Raman Spectroscopy of Carbon Materials : Structural Basis of Observed Spectra 557

The firstand second-order Raman spectral features of graphite and related sp2 carbon materials were examined with laser wavelengths ranging from 293 to 1064 nm. A wide range of carbon materials was considered, including highly ordered pyrolytic graphite (HOPG), powdered and randomly oriented graphite, and glassy carbon prepared at different heat-treatment temperatures. Of particular interest is boron-doped highly ordered pyrolytic graphite (BHOPG), in which boron substitution decreases local lattice symmetry but does not disrupt the ordered structure. New second-order bands at 2950,3654, and -4300 cm-' are reported and assigned to overtones and combinations. The D band at 1360 cm-l, which has previously been assigned to disordered carbon, was observed in ordered boronated HOPG, and its overtone is strong in HOPG. The observed Raman shift of the D band varies with laser wavelength, but these shifts are essentially independent of the type of carbon involved. It is concluded that the D band results from symmetry breaking occurring at the edges of graphite planes in sp2 carbon materials or at boron atoms in BHOPG. The observations are consistent with the phonon density of states predicted for graphitic materials, and the fundamental and higher order Raman features are assignable to theoretically predicted lattice vibrations of graphite materials. The laser wavelength dependence of the D band frequency appears to result from scattering from different populations of phonons, perhaps through a resonance enhancement mechanism. However, the results are inconsistent with resonance enhancement of graphite microcrystallites of varying size.