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Chapter 3 – Raman Spectroscopy of Graphene and Graphene-based Strain Sensors 3.2.5. Sensitivity of Raman bands of graphene to strain Changes to crystal phonons can be caused by external perturbations such as doping,55 strain,256, 263-266 etc. Strain alters/affects the crystal symmetry and its phonon frequency.9 In case of graphene, such changes have huge impact on its Raman bands: The tensile strain causes phonon softening (red shift) and the compressive strain causes phonon hardening (blue shift). The rate of this change is given by Gruneisen parameter (𝛾), and is crucial for quantifying amount of strain in the system.267 First detailed investigation on Raman bands of graphene upon straining and its Gruneisen parameters was reported by Mohiuddin et al.256 Clear G band splitting (G+ and G-) was observed (Figure 3.8a,c) at uniaxial strains greater than 0.4 %, among which one component was polarised parallel (G-) to the strain and another was polarised perpendicular (G+) to the strain (Figure 3.8d). Both the components shifted to lower wavenumber with shift rates of ~ -10.8 cm-1/% strain (G+) and ~ -31.7 cm-1/% strain (G-). Whereas no splitting of the 2D band was observed with a measured shift rate of ~ -64 cm-1/% strain (Figure 3.8b,e). Figure 3.8: a) G and (b) 2D band as a function of uniaxial strain (514 nm). The splitting of G band into G+ and G- was observed for strains > 0.4 %, whereas the splitting was not observed for 2D band. Shift of (c) G+ and G- and (e) 2D band position as a function of uniaxial strain. d) Eigenvectors of G+ and G- modes determined by density-functional perturbation theory, showing G- mode is polarised along the strain axis (higher shift rate), and G+ mode is polarised perpendicular to the strain axis (lower shift rate).256 80PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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