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The use of carbon nanotubes in ultrafast computers and other electronic devices has been stymied because batches of the material contain nanotubes with varying electronic properties. One nanotube is semiconducting, while the next is conducting. Now Northwestern University researchers have developed a reliable and potentially practical way to sort through this mess, segregating nanotubes into precisely the types needed for high-performance electronics. The advance could speed progress toward nanotube computers and has many nearer-term applications, including high-definition displays, devices for nanotoxicity testing, and solar cells.

The new process separates metallic and semiconducting nanotubes. It also segregates them by diameter (another important parameter for reliable computer chips) and eliminates contaminants, such as other forms of carbon. While the researchers expected to be able to sort nanotubes by diameter, the sorting by electronic type came as a surprise, says Mark Hersam, materials-science and engineering professor and one of the Northwestern researchers. “We didn’t believe it at first,” he says.

Carbon nanotubes are appealing candidates for eventually replacing silicon-based computing because of their small size and excellent electronic properties: some are semiconductors–perfect for transistors–and others are metallic conductors and could be useful as wires for connecting transistors. But getting the right electronic type “makes a big, big difference,” says Mildred Dresselhaus, professor of physics and electrical engineering at MIT. Placing metallic nanotubes where there should be semiconducting nanotubes would cause the chip to fail.

So although researchers have been able to painstakingly create logic circuits using carbon nanotubes (see “Carbon Nanotube Computers”), the methods employed to sort them are “all pretty tedious,” Dresselhaus says, and not something that could be scaled up for manufacturing chips with the millions of transistors needed to compete with today’s computers. In addition, past methods have failed to completely separate semiconducting and metallic nanotubes, says Richard Martel, chemistry professor at the University of Montreal. Martel calls the Northwestern researchers’ new approach, described this month in the new journal Nature Nanotechnology, “a breakthrough in the field.”

The researchers begin by adding surfactants to a batch of nanotubes. The surfactants latch on to the nanotubes, but differences in the nanotubes’ size and electronic properties cause the surfactants to assemble in different concentrations and arrangements, which in turn lead to measurable differences in density. These distinct densities can be sorted out using a well-known process called ultra-centrifugation, which involves spinning the materials at ultrafast speeds–up to 64,000 revolutions per minute.

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