July 7, 2011 | 2
The 2010 Nobel Prize in Physics raised the profile of graphene—a super strong one-atom thick sheet of carbon atoms arranged in a hexagonal pattern with countless potential commercial applications. The material can conduct heat and electricity extremely well while also being transparent and highly flexible, making it an ideal candidate for improving fuel cells, electronics and even hygiene products.
Graphene’s ability to deliver on its potential as a game changer in a variety of products depends on two key factors—flatness and purity. Using scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, a team of researchers led by the University at Buffalo (part of the State University of New York) have found that folds and ripples in a graphene sheet and/or chance contaminants from processing—possibly hiding in those wrinkles—disrupt and slow electron flow across the sheet, impairing its conductive properties.
This means simple processing flaws can seriously degrade graphene, according to a study published June 28 in Nature Communications. (Scientific American is part of Nature Publishing Group.) Under ideal conditions, an electron "cloud" lines the surface of graphene samples that enables the high-speed transit of electrons. Wrinkles and imperfections in these samples, however, distort the cloud and create bottlenecks, according to the team, which also included scientists from the National Institute of Standards and Technology (NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory and the SEMATECH research consortium.
Scientists liken graphene’s electrical conductivity to that of copper, and have called graphene’s thermal conductivity the best of any known material. Still, this study suggests that the successful use of graphene in products such as conductive inks, ultrafast transistors and solar panels will likely depend on additional insights into the material’s properties and behavior.
The researchers stopped short of pouring cold water on graphene’s near-term success, and actually suggested a possible solution. NIST researchers were able to make detailed spectroscopic measurements of the graphene samples while heating them and found that the mysterious, disruptive peaks disappeared by the time the sample reached 150 degrees Celsius. This indicates the disturbances in the electron cloud were not chemically bound but rather contaminants absorbed on the surface that could be removed.
In the first image, the red regions depict folds in graphene, whereas the green regions are relatively flat domains. The "hills and valleys" present in the electron cloud can act as speed bumps preventing the flow of charge through the material. Image credit: Brian J. Schultz and Christopher J. Partridge, University at Buffalo
In the second image, the dotted lines show distinctive regions of graphene sloped at different angles. Soft X-rays paint a bird’s-eye view of graphene’s electron cloud. Image credit: Brian J. Schultz