Quantifying defects in graphene via Raman spectroscopy at different   excitation energies
  We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities for a given defect density strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an inter-defect distance ~3nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity. 
L. G. Can\c{c}ado, A. Jorio, E. H. Martins Ferreira, F. Stavale, C. A.   Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo...
cond-mat.mtrl-sci cond-mat.mes-hall