SMALL devices / BIG collaborations

In many key areas of science and technology, nanostructured materials (materials tailored on an atomic scale for particular functions or properties) lie at the heart of future technological developments. UW-Madison's Materials Research Science and Engineering Center (MRSEC) on Nanostructured Materials and Interfaces, directed by ChE Professor Thomas Kuech, is coordinating interdisciplinary research efforts that are at the cutting-edge of this promising field.

To get an idea of the potential of nano-structured materials for technological applications, think of biological systems, which rely exclusively on nanostructured materials to perform the myriad functions of synthesis and replication. DNA, RNA and the proteins they produce are nanostructured materials, whose complex behavior is determined by their atomic structure. Using novel instrumentation and analytical methods, researchers are now able to approach this near-atomic level of control over polymeric and inorganic materials. Potential technological applications of these laboratory-derived nano-structured materials, like the functions of their naturally-occurring cousins, are innumerable.

Graduate student Eric Rehder viewing a scanning tunneling microscope capable of distinguishing silicon and germanium atoms on the surface of thin films. (19K JPG)

"Materials science has pushed the limits of our ability to manipulate, characterize, and exploit matter at the atomic level," says Kuech. "This level of control over materials is at the frontier of our knowledge, and lies at the borders of traditional science and engineering disciplines." Indeed, research supported by the MRSEC involves over 30 faculty and research staff members from six UW-Madison departments and the School of Veterinary Medicine, collaborating closely with scientists from other universities and industry.

Materials research of this scope and complexity would not be feasible under traditional funding of individual research projects. It is for this reason that the National Science Foundation in 1996 established the UW MRSEC with a $10.6 million, five-year grant, with the possibility of renewal. The NSF support is intended to provide maximum flexibility in setting research directions, developing cooperative activities with other institutions and sectors, and responding quickly and effectively to new opportunities in materials research and education that are important to the nation's research and technology base.

Because the MRSEC establishes its own research priorities, it can provide funding to its two large, well-established, interdisciplinary research groups, which focus on different, though interrelated, problems, as well as to a number of smaller, higher-risk research initiatives in emerging areas of interdisciplinary materials research. This funding strategy allows the MRSEC to explore promising new research areas, to focus a broad range of expertise on each research problem, and to encourage sharing of equipment and expertise among the research groups.

Both of MRSEC's largest research groups focus on areas that, despite their technological significance, have not been studied outside of the UW in a comprehensive and fundamental manner. The first group investigates the controlled growth of thin films to produce semiconductors by the process of chemical vapor deposition (CVD). The group uses silicon, germanium and their alloys as model systems. They employ the most advanced surface science capabilities, together with state-of-the-art CVD growth techniques, to study the complete cycle of CVD growth, from precursor chemistry to production of device-quality materials. They have achieved significant breakthroughs in a number of areas including development of a theoretical understanding of many fundamental mechanisms involved in CVD growth, growth of quantum dot structures, and development of novel instrumentation and analytical methods for both atomic-level characterization and direct monitoring of the growth front. The range of disciplines represented in this research group includes chemical engineering, chemistry, electrical engineering, materials science and physics.

The second large research group within the MRSEC focuses on the central problem facing all large-scale applications of high-temperature superconductors: supercurrent transport across grain boundaries in polycrystalline materials. By coupling experimental studies of grain boundary properties with electronic studies and theoretical modeling, the MRSEC has developed the unique capability in Madison of mounting a coordinated, full-frontal attack on the nature of and limitations to supercurrent percolation through polycrystalline high-temperature superconductors. This project involves researchers from the Departments of Physics, Materials Science and Engineering, and Electrical and Computer Engineering, as well as from Mankato State University, the University of Tsukuba (Japan), Clarke-Atlanta University, and the National Institute of Standards and Technology.

A third major research direction has emerged within the MRSEC focusing on the integration of nanostructured surfaces into biological systems. ChE Professors Nicholas L. Abbott, Paul Nealey, and Juan de Pablo, working with researchers in physics, electrical engineering, and the School of Veterinary Medicine, are studying the interaction of three-dimensional substrates having controlled topography on the 1-100nm scale, with similarly-sized protein assemblies, extracellular matrices, microtubules, viruses, and other biological entities that are known to direct the behavior of cells. The group is also investigating diagnostic tools that will permit direct measurement of the response of cells to surfaces. Potential applications of this work include improved cell culture systems, tissues for artificial organs, and materials for prosthetic devices.

The MRSEC is also funding a number of smaller, exploratory research programs with the potential to lead in many fascinating new directions. For example, the goal of one such project is to demonstrate the self-assembly of nanostructures with linear order at stepped surfaces. Researchers in the Department of Physics, together with colleagues at IBM and Lawrence Livermore National Laboratory, have succeeded in preparing extremely straight steps on silicon, and producing linear arrays of 7 nm wide stripes and dots of CaF2 on silicon. The former achievement paves the way for fabricating high-quality wire structures on silicon; the latter suggests that the principle of an inert mask (the equivalent of a photoresist) might work all the way down to a few nanometers. Further research will explore the possibility of using the straight steps achieved on silicon as templates for a variety of adsorbates, from atoms to small organic molecules to polymers and biomolecules, allowing the assembly of organic nanostructures.

In a second smaller project, Professor Kuech and colleagues in chemistry and electrical engineering are studying the precursor chemistry involved in the molecular electrospray deposition of ferroelectrics, a class of materials that possess a rich variety of electro-optic, opto-mechanical, and electro-mechanical attributes not shared by semiconductors. A viable thin film methodology for production of ferroelectrics would permit the design and production of new devices with unique capabilities.

Modern chemical engineering research often intersects strongly with a variety of other disciplines. By encouraging interactions among researchers across the campus, and indeed around the world, UW's MRSEC is contributing in a big way to the atomic-scale universe of nanostructured materials.

By Roger Packard