When graphene is subject to strain equal in all directions, it morphs into a new structure which is mechanically unstable. Using quantum theory and supercomputers, Chris Marianetti, Assistant Professor in Columbia University's Engineering Department of Applied Physics and Applied Mathematics has revealed the mechanisms of mechanical failure of pure graphene under tensile stress.

In 2008, experiments at at Columbia University established pure graphene, a single layer of graphite only one atom thick, as the strongest material known to mankind.

Strongest material

Marianetti's findings are presented in a paper recently accepted for publication in the journal Physical Review Letters.

Marianetti says the failure mechanism is a novel soft-mode phonon instability. A phonon is a collective vibrational mode of atoms within a crystal, similar to a wave in a liquid.

The fact that a phonon becomes ‘soft' under tensile strain means that the system can lower its energy by distorting the atoms along the vibrational mode and transitioning to a new crystalline arrangement.

Under sufficient strain, graphene develops a particular soft-mode that causes the honeycomb arrangement of carbon atoms to be driven towards isolated hexagonal rings. This new crystal is structurally weaker, resulting in the mechanical failure of the graphene sheet, according to a Columbia University press release.

Marianetti added that this is the first time a soft optical phonon has ever been linked to mechanical failure and that therefore it is likely that this novel failure mechanism is not exclusive to graphene but may be prevalent in other very thin materials.

“With nanotechnology becoming increasingly ubiquitous, understanding the nature of mechanical behaviour in low dimensional systems such as graphene is of great importance.

“We think strain may be a means to engineer the properties of graphene, and therefore understanding its limits is critical.”

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