Monday, January 28, 2008

Graphene Transistors

Predicted electronic properties that have made researchers excited about a new material have now been demonstrated experimentally.

By Kevin Bullis

Speedy carbon: Thin ribbons of graphene (left) could be useful for future generations of ultra-high-speed processors (scale bar is 100 nanometers). Graphene is made of carbon atoms arranged in hexagons (right). Credit: Hongjie Dai

A researcher at Stanford University has provided strong experimental evidence that ribbons of carbon atoms can be used for future generations of ultrafast processors.

Hongjie Dai, a professor of chemistry at Stanford, and his colleagues have demonstrated a new chemical process that produces extremely thin ribbons of a carbon-based material called graphene. He has demonstrated that these ribbons, once incorporated into transistors, show excellent electronic properties. Such properties have been predicted theoretically, Dai says, but not demonstrated in practice. These properties make graphene ribbons attractive for use in logic transistors in processors.

The discovery could lead to even greater interest in the experimental material, which has already attracted the attention of researchers at IBM, HP, and Intel. Graphene, which consists of carbon atoms arranged in a one-atom-thick sheet, is a component of graphite. Its structure is related to carbon nanotubes, another carbon-based material that's being studied for use in future generations of electronics. Both graphene and carbon nanotubes can transport electrons extremely quickly, which could allow very fast switching speeds in electronics. Graphene-based transistors, for example, could run at speeds a hundred to a thousand times faster than today's silicon transistors.

But graphene sheets have one significant disadvantage compared with the silicon used in today's chips. Although graphene can be switched between different states of electrical conductivity--the basic characteristic of semiconductor transistors--the difference between these states, called the on/off ratio, isn't very high. That means that unlike silicon, which can be switched off, graphene continues to conduct a lot of electrons even in its "off" state. A chip made of billions of such transistors would waste an enormous amount of energy and therefore be impractical.

Researchers had theorized, however, that it might be possible to dramatically improve these on/off ratios by carving graphene sheets into very narrow ribbons just a few nanometers wide. There had been early evidence supporting these theories from researchers at IBM and Columbia University, but the ratios produced were still much lower than those in silicon.

Dai decided to take a different approach to making thin graphene ribbons. Whereas others had used lithographic techniques to carve away carbon atoms, Dai turned to a solution-based approach. He starts with graphite flakes, which are made of stacked sheets of graphene. Then he chemically inserts sulfuric acid and nitric acid molecules between these flakes and rapidly heats them up, vaporizing the acids and forcing the graphene sheets apart. "It's like an explosion," Dai says. "The sheets go separate ways, and the graphite expands by 200 times."

Next, he suspends the now-separated sheets of graphene in a solution and exposes them to ultrasonic waves. These waves break the sheets into smaller pieces. Surprisingly, Dai says, the sheets fracture not into tiny flakes but into thin and very long ribbons. These ribbons vary in size and shape, but their edges are smooth--which is key to having consistent electronic properties. The thinnest of the ribbons are less than 10 nanometers wide and several micrometers long. "I had no idea that these things could be made with such dimensions and smoothness," Dai says.