Scientia: Research at the University of Tennessee
Behavioral Complex
By CATHERINE LONGMIRE
"At each new level of complexity, entirely new properties appear, and the understanding of these behaviors requires research which I think is as fundamental in its nature as any other."
—P. W. Anderson, Nobel laureate, in More is Different
Distinguished Scientist Ward Plummer sits in his windowed office, surrounded by stacks of books and papers and souvenirs from his travels, including a map of Italy depicting the bicycle tour he completed several years ago. But today his thoughts explore terrain much more challenging than Italy's meandering byways.
The notion of complexity has captured Plummer's imagination—complexity, such as that in systems where small bits of materials can band together to reveal properties they never hinted at as individual pieces. In such systems the reverse may also be true: materials may behave one way when they're in big pieces and completely differently when they're reduced in size.
Complex systems are the starting point for "emergent phenomena," which can be thought of as reorganizing well-understood puzzle pieces to get brand new pictures. To understand complexity is to open up the possibility of new materials, designed atom by atom, that can revolutionize areas as wide-ranging as computing and energy efficiency. Plummer, who was recently elected to the National Academy of Sciences, has built an impressive career on a foundation of tiny bits of matter, studying how they interact.
Simpler Times
Clad in sweater and slacks—a physicist's dress uniform—Plummer, who holds a joint appointment at the University of Tennessee and Oak Ridge National Laboratory, discusses complexity and explains why it is important. "These systems have complex behavior not necessarily predicted by ordinary theory or engineering," he says. "No matter how good you are, you can't predict exactly what's going to happen. This is what makes research in materials so exciting. Something new always seems to happen. "
A national initiative is underway to figure out what might happen next with these complex systems. While science in the 20th century tackled simple systems—synthesizing simple molecules for pharmaceuticals, for example—a Department of Energy report states that the new frontier will take on complicated systems, custom-designed to have specific properties. "Our progress is now limited by our ability to synthesize or fabricate materials with new properties," Plummer says. By playing with nature's smallest components, UT scientists hope to overcome those limits.
The Thin Film Men
Hanno Weitering and Zhenyu Zhang, UT physics professors with joint appointments at ORNL, are part of what Plummer compares to a farm system in baseball—young talent drafted to strengthen the program and assure the future. Both men are in their 40s, unfailingly polite, and well established in their field. Zhang is a fellow of the American Physical Society; Weitering held a chair professorship in his native Netherlands.
In a sparsely furnished office reserved for Zhang's students, the two describe how they remove some of the unpredictability from complex systems in the process of growing stable thin films, microscopically thin layers of material laid down on a substrate; a base that can consist of a metal, ceramic, or semiconductor.
Thin films are a staple of the electronics industry: they represent the top metallic layers on computer chips and the coating on magnetic disks. Yet they can be tricky to make. Weitering simplifies by drawing a comparison to music: If, he begins, you hold a string at both ends and then strum it, you get sound waves. "In principle, there are a large number of tones that you could generate," he says. "Each tone is a different wavelength."
Electrons in a film do the same thing. Sort of. "In very thin metal films there are only a discrete number of tones allowed," Weitering says. If the number of allowable tones, or wavelengths, is exceeded, the films become unstable and are likely to self-destruct. "What he proposed," Weitering says, motioning to Zhang, "is that the same phenomenon actually can be used to play games with controlling the size."
Nano-Magic
What Zhang discovered is the "magic layer." In a material with reduced dimensions—for example, the extreme thinness of thin films—electrons have a higher kinetic energy because they are confined to such a small space. In metals, this pent-up energy overwhelms the energy between the thin film and its substrate, forming a "magic thickness" that dictates the electronic structure, and thus the stability, of the film.
"What you want to do is grow perfectly nice layers," Weitering says, moving to a whiteboard. "Generally that's impossible. What you get is something like this," he explains, drawing a squiggle—"a very irregular landscape. But if you play games with these electrical waves, you can generate structures like this," he says, drawing orderly parallel lines, one on top of the other.
"You can control that," Zhang says, "to the atomic scale." At the atomic scale, as dimensions shrink, the resulting materials have properties different from those they possess when in bulk. Those properties offer many potential uses: They can control friction. They can be catalysts for chemical reactions. And in a March 2006 Nature Physics paper, Weitering's group reported that they can act as superconductors.
"Superconductivity is not yet very practical in nanodevices because it's low-temperature superconductivity," Weitering says, "yet we now understand why superconductivity can be robust at the nanoscale, so now you open up new prospects," including the possibility of one day building ultra-fast quantum computers.
Pocket Full of Sunlight
Just down the hill from Zhang and Weitering, Bin Hu works in a Dougherty Hall office flooded with afternoon sunlight. Fitting, as Hu is something of an expert in sunlight. He came to UT in 2002 and now serves as an assistant professor in the Department of Materials Science and Engineering. Hu's research involves functional polymer synthesis and processing—making large molecules by joining smaller ones. But there's a twist to his approach because, like Weitering and Zhang, Hu works with structures between 1 and 100 nanometers in size.
