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Chapter 3 – Raman Spectroscopy of Graphene and Graphene-based Strain Sensors 3.2.2. Sensitivity of Raman bands of graphene to number of layers Raman spectra of graphene are sensitive to change in number of layers. Ferrari et al. reported the change in 2D band shape, frequency (position) and width (FWHM) with number of layers.48 SLG is characterised by a single Lorentzian peak with an intense and narrow FWHM(2D) ~ 24-30 cm-1. The intensity of 2D band is ~2-4 times more than G band for SLG,48 and can be explained by the triple resonance process (Figure 3.4e).240 This characteristic feature is often used to identify SLG using Raman spectroscopy. As the number of layers increases in Bernal stacked graphene, the evolution of electronic band structure means that different DR process becomes possible, each contributing to different components in the 2D band.48, 240 For instance, in Bernal stacked BLG, four DR processes are possible resulting four components of its 2D band: 2D1B, 2D1A, 2D2A and 2D2B (of which 2D1A and 2D2A have relatively higher intensities than other two) each having FWHM(2D) ~24 cm-1, with higher frequency.48 In Bernal stacked tri-layer graphene (TLG), the 2D bands becomes much broader and splits into 15 components as described by Malard et al.,240 but are usually difficult to deconvolute, so generally fitted with six components with FWHM(2D) ~24 cm-1. For more than 5 Bernal stacked graphene, only two peaks; 2D1 and 2D2, similar to bulk graphite, can be observed.48 Figure 3.5: a) Evolution of Raman spectra at 514 nm (2.41 eV) and 633 nm (1.96 eV) with the number of layers. b) DR process 2D band for BLG. c) The four components of the 2D band in BLG at 514 nm and 633 nm.48 In the case of turbostratic graphene, the electronic structures of the individual layers are decoupled, and so the 2D band retains the single Lorentzian form (Figure 3.6a,b).240, 245-247 However, broadening of 2D band, with FWHM(2D) ~ 40-60 cm-1,243, 245, 248 of turbostratic graphene is generally observed due to its polycrystalline nature.249 76PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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