Nanotechnology: Carbon Nanotube Electronics
Electronic devices built with carbon nanotubes (CNTs) often made headlines in 2001. At that time, researchers were still trying to understand and control basic nanotube properties. But they were also demonstrating that these materials could be integrated into increasingly sophisticated circuitry.
A peek at back issues of C&EN from a decade ago reveals that scientists had already recognized that CNT preparation methods yielded mixtures of metallic and semiconducting tubes. They knew that separating the two varieties to get at the semiconducting tubes is a prerequisite to building reliable circuits.
For example, a team led by Phaedon Avouris, manager of nanoscale science and technology at IBM’s T. J. Watson Research Center, in Yorktown Heights, N.Y., came up with a high-voltage method that selectively oxidizes and decomposes just the metallic tubes. Avouris and his colleagues used that method to construct the first array of CNT-based transistors. Later in the year, the IBM group devised elementary computing circuits known as logic gates from a single CNT bundle.
Also in 2001, Cees Dekker and coworkers at Delft University of Technology, in the Netherlands, showed that, by using a sharp tip to kink a nanotube, they could fashion a CNT transistor that could be switched on and off at room temperature with a single electron. A few months later, they reported that multiple CNT transistors could be controlled simultaneously in a multitransistor logic circuit.
Collectively, these studies helped lay the scientific groundwork needed to shrink electronic circuit components below the size achievable via lithographic patterning—the standard way of fabricating silicon-based electronics.
Avouris noted in 2001 that “carbon nanotubes are now the top candidate to replace silicon when current chip features just can’t be made any smaller.”
Today, CNT-based electronics remain a research project, not a commercial reality, Avouris says, but “the idea is still alive.” His team at IBM is currently studying CNT-based high-frequency transistors that can be used as fast switches for wireless communication.
The biggest obstacle to scaling up CNT-based electronics, Avouris observes, is devising a synthesis method or separation technique that yields very high purity samples of semiconducting nanotubes. Minuscule tube-to-tube variations in diameter and differences in chirality lead to a distribution in CNT bandgaps—a showstopper for logic circuitry.
“The promise is still there,” Avouris says. “It’s the chemistry that isn’t there yet.”