Adrienne Stiff-Roberts: Putting Artificial Atoms to Use

stiffroberts.jpgAdrienne Stiff-Roberts says she didn't do so well as an undergraduate physics student tackling quantum mechanics. "It's one of those topics where, the first time you see it, it's really mind blowing," she says. And yet, today her entire research program is focused on putting the wonders of quantum mechanics to good use.

The shift from struggling to wrap her mind around quantum mechanics, a topic that stressed even Albert Einstein, to embracing it as a research topic came during graduate school at the University of Michigan, Ann Arbor. It was there that she was reintroduced to the topic, not as an abstract science, but as the basis for practical applications. "That's what really attracted me," 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 she embraced quantum mechanics, Stiff-Roberts' work has focused on nanoscale structures known as quantum dots that had only just been demonstrated as she was making her way through graduate school. Quantum dots are smaller than small semiconductor structures that act like artificial atoms because electrons inside them can't 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 hundreds of times smaller than a sliver of a human hair at about 6 to 10 nanometers thick and 10 to 20 nanometers in diameter—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. In this second material, the movement of electrons can be measured as an electrical current. This ability opens up a range of potential applications, from improved night vision using dots tuned to infrared wavelenghts, a key pursuit in the Stiff-Roberts lab, to better solar cells with dots tuned to the range of wavelengths found in sunlight, an endeavor she hopes to pursue in coming years.

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 structures that incorporate dots.

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

Components in existing high-resolution infrared cameras must be cooled to temperatures 100 degrees or more below freezing using liquid nitrogen in containers known as dewars. These dewars add substantial bulk and require refilling that is often logistically challenging.

The current dependence on liquid nitrogen means the cameras can't be as widely used as the military would like. There are much more portable infrared cameras available that operate at warmer temperatures, but the image quality of these cameras is much lower. The detectors Stiff-Roberts is developing would have performance comparable to existing higher-end cameras, but could operate at warm enough temperatures to eliminate the liquid nitrogen need, 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.

Infrared Challenges and Goals
In the development of infrared detectors, one of Stiff-Roberts' main goals is 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 allows 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.

Stiff-Roberts is aiming to develop detectors that are tuned for maximum efficiency with wavelengths in at least one of those windows of interest, and ultimately hopes to devise single devices that can cover all the target windows. "If you can hit multiple wavelengths within those windows then it provides more information," says Stiff-Roberts, "It's like full color as opposed to black and white."

One important technical challenge in developing such quantum dot devices is overcoming a phenomenon known as dark current. This is the flow of electrons out of a detector even when there is no infrared light hitting them, which can ultimately create noise in images produced. This dark current is reduced at lower temperatures, which is why the conventional infrared detectors have to be cooled to such low temperatures.

The dark current problem is much less pronounced in quantum dot detectors because it is harder for electrons trapped in them to leak out, which is why they can operate at higher temperatures. However, the problem has still not been completely solved. "The dark current in the quantum dots still isn't as low as predicted theoretically," says Stiff-Roberts, "so I'm working to better understand the sources of dark curren.

Inorganic Quantum Dots: Putting Strain to Good Use
Stiff-Roberts focuses much of her attention on fabricating quantum dots using the inorganic semiconductor materials indium arsenide and gallium arsenide, which are common in computer chips and related devices. She creates the dots using a process known as strained-layer gmolecular beam epitaxy, which is a method for growing semiconductor crystals that is used to make cell phones, lasers, and LED lights.

Quantum dots can be created using epitaxy by taking advantages of differences between the molecular structures different materials assume as they crystallize. With more basic forms of epitaxy, two materials with strictly complementary crystal structures simply form two uniform, distinct layers—one on top of the other. But in some cases, as with the indium and gallium arsenides, differences between the crystal patterns of each produce strain as bonds form between the molecules in each layer. The relaxation of this strain as the materials "find" a stable configuration enables quantum dot construction.

When Stiff Roberts creates dots using epitaxy, the strain between the indium arsenide and gallium arsenide leads to a stable configuration where tiny quantum dots of indium arsenide in the shapes of pyramids about 10 to 20 nanometers wide grow, and a layer of gallium arsenide is deposited on top.

These indium arsenide dots act like artificial atoms, because electrons in them cannot escape to the surrounding gallium arsenide unless sufficient energy is added, such as in the form of infrared radiation. When electrons are excited enough to escape the dots, they move freely through the gallium arsenide, creating a measurable electric current that can be combined to build detectors, because this current indicates that energy of a known wavelength has just hit the dot.

