Silicon has transformed the digital world, but researchers are still eager to find substances that will make integrated circuits smaller, faster and cheaper. High on the list is graphene—planar sheets of honeycomb carbon rings just one atom thick. This nanomaterial sports a range of properties—including ultrastrength, transparency (because of its thinness) and blisteringly fast electron conductivity—that make it promising for flexible displays and superspeedy electronics. Isolated only four years ago, graphene already appears in prototype transistors, memories and other devices.
But to go from lab benches to store shelves, engineers need to devise methods to make industrial quantities of large, uniform sheets of pure, single-ply graphene. Researchers are pursuing several processing routes, but which approach will succeed remains unclear. “We’ve seen claims by groups that say that they can coat whole silicon wafers with monolayer sheets of graphene cheaply,” reports James M. Tour, a chemist at Rice University. “But so far no one has publicly demonstrated it.”
Making small amounts is surprisingly easy, states graphene’s discoverer, Andre K. Geim of the University of Manchester in England. In fact, “you produce a bit of graphene every time you drag a pencil point across paper,” he notes—the pencil’s graphite is actually a stack of graphene layers. The initial graphene-making methods worked similarly to pencil writing: researchers would abrade some graphite and then search the debris with a microscope for suitable samples or separate individual flakes with sticky tape.
Although most scientists consider such mechanical “exfoliation” techniques to be suited only for making tiny amounts, Geim does not necessarily agree: “Recently the procedure was scaled up to produce as much graphene as you want.” He uses ultrasound to break up graphite into individual layers that are dispersed in a liquid. The suspension can then be dried out on a surface, which leaves a film of overlapping pieces of graphene crystals. Whether these sheets of multiple crystals can work well enough for many applications is uncertain, however, because edge boundaries of individual flakes tend to impede the rapid flow of electrons.
Bigger samples might come from chemical exfoliation. Last May collaborators James P. Hamilton of the University of Wisconsin–Platteville and Jonathan N. Coleman of Trinity College Dublin in
Ireland showed that graphene dissolvesin certain organic solvents. “You place graphite in a bucket, dump in organic liquids that dissolve it,” Hamilton says, “then you remove the solvent and out comes this gray stuff that’s pure graphene.” Hamilton’s start-up company, Graphene Solutions, hopes to convert that graphene into uniform, single-crystal sheets and, ultimately, to commercialize the process.
Other chemical exfoliation techniques are possible. Rod Ruoff, now at the University of Texas at Austin, and his former colleagues at Northwestern University have shown that adding acid to graphite in water can yield graphite oxide that can be separated into individual pieces. Suspended in liquid, the flakes are then deposited onto a substrate to form a film. The addition of other chemicals or heat can drive off the oxygen groups, yielding graphene.
One such oxygen-removing agent is rocket fuel, scientists from Rutgers University found—specifically, vapors of hydrazine, a highly reactive and toxic compound. Last year Yang Yang and Richard B. Kaner of the University of California, Los Angeles, simplified the Rutgers approach by using liquid hydrazine. “We then deposit the pieces onto silicon wafers or other, more flexible substrates,” Yang says. The results are single-layer films composed of many platelets. The pair are now trying to improve the quality of the sheets, as well as find a safer alternative to hydrazine.