Most previous investigations into how graphene deforms were done on flat material, but our new work is concerned with how “wrinkled” graphene deforms, explains team leader Robert Young. “This graphene is not completely flat and we have looked at how a one-atom-thick sheet of the material on a thin polyester plastic film behaves.”

The researchers found that, under mechanical strain, this material deforms very differently to flat graphene on a similar substrate. The material studied has been developed by the company BGT Materials for flexible electrically conductive touch screens in mobile communication devices.

Graphene is a single layer of carbon atoms arranged in a honeycombed lattice. It is the thinnest material known to man and has been attracting the attention of scientists and engineers alike since it was first isolated in 2004 by mechanical exfoliation of bulk graphite. The unique electronic and thermal properties of graphene could come in useful for making a host of novel electronics devices. These include transistors that are faster than any that exist today thanks to the fact that electrons move extremely fast in the material.

Carbon sheet can wrinkle during transfer

Although graphene obtained by mechanical exfoliation is completely flat, this method is not suitable for producing large areas of the material – needed for real-world practical applications. The good news is that graphene can now easily be grown in sheets as large as metres across using a method called chemical vapour deposition (CVD), but the downside is that these sheets are usually polycrystalline and are made up of a patchwork of grains connected by grain boundaries.

When graphene is transferred from the metallic substrate on which it is grown to other substrates (such as silica, boron nitride or plastics) for making transistors and other devices, the monolayer carbon sheet can wrinkle too. Transistors and other devices heat up when they are switched on, and especially at such grain boundary and wrinkle defects. The wrinkles (which look like those seen on tightly pulled plastic cling film) can also adversely affect graphene’s electronic and mechanical properties.

Young and colleagues obtained their results by following the deformation of wrinkled graphene using a technique called Raman spectroscopy, which allowed them to study the phenomenon at the molecular level and observe how the material’s atomic structure responds to mechanical stress.

Applications in flexible displays and touch screens

Wrinkled graphene appears to deform in the same way as the substrate on which it is placed, Young tells, which confirms that the carbon material can be used in flexible electronic displays. But it is not all good news, he adds: the fact that wrinkled graphene is not as stiff as flat graphene on the same substrate means that graphene composites (where the ‘reinforcing’ carbon flakes’ are often wrinkled or crumpled up themselves) might not reinforce the composites as well as expected.

The team, reporting its work in ACS Nano DOI: 10.1021/nn507202c, says that it is now busy looking at how real-world flexible displays made from wrinkled graphene behave under mechanical stress and strain. “In fact, we hope to determine how much they can deform before the graphene film becomes damaged, and how they respond to repeated cyclic deformation under so-called fatigue loading,” reveals Young.

“Such experiments are crucial because these materials are beginning to be used in mobile phones with graphene touch screens and some 30,000 units have recently been released in China.”