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Raman spectroscopy is based on the inelastic light scattering of a laser source in the near IR–UV range. Backscattered photons come out with a lower (ν0νvibr) or higher (ν0 + νvibr) energy with respect to the incoming laser photons (ν0). The energy difference (νvibr) corresponds to the vibrational mode. A Raman active vibrational mode should involve a change in the electric polarizability. This powerful technique is widely adopted in the characterization of carbon-derived materials, both crystalline [e.g., HOPG (highly oriented pyrolytic graphite), nano-graphite] and amorphous (e.g., turbostratic carbon, carbon fibers etc.).

The Raman spectrum of perfect graphite (e.g., HOPG) with large crystalline domains is dominated by a sharp peak centred at 1580 cm−1 (historically indicated as the G peak, first order, see Figure 4.1(a), top), which is attributed to a E2g mode (Figure 4.1(c), part A). In addition, a complex envelope of bands appears in the 2800–2600 cm−1 region (second order), mostly due to combination modes. In particular, two main peaks are observed in the spectrum of perfect graphite, originally indicated as G2 (high frequency side, sharp) and G1 (low frequency side, broad).1–3 

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