Dispersion effects of Raman lines in carbons

Raman spectra of highly oriented pyrolytic graphite (HOPG), natural polycrystalline graphite (PCG), high pressure graphite (HPG), and low-temperature pyrolyzed polyfurfuryl alcohol (PFA) have been measured. Comparing the Raman spectra of each sample illuminated with laser beams of 488.0 nm and 1064.0 nm wavelengths, respectively, unambiguous dispersion effects have been observed for the D′-line in HOPG and in other forms of PCG as well as for their D lines. Comparisons of the intensity ratios of the D′ line to the D line (ID′/ID) of HOPG and polycrystalline indicate that the dispersion effects of the D′ line and the D line may have the same physical origin. In addition, it has also been observed that the G-line position of low-temperature pyrolyzed PFA is sensitive to the excitation wavelength while that of HOPG, PCG, and HPG is not. This phenomenon is attributed to the nanometer size of the microcrystalline graphites by analyzing the factors that may induce such a dispersion effect.