Magnetotransport at the metal-insulator transition in fluorine-intercalated graphite fibers

Abstract

We have investigated the transport properties of high-fluorine-concentration fluorineintercalated vapor-grown graphite fibers (C2 9F and C3,0F). As previously reported, the transport properties of dilute fluorine-intercalated graphite fibers (C F with x )3. 6) exhibit a weak disorder regime with a strong carrier-carrier interaction correction. These corrections to the metallic properties of the C3 6F fibers emphasize the presence of intercalation-induced disorder, which is believed to be associated with the semi-ionic bonding of fluorine in carbon. With increasing fluorine concentration (x (3. 0), the fibers undergo a transition from a metallic to an insulating regime. The temperature dependence of the resistivity of the fibers in the insulating regime is no longer logarithmic at low temperature as is measured for less concentrated samples (x ) 3. 6), but rather fits a two-dimensional Mott law quite closely. From the temperature dependence of the resistivity and the nonlinear I-versus-V relationship at high electric fields, a localization length on the order of (= 1000 A is extracted. This very large localization length shows that the fibers with C2.9F and C3.0F stoichiometries lie close to the metal-insulator transition. For the C2 9F and the C3 OF fibers, the transverse magnetoresistance is negative at low fields, but becomes positive at higher fields, whereas the longitudinal magnetoresistance is positive for all fields. Both the transverse and longitudinal magnetoresistance saturate at high field and low temperature. These features are explained by the superposition of an orbital quantum-interference-induced negative magnetoresistance and a spin-polarization-induced positive magnetoresistance.

A model for disorder in fluorine-intercalated graphite

“…Formation of a covalent C -X bond releases the enormous bonding energy of E C -F = -114 kcal/mole in the case of fluorine, and the lesser bonding energy £c-Br = -65 kcal/mole in the case of bromine. When halogen bonds covalently to carbon, the carbon-carbon bond is changed from sp 2 to sp 3 . Since the sp 2 C -C bonding energy E sp i = -103 kcal/mole is larger than the sp 3 C -C bonding energy E sp i = -8 3 kcal/mole, the actual energy gain Equation (2) was reached by considering that when a halogen atom bonds covalently to carbon, the three neighboring carbon-carbon bonds are transformed from sp 2 bonds into sp 3 bonds.…”

“…After covalent bonding with a halogen atom, the carbon atom is in an sp 3 hybridization state, with bonds to three carbon atoms and one halogen atom. The sp 3 bonded carbon reaches its minimum in energy when the four bonds form a regular tetrahedron, i.e., for an angle of 109°28'. Since the minimum in energy of the neighboring carbons requires 120° angles between bonds, covalent bonding of a carbon to a halogen introduces a large strain into the planar structure.…”

“…The energies of the various carbon-carbon and carbon-halogen bonds used in this model are summarized in Table II. As mentioned above, the resonance energy of graphite is E ies = -6 kcal/mole. Finally, the strain energy can be estimated based on the bonding energy of the C -C sp 3 bonds. If both carbon atoms had sp 3 orbitals pointing toward each other, the sp 3 bonding energy would be -83 kcal/mole.…”

“…Finally, the strain energy can be estimated based on the bonding energy of the C -C sp 3 bonds. If both carbon atoms had sp 3 orbitals pointing toward each other, the sp 3 bonding energy would be -83 kcal/mole. Actually, one carbon has sp 3 orbitals and the other one has sp 2 orbitals, which significantly reduces the overlap of the orbitals of the two carbon atoms, which is described in terms of strain energy.…”

“…7 In contrast, most fluorine GIC's are more resistive than pristine graphite, and beyond a concentration of C4F, they exhibit a semiconducting dependence of resistivity on temperature. 3 For another thing, the metallic bonding of the intercalate to the carbon atoms (through the p z orbitals) is much too weak to disturb the very strong sp 2 in-plane carbon-carbon bonding, 8 so that the very stiff and planar graphene sheets remain essentially undisturbed by the intercalation process. In contrast, this planarity of the carbon planes does not persist after intercalation of fluorine, and TEM micrographs show that the carbon planes in fluorine GIC's are buckled.…”