Understanding Interface Properties of Graphene Paves Way for New Applications

Researchers from North Carolina State University and the University of Texas have revealed more about graphene’s mechanical properties and demonstrated a technique to improve the stretchability of graphene – developments that should help engineers and designers come up with new technologies that make use of the material.

Graphene is a promising material that is used in technologies such as transparent, flexible electrodes and nanocomposites. And while engineers think graphene holds promise for additional applications, they must first 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,” says 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.”

The researchers looked at how a graphene monolayer – a layer of graphene only one atom thick – interfaces with an elastic substrate. Specifically, they wanted to know how strong the bond is between the two materials because that tells engineers how much strain can be transferred from the substrate to the graphene, which determines 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 at various points in the graphene. Strain is a measure of how far a material has stretched.

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 center of the monolayer stretching further than the edges.

“This tells us a lot about the interface properties of the graphene and substrate,” Zhu says. “For the substrate used in this study, polyethylene terephthalate, the edges of the graphene monolayer began sliding after being stretched 0.3 percent of its initial length. But the center continued stretching until the monolayer had been stretched by 1.2 to 1.6 percent.”

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,” was published online Aug. 1 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 National Science Foundation (NSF) and the NSF’s ASSIST Engineering Research Center at NC State.

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Note to Editors: The study abstract follows.

“Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate”

Authors: Tao Jiang and Yong Zhu, North Carolina State University; Rui Huang, University of Texas at Austin

Published: Aug. 1 2013, Advanced Functional Materials

DOI: 10.1002/adfm.201301999

Abstract: We have characterized the nonlinear mechanical response of monolayer graphene on top of polyethylene terephthalate (PET), using in-situ Raman spectroscopy and atomic force microscopy. While interfacial stress transfer leads to tension in graphene as the PET substrate is stretched, retraction of the substrate during unloading imposes compression in the graphene. Two interfacial failure mechanisms, shear sliding under tension and buckling under compression, are identified. Using a nonlinear shear-lag model, the interfacial shear strength is found to range between 0.46 and 0.69 MPa. The critical strain for onset of interfacial sliding is ~0.3%, while the maximum strain that can be transferred to graphene ranges from 1.2% to 1.6% depending on the interfacial shear strength and graphene size. Beyond a critical compressive strain of around -0.7%, buckling ridges are observed after unloading. The results from this work provide valuable insight and design guideline for a broad spectrum of applications of graphene and other two-dimensional nanomaterials such as flexible and stretchable electronics, strain sensing, and nanocomposites.