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  • Synchrotron Imaging Shows How Folds Affect Graphene
Jun 2011
BUFFALO, N.Y., June 30, 2011 — Synchrotron light sources have revealed electron clouds on the surface of graphene, showing how folds and ripples in the material can harm its conductivity.

The research, scheduled to appear June 28 in Nature Communications, was conducted by University at Buffalo chemists along with investigators at the National Institute of Standards and Technology (NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab) and Sematech, a global consortium of semiconductor manufacturers.

Graphene consists of a single layer of carbon atoms linked in a honeycomb-like arrangement. The material’s special structure makes it incredibly conductive: Under ideal circumstances, when graphene is completely flat, electric charges speed through it without encountering many obstacles, said Sarbajit Banerjee, one of the Buffalo researchers who led the study.

Conditions, however, are not always optimal.

Dotted lines show distinctive regions of graphene that are sloped at different angles. Soft x-rays paint a bird's-eye view of the electron cloud of graphene. (Image: Brian J. Schultz, University at Buffalo)

The new images that Banerjee and his colleagues captured show that when graphene is folded or bent, the electron cloud lining its surface also becomes warped, making it more difficult for an electric charge to travel through.

“When graphene is flat, things just kind of coast along the cloud. They don’t have to hop across anything. It's like a superhighway," Banerjee said. “But if you bend it, now there are some obstacles; imagine the difference between a freshly paved highway and one with construction work along the length, forcing lane changes.”

To create the images and understand the factors perturbing the electron cloud, Banerjee and his partners employed two techniques that required use of a synchrotron: scanning transmission x-ray microscopy and near-edge x-ray absorption fine structure (NEXAFS), a type of absorption spectroscopy. The experiments were further supported by computer simulations performed on computing clusters at Berkeley Lab.

The synchrotron imaging was conducted at the Canadian Light Source in Saskatchewan and at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory in New York. NEXAFS was measured at the NIST soft x-ray beamline of the NSLS.

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 graphene. Ideally, for high-performance electronics, one would like a Midwestern topography: completely flat, which would appear all green. (Image: Brian J. Schultz and Christopher J. Patridge, University at Buffalo)

“Using simulations, we can better understand the measurements our colleagues made using x-rays, and better predict how subtle changes in the structure of graphene affect its electronic properties," said David Prendergast, a staff scientist in the Theory of Nanostructures Facility at the Molecular Foundry at Berkeley Lab. "We saw that regions of graphene were sloped at different angles, like looking down onto the slanted roofs of many houses packed close together.”

Besides documenting how folds in graphene distort its electron cloud, the research team discovered that contaminants that cling to graphene during processing linger in valleys where the material is uneven. Such contaminants uniquely distort the electron cloud, changing the strength with which the cloud is bound to the underlying atoms.

Graphene’s unusual properties have generated excitement in industries including computing, energy and defense. Scientists say that graphene’s electrical conductivity matches that of copper, and that graphene’s thermal conductivity is the best of any known material.

But the new study suggests that companies hoping to incorporate graphene into products such as conductive inks, ultrafast transistors and solar panels could benefit from more basic research on the nanomaterial. Improved processes for transferring flat sheets of graphene onto commercial products could greatly increase those products’ efficiency.

“A lot of people know how to grow graphene, but it’s not well understood how to transfer it onto something without it folding onto itself,” Banerjee said. “It’s very hard to keep straight and flat, and our work is really bringing home the point of why that’s so important.”

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absorption spectroscopy
Experimental method of measuring the  transmission of a given sample as a function of the wavelength.
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