Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycombed lattice. Since the material was first isolated in 2004, its unique electronic and mechanical properties, which include extremely high mobility, and high strength, have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future according to some. This is because electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with no rest mass, a property that could allow for transistors that are faster than any existing today.

However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to chemically dope it, but this has to be done carefully so as not to destroy graphene’s unique electronic properties too much.

Plasma-based surface functionalization technique

A team led by Mildred Dresselhaus and Tomas Palacios has now succeeded in p-doping graphene with chlorine using a plasma-based surface functionalization technique. "Compared with other chemical doping methods, the advantages of our approach are very significant," says team member Xu Zhang. "First and foremost, the chlorine-doped graphene keeps a high charge mobility of around 1500 cm2/Vs after the hole doping. This value is impressively high compared to those obtained with other chemical species previously."

The chlorine can also cover over 45% of the graphene sample surface, he adds. This is the highest surface coverage area reported for any graphene doping material until now, according to the researchers.

Density functional theory predicts that a bandgap of up to 1.2 eV can be opened up in graphene if both sides of the sample are chlorinated, and if the amount of chlorine on each side covers 50% of the total sample area. "The 45.3% coverage in single-sided chlorinated graphene observed in our work is thus important and paves the way to ultimately opening up a sizeable bandgap in the material while maintaining a reasonably high mobility," Zhang told nanotechweb.org.

In their work, the researchers studied both "exfoliated" graphene and that obtained using chemical vapour deposition (CVD). They performed the chlorine plasma treatments in an Electron Cyclotron Resonance Reactive Ion Etcher (ECR/RIE) in which chlorine gas was excited into the plasma state by absorbing energy from an in-phase electromagnetic field at a certain frequency. The chlorine plasma was accelerated by applying a DC bias relative to the sample stage. "We carefully optimized both the ECR power and DC bias to control the reaction conditions," explained Zhang, "and the experiments were performed at room temperature."

The p-doped material produced could be used to make all-graphene integrated circuits on a chip and RF transistors, he added. Doping the graphene with chlorine also reduces its sheet resistance, making it suitable for use in electronic circuit interconnects.

The team now plans to dope suspended samples of graphene with chlorine – to access both sides of a sample – and so open up an even bigger electronic band gap.

The present work is detailed in ACS Nano DOI: 10.1021/nn4026756.

Further reading

Nanoengineering graphene with oxygen (Feb 2010)
Stacks open up bandgap (Oct 2011)
Important advance for graphene electronics (Jan 2010)
Tuning a bandgap in graphene (Jun 2009)
Fluorinating graphene the easy way (Apr 2012)
Grain boundary defects affect graphene's strength (Aug 2012)