This work addresses the issue of determining the number of layers in few layers of graphene (FLG) flakes by cross-comparison of several techniques: optical microscopy (OM), atomic force microscopy (AFM) and micro-Raman (μR) spectroscopy. SL were preliminarily identified by μR spectroscopy, which allows an unambiguous distinction between monolayers, bilayers and multilayers from the shape and relative intensity of the G and 2D peaks. It is demonstrated that the thickness of a SL measured by tapping mode AFM with respect to the SiO2 substrate is affected by an “offset” ∼0.3 nm.
This offset is explained in terms of the different adhesion forces between tip and SiO2 and tip and graphene measured by force spectroscopy curves. A calibration curve relating the height of a flake measured by AFM with the number of graphene layer was obtained. Finally, the optical contrast (OC) variations with FLG thickness was measured for different wavelengths in the visible range and for different oxide thicknesses (from 100 to 300 nm). OC was correlated with the number of layers (independently measured by AFM). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)