Despite its amazing electrical properties, the fact that graphene is a semimetal — there is no energy gap between its conduction and valence bands — can be a bit of a handicap. The small bandgap in semiconductors such as silicon (small relative to insulators that is) is essential for the operation of transistors and diodes. Jingwei Bai and co-workers have now shown that shaping a graphene layer into a mesh can open up a bandgap large enough for electrical switching (Nature Nanotech. 5, 190–194; 2010).

Credit: © X. DUAN & S. JIANG

A bandgap has previously been created in graphene by tearing it into ribbons of less than 10 nm in width. The problem with this approach is that it is not easily scalable to produce arrays of devices. Bai et al. constructed their devices using techniques borrowed from, and therefore compatible with, large-scale semiconductor fabrication. A layer of graphene was coated with protective silica upon which lay a polystyrene film with a hexagonal array of cylindrical pores. Bombarding this with reactive ions transferred the pattern into the silica. A mesh was then created by placing the sample into an oxygen plasma to copy the pattern to the graphene.

The team constructed a transistor by placing the graphene mesh onto a silicon substrate and attaching two electrical contacts. A small current flowed through the device when a voltage was applied across the contacts. However, applying a voltage to the substrate 'turned on' the device and enabled a much larger flow, just like in a switch.

The space between the holes in the graphene mesh was crucial to the behaviour of this transistor. Research on graphene ribbons has shown that the bandgap increases with decreasing ribbon width. A similar trend would be expected in the mesh structure, which can be thought of as many ribbons in parallel. This is exactly what the team discovered: the ratio of the on-current to the off-current was 6 in the case of 15-nm-wide channels, but increased to 100 in a device in which the space was just 7 nm.

There are a number of effects that could open a bandgap in this way: localization of the electrons at the newly created edges in the graphene layer or effects owing to electron quantum confinement are just two examples. However, Bai et al. have left a full investigation of the exact origin of the bandgap for future work.