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Researchers Fashion the First Single-Molecule Circuit

NOTgate
Image: IBM RESEARCH

In a remarkable feat of engineering, researchers at IBM have wired up a working computer circuit within a single carbon nanotube. Building on earlier work, Phaedon Avouris and colleagues turned the nanotube¿essentially a sheet of carbon atoms rolled into a supertiny straw¿into a voltage inverter, or NOT gate, one of the three fundamental types of logic gates on which all computers rely. The team presented its findings over the weekend at the 222nd National Meeting of the American Chemical Society and published a paper in the August 26 Web edition of ACS's journal, Nano Letters.

"Carbon nanotubes are now the top candidate to replace silicon when current chip features just can't be made any smaller, a physical barrier expected to occur in about 10 to 15 years," Avouris says. "Such 'beyond silicon' nanotube electronics may then lead to unimagined progress in computing miniaturization and power."

This past April the IBM scientists developed a novel, highly efficient way to create arrays of carbon nanotube transistors. But these nanotubes were what are known as p-type transistors in which, instead of electrons, "holes" that lack electrons carry the current. To make a NOT gate¿which essentially flips an incoming bit of binary code from a zero to a one or vice versa¿requires n-type transistors, in which electrons pass the current. The breakthrough came when Avouris's group recently found that they could convert p-type nanotube transistors into n-type transistors simply by heating them in a vacuum.

Not only were the scientists able to convert an entire nanotube, but they also discovered that they could convert just part of a nanotube. In this way they built the carbon nanotube NOT gate. To make the device, the team first placed the nanotube, shown in blue in the illustration above, over gold electrodes to generate two p-type transistors in series. They then added an insulated layer of PMMA and opened up a window in it to expose part of the nanotube. Potassium that passes through the window converts the one p-type transistor to an n-type transistor.

Of importance, the current in the carbon nanotube NOT gate comes out stronger than it goes in¿a necessary criterion for any circuit design. And because this gain is by as much as a factor of 1.6, Avouris believes that more complex single-nanotube circuits will be possible

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