In general, the optical properties of any molecule are dampened due to aggregation, and particularly planar pi conjugated materials when they are stacked. Similar effects are seen when SWNTs bundle (or debundle)

In a Raman analysis of graphene the intensity of D and G peaks depends directly to defects present on the material, like punctual and line defects, cristallite size, exfoliation degree and number of sheets if you have graphene oxide, the shape of the edges, among others that I forget at this moment. In other words a define and intense D and G peak told us that we have a sample with less defects.

Dear Akibul,

It is related to a change in the density of defects in the lattice. The D-peak is an optical mode corresponding to Phonons in the edge of the Brillouin zone, and according to the selection rules it is forbidden in perfect graphite. This mode is only active in the presence of some disorder/defects. The change in the ID/IG ratio you observe is then likely related to the change in defect density as you reduce the number of layers of the samples.

You can get a detailed explanation of the different Raman contributions in Graphene in the work of Nicola Ferralis (https://arxiv.org/pdf/1005.4900.pdf) and in the paper of A.C. Ferrari: Article Interpretation of Raman Spectra of Disordered and Amorphous Carbon

were a detailed description of ID/IG ratio can be found.

Kind regards,

Julian

You may correlate the changes in the G-band with line broadening and intensity ratios for G, D, and 2D bands in the following short communication. Such changes hint at the level of purity, defect, and disorder present in the Graphene.

https://instanano.com/characterization/experimental/raman-graphene/