Raman Studies of Fluorine-Intercalated Carbon Fibers
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RAMAN STUDIES OF FLUORINE-INTERCALATED CARBON FIBERS A. M. RAO-, A. W. P. FUNG*, M. S. DRESSELHAUS*, G. DRESSELHAUS*, M. ENDOt AND T. NAKAJIMAt *Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 tShinshu University, Nagano 380, Japan tKyoto University, Sakyo-ku, Kyoto 606, Japan
ABSTRACT We report Raman scattering measurements on fluorine-intercalated graphite fibers (CQF) for stoichiometries x =7.8, 4.5 and 2.9. Lorentzian fits to our Raman lineshapes indicate the presence of two lines around 1600 cm- 1 and a broad line around 1355 cm- 1 . The 1355 cm- 1 line is the disorder-induced graphite line and the ratio of the integrated intensity of this line to that of the 1600 cm- 1 doublet (R) provides a measure of the intercalationinduced disorder in the CjF fibers. Both TEM and Raman studies indicate the presence of unintercalated graphitic regions in the CjF fiber. The inverse relation between the average crystal planar domain size L, and the Raman intensity ratio R yields L. ,- 52A for C7.8 F fibers and L. , 40A for C 4.,F and C 2. 9F fibers.
INTRODUCTION Fluorine-intercalated graphite is known to exhibit anomalous behavior relative to other graphite intercalation compounds (GICs), such as a increase in the room-temperature dc resistivity by more than three orders of magnitude with an increase in the fluorine concentration from C10 .sF (p -' 2 x 10 /Atl-cm) to C 2.9 F (p -" 8 x 10' pQf-cm)[1,2]. Electricaltransport experiments[2] on C.F fibers show that the CF fibers undergo a transition from a weakly localized system (x > 3.6) to a strongly localized system (x < 3.6). The occurrence of such localization effects implies the presence of structural defects in the CQF fibers[2]. Experimental Raman spectra on graphite hydrofluorides CFj_8 (HF) [2] show that the Raman-active graphitic mode downshifts in frequency with increasing fluorine concentration, unlike other acceptor intercalation compounds[3]. In this paper we present Raman scattering results for CjF fibers and correlate these results with our high resolution transmission electron microscopy (TEM) images, thereby providing new information on the microstructure of the CF fibers.
EXPERIMENTAL DETAILS Vapor-grown-carbon-fibers (VGCF) that were heat-treated for 15 minutes at 2950°C were then intercalated with fluorine at 200'C in a nickel reactor tube[l]. The sample composition was determined from the weight uptake measurements and the c-axis lattice constant of the fluorine-intercalated fibers was determined by x-ray diffraction measurements. Preliminary TEM observations were made on C 4.5 F fibers using a JEOL 4000 CX instrument with a LaB 6 filament. Lattice fringe images on the periphery of the fluorinated fibers were obtained and the fibers were found to be very stable under beam irradiation and under the high vacuum conditions of the TEM. For the Raman scattering experiment, the Brewster angle back-scattering geometry was used in the Spex 1402 double grating monochromator. Mat. Res. Soc. Symp. Proc. Vol. 209. 91991 Materials Research Soci
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