Fundamentals, overtones, and combinations in the Raman spectrum of graphite

Raman spectra of highly oriented pyrolytic graphite (HOPG) and pyrolytic graphite (PG) have been investigated in the region between 200 and 7000 cm^-1. The high signal-to-noise ratio measurements reveal the existence of new bands in the fundamental and the higher-order regions. The excitation wavelength (λ) dependence of Raman spectra is also studied using 457.9-, 488.0-, and 514.5-nm excitation. It is found that the λ dependence of Raman bands is classified into three types, i.e., upward shifted, insensitive, and downward shifted bands; with the increase of the excitation wavelength. The ratios of the relative intensities of Raman bands against the E_2g mode (1580 cm^-1) between the edge and the basal planes (I¯_edge/I¯_basal) are examined. The fundamental and higher-order modes which become Raman active by degradation of translational symmetry increase their relative intensities in the edge plane. On the other hand, the overtone and combination modes satisfying the wave-vector selection rule by the combination of nonzero wave vectors with opposite sign show similar relative intensities between the basal and the edge planes. The assignments of Raman bands is conducted using the λ dependence and I¯_edge/I¯_basal for HOPG and PG. Most of the Raman bands in the higher-order region can be successfully assigned as the overtones and combinations between G, D, and D′ modes.

It is suggested that the additional fundamental modes exist at 810 and 1080 cm^-1 which are due to features in the density of states other than the 1355- and 1620-cm^-1 modes. These fundamentals show λ dependence opposite to the 1355-cm^-1 mode. Although some of the observed fundamentals or higher-order modes for HOPG (∼1480 and 1754 cm^-1) and PG (1469 cm^-1) in this work cannot be completely explained by the existing density-of-states calculation, it is definitely denied that they are due to adsorbates, oxidized species, and other impurities.