Squeezing light from nanotubes

Read more about nanotech

Researchers at IBM and the University of Toronto are squeezing light out of molecules.

Scientists at IBM Research have discovered a new way to get carbon nanotubes to emit light, a breakthrough that might one day lead to advances in fiber-optic technology.

At the University of Toronto, meanwhile, researchers have managed to produce light by injecting electrons into a polymer embedded with "quantum dots," microscopic crystals made of lead sulfide. Polymers--chemicals made of large molecules in repeated structural units--are being used in research into processor, display and other technologies.

Carbon nanotubes--long, thin strands of specialized carbon molecules--and, to a lesser degree, nanocrystals have become scientific celebrities in recent years because of their unusual electrical, thermal and mechanical properties. Both have emerged as candidates to replace silicon and metal in chip manufacturing a decade or two down the road. In the more immediate future, nanotubes could be employed to create corrosion-resistant paint or to improve fuel cells or batteries.

The research from the two institutions essentially points the way toward another potential application: generating light.

Generating light is not easy or cheap. Current optical equipment does the job, but optical components are difficult to manufacture and as a result expensive. By contrast, semiconductors can be mass-produced cheaply. Unfortunately, researchers have tried, and failed, to get silicon to generate light effectively.

Nano in a box A handy guide to nanotechnology What it is: Nanotechnology is the science of making products out of components measuring 100 nanometers or less. A nanometer is a billionth of a meter. How it works: Below 100 nanometers, matter begins to behave . Electrons go from being like rocks to acting like waves. Silicon becomes an electrical conductor and insulator. The smaller components get, the more pronounced the quantum effects become. Products: Carbon nanotubes, long strands of fullerene carbon, get most of the attention, but standard processors made on the 90-nanometer process, like Intel's , are also examples of nanotechnology. Leading researchers: Rice University's Nobel Prize winner Richard Smalley, IBM's Phaedon Avouris, Hewlett-Packard's Stan Williams, and others at Cornell University, Harvard University and the University of California at Los Angeles. Leading companies: Carbon Nanotechnologies (founded by Smalley), General Electric, HP, IBM, Nanosys.

Several companies are currently trying to find ways to combine optical and silicon technologies. One method, called optoelectronics, involves converting optical signals into electrical signals, which can then be sent through inexpensive silicon. Other techniques involve embedding optical channels into silicon chips to route data faster and reduce energy consumption.

"Our work represents a step towards the integration of many fiber-optic communications devices on one chip," Ted Sargent, the Nortel Networks - Canada Research Chair in Emerging Technologies in the University of Toronto's electrical and computer engineering department, said in a statement. "With this light source combined with fast electronic transistors, light modulators, light guides and detectors, the optical chip is in view."

Besides its potential use in chips, fiber-optic technology also is already used for transmitting information across long-distance telephone lines and in other networks. It carries more information than traditional copper wires, but it's also more expensive and difficult to install.

"The commerical applications will be far away, but definitely this has a potential for great applications" for bridging the optical and electrical fields in communications equipment, David Tomanek, a professor of physics at Michigan State, said of the IBM results. Tomanek is also currently performing nanotube research with NEC. "Japan is very interested in optoelectronics," he added.

"The more near-term application for optical is probably for sensors," said Josh Wolfe, a managing partner at Lux Capital, a venture firm concentrating on nanotechnology. Like everyone else, Wolfe cautioned that commercial applications remain a long way off, but said that researchers are making fairly impressive progress in characterizing the properties of nanotubes.

Let there be light
In IBM's research, the light appears when a negative charge is applied to one end of the nanotube and a positive charge to the other.

Light is created in this manner now in fiber-optic equipment, but the components have to be "doped," or chemically coated, so that the opposing charges will meet. By contrast, nanotubes are so small--measuring about a nanometer, or a billionth of a meter, in diameter--that they are considered one-dimensional objects. No doping is required.

"When electrons and holes (positive charges) come together, they neutralize each other and become light," said Phaedon Avouris, manager of nanoscale science and technology at IBM Research. "A nanotube is the ultimate in confinement. If you place the electrons in one side and the holes on another, they will find each other."

The light emitted by the nanotubes featured a wavelength of 1.5 microns, the same wavelength used in fiber optics today, noted Avouris. That means arrays of light-generating nanotubes have the potential to be used inside fiber-optic cables to transmit data.

The University of Toronto prototype works in a similar fashion. Nanocrystals measuring about 5 nanometers wide sit in deep

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depressions (relatively speaking) in a polymer sheet. When electrons enter the polymer, they fall into the depressions and create light at wavelengths ranging from 1.3 microns to 1.6 microns, or millionths of a meter.

Last year, researchers at Rice University showed how nanotubes can emit light. In Rice's experiments, the nanotubes were suspended in a liquid irradiated with a laser. In other words, the nanotubes were re-emitting externally created light. The results at Rice (which actually makes the nanotubes IBM uses in its experiments) were an important step, Avouris said.

Further details on IBM's research results will come out in an edition of the journal Science on Friday. The University of Toronto published its work in the journal Applied Physics Letters.

While commercialization remains to be seen, the results show the potential versatility of nanotechnology.

"The whole thing is science at this point. We are evaluating how far things can go," Avouris said about carbon nanotubes. "(But) so far, everything is working perfectly."

Other nanotechnology research areas under way at IBM include developing new ways to manufacture carbon nanotubes and creating dense storage devices.