Probing Graphene Edges via Raman Scattering

We present results of a Raman scattering study from the region near the edges of n-graphene layer films. We find that a Raman band (D) located near 1344 cm(-1) (514.5 nm excitation) originates from a region next to the edge with an apparent width of similar to 70 nm (upper bound). The D-band was found to exhibit five important characteristics: (1) a single Lorentzian component for n = 1, and four components for n = 2-4, (2) an intensity I-D similar to cos(4) theta, where theta is the angle between the incident polarization and the average edge direction, (3) a local scattering efficiency (per unit area) comparable to the G-band, (4) dispersive behavior (similar to 50 cm(-1)/eV for n = 1), consistent with the double resonance (DR) scattering mechanism, and (5) a scattering efficiency that is almost independent of the crystallographic orientation of the edge. High-resolution transmission electron microscope images reveal that our cleaved edges exhibit a sawtooth-like roughness of similar to 3 nm (i.e., similar to 20 times the C-C bond length). We propose that in the double resonance Raman scattering process the photoelectron scatters diffusely from our edges, obscuring the recently proposed strong variation in the scattering from armchair versus zigzag symmetry edges based on theoretical arguments.