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Lowly graphite stirs new excitement

July 25, 2006
Special to World Science

Graphite might not sound very exciting. It’s the key ingredient in pencils, those decidedly low-tech writing devices. 

A test electronic device made of graphene, . (Courtesy Georgia Institute of Technology)

But graphite behaves in strange, unexplained ways when carved into sheets one atom thick, scientists have found. And they’re increasingly drawn by the possibility that such sheets might be useful in tiny electronic devices and circuits.

Ultra-thin graphite sheets of this sort, called graphene, are among the hottest new materials under research, according to materials scientists.

Graphene gained prominence when researchers late last year reported exotic electronic behavior in the material. The findings, from two scientific teams, appeared in the Nov. 10 issue of the research journal Nature.

The two groups, one led by Andre Geim of the University of Manchester, U.K. and the other by Philip Kim of Columbia University in New York, study the movements of electrons—the subatomic particles that carry electric charge—in flat materials.

Electrons moving through electrically conducting materials behave differently when they are confined to a two-dimensional “flatland” in very thin slabs or films. Such behaviour has been known for many years in thin metal films. But graphite, a form of carbon, is different, the two teams reported. 

Using sensitive techniques, they separated the individual sheets of carbon atoms that are layered on top of one another in graphite. These layers are electrically conducting because they contain electrons that are free to roam across a layer, they explained. 

But graphene is unlike a two-dimensional metal in its electronic properties, they added. For example, unlike metal, a graphene sheet’s conductivity never falls below a minimum value, even in situations where the sheet should contain no moveable electrons. 

Geim and colleagues wrote that in graphene, the charge-carrying particles act as if they’re without mass, or weightless. In this respect they’re more like photons—the massless particles that convey light—than like electrons, which have a tiny mass.

All this makes graphene a rich system for exploring unusual quantum-mechanical electronic behaviour, the researchers added. Quantum mechanics is a term that describes a range of exotic, often paradoxical behaviors that matter exhibits at the smallest scales.

At the March Meeting of the American Physical Society on March 13 of this year, researchers at the Georgia Institute of Technology in Atlanta, Ga. and the National Centre for Scientific Research in France described additional unusual properties of graphene that they said could be useful in electronics.

Graphene sheets several atoms thick may be able to conduct electricity with almost no resistance, they claimed. Resistance is the extent to which a conducting material hinders the flow of electrons through it—a common property that can result in huge energy losses. Dramatically lowered resistance “will allow the production of very small devices with very high efficiencies and low power consumption,” said Georgia Tech’s Walt de Heer.

This feature is of graphene is tied to another way in which the charge-carrying particles within it act more like photons than electrons, he added. 

Twentieth-century physicists learned that paradoxically, all particles can actually act like either of two seemingly contradictory things—particles or like waves—depending on the type of experiment one performs. But their wave nature becomes less noticeable the heavier they are, which is why the big objects we normally see don’t seem to act like waves. In graphene, the charge-carrying particles’ wave-like properties become prominent, as they are with photons, de Heer said. This produces apparently dramatically different effects.

“This is an entirely different way of looking at electronics,” he said. 

De Heer also noted that graphene is related to another exotic material that has stirred excitement among physicists in recent years, “carbon nanotubes,” sub-microscopic tubes of carbon atoms linked together like rolled-up chicken wire. 

Part of the interest in nanotubes stems from their ability to conduct electricity virtually without resistance, de Heer said. But this advantage is hard to exploit because resistance can appear in features of an electrical circuit, such as at the junctions between different nanotubes.

Graphene may rescue this situation, de Heer claims. 

“Nanotubes are simply graphene that has been rolled into a cylindrical shape,” he said. “Using narrow ribbons of graphene, we can get all the properties of nanotubes because those properties are due to the graphene and the confinement of the electrons, not the nanotube structures.” 

Such ribbons can be produced using standard techniques called microelectronic processing, he added. De Heer and colleagues say they’ve used such techniques to produce graphene with feature sizes about eight times larger than what would be needed. They’re hoping to refine the technique to arrive at smaller and smaller sizes. “We have taken the first step of a very long road,” de Heer said.

More recently, researchers say they have combined graphene with other materials to produce yet new substances that could be useful. In the July 20 Nature, Rodney Ruoff of Northwestern University in Evanston, Ill. and colleagues reported making a new electrically conducting composite material out of graphene embedded in polymers, large molecules made up of smaller, repeating subunits.

Scientists say this could be handy because graphene, for all its advantages, has drawbacks. Graphene by itself is “too soft and flaky” for many applications, wrote Nicholas Kotov of the University of Michigan in Ann Arbor, Mich., in a commentary in the same issue of the journal. 

Combined materials, or composites, such as those produced by Ruoff, could solve the problem if they retain graphene’s distinctive electrical properties, he added. 

The composites are “easy to process using standard industrial technologies,” he wrote, a point that “might sound trivial.” But such matters are crucial, he added, if the technology “is too be applied in the real world.”

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