Abstract
Activated carbon fibers (ACFs) are microporous carbons consisting of a three-dimensional disordered network of nano-graphites with a mean in-plane size of about 30 Å. We investigated the structure, electronic properties and iodine doping effects for ACF samples heat-treated up to 2800°C. The samples heat-treated below 1000°C exhibit Coulomb gap variable-range hopping conduction and the presence of localized spins, suggesting the importance of charging effects and the edge-inherited non-bonding states in nano-graphites, the latter being predicted theoretically. Iodine doping reduces the charging effect due to the dielectric constant enhanced by the iodine that is accommodated in the micropores. Heat treatment above 1300°C changes ACFs from an Anderson insulator to a disordered metal by the development of an infinite inter-nano-graphite percolation path network for electron transport, accompanied by a change from localized-spin magnetism to itinerant electron magnetism. In the metallic regime, carrier scattering is subjected to nano-graphite boundaries in terms of a short range random potential. Iodine-doping introduces ionized impurity scattering, which is caused by the I3- ions generated by the charge transfer from iodine to nano-graphite.
