By Will Soutter
Researchers from Columbia Engineering in New York have disproved claims that graphene loses its strength when fabricated on a large scale.
Defects inherent in large sheets made by CVD (chemical vapour deposition) have been thought to adversely affect the mechanical properties of the "wonder material", but this new research, published in Science, has demonstrated otherwise.
Graphene remains the strongest material ever measured and, as Professor Hone once put it, so strong that "it would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap.” Image credit: Andrew Shea, Columbia Engineering
The team from Colombia Engineering, led by Professors James Hone and Jeffrey Kysar, has been working on this problem for some time. In 2008, their first paper published in Science focused on the intrinsic strength of graphene - this represents the theoretical strength that a completely perfect sheet of graphene would possess.
However, in reality, it is impossible to fabricate perfect sheets of graphene on any sort of usable scale.
The most common process for producing large-area graphene is CVD, in which atoms of carbon are made into a vapour, then deposited onto a metal substrate in a single layer. During this process, the graphene forms in small grains, joined together to form the larger sheet, as Prof. Jeffrey Kysar explained:
Prof. Jeffrey Kysar
“CVD graphene is ‘stitched’ together from many small crystalline grains—like a quilt—at grain boundaries that contain defects in the atomic structure. These grain boundaries can severely 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.”
The Columbia team also managed to uncover what had made CVD graphene so weak in previous studies. They studied the whole fabrication process in detail, and found that the chemical that was often used to etch away the copper substrate under the graphene sheet was actually damaging the graphene itself in the process.
Hone and Kysar's team swapped this damaging etchant for a different chemical, and found that the graphene produced was of a much higher quality, and the strength of the material remained very high.
Results from their experiments showed that large-grained CVD graphene was exactly as strong as tiny flakes - even smaller grained sheets didn't lose more than 10% of their strength.
This discovery resolves a conflict between theoretical predictions and previous experimental evidence - there was no theoretical reason why the grain boundaries in CVD graphene should be as weak as they seemed to be.
Prof. James Hone
“This is an exciting result for the future of graphene, because it provides experimental evidence that the exceptional strength it possesses at the atomic scale can persist all the way up to samples inches or more in size.
"This strength will be invaluable as scientists continue to develop new flexible electronics and ultrastrong composite materials.”
Prof. Hone gives a tour of his graphene synthesis lab at Columbia Engineering