September 16, 2009

Nanotube Roundup: Carbon Nanotubes in Computer Chips, Nanotubes in Solar Cells

1. From MIT, a new technique for growing carbon nanotubes should be easier to integrate with existing semiconductor manufacturing processes.

"Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition of the Carbon Feedstock," Gilbert Nessim, Carl V. Thompson et al, Nano Letters, Aug. 31, 2009

Results: Researchers in the lab of MIT materials science professor Carl V. Thompson grew dense forests of crystalline carbon nanotubes on a metal surface at temperatures close to those characteristic of computer chip manufacturing. Unlike previous attempts to do the same thing, the researchers' technique relies entirely on processes already common in the semiconductor industry. The researchers also showed that the crucial step in their procedure was to preheat the hydrocarbon gas from which the nanotubes form, before exposing the metal surface to it.

Why it matters: The transistors in computer chips are traditionally connected by tiny copper wires. But as chip circuitry shrinks and the wires become thinner, their conductivity suffers and they become more likely to fail. A simple enough manufacturing process could enable carbon nanotubes to replace the vertical wires in chips, permitting denser packing of circuits.

2. Researchers at Cornell University have made a photovoltaic cell out of a single carbon nanotube that can take advantage of more of the energy in light than conventional photovoltaics. Carbon nanotube photovoltaics can wring twice the charge from light.

Researchers led by Paul McEuen, professor of physics at Cornell, began by putting a single nanotube in a circuit and giving it three electrical contacts called gates, one at each end and one underneath. They used the gates to apply a voltage across the nanotube, then illuminated it with light. When a photon hits the nanotube, it transfers some of its energy to an electron, which can then flow through the circuit off the nanotube. This one-photon, one-electron process is what normally happens in a solar cell. What's unusual about the nanotube cell, says McEuen, is what happens when you put in what he calls "a big photon" -- a photon whose energy is twice as big as the energy normally required to get an electron off the cell. In conventional cells, this is the energy that's lost as heat. In the nanotube device, it kicks a second electron into the circuit. The work was described last week in the journal Science.

McEuen cautions that his work on carbon nanotube photovoltaics is fundamental. "We've made the world's smallest solar cell, and that's not necessarily a good thing," he says. To take advantage of the nanotubes' superefficiency, researchers will first have to develop methods for making large arrays of the diodes. "We're not at a point where we can scale up carbon nanotubes, but that should be the ultimate goal," says Lee, who developed the first nanotube diodes while a researcher at General Electric. It's not clear why the nanotube photovoltaic cell offers this two-for-one energy conversion.

3. An international team studying the effects of friction on carbon nanotubes claims that friction can be cut in half when carbon nanotubes are aligned lengthwise rather than transversely.

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