Graphene has exhibited extraordinary properties (exceptionally high electrical and thermal conductivity, high mechanical strength, high optical transparency, etc.), but these can only be observed in thin films of this material, which contain only one or a few layers.
To develop technologies and devices based on the properties of graphene, it is, therefore, necessary to accurately determine the layer thickness of the materials under consideration.
Raman spectroscopy is the most precise method that can be used to provide a fast and non-destructive means of determining whether you are actually handling graphene.
Raman spectroscopy and Raman spectrum of graphene
Raman spectroscopy is a vibrational technique that is extremely sensitive to the geometric structure and bonding within molecules. Even small differences in geometric structure lead to significant differences in the observed Raman spectrum of a molecule.
This sensitivity to geometric structure is extremely useful for the study of different carbon allotropes (e.g. diamond, carbon nanotubes, buckminster fullerenes, carbon nanoribbons, etc.), where the different forms differ only in the relative position of their carbon atoms and the nature of their bonding to each other.
Raman has evolved into an indispensable tool in laboratories pursuing research in the emerging field of carbon nanomaterials.
Raman spectra of graphene and graphite (made up of millions of layers of graphene stacked together) are shown in this figure:
The spectra show a relatively simple structure characterised by two main bands designated as the G and 2D bands (a third band, the D band may also be evident in graphene when there are defects within the carbon lattice).
These differences, while they appear subtle, provide very important information when carefully examined. There are differences in the positions of the bands and the shapes of the 2D bands of the G band and the relative intensity of these bands are also significantly different.
These Raman spectra demonstrate the ability to differentiate between single-layer graphene and graphite. However, Raman’s utility lies in its ability to differentiate single-, double- and triple-layered graphene.
In other words, Raman is able to determine the thickness of the layer, at atomic layer resolution, for graphene layer thicknesses of less than four, i.e. thicknesses that are of interest to the current field of graphene research.