Breaking the Law

PROBLEM: Is the continued growth of supercomputers
challenged by laws of physics?

The ability of Oak Ridge National Laboratory over the last five years to build more powerful supercomputers at an extraordinary pace has been fueled, in large part, by semiconductor manufacturers devising ways to pack more electronic circuits into smaller spaces on silicon microchips. Despite this success, some in the computer industry predict that the drive toward scaled-down silicon microcircuitry will reach its zenith within the next decade as silicon-based electronics collides with fundamental laws of physics that could impose limits on how small silicon-based electronics can be.

In anticipation of silicon's projected demise as the microchip substrate of choice, the search for a technology capable of supporting far greater circuit density has been heating up. One contender, carbon nanotubes, has been the subject of increased scrutiny by Vincent Meunier and his colleagues in ORNL's Computer Science and Mathematics Division and the laboratory's Center for Nanophase Materials Sciences.

"The biggest advantage of carbon nanotubes is that electrons flow through them easily with little resistance," Meunier says. His preliminary work with computer models suggests that computer nanotube-based chips could be smaller, faster, and more efficient than silicon chips. However, when his team attempted to design nanocircuits by stringing nanotubes together, they discovered that electrons treat the intersections between nanotubes as dead-ends.

"Think of ping-pong balls traveling in a small tube," Meunier says. Now imagine that the tube has two branches, like the letter 'Y'. Sometimes the ball may go down one of these branches, but most of the time it will hit the wall between them and bounce back up the tube. Electrons moving through nanotubes behave the same way; when they come to a junction with another tube they are usually reflected back."

To overcome this problem, Meunier's team found that, if the interior surface of the carbon nanotubes is made "rougher" by adding structural defects or impurities, electrons move through the circuit without being affected by transitions between tubes. "We showed that with artificial defects in the nanotubes, we can actually provide more functionality," he says.

Sulfur atoms help to accomplish this trick by adding variety to the normally six-sided ring structure of the tube's carbon molecules. When the sulfur is added to the mix, five- and seven-sided rings appear. These structures not only enable electrons to traverse the molecular network, but they also cause the nano-tubes to curve and branch.

One of the ORNL team's goals is to develop a means of seeding impurities selectively throughout the network to create various types of circuits and to guide electrons through the network on a specific path. "Our experimental collaborators grow these networks using a chemical process called self-assembly," Meunier says. "If we tried to assemble them manually, one tube at a time, it would take forever. We are working on techniques, like chemical vapor deposition, to place the tubes where we want them in what would be, essentially, a process of self-assembly."

In addition to developing methods of producing viable nanocircuits, Meunier's team is also seeking ways to establish functionality between nanocircuits and normal-scale electronic devices. One approach has been to use cobalt nanoparticles to make connections between the nanotubes and copper wire. "The ability to connect nanocircuits and even nanodevices to the real world is a big hurdle," he says. When researchers in the past have tried to do this , the connections have been unsuccessful. Any device on the nano side of the connection would be completely overwhelmed by the effect of the interface."

Despite these challenges, Meunier is optimistic about the potential of using carbon nanotubes for a new generation of computer chips. "Circuitry on silicon chips is created on the micrometer scale. But nanotubes are a thousand times smaller and allow us to pack many more circuits in the same space. If we can use nanocircuits and simultaneously use less energy, we will have found a new pathway to even more powerful computers."

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