Graphene bubbles mimic explosive magnetic field

30 July 2010 by James Urquhart, writing for the New Scientist magazine

Electronics engineers would love to play with super-powerful magnetic fields – if only they weren't so likely to cause explosions. Now it turns out that graphene can mimic the useful effects of such a magnetic field, offering a new way to control the super-material's remarkable electronic properties.

Graphene is a single layer of carbon atoms arranged in a chicken-wire-like hexagonal lattice. It was first isolated in 2004. In the years since, enough has been learned about its electronic and mechanical properties to make it a strong contender for replacing silicon-based semiconductors in future electronic devices.

But although graphene is an excellent conductor, controlling how electrons within the material behave – vital for any electronic device – remains a challenge.

Now Michael Crommie of the University of California, Berkeley, and colleagues have proved in an experiment what others have predicted: put graphene under a particular type of strain and it conjures up a strange quantum effect called a pseudomagnetic field that could be used to control its conductivity.

False friend

The electrons trapped in the pseudomagnetic field move in very small circles, as they would were it possible to expose them to a real magnetic field of sufficient power. The circles are so small that they induce the electrons to segregate into discrete energy levels rather than occupying their typically broad energy bands.

The pseudomagnetic field in Crommie's experiment peaked above 300 tesla, far greater than any lab can achieve for a real steady-state magnetic field. Pulsed electromagnetic coils are able to generate fields as powerful as 100 tesla, but only for fractions of a second – anything higher might risk an explosion.
Chance find

The group chanced upon the discovery while growing graphene on the surface of crystalline platinum sheets at high temperature.

"When we cooled things down we discovered these little bubbles that had formed in the graphene," explains Crommie. Platinum, which has a triangular arrangement of atoms, shrinks more than graphene when cooling, so the platinum pulls the graphene together from three different directions, causing the graphene to wrinkle and form tiny tetrahedral bubbles 4 to 10 nanometres across and a few nanometres tall.

It is because the electrons are trapped inside these bubbles that they move in small circles as they would if exposed to a powerful magnetic field.
Strain gain

Being able to encourage electrons to bunch up at discrete quantum energy levels within the bubbles could have practical applications, says Crommie. "It experimentally shows us a totally new way to control the electronic properties of graphene," he says.

"If I have certain energies where there are no electrons then I can use that as a way of modifying different properties of graphene such as its conductivity." In future it might be possible to strain the graphene sheets carefully enough to control the position of the bubbles and alter the graphene's conductivity in a controlled way.

"These results confirm the view that local strain can be used to significantly impact the electronic and transport properties of graphene," says Vitor Pereira of Boston University. He suggests the finding "is not only another tribute to the richness of this material, but also a promising stride in the still much-unexplored avenue of strain-engineering and electronics in graphene".

Journal reference: Science, www.sciencemag.org/content/329/5991/544