Brighter Nanotubes

Carbon nanotubes are renowned for their strength, small diameter, and stunning electronic properties. They may be a key element in memory and processing chips of the future. Now researchers have made carbon nanotube devices with another highly promising quality: the ability to emit light.

In the past, light-emitting carbon nanotubes were very inefficient at converting electrons into photons – so inefficient that finding applications for them seemed a distant possibility. But in recent findings, announced last week in Science, IBM researchers fabricated nanotube devices that were around 1,000 times more efficient than previous ones at emitting light.

“It’s a pretty big step forward, not only in improving the efficiency and the brightness – it’s a new mechanism, a new way to generate the light that shows direction for further improvement,” says Jia Chen, one of the project’s principle researchers.

The results suggest that nanotubes may some day play a key role in breaking through a major bottleneck in microelectronics. As transistors become smaller, more and more the limiting factor for speed will be the metal wires used to send around the signals. This limitation has researchers pursuing alternative interconnections that use light. To work, however, the electrical signals from the transistor need to be converted into optical ones. This is what IBM’s new nanotube devices are able to do.

Such light-emitting nanotubes could find applications in telecommunications – currently the tubes emit light in the wavelengths used by the telecom industry to send information through optical fibers. The nanotubes, which produce tightly focused light, could also be used for optical probing, manipulation, and spectroscopic analysis at the molecular level.

Of particular interest, according to MIT physicist Mildred Dresselhaus, is that nanotubes emit light in a very specific frequency. This frequency varies with different tube diameters, adds Tobias Hertel, a physics professor at Vanderbilt who studies optical properties of nanotubes, so that the wavelengths they emit can be tuned “just by changing the structure, without any chemical changes.”

The new IBM devices could also be a powerful light source – if they can be assembled into arrays, says Chen. For applications where single nanotubes have to work alone, however, they will need to become more efficient at converting electrons to photons. “The thousand-fold increase is definitely very significant,” says Hertel. However, since the original efficiency was so poor, the “yield is still small.” Indeed, Chen says that some applications will require that efficiency be improved by another factor of 10 to 100.

Chen is encouraged, though, by the recent leap forward, and is working on changes to increase the efficiency. When nanotubes were first coaxed to produce light from electricity, she points out, the prospects seemed dim. But now finding applications appears to be realistic. “It’s not that distant anymore,” she says.