Stretched Carbon Wafer Exhibits Powerful Pseudomagnetic Behaviour, Could Open Door to ‘Strain-tronics’

A UBC researcher is part of an international team that has discovered a remarkable new behavior in graphene, an ultra-thin sheet of pure carbon heralded as a possible replacement for silicon-based semiconductors.

Physicists based at the University of California, Berkeley have discovered that when graphene is stretched in a specific way it sprouts nanobubbles in which electrons behave as if they are moving in a strong magnetic field.

"This gives us a new handle on how to control how electrons move in graphene, and thus to control graphene's electronic properties, through strain," said Michael Crommie, professor of physics at UC Berkeley. "By controlling where the electrons bunch up and at what energy, you could cause them to move more easily or less easily through graphene, in effect, controlling their conductivity, optical or microwave properties."

Crommie and colleagues, which include UBC Assistant Professor Sarah Burke, report the discovery in the July 30 issue of Science.

“Control of electrons is essential for the operation of an electronic device, but that’s been a challenge with graphene,” says Burke, who co-authored the paper while a post-doctoral researcher at Berkeley, and is now jointly appointed with the UBC departments of Physics and Astronomy and Chemistry. “The electrons in graphene behave as if they have no mass and move very differently than they do in a normal material like silicon."

"You can imagine many possibilities utilizing this strain-induced pseudo-magnetic control, including electromechanical transduction and a new area of ‘strain-tronics’. Because this is such new physics, this is really just the beginning.”

The new effect was discovered by accident when Burke and several students in Crommie's lab grew graphene on the surface of a platinum crystal. Graphene is a one atom-thick sheet of carbon atoms arranged in a hexagonal pattern, like chicken wire. When grown on platinum, the carbon atoms do not perfectly line up with the metal surface's triangular crystal structure, which creates a strain pattern in the graphene as if it were being pulled from three different directions.

The strain produces small, raised triangular graphene bubbles 4 to 10 nanometers across in which the electrons occupy discrete energy levels rather than the broad, continuous range of energies allowed by the band structure of unstrained graphene.

Electrons within each graphene nanobubble segregate into quantized energy levels instead of occupying energy bands, as in unstrained graphene. The energy levels are identical to those that an electron would occupy if it were moving in circles in a very strong magnetic field--as high as 300 tesla--which is more powerful than any laboratory can produce except in brief explosions. Magnetic resonance imagers use magnets of less than 10 tesla, while the Earth's magnetic field at ground level is 31 microtesla.

This new electronic behavior was detected spectroscopically by scanning tunneling microscopy. The appearance of a pseudomagnetic field in response to strain in graphene was predicted for carbon nanotubes in 1997 by Charles Kane and Eugene Mele of the University of Pennsylvania. Nanotubes are a rolled up form of graphene.

"Theorists often latch onto an idea and explore it theoretically even before the experiments are done, and sometimes they come up with predictions that seem a little crazy at first, says Crommie. “What is so exciting now is that we have data that shows these ideas are not so crazy."

Within the last year, however, Francisco Guinea of the Instituto de Ciencia de Materiales de Madrid in Spain, Mikhael Katsnelson of Radboud University of Nijmegen, the Netherlands, and AK Geim of the University of Manchester, England predicted what they termed a pseudo quantum Hall effect in strained graphene . This is the very quantization that Crommie's research group experimentally observed. Boston University physicist Antonio Castro Neto, who was visiting Crommie's laboratory at the time of the discovery, immediately recognized the implications of the data, and subsequent experiments confirmed that it reflected the pseudo quantum Hall effect predicted earlier.

Other authors of the report are Niv Levy, now a postdoctoral researcher at the National Institute of Technology and Standards, and UC Berkeley graduate student Kacey Meaker, undergraduate student Melissa Panlasigui and professor Alex Zettl.