Scientists at the National Physical Laboratory (NPL) along with Linkoping University, Sweden, have successfully identified regions of different thicknesses in graphene with the help of Electrostatic Force Microscopy (EFM).
The left hand image is the topography; the middle the topography error image; and right the electrostatic force microscopy image where the tip bias has been switched half way through the image.
Graphene exhibits good performance characteristics only when two or more layers of graphene sheets are combined together. The thicker layers of graphene show properties similar to bulk graphite, while any number of graphene layers can be synthesized together. In order to use graphene in various devices, it is important to distinctly identify one-and two-layer graphene structures in addition to the identification of thicker regions and substrate.
Routine optical microscopy would be sufficient for identifying exfoliated graphene sheets up to a thickness of approximately 100 µm. In The case of epitaxial graphene sheets that are deposited on silicon carbide wafers having a maximum diameter of five inches, routine identification techniques would not be sufficient. In order to address this issue, researchers at NPL have opted to use the EFM technique for identification. EFM is regarded as one of the simplest and most widely available techniques based on scanning probe microscopy. Clear and distinct identification of the graphene layers of varying thicknesses is possible using the EFM technique. The EFM technique is also applicable in favourable industrial environments.
Source: http://www.npl.co.uk/