Novel electronic properties of a nano-graphite disordered network and their iodine doping effects

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 Angstrom. We investigated the structure, electronic properties and iodine doping effects for ACF samples heat-treated up to 2800 degrees C. The samples heat-treated below 1000 degrees 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 degrees 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 I-3(-) ions generated by the charge transfer from iodine to nano-graphite.