribution of fluorine throughout the graphene layers is inhomogeneous, with the formation of both dilute and concentrated islands, about 70 A in diameter. The bonding between carbon and fluorine is predominantly ionic within the dilute islands, whereas it is mostly covalent within the concentrated islands. As a result, the dilute islands contain some conduction electrons, whereas the concentrated islands contain mostly localized electrons. Another feature of the disorder is the waviness of the graphene planes, which increases with increasing fluorine concentration. The disorder is explained by a model of nucleation of a covalent fluorine phase within the ionic matrix at high fluorine concentration, where the waviness results from ^^3 covalent bonds which push a carbon atom out of the graphene planes. The last chapter, which is independent from the main body of this thesis, describes the intercalation of C_{60} with Rubidium, and an Electron Spin Resonance study of the various Rb _{rm x}C _{60} samples prepared. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).