Characterization of Graphene-Polymer Composites with Raman Spectroscopy

Our first post in this two-part series touched upon just a few examples of the tremendous amount of research activity surrounding graphene, an extraordinarily strong, thin, and electrically and thermally conductive “super material” with novel properties and broad potential. Graphene exists as a transparent two dimensional network of carbon atoms. It can exist as a single atomic layer thick material or it can be readily stacked to form stable, moderately thick samples containing millions of layers, a form generally referred to as graphite.

However, the interesting properties exhibited by graphene (exceptionally large electrical and thermal conductivity, high mechanical strength, high optical transparency) are only observed for graphene films that contain one or a few layers. Therefore, developing technologies and devices based upon graphene’s unusual properties requires accurate determination of the layer thickness for materials under investigation. Raman spectroscopy can be employed to provide a fast, non destructive means of determining layer thickness for graphene thin films.

Raman spectroscopy is a vibrational technique that is extremely sensitive to 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 the different allotropes of carbon (i.e. 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 one another. Indeed, Raman has evolved into an indispensable tool in laboratories pursuing research into the nascent field of carbon nanomaterials.

In the study of graphene, the utility of Raman lies in the ability to differentiate single, double, and triple layer graphene. In other words, Raman is capable of determining layer thickness at atomic layer resolution for graphene layer thicknesses of less than four layers (i.e. at thicknesses that are of interest to the present field of graphene research).

A few things should be considered when selecting a Raman instrument for graphene characterization.

Sample size: Graphene samples are usually very small, so it is important to select a Raman instrument with microscopy capabilities.
Excitation laser to selection: While graphene measurements can be made successfully with any of the readily available Raman lasers, it is also necessary to consider the substrate that the graphene will be deposited on.
Wavelength calibration: Relatively small wavenumber shifts can have significant impact on the interpretation of the Raman spectra, so it is critical to have a robust wavelength calibration across the entire spectrum.
Wavenumber precision: Choose an instrument with high wavenumber precision to insure that small wavenumber shifts that are observed when altering the sample are in fact representative of changes in the sample rather than representative of measurement variability from the instrument. There is a common myth that it is necessary to use high resolution in order to achieve high wavenumber precision. Not only is this incorrect, but high resolution will actually add considerable noise to the spectrum which will add to the wavenumber variability.
Laser power: It is crucial to have very precise control of your laser power at the sample and to be able to adjust that laser power in small increments. This controls temperature related effects and provides flexibility to maximize Raman signal without sample heating or damage from the laser.
Software: Select a Raman microscope that has an automated stage and associated software to generate detailed Raman point maps.

Read study results evaluating the use of Raman spectroscopy to measure graphene and identify a particle with a particular thickness at a specific point.