Jun 10, 2010
AFM tip 'writes' graphene nanowires
A new, simple one-step process to create nanocircuits from "wonder material" graphene has been developed by researchers in the US and France. The new technique, which involves "writing" electrically conducting nanowires onto graphene oxide using a heated atomic force microscope tip, could be ideal for making flexible nanoelectronic devices.
Graphene is a flat sheet of carbon just one atom thick arranged in a honeycombed lattice. It could replace silicon as the electronic material of the future thanks to its unique physical and mechanical properties. For example, it could be used to make ultrafast transistors because electrons move through it at extremely high speeds.
Single sheets of graphene are also transparent to light, a property that could be exploited to make displays, touch-sensitive screens and solar cells. Indium tin oxide is currently used for such applications but graphene would allow for lower-cost components and flexibility. Graphene might even help increase the charge density stored on capacitors because of the very high surface area of electrodes made of the material, and its low mass means that it is perfect for portable devices.
To make such devices, however, scientists and engineers need to produce graphene circuits on a large scale using reproducible and reliable techniques – something that is lacking at present. For example, existing methods "cut" nanowires out from a sheet of graphene and re-assemble them to make circuits. This "bottom-up" approach is similar to the way traditional devices are made – stacks of silicon-based, or other electronic, material are used to create boundaries between the conducting, semiconducting and insulating part of a working device.
But things may be a lot simpler where graphene is concerned. Paul Sheehan of the Naval Research Laboratory in Washington, Elisa Riedo at GeorgiaTech and colleagues have now exploited the fact that graphene oxide, which is an insulator, converts back to conducting graphene when heated. However, instead of heating the whole sample, the researchers used a hot AFM tip to convert very narrow nanoribbons, measuring just 12 nm across, into reduced graphene. The technique is precise, which means that the rest of the graphene oxide sample remains insulating. Writing with different tip temperatures, from 130 °C upwards, also allows the electronic properties of the nanowires to be tuned over four orders of magnitude, making the wires more or less conducting.
"The beauty of our technique is that we have devised a simple, robust and reproducible technique that enables us to change an insulating sample into a conducting nanowire," said Sheehan, who heads the Surface Nanoscience and Technology Center at the NRL. "The flexibility of the technique also allows the nanowires to be written before or after transfer of the graphene to a receiving material," he told nanotechweb.org.
"The process – called thermochemical nanolithography – of changing the chemistry of a material with a hot nanotip is very promising and we are able to write semiconducting or metal lines in an insulating matrix by using an array of tips heated at different temperatures," added Riedo.
According to the team, which includes researchers from the University of Illinois and the Institut Néel in Grenoble, the nanowires conduct better than doped amorphous silicon and compare well to most doped polymer conductors. And, the most interesting is that all of the conduction occurs in a single layer of atoms in the sample.
Although it is difficult to see far down the road, the new technique could be extended so that arrays of AFM tips rapidly write circuits at high rates across graphene wafers on-demand. Graphene could be a sort of nanoscale "electronic breadboard", says Sheehan.
"Graphene and chemically modified graphenes, like graphene oxide, are exceptionally promising materials," he added. "So much is known about carbon chemistry and so much has been learnt recently about carbon nanotubes that yet has to be applied to graphene. I think this will be rich and surprising field for a while to come."
The researchers are now extending their work to single graphene sheets that have been transferred onto silicon wafers.
The results were published in Science.
About the author
Belle Dumé is contributing editor at nanotechweb.org