points of light

Nanodots glow in the dark—or eliminate the dark entirely.

By Jeffrey Winters, Supplement Editor

The past decade has seen a small revolution in the way we produce and capture light. Light-emitting diodes, once confined to pocket calculators and indicators on electronic equipment, have become cheap enough to be used in an ever-widening range of lighting applications. In New York City and elsewhere, for instance, LEDs have replaced conventional light bulbs in traffic signals. And advances in charge-coupled devices have enabled digital cameras to all but replace film-based models. Even phones can now take pictures.

But these technologies may soon seem old-fashioned, if some recent laboratory breakthroughs come to fruition. We may soon be using nanotechnology-based devices to efficiently light our houses—and to see in the dark.

Lighting creates a huge energy demand (20 percent of worldwide electricity demand, by one estimate), but it is an area where efficiency is rarely a prime consideration. Conventional incandescent light bulbs are notorious energy wasters—as much as 90 percent of the electricity flowing through their filaments is turned into heat. (There's enough heat from a small light bulb, after all, to power a child's oven.) More efficient fluorescent lighting has been successfully adopted for institutional situations—offices, hospitals, and so on—but is often seen as too harsh for home use, or for many retail applications. And durable LEDs have up-front costs that price them out of most potential markets.

Researchers have been investigating the use of nano-crystals in lighting applications and have found that they hold the promise of ultraefficient lighting. These crystals, called quantum dots, are bits of semiconductor material such as cadmium or selenium just one nanometer across. The material is engineered to absorb ultraviolet radiation and emit that energy as visible light—much the same way that the coatings in fluorescent tubes do. Because of their size, however, quantum dots can work more efficiently than fluorescent coatings. And unlike conventional coatings, the color the dots emit is controlled by their size. That means the same material could be used to make blue, green, or red lights.

A team of researchers at Los Alamos National Laboratory in New Mexico wanted to see if they could create an even more efficient means of energizing the nanocrystals than by putting them under UV lamps. The team, led by Marc Achermann, created a thin layer of nanocrystals atop a so-called quantum well: a multi-layer film three nanometers thick, designed to emit ultraviolet radiation. Because of the closeness of the quantum well and the quantum dots, the energy was able to travel from one to another without having to be emitted in the form of light. This, plus the fact that quantum wells can be excited directly with electricity, means that a nanocrystal-quantum well combination could be almost 100 percent efficient.

Another, almost opposite use of quantum dots has been developed by researchers at the University of Southern California in Los Angeles and at the University of Texas at Austin. They built a nanocrystal-based device that could help improve night vision goggles.

Air absorbs most wavelengths of infrared radiation; only that which falls in the 8- to 12-micrometer range of wavelengths can be used for night vision purposes. Finding material that can tune into those wavelengths has been a bit of a problem, and night vision systems are, as a consequence, expensive.

Using quantum dots could help on that score, as it is expected that they will become increasingly inexpensive to produce. But work up to now has yielded infrared detectors 100 times less sensitive than state-of-the-art QWIP night vision systems. QWIP (for Quantum Well Infrared Photodetector) systems use quantum wells in their imaging hardware.

Anupam Madhukar of USC and Joe Campbell of Texas designed self-assembling quantum dots made of indium, gallium, and arsenic that were especially tuned to the key infrared wavelengths. In benchmark tests, a quantum dot-based device (pictured above) detected infrared radiation nearly as well as sensors made from quantum wells. But unlike infrared detectors using quantum well technology, these nanocrystals can absorb light shining straight down on them, meaning night vision systems based on quantum dots could be simpler and less expensive.

Cheaper, more reliable infrared vision plus cheaper, more efficient lights: The night may never be the same.



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