In a new study, published in Science, Columbia Engineering researchers demonstrate that graphene, even if stitched together from many small crystalline grains, is almost as strong as graphene in its perfect crystalline form. This work resolves a contradiction between theoretical simulations, which predicted that grain boundaries can be strong, and earlier experiments, which indicated that they were much weaker than the perfect lattice（格子）. Graphene consists of a single atomic layer of carbon, arranged in a honeycomb lattice. "Our first Science paper, in 2008, studied the strength graphene can achieve if it has no defects -- its intrinsic strength," says James Hone, professor of mechanical engineering, who led the study with Jeffrey Kysar, professor of mechanical engineering. "But defect-free, pristine¹ graphene exists only in very small areas. Large-area sheets required for applications must contain many small grains connected at grain boundaries, and it was unclear how strong those grain boundaries were. This, our second Science paper, reports on the strength of large-area graphene films grown using chemical vapor² deposition³ (CVD), and we're excited to say that graphene is back and stronger than ever."

 

The study verifies that commonly used methods for post-processing CVD-grown graphene weaken grain boundaries, resulting in the extremely low strength seen in previous studies. The Columbia Engineering team developed a new process that prevents any damage of graphene during transfer. "We substituted a different etchant and were able to create test samples without harming the graphene," notes the paper's lead author, Gwan-Hyoung Lee, a postdoctoral fellow in the Hone lab. "Our findings clearly correct the mistaken consensus⁴ that grain boundaries of graphene are weak. This is great news because graphene offers such a plethora⁵ of opportunities both for fundamental scientific research and industrial applications."

 

In its perfect crystalline form, graphene (a one-atom-thick carbon layer) is the strongest material ever measured, as the Columbia Engineering team reported in Science in 2008 -- so strong that, as Hone observed, "it would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap." For the first study, the team obtained small, structurally⁶ perfect flakes⁷ of graphene by mechanical exfoliation, or mechanical peeling, from a crystal of graphite. But exfoliation（剥落） is a time-consuming process that will never be practical for any of the many potential applications of graphene that require industrial mass production.

 

Currently, scientists can grow sheets of graphene as large as a television screen by using chemical vapor deposition (CVD), in which single layers of graphene are grown on copper⁸ substrates in a high-temperature furnace. One of the first applications of graphene may be as a conducting layer in flexible displays.

 

"But CVD graphene is 'stitched' together from many small crystalline grains -- like a quilt -- at grain boundaries that contain defects in the atomic structure," Kysar explains. "These grain boundaries can severely⁹ limit the strength of large-area graphene if they break much more easily than the perfect crystal lattice, and so there has been intense interest in understanding how strong they can be."

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