Flower flaws ease stressed out graphene

GEORGIA TECH (US) — Flower-like defects may relieve stress in graphene sheets by allowing atoms to spread out and occupy more space without tearing.

Beyond its ability to conduct electrons almost without resistance, graphene has amazing mechanical properties, including high strength that could one day make it useful in lightweight, robust structures—but flaws could detract from its electronic and mechanical properties.

“For an engineer interested in the mechanical properties of graphene to create atom-thick membranes, for instance, it would be very important to understand these kinds of properties, which could give rise to plastic deformation of the material,” says Phillip First, professor of physics at Georgia Institute of Technology.

“For instance, it may be that these defects are just one part of the kinetic pathway to failure for a strained sheet of graphene.”

The new research is published in the journal Physical Review B.

For electronic applications, the defects could deflect electrons and cause backscattering that would increase the resistance of the material – like a rock in a stream slows the flow of water.  However, improved growth techniques developed since the defect study began may eliminate that concern.

“With the growth techniques that have now been developed using silicon carbide, we typically do not see these defects,” he says.  “The defects occur on material that we know to be of a lower quality because of the growth conditions or substrate preparation.”

Defects can appear due to the movement of carbon atoms at high temperatures. Rearrangements of graphene that require the least amount of energy involve switching from the standard six-member carbon rings to structures containing either five or seven atoms.   Researchers have discovered that stringing five- and seven-member rings together in closed loops creates a new type of defect or grain boundary loop in the honeycomb lattice.

The fabrication process plays a big role in creating the defects.

“As the graphene forms under high heat, sections of the lattice can come loose and rotate,” says Eric Cockayne, a researcher at the National Institute of Standards and Technology (NIST).

“As the graphene cools, these rotated sections link back up with the lattice, but in an irregular way.  It’s almost as if patches of the graphene were cut out with scissors, turned clockwise, and made to fit back into the same place.  Only it really doesn’t fit, which is why we get these flowers.”

So far, only the flower defect, which is composed of six pairs of five- and seven-atom rings, has been observed.  Modeling of graphene’s atomic structure suggests there might be a veritable bouquet of flower-like configurations.  These configurations—seven in all—would each possess its own unique mechanical and electrical properties, Cockayne explains.

The researchers hope to next learn whether formation of the defects can be controlled and to clarify the role they play in the material’s mechanical properties.

Flower-like defects in graphene. (Credit: Eric Cockayne/NIST)

“Graphene is strong and light, so the mechanical properties are of great interest,” he notes.  “Understanding just how it rips apart is an interesting question that has important implications.  But even with these defects, graphene is still spectacularly strong.”

The research was funded in part by the Semiconductor Research Corporation and by the National Science Foundation.

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