Even Graphene Has Weak Spots
New work by theorists at Rice and Tsinghua universities shows defects in polycrystalline forms of graphene will sap its strength. The illustration from a simulation at left shows a junction of grain boundaries where three domains of graphene meet with a strained bond in the center. At right, the calculated stress buildup at the tip of a finite-length grain boundary. (Credit: Zhiping Xu/Tsinghua University)
Graphene, the single-atom-thick form of carbon, has become famous for its extraordinary strength. But less-than-perfect sheets of the material show unexpected weakness, according to researchers at Rice University in the U.S. and Tsinghua University in China. 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 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.