Even graphene has weak spots

RICE (US) — Even the Superman of materials has its kryptonite—defects in polycrystalline graphene will sap its strength.

The unexpected weakness 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, report researchers.

At these points, under tension, polycrystalline graphene has about half the strength of pristine samples of the material.

New research shows defects in polycrystalline forms of graphene will sap its strength. The illustration from a simulation (above) shows a junction of grain boundaries where three domains of graphene meet with a strained bond in the center. Below, the calculated stress buildup at the tip of a finite-length grain boundary. (Credit: Zhiping Xu/Tsinghua University)


The new calculations 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, says Boris Yakobson, theoretical physicist at Rice University.

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 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.

“In the past, people studying what happens at the grain boundary looked at it as an infinite line,” Yakobson says. “It’s simpler that way, computationally and conceptually, because they could just look at a single segment and have it represent the whole.”

But in the real world, “these lines form a network. Graphene is usually a quilt made from many pieces. I thought we should test the junctions.”

As reported in the journal Nano Letters, the researchers determined through molecular dynamics simulation and “good old mathematical analysis” that in a graphene quilt, the grain boundaries act like levers that amplify the tension (through a dislocation pileup) and concentrate it at the defect either where the three domains meet or where a grain boundary between two domains ends.

A crack goes a long way

“The details are complicated but, basically, the longer the lever, the greater the amplification on the weakest point,” Yakobson says. “The force is concentrated there, and that’s where it starts breaking.”

“Force on these junctions starts the cracks, and they propagate like cracks in a windshield,” says Vasilii Artyukhov, a postdoctoral researcher and co-author of the paper.

“In metals, cracks stop eventually because they become blunt as they propagate. But in brittle materials, that doesn’t happen. And graphene is a brittle material, so a crack might go a really long way.”

Conceptually, the calculations show what metallurgists recognize as the Hall-Petch Effect, a measure of the strength of crystalline materials with similar grain boundaries.

“It’s one of the pillars of large-scale material mechanics. For graphene, we call this a pseudo Hall-Petch, because the effect is very similar even though the mechanism is very different.

“Any defect, of course, does something to the material,” Yakobson says. “But this finding is important because you cannot avoid the effect in polycrystalline graphene. It’s also ironic, because polycrystals are often considered when larger domains are needed.

“We show that as it gets larger, it gets weaker. If you need a patch of graphene for mechanical performance, you’d better go for perfect monocrystals or graphene with rather small domains that reduce the stress concentration.”

Researchers from Tsinghua University were co-authors of the study, which was funded by the Air Force Office of Scientific Research, the National Science Foundation, the National Natural Science Foundation of China, the Tsinghua University Initiative Scientific Research Program, and Tsinghua National Laboratory for Information Science and Technology of China.

Source: Rice University