—by Marilyn Davis
 

Building new materials at the molecular scale is the province of two SIUC chemists recently recognized with major National Science Foundation grants.
 

Materials chemists Daniel Dyer and Shaowei Chen are skilled at working small.

Much of their research is in the area of nanotechnology: extremely small-scale materials with intriguing properties and many possible applications, from minuscule electrical circuits to exquisitely sensitive sensors for manufacturing and medicine.

"This nanotechnology is of great interest because we’re reaching the limits of where we can go with computer chips and such," says Dyer. "Scientists are predicting a fundamental revolution in dimension scale, going from a micron scale to a nanometer scale."

A micron is one-thousandth of a millimeter. Take one-thousandth of a micron, and you have a nanometer. It takes sophisticated imaging instruments to peer into the nanoscale world, roughly defined as objects less than 100 nanometers in size.

The two assistant professors are thinking big about this small world, and both recently landed major funding as a result. The National Science Foundation awarded them prestigious CAREER awards—five-year grants made to promising young scientists who integrate teaching with their research.
 

Shaowei Chen makes and studies new materials called nanoscale composites: tiny metal or semiconductor particles coated with organic molecules. You can think of such a nanoparticle as resembling a clove-studded orange, where the orange is the metal core and the cloves are the organic molecules. But Chen’s composite nanoparticles are almost unimaginably small: 1-3 nanometers in diameter. 

These nanoparticles behave like jumbo molecules—a state of matter in-between atoms and bulk material, with its own distinctive properties. Furthermore, says Chen, those properties can be changed by tinkering with the nanoparticles’ size and shape, not just their composition.

Chen is most interested in the particles’ electrical properties. Given the right conditions, electrons can tunnel through the organic coating into or out of the metal core. The coating can be tailored to control this current flow, as well as to control other properties, such as the color of light the particles emit. The latter could be useful in lasers and biological sensors.

Chen’s CAREER award will help fund his efforts to reliably fabricate, control, and analyze assemblies of such nanoparticles. One fabrication method, self-assembly, involves chemical manipulation to coax the particles into binding themselves in an orderly fashion to a metal plate. The nanoparticles themselves also can be linked together with certain kinds of organic molecules to create what Chen calls a super-lattice: a large patch of networked particles. Chen is tailoring these assembly techniques for his own purposes. 

After fabricating different kinds of assemblies with different kinds of nanoparticles, Chen and his lab team analyze their physical and electrochemical properties. Chen began by making nanoparticles with gold cores, since gold is highly stable. He’s since branched out to palladium, silver, platinum, copper, and nickel. The lighter and more reactive the metal, the tougher it is to work with. But different metals offer different properties that might be useful for nano-devices.

The metal core also can be an alloy, such as a mix of gold and platinum. "These can be used for very efficient catalysts," Chen explains. Because a nanoparticle is so big and complex compared to an ordinary molecule, it has a lot of surface area and many edges. These edges are active sites for catalysis.

By experimenting with how you make the nanoparticles, Chen says, "you can artificially create a large number of active sites. This would be important for people making fuel cells or batteries. There’s a lot of emphasis now on nanomaterial applications in catalysis, and this is one direction we’re going in."
 

Making nanoparticles that function as tiny electrical capacitors—devices able to store and release energy—is another goal of Chen’s lab. Applying a negative or a positive voltage to these nanoparticles in an ion-containing solution will charge or discharge the particles. 

"Since the nanoparticles are so small, even a single electron coming in or out of them generates a huge potential difference," says Chen. "This is very important in electronic communications, where the trigger signal is basically a voltage change."

Because the electrical conductivity of organic compounds is very low, the organic coating limits the ability of the nanoparticles to store electrons. And that allows researchers to control the particles’ charging at a very fine scale—even on an electron-by-electron basis.

Each additional electron causes a big, well-defined jump in the energy state. This quantum energy effect is observable in bulk materials only by cooling those materials to near absolute zero. But in nanoparticles, it occurs at room temperature—opening the door to commercial applications.

Chen’s group is working to understand how they can precisely control the charging of the particles by altering the coating and the particle size and shape. These factors affect things like the speed of the electron transfer and how many electrons can be introduced to or removed from the particles. For example, using different molecules for the organic coating or manipulating the bonds in those molecules affects the particles’ electrical conductivity. 

Chen also is experimenting with different ion solutions to drive the charging process. Last year, he was the first scientist to show that nanoparticles could function as capacitors in water-based solutions, not just in organic solutions. 

"In aqueous solutions, the particles behave very differently—like molecular-scale diodes," Chen says. They allow only positive current through, in effect changing alternating current to direct current.

Scientists have much to learn about these nanoscale particles. The work is painstaking—making particles so small is very difficult—but someday it could yield a big payoff.

"At the moment, we’re mostly focusing on fundamentals, laying the foundation for applied work," says Chen. "It’s a long shot, but I think it’s very promising. The potential for applications is huge. Eventually something very interesting and unique will come out of it."
 

Meanwhile, just down the hall from Chen in SIUC’s Neckers Building, Daniel Dyer is creating thin films and ultra-thin films from organic polymers (long, chain-like molecules) and liquid crystals. Thin films are measured in microns; ultra-thin films can get down to the nanoscale.

