RAMAN-SCATTERING AND TRANSMISSION-ELECTRON-MICROSCOPY STUDIES OF FLUORINE-INTERCALATED GRAPHITE FIBERS CXF (7.8-GREATER-THAN-OR-EQUAL-TO-X-GREATER-THAN-OR-EQUAL-TO-2.9)

Raman-scattering and transmission-electron-microscopy (TEM) experiments are reported which provide information on the microstructure of fluorine-intercalated graphite fibers (CxF) for stoichiometries in the range 7.8 greater-than-or-equal-to x greater-than-or-equal-to 2.9. Lorentzian fits to our Raman spectra indicate the presence of a doublet near 1600 cm-1 and a broad line near 1360 cm-1. An anomalous frequency downshift of the doublet was observed in CxF fibers upon increase in the intercalate concentration, unlike the behavior observed in other acceptor graphite intercalation compounds. This downshift in frequency of the doublet can be understood in terms of a decrease in the hole concentration in the graphene layers with increasing fluorine concentration, resulting in an increase in the in-plane lattice constant and a decrease in the in-plane mode frequency. These Raman results are consistent with high-resolution TEM experiments on the same fibers. Lattice fringe images of CxF fibers show wavy fringes, and the waviness is identified with semi-ionic (covalent) bonding. Evidence for unintercalated graphite regions is also presented. As the fluorine concentration of the CxF fibers increases to high values, the amount of waviness increases.