The force needed to slide sheets of graphene across each other has been measured using a new technique that involves blowing air bubbles made of the material. Developed by Zhong Zhang of the National Center for Nanoscience and Technology in Beijing and colleagues in China and the US, the technique was also used to measure the force needed to slide graphene across a surface of silicon dioxide.

Graphene is a layer of carbon just one atom thick that has a wide range of potentially useful electronic and mechanical properties. Developing practical graphene-based devices will require an understanding of how well graphene layers stick to each other, and also how they stick to popular substrates such as silicon dioxide. This stickiness is expressed as the shear resistance – the minimum force required to slide one layer over another. This quantity is not well known for graphene because it is extremely difficult to measure for a material just one atom thick.

Tiny bubbles

Now, Zhang and colleagues have adapted a technique called the blister test for use on graphene. One of their measurements involves a single layer of graphene on a silicon-dioxide substrate with micron-sized holes in it. The air pressure in the holes is increased, causing the graphene sheet to form tiny bubbles in the regions above the holes. The size of each bubble is monitored using atomic force microscopy (see figure). As the bubble pushes up, the surrounding graphene on the substrate is pulled and stretched towards the hole – creating a zone where shear occurs. Raman spectroscopy is used to measure the stretching of graphene's carbon bonds in this "shear zone" as the size of the bubble increases. A similar measurement is made on two layers of graphene on the substrate, which has an additional graphene-on-graphene shear zone that can be analysed.

Small resistance

The team found that the shear resistance between layers of graphene was relatively small at 40 kPa. The value for graphene on silicon dioxide was about 40 times greater at 1.64 MPa. Writing in Physical Review Letters, Zhang and colleagues say that their technique could be used to study the interfaces between other 2D materials.