Raman-scattering and transmission-electron-microscopy studies of fluorine-intercalated graphite fibersCxF (7.8≥x≥2.9)

Abstract

Raman-scattering and transmission-electron-microscopy (TEM) experiments are reported which provide information on the microstructure of fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) for stoichiometries in the range 7.8≥x≥2.9. Lorentzian fits to our Raman spectra indicate the presence of a doublet near 1600 ${\mathrm{cm}}^{\mathrm{-}1}$ and a broad line near 1360 ${\mathrm{cm}}^{\mathrm{-}1}$. An anomalous frequency downshift of the doublet was observed in ${\mathrm{C}}_{\mathit{x}}$F 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 ${\mathrm{C}}_{\mathit{x}}$F 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 ${\mathrm{C}}_{\mathit{x}}$F fibers increases to high values, the amount of waviness increases.