Differential reflectance contrast technique in near field limit: Application to graphene

We report on differential reflectance contrast (DRC) sub-microscopic images measured of graphene layers exfoliated on a SiO2/Si substrate by using a near field scanning optical microscope (NSOM) with a spatial resolution of 40 nm. In general, high-quality mechanically exfoliated graphene flakes have sizes of some micrometers and exhibit a distribution of different thicknesses; thus an approach to characterize the topography of the flakes in the sub-micrometric regime is fundamental. DRC in the near field limit is a very useful technique to characterize the flakes in the sub-microscopic scale. The DRC signal is obtained by taking the numerical difference between the reflectivity coming from a region with no graphene (substrate) and a region containing a graphene layer. It is shown by a multiple reflection model (graphene/SiO2/Si) and spectroscopic ellipsometry measurements that the optical contrast in such system can be modulated by changing the thickness of the SiO2 layer or/and the wavelength of the incident light. The results open the possibility to use this optical technique for the thicknesses characterization in the sub-micrometer scale of 2D materials.