Graphene is a promising nanomaterial for making tiny electronic devices because it is both a semiconductor and a very good electrical conductor. However, researchers need to be able to engineer and control the material's electronic structure before real-world applications see the light of day. There are various ways that this could be done, including opening up bandgaps in the material, introducing gap states using defects and controlling conductivity – by either chemical treatment or bombarding the structure with particles.

Now, Klaus Leifer of Uppsala University in Sweden and colleagues have shown that adding defects to graphene can increase its conductivity by as much as an order of magnitude (http://arxiv.org/abs/0905.1346). The researchers added the defects by simply exposing the samples to hydrochloric acid.

Counter-intuitive result
The result is counter-intuitive because defects normally reduce the conductivity of a material by acting as scattering centres. Indeed, Leifer and colleagues found that defect concentrations of between 0.01 and 0.05% increased graphene's resistivity. However, the conductivity was enhanced for smaller concentrations thanks to an increase in the local density of states adding electronic levels at the Fermi energy. According to the team, these additional mid-gap states create a region in the material that acts like a metal.

The researchers confirmed their results using both theory and experiments. For the theory part, they used a Green's function approach and numerical density functional theories to calculate graphene's band structure as well as its local density of states. These were then used to calculate the conductivity of graphene as a function of defect concentration.

In the experimental work, the team deposited graphene sheets onto tungsten substrates. The nano-sheets stick out from the substrate by about a micron, which means that they can be contacted with a nano-manipulator sharpened to a point just 10 nm across. Both the manipulator and substrate were then connected using cables and the electrical properties of the graphene measured using a Keithley 6430 source meter.

"Our work shows a scheme that leads to a permanent modification of graphene's bandstructure, which is an important step in the effort to tailor the material's electronic structure," Leifer told nanotechweb.org. "This modification can lead to electronic junctions being created in graphene, something that is needed to make electronic devices."

The team is now preparing an in situ platform to modify graphene both chemically and through particle bombardment.