Electronics made of graphene–sheets of carbon just an atom thick–could be considerably faster than those made of silicon. But making transistors from the material at the large scale necessary for industrial production has proved challenging. Now two separate research groups have devised simple methods for unfurling carbon nanotubes–a form of carbon that is easier to produce–to make “nanoribbons” of graphene. Both methods are being licensed for commercial development, and one has been used to produce the best-performing graphene transistor yet.
Transistor materials must be semiconducting–that is, it must be possible to switch them between more or less conducting states. While graphene sheets conduct electrons very rapidly, the way that electrons move through this two-dimensional material means that graphene can’t switch between conducting states unless it’s stacked in layers or made into very narrow, long ribbons. Layered graphene devices require more power than their silicon equivalents, and although nanoribbon devices have the same power requirements as silicon, they are more difficult to make.
In a paper published today in the journal Nature, two research groups describe methods for making graphene nanoribbons by slicing open carbon nanotubes (single-atom-thick sheets of carbon rolled into layered, strawlike shapes). One research group was led by James Tour, a professor of chemistry and computer science at Rice University, the other by Hongjie Dai, a professor of chemistry at Stanford University.
The Rice researchers exposed their carbon nanotubes to sulfuric acid and potassium permanganate, a strong oxidizing agent. This led to a reaction in which a bond between two carbon atoms in the nanotube was broken, and each carbon atom bonded with an oxygen atom instead. The oxygen atoms created a strain that caused the nanotubes to unzip. “They’re sticking out of the tube, hitting each other, really straining the adjacent carbon-carbon bonds,” says Tour. This reaction slices each nanotube layer open in one clean cut because (chemically speaking) it’s more favorable for the reaction to propagate along the length of the tube than for distant bonds to be oxidized. The width of the ribbons is determined by the diameter of the nanotubes used to make them. “Most methods give you pico- to nanograms of nanoribbons,” says Tour. “This method could easily be scaled to kilos.”
“I’m amazed that this actually works,” Dai says of Tour’s technique. “In our method, we have to protect the side of the tube.” Unlike Tour’s approach, which takes place in solution, Dai’s must be performed on a surface. The Stanford researchers first spin nanotubes and a polymer to create a film in which the polymer covers up part of the tubes. The film is then peeled away and exposed to argon plasma in a reaction chamber, which etches away the exposed carbon atoms. The size of the resulting graphene nanoribbons can be controlled by varying the thickness of the polymer film and the reaction times involved.
The two papers “show there is a path to create graphene from nanotubes,” says Yu-Ming Lin, a researcher in the nanoscale science and technology group at IBM’s Watson Research Center, in New York. Dai developed the previous standard for making graphene nanoribbons: breaking graphene sheets into smaller pieces, including nanoribbons, by exposing them to intense sound waves (a relatively low-yield method). “There are pros and cons for each [new] method,” says Dai.
Tour’s unzipping method yields graphene in bulk, which is an advantage from a manufacturing perspective. But “[Dai]’s going to have better control,” admits Tour. The width of the Rice group’s nanoribbons is determined by the diameter of the nanotubes that they come from. In contrast, using the Stanford team’s technique, it’s possible to finely control the width of the nanoribbons. In today’s publication, Dai and his colleagues describe nanoribbons six nanometers wide, but he says that they have subsequently made narrower and more semiconducting ones. “There might be an optimum width; that needs to be investigated,” he says.
Tour’s nanoribbons are easy to process because they are graphene oxide, which is soluble in water. “You can use shear force to align them like logs in a river lining up in parallel,” says Tour. “You can paint them down, and they will align.” Tour adds that the nanoribbons can be made into devices using ink-jet printing. Once the ribbons are in place on a chip, they’re treated with hydrogen at high heat to remove the oxygen at their edges and turn them into semiconductors. Without this step, the ribbons are insulators.
The Stanford research was funded by Intel, and Tour says that he is in talks with companies interested in licensing his manufacturing method as well as devices made with the nanoribbons.