Stacking Graphene Enhances Its Performance

Stacking 100 graphene layers together helps gets the most out of graphite, researchers in South Korea have found.

AsianScientist (Jul. 3, 2019) – Fragile sheets of graphene become strong and stiff when stacked and heated, according to a study published in Advanced Materials.

Although both diamond and graphite are composed of only carbon atoms, their different arrangement of atoms causes distinct properties. In graphite, the carbon atoms are arranged in layers called graphene. In each layer, carbon is bonded to three other carbon atoms with an appearance similar to honeycomb. This unique crystal structure enables graphite to have high conductivity for both electricity and heat, as well as absorb light.

Scientists at the IBS Center for Multidimensional Carbon Materials (CMCM) have developed a new carbon material related to graphite, but exceeds its mechanical and thermal performance. To accomplish this they grew individual layers of graphene on large-area copper foils as single atom-thick layers. They then took the layers and stacked them one-by-one to create a large film consisting of 100 layers. This ‘hand-made’ approach combated nature’s way of producing well-ordered structures by causing each layer to be rotated in a random direction. The resulting material was found to outperform natural and synthetic graphites, possessing both higher mechanical strength and thermal conductivity.

Graphene has been often predicted to be an excellent building block for mechanically strong films due to its very promising mechanical properties. However, large size graphene-films have not yet achieved the very high predicted strengths.

“This is in part because until now very small platelets of graphene have been used,” explained CMCM director and corresponding author Professor Rod Ruoff. “When they are stacked and overlapped, the film inevitably contains many boundaries and voids that cause weakness. We are using much larger sized sheets grown by chemical vapor deposition to avoid this problem.”

As graphene layers are only one atom thick, they are very fragile. This resulted in the introduction of many defects and tears during the stacking process. Right after the stacking, the 100-layer film is defective and has a very poor performance. The scientists needed to find a way to restore the optimal properties of individual graphene.

“It turns out the best way is to heat the film to very high temperatures, up to 2,800°C. At extreme temperatures the carbon atoms have enough energy to find their stable state, causing a ‘healing’ of the individual layers,” said Dr. Wang Bin, the first author of the article.

The heat-treatment boosted the thermal conductivity and mechanical performance of the film to an extent that exceeded even the best of commercial high quality graphites. When testing at micrometer length scales, the film has a strength of almost 6 GPa, almost 60 times graphite and similar to carbon fibers. At centimeter length scales, the scientists measured 0.6 GPa which is still six times higher than the best commercial graphites. Thermal conductivity of these films was also exceptionally high, around 20 percent higher than the best graphite samples and even higher than single crystal diamonds.

“We attribute the high performance of these films to a synergy of the high-temperature healing effect and the unique layer stacking of the film,” study co-corresponding author Dr. Benjamin Cunning.

It is still a laborious task to stack 100 individual graphene sheets, but there is hope.

“There is a significant research effort towards the commercialization of graphene. It will be only a matter of time before graphene transfer can be automated and films such as what we have prepared in the lab could become realized,” said Ruoff.

“Companies around the world are now growing graphene with enormous dimensions, we may see these many layer films soon. Of course—we hope that our colleagues around the world who specialize in applications will be inspired by our team effort here.”



The article can be found at: Wang et al. (2019) Ultrastiff, Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order.

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Source: Institute for Basic Science; Photo: Shutterstock.
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