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Materials: Carbon Nanotubes

07.01.03

Silicon’s likely successor, and much more.

It’s the Clark Kent of microelectronics. In the early 1990s, scientists at the NEC Fundamental Research Laboratory in Tsukuba, Japan, discovered a tiny graphitelike structure with the most beguiling dual identity. Sometimes it’s a metal, and sometimes it’s a semiconductor. It can serve as a wire, transporting current from one place to another, and it can also serve as a transistor, using changes in current to store information.

This microscopic structure, known as a carbon nanotube, could be the secret to extending Moore’s Law—which predicts that the number of transistors on the fastest CPUs will double every 18 months—beyond the limits of today’s silicon microprocessors (quite a feat in itself). “This is our best hope for the next generation of electronics,” says Jie Liu, a Duke University chemist at the forefront of carbon nanotube research. It is also the basic building block for all sorts of future products, from flat-panel displays and long-lasting batteries to fishing poles and satellite cables (pound for pound, nanotubes are 10 to 100 times as strong as steel).

AMD, IBM, and Intel will continue to improve silicon-based CPUs for at least another decade (see “Extreme Ultraviolet Lithography”). But when they are unable to shrink silicon transistors any further, they may abandon silicon altogether and move on to completely new materials.

Only 1/100,000 the thickness of a human hair yet exceedingly durable, a carbon nanotube is akin to graphite—a sheet of carbon atoms arranged in a tight honeycomb pattern. Your pencil tip consists of stack after stack of such microscopic sheets. Carbon nanotubes are formed when the sheets of atoms are rolled into cylinders. “They look a lot like hollow cigars,” says IBM researcher Joerg Appenzeller.

When carbon atoms assume a certain arrangement along the length of a tube, the nanotube behaves like a semiconductor. In a different arrangement, it becomes a metal. Semiconductors conduct current at certain voltages but not others. They are used to build transistors, in which processors store information. When one voltage is applied, current flows freely through the nanotube, and the transistor turns on. When a different voltage is applied, the current stops, and the transistor turns off. Metals, which conduct at any voltage, are used to build the wires that connect transistors.

In theory, you could build an entire microprocessor from carbon nanotubes. Its parts would be far smaller—and thus far faster—than the copper wires and silicon transistors used today.

Nanotubes are the by-products of various chemical reactions. Scientists can easily grow them on a substrate by reproducing these reactions, but they’re struggling to arrange nanotubes in complex circuit patterns.

Researchers are still seeking answers. “How do you control their physical properties? How do you grow them in the right place? How do you connect them?” asks Bob Gassar, director of components research at Intel. “Those are not trivial problems, and they may never be solved.”

Carbon nanotubes show promise for an extraordinary variety of products. IBM recently demonstrated a carbon nanotube that produces infrared light. Motorola and Samsung are working on carbon nanotubes for flat-panel displays. Nantero is developing nanotube-based memory. And researchers at the University of North Carolina have shown carbon nanotube batteries to hold twice as much energy as conventional batteries.

Intel has just launched a research program on carbon nanotubes, which means the company believes there’s a good chance they’ll be used in production-level processors within the next ten years. A dual identity has its advantages.