Graphene, an atom-thick carbon sheet, is a promising replacement for silicon in electronic circuits, since it transports electrons much faster. IBM researchers have already made transistors, the building blocks of electronic circuits, with graphene that work 10 times faster than their silicon counterparts. But to make these transistors, researchers first have to alter the graphene's electronic properties by cutting it into thin ribbons, which are then incorporated into devices. Researchers have made these nanoribbons with lithography, with chemical solution-based processes, or by unzipping carbon nanotubes.
In the new Science paper, researchers at the Georgia Institute of Technology and the U.S. Naval Research Laboratory instead "write" such nanoribbons on a surface rather than cutting graphene. The researchers start with a graphene oxide sheet, which doesn't conduct electric current. When they pull an AFM tip heated to between 150 °C and 1060 °C across the sheet, oxygen atoms are shed at the spots that the tip touches. This leaves behind lines of almost-pure graphene that are 10,000 times more conductive than the surrounding graphene oxide.
"It's a fast, reproducible technique, it's one-step, it's simple," says Paul Sheehan, who led the work at the Naval Research Laboratory. "Instead of putting down resist and trying to cut graphene in different ways, you can use local heat and write the lines exactly where you want them." Sheehan says that an array of thousands of AFM tips could sketch circuits on graphene oxide at the same time.
Lithographic methods to make nanoribbons are cumbersome and expensive, says Jing Guo, an electrical and computer engineering professor at the University of Florida in Gainesville. These methods can also create ribbons with rough edges, which affect graphene's electronic properties and result in low-quality transistors. "This is a new way to [make nanoribbons] that's very simple and reliable and potentially scalable to large scale," he says. "You basically have a paper and take a pencil to scratch it, and you have a very narrow line."