Photoconductivity of activated carbon fibers

Abstract

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.

Raman scattering and electrical conductivity in highly disordered activated carbon fibers

“…The average platelet thickness of about 10 A determined by the simple Scherrer analysis25 should also be considered only as an order-of-magnitude estimate. 9 The Transmission Electron Microscopy (TEM) picture shown in Fig. 8, though for a pitch-based ACF, is believed to be typical of the microstructure of ACF's.…”

“…Both the charge transfer effect and the chargetransfer-induced lattice contraction effect, as similarly observed in acceptor-doped Graphite Intercalation Compounds, 11 -26 could still play some role in producing a frequency shift of the graphitic peak in ACF's since ACF's are observed to be p-type at room temperature. 9 The calculations for the frequency shift in Ref. 26, however, cannot be taken literally for ACF's that have localized states instead of conduction states.…”

“…Before we discuss our conductivity results, x-ray 9 and TEM results are reviewed, along with the proposition of two simple models that can explain the high SSA observed in ACF's, so that a picture of the microstructure of ACF's can be drawn and then used to analyze the transport results. First, the very presence of a well-resolved graphitic peak in the Raman spectra indicates that the microcrystallites are quite graphitic in structure.…”

“…Di Vittorio 8 made a preliminary survey of the temperature dependence of the dc electrical conductivity, the magnetoresistance, the thermal conductivity, and the thermopower of pitch-based ACF's with specific surface area (SSA) of 1000 m 2 /g. Kuriyama and Dresselhaus 9 have done a more thorough study of the dc conductivity and the photoconductivity over a temperature range of -30 K < T < 300 K for phenolic ACF's with SSA's ranging from 1000 m 2 /g to 2000 m 2 /g. The conduction mechanism in disordered solids is generally known to be governed by hopping.…”

Photoconductivity of carbon aerogels

“…1^5 These observations provide useful information for understanding the electronic properties, especially for the structural defects, the carrier transport, and the recombination. So far, photoconductivity in vapor-grown carbon fibers, 1 evaporated carbon films, 2 pyrolyzed cellophane films, 3 and activated carbon fibers (ACF) 4 ' 5 has been reported. Kuriyama and Dresselhaus 4 ' 5 measured the temperature dependence of the photoconductivity and its decay time in ACF's, and explained their experimental results in terms of localized states and hopping conduction.…”