Writing Circuits on Graphene

Using a heated atomic force microscope tip, researchers have drawn nanoscale conductive patterns on insulating graphene oxide. This simple trick to control graphene oxide's conductivity could pave the way for etching electronic circuits into the carbon material, an important advance toward high-speed, low-power, and potentially cheaper computer processors.

Hot wire: An AFM tip heated to over 150 °C can etch an insulating graphene oxide surface to create thin conductive nanoscale wires.

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."
The researchers wrote lines as narrow as 12 nanometers across and at speeds of up to 0.1 millimeters per second. The writing speed increased with temperature. "It is exciting to see that this conversion can be done and controlled at the nanoscale," says Yu-Ming Lin, a nanoscale science and technology group researcher at IBM's Watson Research Center in Yorktown Heights, NY. "This is an important step for graphene-based [electronics]."

Starting with graphene oxide sheets rather than graphene is easier and cheaper, says Elisa Riedo, a physics professor at Georgia Tech who led the work with Sheehan. Pristine graphene sheets are typically obtained by mechanically separating flakes from graphite or by growing graphene on two-inch silicon carbide wafers. "Graphene oxide was cheaper to produce in large areas compared to graphene," Riedo says. "It's a different path to arrive to graphene."

The researchers plan to make transistors using their technique, but they might need additional processing first, says Yanwu Zhu, a graphene researcher at the University of Texas at Austin. For one thing, they will have to find a way to remove graphene oxide remnants from the conductive ribbons. 

By Prachi Patel
From Technology Review


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