Inter-valley phonons

The second-order Raman spectra from 1,700 to 3,300 cm are relatively weak and broad compared to the first order Raman spectra. There are many papers devoted to the second-order Raman processes in graphite [49, 59-62] and carbon nanotubes [8, 34, 63]. The interpretation of the spectral behavior of the second-order bands in terms of DR have been done [3, 64, 65]. The corresponding phonon is involved in an inter-valley scattering process of in-plane modes (LO or TO) around the K-points in the two-dimensional (2D) Brillouine zone (BZ, Fig. 7.4) [66, 67]. 

In conclusion, the D and 2D peak are activated by an inter-valley resonant Raman scattering process. Consequently, these peaks are strongly sensitive to changes in the electronic structure, i.e. the phonon probed by Raman spectroscopy strongly depends on the electronic band structure. Thus, every change in the electronic structure will be reflected in a change of the D and 2D peaks. This effect allows identification of graphene. 

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