Dyer is trying to make films that have a special characteristic called polar order. Like tiny batteries, the molecules in the materials he works with have dipoles: a negative end and a positive end. The dipoles usually are randomly oriented. In some materials, however, such as a crystal or liquid crystal, dipoles have some order: they all align along the same axis. And in a so-called polar material, the dipoles not only align the same way, but their negative ends also point the same direction. 

Think of a box of crayons, Dyer suggests. If you tossed the crayons out onto the floor they would be randomly oriented. If you put the crayons back into their places in the box, they would be aligned the same way, parallel to each other. And if you put them back so neatly that the tips all pointed up, that would be analogous to polar order.

Liquid or rubbery materials with dipoles have polar order when an electric field is applied; that phenomenon is what makes liquid crystal displays possible. But if a material has inherent polar order, other electronic properties come into play and other interesting applications become possible.

Dyer explains that some inorganic materials will spontaneously crystallize with polar order. "We’re interested in creating organic materials that have this type of orientation," he says, "and we want to do it via a self-assembly approach so that we can simply take a material, heat it up, cool it down, and it will spontaneously organize into the structure that we want."

Organic polymers and liquid crystals with polar order would offer some striking benefits for silicon technology. For example, it’s much easier to coat a silicon chip with an organic compound than with an inorganic compound. If you could get those organic molecules to orient in the particular direction you wanted, the advantages would be huge.

Another benefit, Dyer says, is that chemists know a lot about organic compounds and how to predict and tailor their properties. Consequently, there would be many possible applications. "It’s much more difficult to synthesize new inorganic materials," he says.
 

Dyer’s lab is working toward organic thin films with three properties that would be highly useful in commercial devices. He says, "We’re exploring structure/property relationships: how does the structure of the molecules affect the global properties of the bulk material?"

One property is pyroelectricity—a change in a material’s electrical response due to a change in temperature. Pyroelectric thin films could be used to make sophisticated infrared sensors for military applications and for materials analysis. 

A second property is piezoelectricity—a change in a material’s electrical response due to a change in pressure. Piezoelectric materials are widely used—for example, in microphones—but almost all of them are inorganic. Piezoelectric organic biosensors that could detect the binding of biological molecules would be valuable for medical and environmental analyses. ("The Holy Grail of biosensors would be the ability to detect a single molecule," Dyer says.) Piezoelectric thin films also could be used to make better vibration sensors for monitoring stress on a bridge or airplane. 

The third property, called nonlinear optics, involves the ability to modulate, or change, the frequency of light. Nonlinear optical devices (NLOs) can change red laser light into blue, for example. Improved NLOs could more quickly translate streams of electrons from computers into streams of photons for transmission across fiber optic lines. And organic NLOs (virtually all NLOs are now made of inorganic materials) could have useful new characteristics.

To make polymer thin films with one or more of these three properties, Dyer must create polymer chains that will grow (self-assemble) from a surface with polar order. In other words, each chain’s chemical units (the dipoles) would all orient the same direction as they link up. Accomplishing this involves manipulating the chemical bonds holding the links together.

"By controlling the thickness of the films, we can use them for a variety of applications," Dyer says. He also hopes to develop new ways of patterning surfaces with these thin-film polymers at nanometer scales, so that they could be used for nanocircuitry and nanosensors.

"I’m more or less like a builder," he says. "I design the compounds, and then I collaborate with other people who study their properties." He has enlisted colleagues from various countries to help with this research.
 

A lot of collaboration goes on inside Dyer’s lab as well. Both Dyer and Shaowei Chen are firm believers in the benefits of involving undergraduates in research. In their labs, undergraduates work alongside of graduate students, postdoctoral fellows, and visiting scientists.

Dyer is planning a materials chemistry course that would bring together students from chemistry, physics, and engineering to tackle problems from their different viewpoints. He also wants to offer a summer research workshop where interdisciplinary groups of students would work with industrial partners to find solutions to real-world problems. Both the course and the workshop would replicate the team-based approach of a large industrial company such as IBM or Hewlett-Packard.

"Organic chemists are trained to think about how to make molecules and the properties of those molecules," says Dyer. "But when working with materials chemistry, organic chemists must think about the bulk properties [too]. For this, they can draw from the knowledge that physicists and engineers have."

As part of the teaching component of his CAREER award, Chen will incorporate nanotech research in an undergraduate materials chemistry lab course. He also stresses the need for recruiting and mentoring undergraduate students to do independent research in nanotechnology. 

A very young research area, nanotech is in its frontier days, with scientists and engineers exploring various directions and the future largely unpredictable but highly promising. As Chen points out, "We need to nurture new talent to sustain its growth."

Both Dyer and Chen received seed funding from SIUC’s Materials Technology Center (see sidebar) and Office of Research Development and Administration. Chen also holds grants from the Office of Naval Research, the Petroleum Research Fund, and Research Corporation, and Dyer holds grants from the Petroleum Research Fund, 3M Corporation, and Oak Ridge Associated Universities. 

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