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Chapter 3 – Raman Spectroscopy of Graphene and Graphene-based Strain Sensors is a double resonance (DR) process (Figure 3.4b):240 the excited charge carrier undergo one-elastic scattering by a defect and one-inelastic scattering by an iTO phonon, satisfying the momentum conservation. The second-order 2D band is the overtone of D band, appearing approximately at twice the frequency of the D band, is also activated by DR mechanism. However, with 2D band, the excited charge carrier undergoes two-inelastic scattering process by iTO phonons (Figure 3.4d) before momentum is conserved. Hence, unlike the D band, 2D band doesn’t need a defect to be active, and is always present in graphitic systems with hexagonal carbon symmetry. Figure 3.4: a) Schematic representation of A1g breathing mode.9 b, c) Second order DR process involving one phonon for the D band (intervalley process) and D’ band (intravalley process), respectively. d) Second order DR process involving two phonons for the 2D band. e) Two phonon involved triple resonance process in SLG.240 The D band also gives rise to another disorder band: D’ band at ~1620 cm-1 (close to G band). This D’ band is analogous to the D band with respect to DR mechanism, in which D’ band corresponds to “intravalley” scattering (K to K) (Figure 3.4c), whereas D band is “intervalley” scattering (K to K’) (Figure 3.4b) process.240 The D band and the D’ band are highly sensitive to the disorder or defect, so in highly crystalline SLG these bands do not appear apart from the edges (Figure 3.2b).48, 240 Due to the linear electron dispersion near the Fermi level, the D band, D’ band and 2D bands are dispersive with different laser energies. They show a blue shift in their frequency with increasing laser excitation energy: D band – 50 cm-1/eV,243 D’ band – 10 cm-1/eV,243 and 2D band – 88-100 cm-1/eV.243, 244 75PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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