Photoconductivity of Highly Disordered Carbon Fibers
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PHOTOCONDUCTIVITY OF HIGHLY DISORDERED CARBON FIBERS KAZUYOSHII KURIYAMA*, MILDRED. S.DRESSELHAUSS* ISumitomo Metal Industries Ltd., Sunayamal6, Hlasaki, Kashima, Ibaraki, Japan "M=assachusetts Institute of Technology, Electrical Engineering and Physics, 77 Massachusetts Avenue, Cambridge, NA 02139 ABSTRACT A highly disordered carbon material, activated carbon fibers, is investigated through bulk conductivity and photoconductivity measurements. This material has a high density of defect states introduced by an aqtivation process that leads to a huge specific surface area of up to 2000m •/g. The conductivity increases by a factor of 4-10 with increasing temperature from 30K to 290K. In contrast, the photoconductivity decreases by a factor of three with increasing temperature. The1 relaxation time of the photoconductivity is rather long(on the order 1O0 sec), indicating that the recombination process proceeds through localized states. flopping processes are used to interpret the transport properties of this material. INTRODUCTION Photoconductivity data provide useful information about defect states in disordered materials because the relaxation time, temperature dependence and photon intensity dependence reflect the properties of the defect states. In carbon materials, the photoconductivities for an evaporated carbon film[l] and for vapor-grown carbon fibersE2] have been reported. The material investigated in this paper is actiyated carbon fibers(ACFs) with a huge specific surface area(SSA) up to 20O0mZ/g, where a high density of dangling bonds, vacancies and other defects are contained. These defects will cause a wide range Anderson localization of carriers in this disordered material [3,4,5], which consists basically of 2-dimensional graphene lay~er fragments. In the present paper, •phenol-derived ACFs with SSA=I0OO- 2OOm'/g and AC~s with the same SSA(IOOOm Z/g) but derived from three different precursors are used for the conductivity and photoconductivity measurements. The temperature dependence of the bulk conductivity and photoconductivity, decay time of the photoconductivity and photon intensity dependence of the photoconductivity are examined in order to understand the transport properties. SAMPLES AND EXPERIMENTAL DETAILS The studied ACFs are derived from PAN[6], cellulose[7] and phenol[8] precursors, which are activated at temperatures 1100-1400K in 0?. H20, C02 or other oxidizing atmospheres. In contrast with heat treatments in vacuum, a consequence of this heat treatment in an oxidizing atmosphere is to introduce disorder into the material. After the heat treatment processing, th9 ACFs have a diameter of around lOum. a conductivity of thQ order 101 S/cm at room temperature and a specific surface area up to 2000m £/g. The str ucture of the fibers is almost amorphous with an estimated Lc of 911 A(about three interlayer spacings), according to X-ray diffraction patterns as shown in Fig.1I. The dominant conduction type is found to be p-type at room temperature with a rough measurement of the thermoelectric pow
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