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Chapter 3 β Raman Spectroscopy of Graphene and Graphene-based Strain Sensors Whereas the 2D band position was shown to blue shift (10-20 cm-1) with increasing number of turbostratic layers (Figure 3.6a).247, 248 Figure 3.6: a) Raman 2D band of SLG, and turbostratic BLG and TLG of epitaxial graphene grown on SiC (488 nm).242 b) Raman spectra of CVD grown BLG and FLG.244 c) πΌπΊ βπΌ2π· vs number of layers in MC graphene.250 d) Change in G band position with number of layers (Inset show the change in the G band intensity and position).250 Hence when dealing with epitaxial grown graphene, precise determination of number of layers using 2D band shape can be challenging. Therefore intensity of 2D band to G band, πΌ2π·βπΌπΊ, is normally considered for turbostratic graphene. In general, πΌ2π·βπΌπΊ > 1 is considered as SLG, and πΌ2π·βπΌπΊ ~ 1 is evidenced as turbostratic FLG.240, 245 The G band is also sensitive to number of layers and its intensity has been shown to increase with number of layers.250, 251 This method was followed by Ni et al. to determine number of layers in their experiments.251 Graf et al. have shown that the intensities of G band to 2D band, πΌπΊβπΌ2π· (inverse of πΌ2π·βπΌπΊ), increases with number of layers (Figure 3.6c).250 They have also showed that the G band position shifts to lower wavenumber as the number of layers increases (Figure 3.6d). Similar results were reported by Das et al.252 3.2.3. Sensitivity of Raman bands of graphene to disorders The otherwise forbidden D and Dβ bands appear in Raman spectra of graphene when Raman active defects such as edges, vacancies or sp3-type are present in graphene lattice.9, 57, 253 However, Raman inactive defects such as charged impurities,55, 254 intercalants,255 and strain,256, 257 do not generate D band, but affects the G and 2D bands characteristics.9 The edge of graphene acts as defects, generating the D band, except for perfectly aligned zigzag edge.56 Whereas, the Dβ band appears for both zigzag and armchair edges, 77PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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