Big potential in a tiny discovery

Paradoxically enough, sometimes “big science” involves very small objects. Particle accelerators are one good example. Another is the burgeoning field of nanotechnology: the technology of very small things.

January was a very big month for very small things at the University of Toronto, with two related announcements about a new breakthrough that could give us cheap, clean, inexhaustible energy, plus let us see in the dark, diagnose cancer in its earliest stages, and more.

U. of T. professors Ted Sargent and Eugenia Kumacheva announced in the magazine Nature Materials on January 9 that they have developed a substance that can sense and harness the sun’s energy at certain wavelengths in the infrared. It consists of “quantum dots,” extremely tiny particles of the semi-conductor lead sulfide, only two, three or four nanometres (one thousandth-millionth of a metre) in size–depending on what wavelengths they want the particles to be sensitive to. These dots are then wrapped in eight carbon atoms strung together in a chain.

These nanoparticles exhibit the photoelectric effect: when light strikes them, they give off electrons. Capture those electrons and let them flow through a circuit, and you’ve got an electrical current.

The nanoparticles are not hard to make, which is one thing that’s special about them. Another reason they’re special is that they’re so small they can remain dispersed in everyday solvents, just like the particles in paint that give it its color–resulting in a “spray-on” technology.

But the most important reason they’re special is that they exhibit the photoelectric effect not just when struck by visible light, but also when struck by the longer wavelengths of light that we can’t see: infrared light. Much of the sun’s energy striking the earth falls into these longer wavelengths, which we can only sense as heat.

One of the goals of current photovoltaic research is to replace the familiar, but rather ugly and clunky, crystalline solar panels with sheets of clear plastic that could be molded onto a roof or the side of a building (or anything else). Unfortunately, the photovoltaic plastics researchers have come up with so far are very inefficient, converting only between three and 12 percent of the sunlight that falls on them into electricity. One reason is that they are only sensitive to visible light. When the Toronto researchers combined their new nanoparticles with one of these photovoltaic polymers, however, it produced electricity from infrared light, too.

Peter Peumans, a professor at Stanford University who has examined the University of Toronto work, believes that a polymer incorporating the new nanoparticles could eventually be as much as 30 percent efficient. Not only that, but unlike visible light, infrared lingers after dark, because everything that’s warm gives off infrared radiation as heat, so such a polymer would produce some electricity even at night.

That’s the key to another possible application of the new nanoparticles, announced in the January issue of the journal Optics Letters: to create inexpensive, spray-on infrared amplifiers. Suspended in water, rather than in a polymer, and applied in a thin coat to glass (by the simple expedient of putting a drop of the suspension on a piece of glass and then letting it dry), then excited by an intense laser, the nanoparticles doubled the power of a light beam every 30 microns.

In other words, by coating a lens with a film a tenth of a millimeter thick and powering it with a laser, you could enhance infrared images tenfold. The material could be coated onto a digital camera’s imaging chip to make it infrared-sensitive, put on a window to reveal the infrared world on the other side, and, of course, make night-vision equipment much cheaper and less complex, making it more affordable for search-and-rescue work, anti-terrorism operations, and looking for a kitten in the backyard at three in the morning.

It could even have medical applications, allowing a new form of non-invasive test for diagnosing cancer before it has a chance to progress.

As for the infrared-sensitive photovoltaic material, it not only opens the door to more energy with less pollution, but to novel devices like clothes that can power a handheld computer or cell phone: wireless power to go with our wireless communications.

We could start to see commercial applications of these discoveries in just three to five years, analysts figure.

Pretty big results from something so small!

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