Pulling apart graphene nanoribbons under the right conditions should make it possible to select their edge properties.

This is important because the way atoms line up along the edge of a ribbon of graphene—the atom-thick form of carbon—controls whether it’s metallic or semiconducting. Current passes through metallic graphene unhindered, but semiconductors allow a measure of control over those electrons.

Since modern electronics are all about control, semiconducting graphene (and semiconducting two-dimensional materials in general) are of great interest to scientists and industries working to shrink electronics.

TWO TYPES OF ZIGZAGS

In the new work, which appears in the journal Nanoscale, the team used computer modeling to show it’s possible to rip nanoribbons and get graphene with either pristine zigzag edges or what are called reconstructed zigzags.

Perfect graphene looks like chicken wire, with each six-atom unit forming a hexagon. The edges of pristine zigzags look like this: /\/\/\/\/\/\/\/\. Turning the hexagons 30 degrees makes the edges “armchairs,” with flat tops and bottoms held together by the diagonals.

The electronic properties of the edges are known to vary from metallic to semiconducting, depending on the ribbon’s width.

“Reconstructed” refers to the process by which atoms in graphene are enticed to shift around to form connected rings of five and seven atoms. The Rice calculations determined reconstructed zigzags are the most stable, a desirable quality for manufacturers.

All that is great, but one still has to know how to make them.

“Making graphene-based nano devices by mechanical fracture sounds attractive, but it wouldn’t make sense until we know how to get the right types of edges—and now we do,” says ZiAng Zhang, graduate student at Rice University and the paper’s lead author.

THE RIGHT AMOUNT OF HEAT

Physicist Boris Yakobson, Zhang, and postdoctoral researcher Alex Kutana used density functional theory, a computational method to analyze the energetic input of every atom in a model system, to learn how thermodynamic and mechanical forces would accomplish the goal.

Their study reveals that heating graphene to 1,000 kelvins and applying a low but steady force along one axis will crack it in such a way that fully reconstructed 5-7 rings will form and define the new edges. Conversely, fracturing graphene with low heat and high force is more likely to lead to pristine zigzags.

Yakobson is professor of materials science and nanoengineering and a professor of chemistry.

The Air Force Office of Scientific Research and NASA funded the research. The researchers used the National Science Foundation-supported DAVinCI and SUGARsupercomputer clusters administered by Rice’s Ken Kennedy Institute for Information Technology.