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 m 2 /g to 3000 m 2 /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 E 2g2 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 A. 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[(r o /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.

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The conductivity and photoconductivity are measured on a high-surface-area disordered carbon material, i.e., activated carbon fibers, to investigate their electronic properties. This material is a highly disordered carbon derived from a phenolic precursor, having a huge specific surface area of 1000-2000 m 2 /g. Our preliminary thermopower measurements show that the dominant carriers are holes at room temperature. The x-ray diffraction pattern reveals that the microstructure is amorphous-like with L c -10 A. The intrinsic electrical conductivity, on the order of 20 S/cm at room temperature, increases by a factor of several with increasing temperature in the range 30-290 K. In contrast, the photoconductivity in vacuum decreases with increasing temperature. The magnitude of the photoconductive signal was reduced by a factor of ten when the sample was exposed to air. The recombination kinetics changes from a monomolecular process at room temperature to a bimolecular process at low temperatures, indicative of an increase in the photocarrier density at low temperatures. The high density of localized states, which limits the motion of carriers and results in a slow recombination process, is responsible for the observed photoconductivity.

The electronic transition from localized to delocalized states of carriers in a disordered carbon material is investigated by photoconductivity measurements. Phenol-derived activated carbon fibers, where the carriers are strongly localized due to disorder, are heat treated in the range 300–2500 °C to give rise to the insulator-metal transition. Dark conductivity, Raman spectra, and x-ray diffraction patterns are also measured to characterize their structural changes. As a result, the transition temperature was determined to be rather low, around 1000 °C, considering the rapid decrease in the photoconductivity above this temperature. This decrease was ascribed to a fast recombination between the photoexcited carriers and the delocalized carriers generated by heat treatment.

An alternative expression for the temperature dependence of hopping conductivity is proposed. A conduction model is proposed based on a collection of many independent Arrhenius-type processes. The density in the material considered has a A-shaped distribution as a function of activation energy, for example Gaussian, isosceles, and scalene distributions. The validity of the model has been checked with the electrical-conductivity data of disordered carbon fibers which show a metal-insulator transition. The result is that the conductivity data between 4 and 250 K fit well to the form T [1exp( E/kT-)]', where T is temperature and E activation energy, and is related to the degree of disorder in the system. This form is the simplest form derived from the isosceles distribution; however, a better fit is obtained from the scalene distribution with more complex form. By using the proposed conduction model, the activation energy E is found to decrease systematically as the insulator-metal transition is approached by heat treatment. Also this model can yield the widely observed fractional temperature dependences of hopping conduction: x = 1, 2, -, ', and -' in the conventional form exp -( To/T) .