Graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice. It is the thinnest elastic material known and has attracted the attention of scientists and engineers alike since it was discovered in 2004 thanks to its unique electronic and mechanical properties that make it useful for a host of device applications. The material could even replace silicon as the electronic material of choice in the future thanks to the fact that electrons travel ballistically through the material at extremely high speeds. This is because they behave like relativistic particles that have no rest mass.

Pre-existing strains
However, graphene contains ripples, similar to those seen on tightly pulled plastic cling film. These ripples exist because of pre-existing strains in the material and can strongly affect its electronic properties by inducing effective magnetic fields and changing local potentials.

Jeanie Lau, Chris Dames and colleagues now say that they can directly observe and control the creation of one- and two-dimensional ripples in graphene sheets stretched across a pair of parallel trenches. The researchers heated up the material and found that the ripples disappeared thanks to graphene's negative thermal expansion coefficient, which means that the material actually contracts on heating, unlike most materials that expand. When the sheets are cooled down to room temperature again, the ripples reappear, with ridges at right angles to the edges of the trenches.

“This effect is similar to what happens to a piece of thin plastic wrap that covers a container and is heated in a microwave oven,” said Lau. Using its technique, the team was able to control the orientation, wavelength and amplitude of the ripples.

The work also actually provides the first experimental evidence of graphene's negative thermal expansion coefficient, something that had only been predicted before. "It is this peculiar feature of graphene that allows us to control the ripples' amplitude and orientation," she told nanotechweb.org.

The results could lead to an improved understanding of suspended graphene devices, say the researchers, and is a first step in controlling thermally induced stress in graphene. The ripples are extremely important for graphene-based electronics because the material's ability to conduct electricity is expected to vary with the local shape of the membrane, explained Lau. “For instance, the ripples may produce effective magnetic fields than can be used to steer and manipulate electrons in a nanoscale device without an external magnet.”

The researchers observed their samples using scanning electron and atomic force microscopes. The experimental set-up consisted of a built-in heating stage that allowed them to image graphene while it was being heated and cooled.

The work was published in Nature Nanotechnology.