Spatial dependence of observed Raman frequencies and disorder in graphene monolayers

A micro-Raman investigation was carried out on several flakes of monolayer (1 L) graphene obtained by the micro-mechanical exfoliation technique and, then, put on a c-Si wafer coated by a SiO2 thin film. Some of the 1 L zones show a remarkable dispersion of the 2D-overtone wavenumber as a function of the position within the graphene sheet, and, in some case, a significant broadening of the E2g phonon (G band) is associated to this wavenumber shift of 2D-band. Such effects were studied, in particular, for a 1 L zone characterized by a rather strong lattice disorder, as revealed by the strong D/G band intensity ratio, and for other zones quite ordered, showing a vanishing intensity of the D band. Moreover, by moving along different directions within 1 L graphene sheets, different trends for 2D wavenumber and E2g phonon bandwidth vs. position were observed. All these reported behaviours are explained in terms of different distributions of intrinsic uniaxial strain occurring within the 1 L graphene sheets.