Andre Geim and Konstantin Novoselov win 2010 physics Nobel for graphene

Andre Geim and Konstantin Novoselov are the winners of this year’s Nobel Prize in Physics. Six years ago the two researchers discovered how to make graphene, a honeycomb sheet of carbon atoms just one atom thick. Both researchers are based at the University of Manchester in the UK, where they did the prize-winning work.

Geim and Novoselov’s method is simple and cheap. By applying Scotch tape to graphite, they could pull off thin flakes that consist of one, several, or many layers of graphene. To locate the rare one-layer flakes, they took advantage of an optical effect: If the flakes are deposited on silicon dioxide substrate of just the right thickness, one-layered graphene reveals itself through interference fringes.

Thanks to its two-dimensionality and to the symmetry and strength of its lattice, graphene has a host of fascinating electronic properties. Theorists had anticipated some of them decades ago, but by showing physicists that making the material is feasible and straightforward, Geim and Novoselov touched off, and participated in, an explosion of experimental and theoretical work.

The feverish activity continues. As of today, 1476 papers with “graphene” in the title have appeared in Physical Review Letters, the world’s most prestigious physics journal. All but 21 of them came out after Geim and Novoselov’s 2004 discovery paper.

Interest in graphene isn’t limited to its fundamental properties. The material is also a candidate for replacing silicon as a basis of faster, more powerful electronics. Already, 343 papers about graphene have appeared in Applied Physics Letters. Carbon-based electronics is an active area of research at IBM, Samsung, and other device manufacturers.

Opportunity, serendipity, and luck

In an interview last year, Novoselov recounted how he and Geim made their discovery. The project began as a long-shot attempt to find a metallic semiconductor. On paper, graphene was a promising candidate. The challenge was to make it.

The first step toward that goal occurred when a member of Geim’s lab, Oleg Shkliarevskii, reminded Geim and Novoselov that he and his fellow electron microscopists routinely make thin samples by applying Scotch tape to a material then peeling it off.

The next, and crucial step, occurred by chance. As a mount for the peeled-off graphite flakes, Geim and Novoselov chose a 300-nanometer-thick substrate of silicon dioxide. The thickness is fairly standard, but, as the researchers were to find out later, if it had been off by more than just 5%, they would not have seen the revelatory interference.

One of the first experiments that Geim and Novoselov did was to confirm graphene’s most important electronic property: the unusual relationship between the energy and momentum of its charge carriers. In a crystal, the combination of the atoms’ energy levels and the lattice’s three-dimensional structure compels electrons to occupy bands in an energy-momentum diagram. For a given energy, only certain values of momentum are allowed.

Silicon and other semiconductors have more or less the same band structure: a valence band, in which electrons are tied to their atoms, and a more energetic conduction band, in which electrons can move freely as if they were in a metal. A narrow energy gap of varying width separates the two bands. At low temperature, the most energetic electrons are stuck in the valence band. But heat or voltage can give them enough extra energy to jump across the gap, turning the material from an insulator into a conductor.

Graphene’s band structure consists not of wavy, gap-separated bands but of two cones--one upright, the other upside down--that meet at their apexes. The cones’ straight sides imply that the electrons will behave like massless particles and whizz through the material ballistically, as if they were photons travelling in free space.

Geim and Novoselov confirmed graphene’s band structure by measuring its conductivity as they varied a voltage applied perpendicular to the sheet. Other experiments followed, including the demonstration that graphene exhibits a quantum Hall effect at room temperature.

One of graphene’s surprising properties is mechanical. Theory says a sheet of material one atom thick is unstable above absolute zero. The slightest amount of thermal energy causes the sheet to buckle. Graphene is no exception, but the carbon--carbon bonds are strong enough to limit the buckling to waves no higher than 10 nm.

It’s too early to say whether graphene could end up being useful. Exploiting its unusual electronic properties could prove too difficult to pull off in a cost-effective way. Still, the research that Geim and Novoselov’s discovery spawned has been remarkably diverse and fruitful.

When asked about what he’d tell the public about his work, Novoselov replied: “That science should be fun, and you don’t always need to do expensive multi-million dollar experiments to be on the cutting edge of research.”

Charles Day

Further reading

• The discovery paper: K. S. Novoselov et al., Science 306, 666 (2004) .
• Review of graphene’s properties: “Graphene: Exploring Carbon Flatland,” A. K. Geim and A. H. MacDonald, PHYSICS TODAY, August 2007, p. 35 .
• News story about graphene’s quantum Hall effect: PHYSICS TODAY, January 2006, p. 21.