Graphene wrinkles create semiconductor behaviour

Graphene might be the flattest material known to humanity, but it isn’t actually that flat. Like many things that are flat in theory it is prone to crinkling and wrinkling. This can be a problem, but it now also seems that it can be an opportunity. Researchers at RIKEN, the Japanese applied research organisation, have found that when graphene wrinkles, this changes the way that electrons pass through the material; moreover, they’ve used the tip of a scanning tunnelling electron microscope (STEM) to manipulate the structure of the material, which provides a physical method to alter its electrical conducting properties.

The team, led by Yousoo Kim, was trying to grow graphene on a substrate of single-crystal nickel using chemical vapour deposition with acetylene, but found that instead of generating graphene on the surface, they were making a nickel carbide. To avoid this, they tried cooling the nickel surface rapidly, which worked, but generated a graphene film with 5nm-wide microwrinkles in it.

What was surprising about these wrinkles was that they made the graphene behave like a semiconductor. Perfectly flat graphene is a very strong conductor of electricity, but in a semiconductor there is an energy difference between valence electrodes (the ones that hold a structure together) and conducting electrodes, which flow through the material. Only electrons with enough energy to bridge that ‘band gap’ can move through the substance.

The team explains in the journal Nature Communications that the three-dimensional shape of the wrinkles might be able to traps electrons owing to a phenomenon called ‘quantum confinement.’

This means that it could be possible to make a graphene semiconductor by wrinkling the material, meaning it could be used to make electronic devices. “Up until now, efforts to manipulate the electronic properties of graphene have principally been done through chemical means, but the downside of this is that it can lead to degraded electronic properties due to chemical defects,” Kim said. “Here we have shown that the electronic properties can be manipulated merely by changing the shape of the carbon structure. It will be exciting to see if this could lead to ways to find new uses for graphene.”