Thursday, April 17, 2008

Atom-thick material runs rings around silicon

19:00 17 April 2008
Chunks of this atom-thick material just one nanometre across can function as transistors - the devices central to computing power (Image: Manchester University Mesoscopic Physics Group)
Chunks of this atom-thick material just one nanometre across can function as transistors - the devices central to computing power (Image: Manchester University Mesoscopic Physics Group)

A leading contender to replace silicon as the basis for computing has made another step forward.

Transistors one atom thick and ten atoms wide have been made by UK researchers. They were carved from graphene, predicted by some to one day oust silicon as the basis of future computing.

For 40 years computing has been dominated by a rule of thumb named Moore's law, which predicts that the number of transistors on a chip will double roughly every two years.

Yet silicon, the material that has so far been used to keep up with Moore's law cannot form stable structures below 10 nanometres in size. And today's newest chips already have features just 45 nm across. The hunt is on for a replacement for silicon.

Graphene, a material made from flat sheets of carbon in a honeycomb arrangement is a leading contender. A team at the University of Manchester, UK, have now used it to make some of the smallest transistors ever. Devices only 1 nm across that contain just a few carbons rings.

Previous graphene transistors were significantly bigger – ribbons 10 nm across and many times longer.

Small proportions

"A big question has been which material to use for smaller transistors," says Kostya Novoselov, who with project co-leader Andre Geim discovered graphene in 2004. "This is one of the smallest transistors at the moment."

Graphene's carbon-carbon bonds are among the strongest in nature, and its honeycomb-like structure (see image, right) allows electrons to travel very rapidly. It also exhibits bizarre electrical properties that have fuelled an explosion of interest in the material.

Yet making transistors from graphene has proved difficult. The material usually lacks the switchable conductivity that transistors need to control electric current.

Novoselov and colleagues found that cutting small "quantum dots" of graphene can give it that property. Dots just a few nanometres across trap electrons thanks to quantum effects that become dominant at such small scales.

Tiny transistor

Applying a magnetic field to the smallest dots lets current flow again, making a switchable transistor. The smallest dots that worked as transistors contained as few as five carbon rings – around 10 atoms or 1nm wide.

There are other kinds of prototype transistors in this size range. But they usually need supercooling using liquid gas, says Novoselov. The new graphene devices work at room temperature.

Such prototypes are typically made by building one atom at a time, or wiring up individual molecules. Those approaches are complex and impractical, Novoselov says.

By contrast, the graphene transistors were made in the same way that silicon devices are, by etching them out of larger pieces of material. "That's their big advantage," he says.

Amazing result

"The most amazing result for me is that they were able to obtain quantum dots as small as 1 nm," says Antonio Castro Neto of Boston University, US. "This is shocking." "If you try to reduce the dimensions of any other structure, the structure would disintegrate before you reach these dimensions," Neto adds.

"There is no doubt in my mind that these structures can be used for technological applications," he says. "The electronic flexibility and structural stability, fundamental for modern device development, are unmatched in any other material on Earth." But working out how to manufacture graphene devices on a practical scale remains a challenge, he concludes.

Journal reference: Science (DOI: 10.1126/science.1154663) Link


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