Raman scattering and electrical conductivity in highly disordered activated carbon fibers

Because of their unusually large specific surface area (SSA), Activated Carbon Fibers (ACF's) have a huge density of micropores and defects. The Raman scattering technique and low-temperature dc electrical conductivity measurements were used as characterization tools to study the disorder in ACF's with SSA ranging from 1000 m²/g to 3000 m²/g. Two peaks were observed in every Raman spectrum for ACF's and they could be identified with the disorder-induced peak near ∼1360 cm⁻¹ and the Breit–Wigner–Fano peak near ∼1610 cm⁻¹ associated with the Raman-active E₂g₂ mode of graphite. The graphitic nature of the ACF's is shown by the presence of a well-defined graphitic structure with L_a values of 20–30 Å. We observed that the Raman scattering showed more sensitivity to the precursor materials than to the SSA of the ACF's. From 4 K to room temperature, the dc electrical resistivity in ACF's is observed to follow the exp [(T₀/T)^1/2] functional form and it can be accounted for by a charge-energy-limited tunneling conduction mechanism. Coulomb-gap conduction and n-dimensional (n ≤ 3) variable-range hopping conduction models were also considered but they were found to give unphysical values for their parameters.