Stiff-Roberts, while working on her Ph.D. at the University of Michigan, Ann Arbor, was the first person to exploit these properties of quantum dots to use them as an infrared detector and actually produce images. She continues to explore quantum dots constructed using molecular beam epitaxy as a promising basis for eventual commercial detectors, but she is also exploring other possibilities.

Hybrid Nanocomposites: Putting Quantum Dots in a New Mix
Another possible means Stiff-Roberts is exploring for fabricating quantum dots begins with commercially available inorganic quantum dots made of cadmium selenide or lead selenide. Stiff-Roberts uses different methods for embedding these dots in organic plastics with hopelessly complex names such as poly[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene, or MEH-PPV if that seems like it takes a tad to long to remember. These specialized plastics function similar to semiconductors because they are intentionally designed to conduct electricity. Structures that combine the dots and conducting plastics, called hybrid organic-inorganic nanocomposites, may also prove useful as infrared detectors.

Because different types of quantum dots are excited by different wavelengths of infrared or other radiation, Stiff-Roberts is working with different combinations in hopes of building devices that can detect the different ranges of wavelengths that would be most useful. A given type of dot can also be tuned to different sensitivities by manipulating such factors as, with the hybrid nanocomposites, the type of organic polymer in which the dots are embedded.
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MAPLE: The Benefits of Complete Control
One of the ways Stiff-Roberts is manipulating and tuning dots, especially those in the hybrid nanocomposites, is by developing new methods for producing these composites. She and her colleagues have already developed variations on an existing manufacturing technique, known as pulsed laser deposition, that is providing unprecedented control of the process.

Hybrid composites are typically created using methods such as spin casting, drop casting, or even ink jet printing, all of which involve spraying down layers of material on top of each other. One problem is that each individual material must be added in a solvent solution, and the solvent for one material might eat away another layer when trying to combine organic and inorganic materials, as is Stiff-Roberts' goal. These methods also limit how thin layers can be. Stiff-Roberts has advanced a new technique that is enabling fabrication of much more precisely controlled hybrid nanocomposites than previously possible. The base method is known as Matrix Assisted Pulse Laser Evaporation, or MAPLE. The process involves placing materials of interest in solution—the matrix from the title—in a vacuum chamber, then targeting the solution with a laser that evaporates it. The matrix solution absorbs much of the laser's energy, preventing the degradation of the nanocomposite materials.

One trick Stiff-Roberts has advanced is solidifying the solvent mix using liquid nitrogen, which preserves the solution during the evaporation process. Over the course of hours, as the solution evaporates into the chamber, the target materials are deposited on the device base material, or substrate, while the solvent is pumped away from the chamber by the vacuum action.

Stiff-Roberts uses MAPLE to deposit a target of the inorganic quantum dots mixed with an organic polymer for deposition of a combined, single layer of nanocomposite on the substrate. Another approach she uses is to deposit the dots and one or more polymers separately, creating more layered structures. Deposition of the materials can be precisely controlled by the amount of time the laser is used, making layers with thicknesses measured in nanometers possible.

The form of MAPLE that Stiff-Roberts has developed allows her to precisely control deposition of plastics and dots to explore their combined benefits in achieving goals for detecting specific infrared wavelengths. But, while infrared detection is the key focus for Stiff-Roberts for now, she is moving toward applying similar techniques to a range of other applications.

How to Build a Better Solar Cell
Stiff-Roberts is already looking into the possibility of applying MAPLE methods to the construction of cheaper and more efficient solar cells that could make wider use of solar power more feasible.

The best solar cells available today have impressive efficiency in the 40 percent range, meaning that 40 percent of the sun's energy hitting the cells is actually converted into electricity. The problem is that these cells are extremely expensive to produce, so pricey in fact that they are typically only used for space applications. The more widely available, commercially produced cells are created using basic semiconductor technologies and offer efficiencies of at best 20 percent.

Researchers are exploring other fabrication techniques, including some related to how Stiff-Roberts constructs her hybrid nanocomposites, but such research has been confined mainly to techniques that offer less control than MAPLE, resulting in efficiencies of only about 6 percent. One way to improve the efficiency of these devices may be to combine layers of solar cells, but more conventional methods for doing this have only allowed combination of two cells. Stiff-Roberts believes the improved control offered by MAPLE could make it possible to increase efficiency by combining many more layers. These layers could also be composed of different materials that might tune the sensitivity of a given device to more wavelengths of sunlight, further increasing efficiency.

Besides working with solar cells, Stiff-Roberts also hopes to eventually begin research with materials that have magnetic, pressure, and other sensitivities to enable creation of new sensory devices.