Stretchiness measurement could improve graphene reach

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North Carolina State University and University of Texas researchers have demonstrated a technique to improve graphene’s stretchability, a development that could help engineers create new technologies with the material.

First isolated at Manchester University in 2004, graphene is a promising material for use in technologies such as transparent, flexible electrodes and nanocomposites. Engineers believe the material holds promise for additional applications, provided they have a better understanding of its mechanical properties, including how it works with other materials.

‘This research tells us how strong the interface is between graphene and a stretchable substrate,’ said Dr. Yong Zhu, an associate professor of mechanical and aerospace engineering at NC State and co-author of a paper on the work. ‘Industry can use that to design new flexible or stretchable electronics and nanocomposites. For example, it tells us how much we can deform the material before the interface between graphene and other materials fails. Our research has also demonstrated a useful approach for making graphene-based, stretchable devices by ‘buckling’ the graphene.’

According to NC State, the researchers looked at how a graphene monolayer – a one atom thick layer of graphene– interfaces with an elastic substrate. Specifically, they wanted to know how strong the bond is between the two materials as this will reveal how much strain can be transferred from the substrate to the graphene, thereby determining how far the graphene can be stretched.

The researchers applied a monolayer of graphene to a polymer substrate, and then stretched the substrate. They used a spectroscopy technique to monitor the strain - the measure of how far a material has stretched - at various points in the graphene.

Initially, the graphene stretched with substrate. However, while the substrate continued to stretch, the graphene eventually began to stretch more slowly and slide on the surface instead. Typically, the edges of the monolayer began to slide first, with the centre of the monolayer stretching further than the edges.

‘This tells us a lot about the interface properties of the graphene and substrate,’ Zhu said in a statement. ‘For the substrate used in this study, polyethylene terephthalate, the edges of the graphene monolayer began sliding after being stretched 0.3 per cent of its initial length. But the centre continued stretching until the monolayer had been stretched by 1.2 to 1.6 per cent.’

The researchers also found that the graphene monolayer buckled when the elastic substrate was returned to its original length. This created ridges in the graphene that made it more stretchable because the material could stretch out and back, like the bellows of an accordion.

The technique for creating the buckled material is similar to one developed by Zhu’s lab for creating elastic conductors out of carbon nanotubes.

The paper, Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate, has been published online in Advanced Functional Materials.

Lead author of the paper is Dr. Tao Jiang, a postdoctoral researcher at NC State. The paper was co-authored by Dr. Rui Huang of the University of Texas. The research was funded by the US National Science Foundation and the NSF’s ASSIST Engineering Research Center at NC State.