Better Off Bumpy

A structural model of a graphene membrane. Courtesy of Jannik Meyer, University of California, Berkeley

Thursday, March 1st, 2007--

At one atom thick, graphene is as thin as it gets, but it’s not flat, according to a study published this week in the journal Nature. Graphene—like graphite but thinner—is a single layer of carbon atoms, arranged in a honeycomb structure.

“If you write with pencil, you probably cleave out individual planes of graphene from the graphite,” says Jannik Meyer, a post-doctoral researcher at the University of California, Berkeley and an author on the study, “but you usually don’t see those because they are too thin.”

In this experiment, Meyer and his colleagues suspended a single layer of graphene from microscopic scaffolding and watched to see if the graphene broke apart. Physics theory predicts that two-dimensional stability is impossible because changes in temperature within the substance should create vibrations that would break up the two-dimensional sheet. Until now, experiments have confirmed the theory: stability requires either dozens of layers of atoms, or a single layer of atoms nestled in an existing three-dimensional structure. Yet, this experiment revealed that graphene held together as a single isolated layer.

What’s the secret to its stability? It seems to be the bumps. Close inspection of a graphene film revealed that it was covered with ripples. “If it’s curved, it cannot vibrate so much if there are thermal fluctuations inside it. It makes sense for it to assume shape,” says Meyer. Because graphene is bumpy, and therefore not a perfectly smooth two-dimensional crystal, Meyer says the physics theory still holds.

Meyer and his colleagues detected the bumpiness by shining electrons at the graphene. The way the electrons bounced off the surface, the diffraction pattern, indicated that the surface was rough. But graphene is only bumpy when it stands alone: if you stack a few graphene layers on top of each other, the surface becomes perfectly smooth again.

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--Flora Lichtman