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Mar 30th, 2013
 
Rice, Tsinghua theorists find junctions in polycrystalline graphene sap strength of super material
 
Graphene, the single-atom-thick form of carbon, has become famous for its extraordinary strength.
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  • Less-than-perfect sheets graphene show unexpected weakness, compared to perfect graphene.

But less-than-perfect sheets of the material show unexpected weakness, according to researchers at Rice University in Houston and Tsinghua University in Beijing.

The kryptonite to this Superman of materials is in the form of a seven-atom ring that inevitably occurs at the junctions of grain boundaries in graphene, where the regular array of hexagonal units is interrupted. At these points, under tension, polycrystalline graphene has about half the strength of pristine samples of the material.

Calculations by the Rice team of theoretical physicist Boris Yakobson and his colleagues in China were reported this month in the American Chemical Society journal Nano Letters. They could be important to materials scientists using graphene in applications where its intrinsic strength is a key feature, like composite materials and stretchable or flexible electronics.

Graphene sheets grown in a lab, often via chemical vapor deposition, are almost never perfect arrays of hexagons, Yakobson said. Domains of graphene that start to grow on a substrate are not necessarily lined up with each other, and when these islands merge, they look like quilts, with patterns going in every direction.

The lines in polycrystalline sheets are called grain boundaries, and the atoms at these boundaries are occasionally forced to change the way they bond by the unbreakable rules of topology. Most common of the “defects” in graphene formation studied by Yakobson’s group are adjacent five- and seven-atom rings that are a little weaker than the hexagons around them.

The team calculated that the particular seven-atom rings found at junctions of three islands are the weakest points, where cracks are most likely to form. These are the end points of grain boundaries between the islands and are ongoing trouble spots, the researchers found.

 

 
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