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Hidden complexity in a pencil mark

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Callie JonesWe take the humble pencil for granted as a writing tool to grab when we need to make some notes or a shopping list. Yet at one time, the simple pencil was a must-have, high-tech gadget and so highly prized that it was once even banned from export as a strategic military asset since it had been found to make an ideal lining in casting moulds for cannonballs. Today, the pencil is making a new mark owing to its content of graphene, a material that is finding uses in physics and nanotechnology.

Graphene comes from graphite, the “lead” in a pencil. Graphite has a three-dimensional structure in which carbon atoms are stacked on top of one another in flat layers. This structure was identified several centuries ago and, not surprisingly, physicists wanted to try splitting this material into its constituent sheets to study its physical properties and behaviour.

Until recently, however, all attempts had ended in failure. One early approach involved inserting various molecules between the atomic planes of carbon in graphite to drive the planes apart, while another involved splitting graphite crystals into thin wafers by rubbing or scraping them against another surface. However, these methods produced material that was still graphite — particles in the first case, thin sheets in the second. By 1990, graphite films made up of fewer than 100 atomic planes and thin enough to be optically transparent had been isolated, but many scientists believed that a single layer would be too thin and too unstable to exist.

In 2004, Andre Geim and Konstantin Novoselov, two scientists working at Manchester University, solved the problem using ordinary adhesive tape. They simply stuck a flake of graphite debris on to plastic adhesive tape, folded the sticky side of the tape over the flake and then pulled the tape apart, cleaving the flake in two. As they repeated the process, the resulting fragments grew thinner, and on examination some were found to be only one atom thick.

This material is graphene, a two-dimensional crystal that resembles chicken wire. According to a 2008 article in Scientific American, the pure crystal conducts electricity faster at room temperature than any other substance. Engineers are scrutinising it for possible application in ultra-fast transistors and quantum dot computers.

For now, this peculiar atomic structure is enabling physicists to test exotic phenomena previously thought to be observable only in black holes and high-energy particle accelerators.

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