Our last article discussed Graphene, a strong, thin, and electrically and thermally conductive “super material” that has exceptional barrier properties and may be used as a barrier layer in a variety of applications, such as an anti-corrosive metal coating.
We conducted an experiment to present preliminary work investigating the protective capabilities of graphene employing the use of rapid Raman imaging.
Raman spectroscopy is a laser based scattering spectroscopy providing detailed molecular information that can be used to identify the chemical nature of a material. Raman is ideally suited for graphene due to the structure and bonding found in the material. Single layer graphene is a two-dimensional material composed exclusively of sp2 bonded carbon. The sp2 bonded carbon gives an extended network of highly polarizable π bonds which results in an extremely intense Raman signal.
The material we used was a fresh layer of graphene on copper (Cu), obtained via chemical vapor deposition (CVD). The sample was annealed in air at 200 °C for one hour prior to measurement. Before annealing, the sample had a bright, shiny copper appearance. After annealing, the sample contained dark point and line features.
A 10,000 spectra Raman chemical image was obtained in 50 minutes, from the annealed sample, using 2mW 455 nm excitation and a 100× objective. 455 nm laser excitation is ideal for graphene samples that have been deposited on to Cu as it does not exhibit the fluorescence that is observed for Raman measurements using 532, 514.5 or 633 nm excitation. The absence of fluorescence greatly simplifies the interpretation of the Raman results.
Each pixel in the chemical image represented the Raman intensity integrated over 250 nm of stage movement. The optical image, the Raman chemical image overlay, and points in the Raman spectrum were present for the point-like and line-like dark features. The Raman chemical images were based upon a color intensity scale where blue represents low Raman intensity and red represents high Raman intensity. The Raman imaging software allowed for a number of ways to view the chemical image. In this case, the color intensity image was based upon the peak height of the band occurring at 685 cm-1. This band location in the chemical image coincided with the dark feature and thus identified them as oxidized Cu.
For oxidation to occur on graphene coated Cu, as in this sample, oxygen must penetrate the graphene film barrier. Since oxygen would be too large to permeate through graphene it is reasonable to assume that it came into contact with the Cu substrate at point defects or tears in the graphene.
To verify this, the Raman chemical images were re-analyzed so as to base the color scale on the D band of graphene. The D band would show appreciable intensity when defects are present that disrupt the sp2 network of bonds and can allow a route for oxygen to permeate across the barrier to cause Cu oxidation. The chemical images were re-analyzed to display the image intensity color scale as based upon the intensity of the D band of graphene. It was seen that the D band intensity also coincides with the dark features in the optical image just as it did for the Cu oxide band. The D band intensity occurredat point defects, tears, or grain boundaries of graphene which is consistent with oxygen making its way across the graphene via these channels.
A closer look at the two sets of chemical images revealed that the D band based features were smaller (i.e. narrower) than the Cu oxide band features. This broadening of the oxidation beyond the D band was most likely the result of oxygen migrating underneath the surface of the graphene once it had made its way across the graphene via the point defects, tears, or at grain boundaries. This work questioned whether the heating process or the heating in air caused the defects and thus, the route of oxidation. Future studies will be pursued to address the important issue of the temperature stability graphene coatings.
We concluded that Raman imaging can be used to investigate failure points in the protective barriers that were induced with high temperature annealing of single layer graphene coated Cu. In this study Raman imaging revealed the chemical nature of the discoloration and insight into the mechanism underlying the behavior.
To see the spectra, images, and more details about this experiement, read the application note: Graphene Protective Coating Capabilities Investigated by Raman Rapid Chemical Imaging.)
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