16 April 2009—Graphene, a one-atom-thick sheet of carbon with remarkable electrical properties, shows promise for future generations of high-speed transistors. It may have uses as diverse as the production of sensors or as scaffolding for tissue regeneration. But research is still in the early stages, in part because it’s so difficult to produce large quantities of graphene.
Now two research groups are reporting ways to make graphene ribbons, ranging in width from a few nanometers to a few hundred nanometers. The width matters because it, along with the shape of the edges of the ribbons, affects the conductivity of the graphene; ribbons narrower than about 10 nm confine the movement of electrons and act as semiconductors, while wider ribbons act as metallic conductors. Both methods start with carbon nanotubes and ”unzip” them to form flat ribbons of graphene.
One method, developed by James Tour and fellow chemists at Rice University, in Houston, treated the carbon nanotubes with concentrated sulfuric acid and the oxidizing agent potassium permanganate, then heated them to between 55 °C and 70 °C. The process created a hole in the nanotube, which then expanded, unzipping the tube to form a ribbon. ”Under the acidic oxidation conditions, the tubes seem to cut, in large part, along the linear longitudinal direction,” says Tour, a professor of chemistry, materials science, and computer science. ”It’s easy bulk chemistry.”
For some purposes, such as making a material that’s water soluble, the resulting graphene oxide may be desirable, Tour says. But if the researchers want to get rid of the oxide, there are simple methods, such as treating the ribbons with hydrazine or annealing the graphene under hydrogen. The width of the ribbons depends on the diameter of the nanotubes that go into the process. With multiwalled nanotubes made up of 15 to 20 concentric cylinders, and with diameters ranging from about 40 to 80 nm, the researchers produced several layers of ribbons ranging from about 100 to 250 nm in width and about 4 micrometers long.
To get narrower semiconducting ribbons, the team tried starting with single-wall carbon nanotubes but found the resulting ribbons were too entangled to be of any use. ”They were like noodles,” Tour says. However, since submitting the research—which was published this week in Nature—the team has been developing methods to counter that effect, such as adding a thin coating of a surfactant. ”We solved part of the problem already,” Tour says.
The type of graphene ribbon the Rice team produced would be appropriate for conductive or semiconducting thin films and could prove to be a cheaper substitute for monocrystalline silicon in photovoltaics. The ribbons could also be made into liquid crystals and extruded through a pore to produce lightweight wires. But because there’s no easy way to accurately place a single ribbon of graphene on a substrate, the Rice method will be of little help in making most discrete devices, Tour says.
At Stanford, a team led by chemist Hongjie Dai placed multiwalled carbon nanotubes on a silicon substrate, then coated them with a polymer and baked them. When the team peeled the polymer off the silicon, it came away with the nanotubes embedded within it. The team then exposed the material to a 10-watt argon plasma. Ten seconds of exposure converted about 20 percent of the nanotubes into graphene ribbons, ranging in width from about 20 to 40 nm. Longer exposures cut more nanotubes, which were embedded deeper in the polymer. The researchers then dissolved away the polymer, leaving only the ribbons.
While the Rice method worked well on cheaper nanotubes that already had a relatively high level of defects, the Stanford method was better with highly crystalline nanotubes. The Stanford method also produced few defects and clean edges, important for optimizing the conductive properties of the ribbons.
The research is still in the early stages, and more work is needed to gain better control of the widths and the edge patterns of the ribbons, which also entails having more control over the size and uniformity of carbon nanotubes, according to Max Lemme, a Humboldt Research Fellow at the Center for Nanoscale Systems at Harvard University, who is working on making transistors from graphene. He wonders how applicable to actual semiconductor manufacturing the work will be. ”The two methods are exciting research results that will lead to better understanding of [graphene nanoribbon] electronics, but I don’t really see them as the grail in the quest for a graphene deposition method relevant to the classic semiconductor industry,” Lemme says.
About the Author
Neil Savage writes from Lowell, Mass., about lasers, LEDs, optoelectronics, and other technology. In March 2009, he reported on amazingly strong carbon nanotube–based artificial muscles.