A research team led by University at Buffalo (UB) chemists has used synchrotron light sources to observe the electron clouds on the surface of graphene, producing a series of images that reveal how folds and ripples in the material can harm its conductivity.
The 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.
Graphene’s special structure—a single layer of carbon atoms linked in a honeycomb-like arrangement— 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 UB researchers who led the study published in the journal Nature Communications. But conditions are not always optimal.
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. 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. When we imaged the electron cloud, you can imagine this big fluffy pillow, and we saw that the pillow is bent here and there.—Sarbajit Banerjee
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.
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. We saw that regions of graphene were sloped at different angles, like looking down onto the slanted roofs of many houses packed close together.—David Prendergast, a staff scientist in the Theory of Nanostructures Facility at the Molecular Foundry at Berkeley Lab
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.
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. It’s very hard to keep straight and flat, and our work is really bringing home the point of why that’s so important.—Sarbajit Banerjee
The research was conducted by UB, 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.
We place a premium on the power of collaboration, and this is a great example of the benefits associated with that philosophy. The unique expertise of each of the four collaborative entities has come together to forge a new understanding of subtle functionalization variations of surface graphene atoms. Our findings represent another important step toward potential industrial applications such as low-cost broadband radio frequency (RF) devices, and correlation of NEXAFS with Raman spectroscopy which may enhance monitoring capabilities for graphene as a replacement for large area organic LED displays.—Pat Lysaght, SEMATECH Front End Processes, senior member technical staff
Synchrotron imaging was conducted at the Canadian Light Source in Saskatchewan in Canada and at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory in New York State. NEXAFS was measured at the NIST soft X-ray beamline of the NSLS.
Portions of this work conducted at the Molecular Foundry were supported by the Department of Energy Office of Science. Additional support for the project came from a New York State Energy Research and Development Authority (NYSERDA) grant disbursed through Graphene Devices, a Western New York start-up company exploring ways to optimize production of graphene using processes that Banerjee and UB colleagues invented.
Brian J. Schultz, Christopher J. Patridge, Vincent Lee, Cherno Jaye, Patrick S. Lysaght, Casey Smith, Joel Barnett, Daniel A. Fischer, David Prendergast and Sarbajit Banerjee (2011) Imaging local electronic corrugations and doped regions in graphene. Nature Communications 2, 372 doi: 10.1038/ncomms1376