Ripping graphene nanoribbon edges converts the material from conductive to semiconducting

January 26, 2015

Graphene nanoribbon properties can be tuned by pulling them apart with the right force and at the right temperature. The illustration shows the crack at the edge that begins the formation of five- and seven-atom pair. (Credit: ZiAng Zhang/Rice University)

Theoretical physicists at Rice University have figured out how to custom-design graphene nanoribbons by controlling the conditions under which the nanoribbons are pulled apart to get the edges they need for specific mechanical and electrical properties, such as metallic (for chip interconnects, for example) or semiconducting (for chips).

The new research by Rice physicist Boris Yakobson and his colleagues appeared this month in the Royal Society of Chemistry journal Nanoscale.

Customized ripping

Perfect (pristine) graphene is conductive and looks like chicken wire, with each six-atom unit forming a hexagon, with edges that are zigzags like this: /\/\/\/\/\/\/\/\ .

Turning the hexagons 30 degrees makes the edges “armchairs,” with flat tops and bottoms held together by the diagonals, making the nanoribbons both semiconducting and more stable.

The researchers 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 revealed that heating graphene to 1,000 kelvin 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 Rice’s Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry.

The Air Force Office of Scientific Research and NASA funded the research.

Abstract for Edge reconstruction-mediated graphene fracture

Creation of free edges in graphene during mechanical fracture is a process that is important from both fundamental and technological points of view. Here we derive an analytical expression for the energy of a free-standing reconstructed chiral graphene edge, with chiral angle varying from 0° to 30°, and test it by first-principles computations. We then study the thermodynamics and kinetics of fracture and show that during graphene fracture under uniaxial load it is possible to obtain fully reconstructed zigzag edges through sequential reconstructions at the crack tip. The preferable condition for this process is high temperature (T ~ 1000 K) and low (quasi-static) mechanical load (KI ~ 5.0 eV Å−5/2). Edge configurations of graphene nanoribbons may be tuned according to these guidelines.