Scientists have known for a long time that nanostructured polymers could exist, he explains. "It's relatively easy to fabricate nanostructures in polymer materials," he says. "But it is a big challenge to obtain the desired properties from these fabricated nanostructures."
Understanding the quantum effects—how particles work at such a small scale—could enhance the electronic and optical properties already understood in large-scale and more traditional devices and materials. Such understanding could also allow us to see new phenomena, Hu says. To demonstrate, he slowly opens the top drawer of his desk and pulls from it a 9-volt battery and what appears to be half of a microscope slide. By gently tapping the slide against the battery's terminals, he illuminates a small green patch that looks like the map of a tiny island.
Hu explains that the light-emitting device is really two pieces of glass with a polymer sandwiched between them. "This device has two functionalities," he says. "If you have electricity, this device converts electricity into light. If you have sunlight, this can convert sunlight into electricity." Hu explains that the light-emitting or sunlight-absorbing device can be created on a flexible conducting plastic.
Flexible lightweight solar cells are of keen interest to the United States Air Force, which sponsors one of Hu's projects. An average soldier carries 155 pounds, he says. "Some of that weight is batteries to power up electronic components," he explains. "Ultimately, we'd like to use solar cells to replace the batteries." But applications like this will take some work, (solar cells, for example, says Hu, still need improved efficiency)but tweaking properties in these reduced-dimension materials holds great potential.
And Ward Plummer sees UT's young scientists leading the way in exploiting that potential. He maintains that the university's most important contribution lies not in the number of papers published but in the continued development of young faculty members. "Our strength here is that we're very good in many areas," he says. "We have the ability to grow and characterize our own materials. Many institutions don't. If we keep dabbling, great things are going to happen."
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For more information, contact Ward Plummer, e-mail eplummer@utk.edu; Hanno Weitering, e-mail hanno@utk.edu; Zhanyu Zhang, e-mail zhangz@ornl.gov; Bin Hu, e-mail bhu@utk.edu.
The Spin Doctors
Legend holds that J. J. Thomson, discoverer of the electron in 1897, had a favorite toast: "To the electron—may it never be of any use to anybody."
The good professor might be disappointed. The electron has become the foundation for multibillion-dollar industries that have changed forever the way people travel, work, communicate, and play. And science isn't done with it yet. The electron still has untapped potential for researchers like Jian Shen, who makes magnetic nanostructures.
Among these are nanodots, which are so small that 100,000 of them would fit on the head of a pin. These tiny wonders have the potential to improve the efficiency of lasers or the memory in electronic devices. Memory efficiency is the reason that scientists like Shen are so interested in the magnetic properties of their creations.
"The greatest example is the so-called giant magnetoresistance," says Shen, who holds an adjunct position in the University of Tennessee's Department of Physics.
In the late 1980s, researchers discovered that materials made of thin alternating layers of various metals showed a significant change in resistance in the presence of low magnetic fields. Within a decade, Shen says, all computers were using this effect, which dramatically increased their storage density.
It also opened up a new avenue for scientists to follow. "What's more important is that people are thinking about moving into a new era—the era of so-called ‘spintronics'—rather than conventional electronics," Shen says.
Historically, electronic devices from the vacuum tube to the microchip have been designed to exploit the electron's charge to make them work. Semiconductors depend on precise control of electric charges to function properly. But as devices shrink and more transistors are squeezed onto a chip, they become less efficient, not to mention hotter.
"Spintronics" (spin electronics) offers a cooler alternative. Electrons are restless, constantly spinning either up or down. Their electromagnetic spin can be corralled faster and with lower energy costs than conventional electric charges, and manipulating spin allows scientists to control the properties of a material.
Like Shen, Bin Hu, an assistant professor in UT's Department of Materials Science and Engineering, builds some interesting nanostructures of his own, in the form of polymers. He uses spintronics to control the opto-electronic functionalities of his polymers. Polymer materials have both singlet (where all electrons are antiparallel-paired) and triplet (where all electrons are parallel-paired) states under electrical excitation.
In polymer fluorescent light-emitting diodes, only the singlet state emits light; the triplet state does not. The ratio of singlet to triplet states, then, determines the polymer's light-emission efficiency. "The coexistence of singlet and triplet states is an intrinsic property," Hu explains. "The challenge is for us to tune the relative ratios of those spin-dependent states."
One method is to inject spin-polarized charge carriers into the polymer to control the spin orientations and consequently increase singlet formation in these polymer materials. "These days there's actually a very hot topic in the community called ‘spin transfer effect,' " Shen says. "What that means is that you can actually manipulate the spin of the atoms not by the magnetic field, but simply by electrical current."
Shen explains that the simplest way to manipulate spin is by using the magnetic field, because spin follows the magnetic field. But this approach can be inconvenient because with some devices, manipulating the magnetic field might affect other functions. Using the electrical current instead is attractive because almost all such devices already have current running through them.
No matter how they get there, scientists know that spintronic devices promise to be smaller, faster, and more energy-efficient than their predecessors. And that's good news, no matter how you spin it.
— C. L.
For more information, contact Jian Shen, e-mail shenj@ornl.gov; or Bin Hu, e-mail bhu@utk.edu.