We present Raman spectroscopy measurements on single-and few-layer graphene flakes. Using a scanning confocal approach we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double-and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab-initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry. The interest in graphite has been revived in the last two decades with the advent of fullerenes 1 and carbon nanotubes. 2 However, only recently single-and few-layer graphene could be transferred to a substrate. 3 Transport measurements revealed a highly-tunable two-dimensional electron/hole gas of relativistic Dirac Fermions embedded in a solid-state environment. 4,5 Going to few-layer graphene, however, disturbs this unique system in such a way that the usual parabolic energy dispersion is recovered. The large structural anisotropy makes few-layer graphene therefore a promising candidate to study the rich physics at the crossover from bulk to purely twodimensional systems. Turning on the weak interlayer coupling while stacking a second layer onto a graphene sheet leads to a branching of the electronic bands and the phonon dispersion at the K point. Double-resonant Raman scattering 6 which depends on electronic and vibrational properties turns out to be an ingenious tool to probe the lifting of that specific degeneracy.We report on Raman mapping of single-and few-layer graphene flakes resting on a silicon oxide substrate. Lateral resolution of 400 nm allows to address neighboring sections with various layers of graphene down to a single graphene sheet, previously determined with the scanning force microscope (SFM). We find that the integrated G line signal is directly correlated with the thickness of the graphitic flake and is shifted upward in frequency for double-and single-layer graphene compared to bulk graphite. The mapping of the peak width of the D' line shows a strong contrast between single-and few-layer graphene. Such a pronounced sensitivity to the transition to the very last layer offers an optical and nondestructive method to unambiguously detect single-layer graphene. In addition, we locally resolve the structural quality of the flake by investigating the D band, which is related to elastic backscattering. The map of the integrated D line signal of a graphitic flake with doubleand single-layer sections shows that the inner part of the flake is quasi defect free, whereas edges and steps serves as scatterers. Finally, we explain the splitting of the D' line as a function of the number of graphene layers with...

We report ab initio calculations of the phonon dispersion relations of the single-layer and bulk dichalcogenides MoS 2 and WS 2 . We explore in detail the behavior of the Raman-active modes A 1g and E 1 2g as a function of the number of layers. In agreement with recent Raman spectroscopy measurements [C. Lee et al., ACS Nano 4, 2695 (2010)], we find that the A 1g mode increases in frequency with an increasing number of layers while the E 1 2g mode decreases. We explain this decrease by an enhancement of the dielectric screening of the long-range Coulomb interaction between the effective charges with a growing number of layers. This decrease in the long-range part overcompensates for the increase of the short-range interaction due to the weak interlayer interaction.