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Ironing the Wrinkles Out of Graphene

Over the past few years, graphene has been hailed for its potential uses in such wide-ranging medical device applications as antimicrobial technologies and electronics. Indeed, graphene's electrical capabilities have been heralded far and wide, not least in blogs and editorials featured in Medical Product Manufacturing News. as "Graphene-Based Paper Demonstrates Antibacterial Properties," "Graphene Arrays Could Revolutionize Electronics," and "Going Gaga Over Graphene."

A map showing the electronic cloud of a graphene sheet. The red regions show folds in the graphene, while the green areas are relatively flat. (Image by Brian Schultz and Christopher Patridge, University at Buffalo)

But this wonder substance is not without its wrinkles. Using a combination of sophisticated computer modeling and advanced materials analysis techniques at synchrotron laboratories, a research team led by the University at Buffalo (UB) and including the National Institute of Standards and Technology (NIST; Gaithersburg, MD), the molecular foundry at Lawrence Berkeley National Laboratory (Berkieley, CA) and Sematech (Albany, NY) has demonstrated how some relatively simple processing flaws can seriously degrade graphene's electrical properties.

Essentially a one-atom thick sheet of carbon atoms arranged in a hexagonal pattern, graphene is extremely strong, conducts heat very well, and has high electrical conductivity. Moreover, it is both flexible and transparent. Graphene's electrical properties, however, apparently depend greatly on the material's flatness and purity.

The researchers demonstrate how both wrinkles in the graphene sheet and/or chance contaminants from processing--possibly hiding in those folds--disrupt and slow electron flow across the sheet. The results could be important for the design of commercial manufacturing processes that exploit the unique electrical properties of graphene. At least in the case of contaminant molecules, the scientists suggest that simply heating the graphene could solve the problem.

Using x-rays, the UB team produced images that show an electron 'cloud' lining the surface of graphene samples--the structure responsible for the high-speed movement of electrons across the sheet. The images also show how wrinkles in the sheet distort the cloud and create bottlenecks. Spectrographic data showed anomalous 'peaks' in some regions that also corresponded to distortions of the cloud. The NIST researchers, using a dedicated materials science 'beam line' at the National Synchrotron Light Source (NSLS; Upton, NY), contributed a sensitive analysis of spectroscopic data confirming that these peaks were caused by chemical contaminants that adhered to the graphene during processing.

After making detailed spectroscopic measurements of the graphene samples while heating them, the NIST synchrotron methods group found that the peaks disappeared by the time the sample reached 150°C. This phenomenon showed that these particular disturbances in the electron cloud were due to contaminants and that there is a way to get rid of them, according to Dan Fischer, leader of the NIST group. "They're not chemically bound, they're just physically absorbed on the surface, and that's an important thing. You have a prescription for getting rid of them," Fischer adds.

"When graphene was discovered, people were just so excited that it was such a good material that people really wanted to go with it and run as fast as possible," comments Brian Schultz, one of three UB graduate students that worked on the project. "But what we're showing is that you really have to do some fundamental research before you understand how to process it and how to get it into electronics."

"This is the practical side of using graphene," Fischer asserts. "It has all these remarkable properties, but when you go to actually try to make something, maybe they stop working, and the question is: why and what do you do about it? These kinds of extremely sensitive, specialized techniques are part of that answer."

For more information on this study, see the UB news announcement "Researchers Image Electron Clouds on the Surface of Graphene, Revealing How Folds in the Remarkable Material Can Harm Conductivity."

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