Materials: Spotting graphene's speed bumps
X-ray microscopic image of graphene taken at the CLS. The red regions depict folds in graphene, whereas the green regions are relatively flat domains. The “hills and valleys” can act as speed bumps preventing the flow of charge through graphene. An all green topography would be preferred for high-performance electronics.
No two-dimensional material has piqued as much scientific interest as graphene, the subject of the 2010 Nobel Prize in Physics. Hailed as a miracle material, graphene is a molecular sheet composed of a single layer of carbon atoms. It is noted for its excellent electrical and thermal conductivity and its seemingly endless applications from ultrafast electronics to flexible solar panels. Subtle imperfections, however, can drastically mute graphene's prized properties. Dr. Sarbajit Banerjee and his research team at the University at Buffalo are using the Canadian Light Source to hone in on graphene's topography, producing for the first time images of the folds and ripples in the electron cloud that surround the nanomaterial and affect its conductivity.
For as long as graphene has been grown in labs, one of its most studied properties has been the highly conductive nature of the nanomaterial. Because of this feature, graphene has the potential to revolutionize electronics manufacturing. Scientists, however, have not been able to get the material to reach its peak potential in conductivity, which has been theorized to be the highest known. Thanks to data obtained at the CLS, Banerjee and his team have found that kinks and folds in the nanomaterial act like speed bumps or potholes in the electrons’ paths, slowing them down.
“One of the problems that people have suspected for awhile is that graphene is not entirely flat,” says Banerjee.
Because graphene is a one-of-a-kind material, any non-conformity can prove to be very problematic. Graphene is grown in the lab as very thin sheets (one millimetre of pencil lead contains approximately 3 million layers of graphene). Because graphene is only one atom thick, any of those atoms can interact with the environment.
“Graphene is a single layer of carbon where every atom lies on the surface. So every atom interacts with the outside world, so it can interact with and be contaminated by something else. It turns out that this happens often during processing,” explains Banerjee. “The electrons have a strange interaction with the atoms such that they are able to zip around without being scattered off by the atom. You can almost imagine a charge zipping through graphene like it is levitating without encountering barriers.”
The incredibly high speed at which these electrons can move is one of the key characteristics that makes graphene so attractive for next generation electronics, but contamination and the development of kinks prove to be detrimental to the movement of the electrons. When hills and valleys occur they create barriers, or domains, within the sheet. The electrons have to jump from domain to domain causing them to slow down. Additionally, the valleys are also problematic as contaminants that stick to the graphene often take shelter in these dips.
Banerjee believes that a majority of the irregularities occur at the transfer stage of the production process. After the graphene is grown, it must be transferred from the surface on which it was grown for use. So researchers will have to develop better transfer techniques in order to keep the sheets as flat as possible.
“It’s kind of like saran wrap that you have stretched across something flat, like a box, and as you take the box away the wrap starts to crumple. So like the saran wrap, the graphene sheets that we grow on a metal foil starts to crumple ever so slightly when we transfer it,” says Banerjee.
Though research into the real world applications for graphene is still in its infancy, there is no disputing that it will have major implications for the electronics industry. Some of the possible applications range from ultra small transistors, to tough, flexible touch screens and light panels, to replacing silicon chips; it even has the potential for use in solar cells. Graphene will be leaving a footprint far larger than its size on everyday life.
“When you start scaling down computer processors to smaller and smaller sizes, you need high performance components that are very small, incredibly thin, and can carry a lot of charge quickly,” says Banerjee.
Reference: Schultz, B.J. et al., Imaging local electronic corrugations and doped regions in graphene. 2011. Nature Communications, DOI: 10.1038/ncomms1376
Last modified: 2012-01-19 17:01:55