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

Activated carbon fibers (ACFs) are microporous carbons consisting of a thre e-dimensional disordered network of nano-graphites with a mean in-plane siz e of about 30 Angstrom. We investigated the structure, electronic propertie s 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 varia ble-range hopping conduction and the presence of localized spins, suggestin g the importance of charging effects and the edge-inherited non-bonding sta tes in nano-graphites, the latter being predicted theoretically. Iodine dop ing reduces the charging effect due to the dielectric constant enhanced by the iodine that is accommodated in the micropores. Heat treatment above 130 0 degrees C changes ACFs from an Anderson insulator to a disordered metal b y the development of an infinite inter-nano-graphite percolation path netwo rk for electron transport, accompanied by a change from localized-spin magn etism to itinerant electron magnetism. In the metallic regime, carrier scat tering is subjected to nano-graphite boundaries in terms of a short range r andom potential. Iodine-doping introduces ionized impurity scattering, whic h is caused by the I-3(-) ions generated by the charge transfer from iodine to nano-graphite.