Monday, June 21, 2010
See through: Researchers have created a flexible graphene sheet with silver electrodes printed on it (bottom) that can be used as a touch screen when connected to control software on a computer (top).
Graphene, a sheet of carbon just one atom thick, has spectacular strength, flexibility, transparency, and electrical conductivity. Spurred on by its potential for application in new devices like touch screens and solar cells, researchers have been toying with ways to make large sheets of pure graphene, for example by shaving off atom-thin flakes and chemically dissolving chunks of graphite oxide. Yet in the thirty-some years since graphene's discovery, laboratory experiments have mainly yielded mere flecks of the stuff, and mass manufacture has seemed a long way away.
"The future of the field certainly isn't flaking off pencil shavings," says Michael Strano, a professor of chemical engineering at MIT. "The large-area production of monolayer graphene was a serious technological hurdle to advancing graphene technology."
Now, besting all previous records for synthesis of graphene in the laboratory, researchers at Samsung and Sungkyunkwan University, in Korea, have produced a continuous layer of pure graphene the size of a large television, spooling it out through rollers on top of a flexible, see-through, 63-centimeter-wide polyester sheet.
"It is engineering at its finest," says James Tour, a professor of chemistry at Rice University who has been working on ways to make graphene by dissolving chunks of graphite. "[People have made] it in a lab in little tiny sheets, but never on a machine like this."
The team has already created a flexible touch screen by using the polymer-supported graphene to make the screen's transparent electrodes. The material currently used to make transparent electronics, indium tin oxide, is expensive and brittle. Producing graphene on polyester sheets that bend is the first step to making transparent electronics that are stronger, cheaper, and more flexible. "You could theoretically roll up your iPhone and stick it behind your ear like a pencil," says Tour.
The Korean team built on rapid advances in recent months. "The field really has advanced in the past 18 months," says Strano. "What they show here is essentially a monolayer over enormous areas--much larger than we've seen in the past."
Roll and reel: A freshly made sheet of graphene is transferred onto a polyester sheet as it passes between hot rollers.
Last year, Rodney Ruoff and his team at the University of Texas in Austin showed that graphene could be grown on copper foil. Carbon vaporized at 1,000 degrees would settle atom-by-atom on the foil, which was a few centimeters across. Byung Hee Hong, a professor at Sungkyunkwan University and corresponding author on the paper, says the use of a flexible base presented a solution to the graphene mass-manufacturing dilemma.
"[This] opened a new route to large-scale production of high-quality graphene films for practical applications," says Hong. "[Our] dramatic scaling up was enabled by the use of large, flexible copper foils fitting the tubular shape of the furnace." And the graphene sheets could get even bigger. "A roll-to-roll process usually allows the production of continuous films," says Hong.
In Hong's method, a sheet of copper foil is wrapped around a cylinder and placed in a specially designed furnace. Carbon atoms carried on a heated stream of hydrogen and methane meet the copper sheet and settle on it in a single uniform layer. The copper foil exits the furnace pressed between hot rollers, and the graphene is transferred onto a polyester base. Silver electrodes are then printed onto the sheet.
The technique shows some potential to be scaled up for mass production. "They particularly show that they are able to grow the graphene [in a way] that is compatible with manufacturing," says Strano. "It's a very economical way to manufacture materials."
Hong sees application for the method in the production of graphene-based solar cells, touch sensors, and flat-panel displays. But he says products will be a while in coming. "It is too early to say something about mass production and commercialization," he says. Current manufacturing processes for indium tin oxide use a spreading technology that is different from roll-to-roll printing. "However, the situation will be changed when bigger flexible-electronics markets are formed in the near future," Hong says.