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Adrienne Stiff-Roberts struggled when she was first introduced to the fun-house physics of quantum mechanics. But when she encountered them again as the basis for practical applications during graduate school, the light went on.

“It’s one of those topics where, the first time you see it, it’s really mind-blowing,” she says. “I started to enjoy it much more after learning how, with special materials, you can use the principles of quantum mechanics to actually enhance the performance of different devices for all sorts of applications.”

Ever since, Stiff-Roberts’ work has focused on nanoscale structures known as quantum dots which may be used to dramatically improve night vision technology, infrared sensing and solar cells.

A quantum dot is a tiny semiconductor structure that acts like an artificial atom because the electrons inside it cannot escape without an influx of energy.

If the materials used in quantum dots were thicker, or the dots themselves were wider, electrons could travel freely. But because the dots are so small — typically 6 to 10 nanometers thick and 10 to 20 nanometers in diameter, about the size of 200 hydrogen atoms — the electrons are instead governed by quantum mechanics.

Just as electrons can jump from one electron shell in an atom to the next higher shell if energy is added, electrons can jump out of a quantum dot into surrounding material when excited, for instance by energy from infrared light. The movement of electrons into this surrounding material can be measured as an electrical current, which is how Stiff-Roberts sees them becoming ultra-fine sensors of various kinds of electromagnetic energy.

Stiff-Roberts’ lab is involved in all phases of quantum dot development, including work with dots she creates herself, experimentation with dots that are commercially available, and the development of new techniques for building composite materials that incorporate dots.

One of Stiff-Roberts’ primary research pursuits is devising ways to use quantum dots for improved infrared detectors in cameras. Infrared cameras are of special interest to the military because humans and many objects give off substantial infrared radiation — heat — even in the dark, and this radiation travels easily through the atmosphere, even penetrating clouds and smoke. There is also potential for using infrared detection in medical scanning, space science and atmospheric monitoring.

The IR detectors in existing high-resolution infrared cameras must be cooled to temperatures 100 degrees or more below freezing using liquid nitrogen, which requires bulky storage tanks and logistically challenging refilling. That makes them impractical for the military, which instead uses portable infrared cameras that operate at warmer temperatures, but with much poorer image quality.

Quantum dot IR detectors would perform like the higher-end cameras, but without the need for liquid nitrogen, enabling drastic shrinkage and simplification. “If you can eliminate that need for cooling, you might even be able to have individual soldiers with these better cameras,” she says.

Stiff-Roberts is trying to design quantum dot devices that respond to specific windows of infrared light. Targets include those wavelengths that aren’t absorbed by water and carbon dioxide in the atmosphere, which would allow imaging through clouds; and wavelengths that can travel through smoke without significant absorption, which opens the possibility of clear imaging during fiery battles, among other possibilities.

She ultimately hopes to devise single devices that can cover all the target windows. “If you can hit multiple wavelengths, it’s like full color as opposed to black and white,” says Stiff-Roberts.

She grows her own quantum dots at Duke with a process that is used commercially for growing the semiconductor crystals found in cell phones, lasers, and LED lights. The result is a collection of pyramids just 10-20 nanometers wide, made of an exotic material called indium arsenide. Covering them is a thin blanket of gallium arsenide, the conducting material where any electrons that escape the quantum dot pyramid will be detected as current.

Stiff-Roberts is also embedding commercially available quantum dots in an organic plastic called, incredibly, poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene, or MEH-PPV for short. The plastic is itself semiconductor material, and her hope is that the hybrid dot-and-plastic composite can be turned into a detector device.

To make these hybrid composites, she and her colleagues are using a new technique called Matrix Assisted Pulse Laser Evaporation, or MAPLE, in which laser light triggers when and how materials are deposited on a base. The technique gives researchers the ability to precisely control the formation of dots and polymer structures, resulting in a complex pattern of multiple materials in layers that are only nanometers thick.

Stiff-Roberts is now using MAPLE to precisely control the deposition of plastics and dots and explore how well these composite devices can detect specific infrared wavelengths.

The technique may also open the door to a new generation of solar cells. The efficiency of existing photovoltaic (sunlight-to-electricity) cells could be improved by layering them, but conventional methods have only achieved two layers. MAPLE might make it possible to combine many more layers and to tune the sensitivity of a given device to more wavelengths of sunlight, further increasing efficiency.

Beyond solar cells, Stiff-Roberts also hopes to begin research with materials that have magnetic, pressure, and other sensitivities to create new sensory devices.

Mark Schrope is a freelance writer in Florida

A longer version of this story appears on the Pratt School of Engineering website.

Blog: Daily Mail Online