Electron superhighway: can graphene overtake silicon as the essential ingredient of computer chips?

Science News,  Sept 29, 2007  by Davide Castelvecchi

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"Graphene has always been before our eyes but no one ever tried to look," says Andre Geim, a physicist at the University of Manchester in England. A single-atom-thick, chicken wire web of carbon atoms, graphene forms the layers that stack up to make the graphite found in pencil lead and carbon soot.

However mundane the stuff may be, physicists have long predicted that if it were possible to isolate single graphene sheets, they would be sturdier than diamond and would have almost preternatural abilities to manipulate electrons. That could make graphene a better material than silicon for making computer chips. Until recently, though, no one had been able to isolate graphene sheets, let alone do anything useful with them.

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In 2004, Geim and his collaborators startled the physics community by announcing that they had peeled graphene layers off graphite using common adhesive tape. The discovery raised a buzz in physics circles reminiscent of the excitement that greeted carbon nanotubes a decade ago.

In fact, graphene is nothing but a large, unrolled carbon nanotube, and the two materials share many qualities, including strength and conductivity.

Though promising, nanotubes have proved devilishly difficult to assemble into circuits. Nanotubes don't readily connect to one another, and attaching them to metal contacts creates spots where electrons tend to scatter, dissipating energy as heat.

Graphene, on the other hand, comes in sheets. It may be possible to etch graphene circuits, just as circuits are now etched into silicon wafers. Forming circuits from one sheet of graphene could be much easier than assembling them from nanotube pieces. "We want to be able to use the essential properties of carbon nanotubes in a material that can be patterned easily," says Walt de Heer of the Georgia Institute of Technology in Atlanta. "It could realize the dream people had of carbon-nanotube electronics."

Graphene circuits could in principle work efficiently even with components measuring only a few atoms across--scales that can't be achieved with ordinary semiconductors. In recent months, scientists have learned how to make graphene-based transistors and diodes--the basic elements of computer chips. And they have begun trying to connect graphene to other materials, including carbon nanotubes.

But that's only a beginning. If graphene is to replace silicon one day, scientists and engineers will have to figure out how to manufacture large numbers of circuits with nearly atomic precision.

CAUGHT ON TAPE Geim's adhesive tape stratagem could hardly be the basis for a new chip-fabrication plant, but it continues to be researchers' favorite way of making graphene for experimentation.

Anyone who uses a pencil is likely to leave some single-layer graphene flakes scattered on paper, he says. The graphene sheets in graphite are bound to one another only by weak electrostatic forces. That's why pencil lead is so soft.

After gently rubbing graphite on a silicon-oxide crystal, Geim stuck strips of tape on the carbon debris, hoping that when he peeled off the tape, thin stacks of a few graphene sheets would stick to it. To further pry apart the sheets, he repeatedly folded the pieces of tape, sticky sides together, and peeled them open again. Then, by dissolving the tape in a solution, he let the graphene flakes settle onto the surface of a silicon-oxide crystal.

Through an ordinary microscope, Geim spotted graphene stacks of varying thicknesses stuck to the crystal's surface. The translucent flakes created rainbows of colors "like oil on the surface of a rain puddle," he says. With a bit of experience, Geim learned how to recognize single sheets by their colors. "If it's blue or red, you know it's thick," he says. To find single layers, "you look for another shade of purple" (SN: 10/23/04, p. 259; 8/13/05, p. 110).

To confirm that they had actually found single sheets of graphene, Geim and his collaborators tested how the flakes conducted currents. Measurements showed that electrons were able to travel microns--enormous distances by atomic-scale standards--without bumping into atoms.

These findings confirmed crucial predictions about single-layer graphene. In graphene sheets, as in carbon nanotubes, each carbon atom binds strongly to three neighboring atoms, creating a web of hexagons resembling chicken wire. In addition, the atoms form bonds by sharing electrons from barbell-shape orbitals that are perpendicular to the chicken wire plane. These sideways orbitals fuse with their neighbors, creating veritable electron superhighways above and below the graphene plane.

In 2005, Geim and his colleagues made another important discovery. Placing graphene samples in magnetic fields whose intensities the researchers ratcheted up, they saw the electrical resistance increasing in discrete steps, a phenomenon known as the quantum Hall effect. Around the same time, a group led by Philip Kim of Columbia University made the same discovery after learning of Geim's tape-peeling technique.