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Materials Science and Engineering

Soonjoo Seo and Paul Evans

Soonjoo Seo and Assistant Professor Paul Evans (large image)

From defect to effect: Optimizing an organic semiconductor material

Most people think of “organic” and “electronic” as contradictory. For Assistant Professor Paul Evans, they work in harmony.

Evans and graduate student Soonjoo Seo are studying organic semiconductors useful for transistors, diodes and solar cells as low-cost alternatives to conventional semiconductors. Organic semiconductors can be more stable in their interactions with air, water and other environmental elements than silicon, giving them great potential as sensors and in other applications.

Defects in crystal structure can hinder material performance. Researchers don’t know why. “Researchers know that somehow the materials that have lots of defects are worse, but they know so little about what the defects are, it’s hard to know what makes them worse,” says Evans.

One common defect occurs when a molecule is missing from the crystal structure, a phenomenon known as a vacancy. Using a scanning tunneling microscope, Evans and Seo can visualize vacancies on the surface of a thin film of pentacene, a model organic semiconductor. They have found that the molecules near the vacancies are slightly shifted, relaxing around the open space.

“That turns out to be relevant to where the electronic energy levels are in these crystals,” says Evans.

With a better understanding of what the defects are and how they alter the properties of the crystal, researchers could develop strategies for growing the crystals to minimize unwanted defects—or use them advantageously.

For example, Evans is collaborating with Assistant Professor Padma Gopalan to induce a specific “defect” by creating an interface between pentacene and the carbon molecule C60. The defect changes the electronic dynamic and enables applications like organic solar cells, opening the door to technologies where organic materials are not only components of electronic devices, but also help produce energy.

What’s that molecule?
Future biosensors may have the answer

Like voice-sensing technology that can recognize an individual speaker, Assistant Professor Izabela Szlufarska hopes to perfect biosensors that can identify a specific molecule.

Each biosensor resonates at a characteristic frequency. When a molecule touches a biosensor, the frequency changes. Therefore, researchers know that when the resonance changes, a molecule is present—but they don’t know which molecule, like a sound sensor that registers a voice but can’t identify the speaker. To unlock the science that could make biosensors more sensitive and specific, the National Science Foundation granted Szlufarska a Faculty Early Career Development Award.

Szlufarska is using molecular dynamics simulations to study how resonance frequency depends on the type of molecule that the sensor detects. Working with carbon-coated sensor surfaces developed by Chemistry Professor Robert Hamers, Izabela Szlufarska studies specific molecular systems to determine the mechanisms responsible for energy dissipation at the sensor interface, which causes frequency changes. Armed with that knowledge, she can then create models that quantify the amount of resonance for specific molecules, like an acoustic expert could map the unique timbre of a person’s voice. Her models will enable researchers to predict relative frequency changes caused by a specific target molecule. “The ability to predict relative resonance shifts will enable design of structures capable of real-time biosensing,” says Szlufarska.

Real-time biosensors have many applications in health-care, since carbon-based sensors could be implanted in the body to monitor levels of a specific protein or hormone. They also could have applications for the military and national defense.

For ultra-high-temperature alloys,
new coating turns up the heat

Researchers in Professor John Perepezko’s lab have shown their new oxidation-resistant coating can take the heat. Ultra-high-temperature metals and alloys treated with this coating could enable new components for technologies ranging from airplane brakes and turbines to space vehicles.

Today, efficient combustion, greater energy efficiency and reduced emissions are the gold standard—and engines and turbines functioning at higher temperatures can accomplish all three. However, to reach those high temperatures, engine components, for example, have to be made of materials that can withstand extreme heat and pressure.

Nickel-based alloys are the industry standard for high-temperature applications, but current materials are at the limits of their potential, prompting researchers to investigate other alloys. Molybdenum alloys, particularly those with silicon and boron (Mo-Si-B), show great promise for ultra-high-temperature applications, since they maintain their strength at much greater temperatures and pressures than nickel alloys. However, even Mo-Si-B materials show some oxidation under the extreme conditions they might face in aerospace applications.

Perepezko and his students have developed a surface coating for Mo-Si-B alloys that prevents cracking, peeling, delamination and oxidation, even under extreme temperature and pressure conditions.

“According to the models, these coatings should work at very high temperatures—up to 1,800 degrees Celsius,” says Perepezko. “That’s like a tungsten filament in a light bulb. That’s hot.”

Surprisingly, the researchers’ tests exceeded what their models predicted. Oxygen torch tests have shown that both the metal and coating survive with no sign of wear at more than 2,000 degrees Celsius (about 3,600 degrees Fahrenheit).

“This is a good example of true discovery, which you wouldn’t get with any computer simulation,” says Perepezko.

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