Boffins toy with carbon nano-ribbons

Transistors built from graphene show promise for faster computers

Ian Williams, vnunet.com 29 May 2008

Chemists at Stanford University have developed a new way to make transistors out of carbon nano-ribbons, which could be used to make cooler and faster computer chips.

The researchers, led by chemistry professor Hongjie Dai, have made a field-effect transistor with graphene that can operate at room temperature.

Field-effect transistors are widely used in computer components and act as data carriers from one place to another within a circuit.

Unlike a traditional transistor, in the presence of an electric field a charged metal plate can draw positive and negative charges in and out of the semiconductor.

This allows the current to pass through or be blocked, which in turn controls how the devices can be switched on and off thereby regulating the flow of data.

Previous graphene transistors have been made with wider nano-ribbons or thin films of graphene, but require much lower temperatures to operate.

Professor Dai's group succeeded in making graphene nano-ribbons fewer than 10 nanometres wide, which allows them to operate at higher temperatures.

"For graphene transistors, previous demonstrations of field-effect transistors were all done at liquid helium temperature, which is 4 Kelvin [-452 Fahrenheit]," he said.

"People had not been able to make graphene nano-ribbons narrow enough to allow the transistors to work at higher temperatures until now."

Although several researchers have shown that carbon nanotubes significantly outperform silicon, not all of the tubes are semiconducting.

"Depending on their structure, some carbon nanotubes are born metallic, and some are born semiconducting," explained Professor Dai.

"Metallic nanotubes can never switch off and act like electrical shorts for the device, which is a problem."

However, the narrow graphene nano-ribbons created by Professor Dai's novel chemical technique are always semiconductors.

"This is why structure at the atomic scale, in this case width and edges, matters," he said.

As chip makers begin to encounter problems with the further shrinking of silicon-based components, some are considering graphene as a possible alternative.

But Professor Dai is quick to point out that, although it could be a useful material for future electronics, it will not replace silicon any time soon.

The work is described in a paper published in the 23 May issue of Physical Review Letters.

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