Columbia researchers find graphene can't cope with stress

Graphene, a one-atom-thick layer of carbon, is considered the strongest material known to mankind. It has found countless applications in the field of nanotechnology, including the manufacturing of stronger-than-steel-by-a-hundredfold nanotubes. However, Assistant Professor Chris Marianetti at Columbia University has exposed a fundamental structural weakness of graphene that leads to its possible mechanical failure under strain, and could change the way we use this and other materials to build nanotech devices.

Using quantum theory and aided by supercomputers, Marianetti has discovered that when pure graphene is subject to equal strain in all directions, it morphs into a new structure that is mechanically unstable – the honeycomb arrangement of carbon atoms in the graphene sheet transforms into a series of isolated hexagonal rings that is structurally weaker, causing a mechanical failure of the material.

At any temperature above absolute zero, all the atoms in a crystal vibrate with a certain intensity – the higher the temperature, the stronger the vibrations. The team led by Marianetti found that under isotropic stress, a phonon (the collective vibrational mode of atoms within a crystal) is altered and becomes "soft." The system then distorts its atoms along the vibrational mode and transitions to a new arrangement that is structurally weaker.

This is the first time a soft optical phonon has been linked to mechanical failure, and opens the way to further research that should ascertain whether, as the team suspects, this failure mechanism is present in other very thin materials as well. Strains may even be a means to engineer the properties of graphene, so understanding its limits is critical.

The Columbia research was funded by the National Science Foundation. A paper detailing the research is due to be published soon in the journal Physical Review Letters.