Raman spectroscopy has historically played an important role in the structural characterization of graphitic materials, in particular providing valuable information about defects, stacking of the graphene layers and the finite sizes of the crystallites parallel and perpendicular to the hexagonal axis. Here we review the defect-induced Raman spectra of graphitic materials from both experimental and theoretical standpoints and we present recent Raman results on nanographites and graphenes. The disorder-induced D and D′ Raman features, as well as the G′-band (the overtone of the D-band which is always observed in defect-free samples), are discussed in terms of the double-resonance (DR) Raman process, involving phonons within the interior of the 1st Brillouin zone of graphite and defects. In this review, experimental results for the D, D′ and G′ bands obtained with different laser lines, and in samples with different crystallite sizes and different types of defects are presented and discussed. We also present recent advances that made possible the development of Raman scattering as a tool for very accurate structural analysis of nano-graphite, with the establishment of an empirical formula for the in- and out-of-plane crystalline size and even fancier Raman-based information, such as for the atomic structure at graphite edges, and the identification of single versus multi-graphene layers. Once established, this knowledge provides a powerful machinery to understand newer forms of sp² carbon materials, such as the recently developed pitch-based graphitic foams. Results for the calculated Raman intensity of the disorder-induced D-band in graphitic materials as a function of both the excitation laser energy (E_laser) and the in-plane size (L_a) of nano-graphites are presented and compared with experimental results. The status of this research area is assessed, and opportunities for future work are identified.

This work presents a systematic study of the ratio between the integrated intensities of the disorder-induced $D$ and $G$ Raman bands $(I_D∕I_G)$ in nanographite samples with different crystallite sizes $\left(L_a\right)$ and using different excitation laser energies. The crystallite size $L_a$ of the nanographite samples was obtained both by x-ray diffraction using synchrotron radiation and directly from scanning tunneling microscopy images. A general equation for the determination of $L_a$ using any laser energy in the visible range is obtained. Moreover, it is shown that $I_D∕I_G$ is inversely proportional to the fourth power of the laser energy used in the experiment.

The origin and dispersion of the anomalous disorder-induced Raman band $(D$ band) observed in all ${\mathrm{sp}}^2$ hybridized disordered carbon materials near 1350 ${\mathrm{cm}}^{-1}$ is investigated as a function of incident laser energy. This effect is explained in terms of the coupling between electrons and phonons with the same wave vector near the K point of the Brillouin zone. The high dispersion is ascribed to the coupling between the optic phonons associated with the D band and the transverse acoustic branch. The large Raman cross section is due to the breathing motion of these particular phonons near the K point. Our model challenges the idea that the Raman D peak is due to laser-energy-independent features in the phonon density of states, but rather is due to a resonant Raman process.