Published on Friday May 18, 2012
Materials scientists and engineers seek to improve upon existing materials-the things that make up the physical world around us-and invent new ones that fit a variety of optimal criteria. This could mean making a computer faster, a building stronger, or a car safer, for example. As materials scientists and engineers become better at designing these materials at the smallest scale, for example atom-by-atom, targeted properties needed for next-generation technologies will be able to be quickly obtained.
Assistant professor James Rondinelli in Drexel University's Department of Materials Science and Engineering is one of 51 awardees, out of a total 560 applicants, of the Defense Advanced Research Projects Agency (DARPA) Young Faculty Awards (YFA) working to design materials at the atomic level. The DARPA YFA program provides funding, mentoring, and industry exposure to rising stars in junior faculty positions with an emphasis on Department of Defense and national security needs. With the $300,000 two-year grant, "Seizing the third dimension in correlated oxide thin films," Rondinelli hopes to identify routes by which to control the atomic structure (atom positions and lattice constants) in thin films of functional electronic materials.
Technologically important materials, including silicon and compound semiconductors, are fabricated in thin layers or films on a support material, or substrate, comprised of chemically different elements. In nearly all cases, this means that the crystal lattice - the fundamental building block which tiles space to create a macroscopic crystal - of the functional film is larger, or smaller, than the substrate. The discrepancy between the sizes of the two crystal lattices produces a mechanical strain, imposed by the substrate, onto the film.
Such strains induced by the support material are highly useful in tailoring, or controlling, the atomic structure, and, in turn, the functional properties of materials. In correlated oxides - energy-critical materials where the electron motion behaves in a highly non-trivial fashion - the strains can produce dramatic changes: a metallic material, for example, can be turned into an insulator.
At present, the strain experienced by a thin film, however, is largely limited to and imposed only in two directions in the film, often referred to as the basal plane. The out-of-plane lattice parameters and interatomic spacing along the third direction only relax in response to the basal plane stresses, rather than being directly controlled by design.
"My job is to figure out how to achieve deterministic control in [that third] direction normal to the film-substrate interface, which has hitherto evaded experimental realization, yet is crucial to creating correlated properties-by-design," says Rondinelli.
How exactly will that be achieved?
"We'll begin from a crystal-chemistry engineering approach," he says.
Rondinelli will use sophisticated computational tools to evaluate how the ordered and layered arrangement of A-site cations with different valence states perturb the electrostatic electron-lattice interactions in the system-all in advance of experimental synthesis. In other words, he will simulate whether artificial chemical strains can be used to control the atomic structure in the direction perpendicular to the basal plane.
So what if one could tailor the out-of-plane strain in a thin film, or as Rondinelli says, "seize the third dimension," and the in-plane strain from the substrate at the same time?
Many of the novel properties of correlated oxides, including metal-insulator transitions, colossal magnetoresistance, and even superconductivity would be reliably integrated into high-performance non-linear optical (laser), sensor and security applications.
While it's unclear if this research will have immediate impact, "it is an important step towards absolute materials tunability at the atomic level," says Rondinelli.
Rondinelli joined Drexel's Department of Materials Science and Engineering as an assistant professor this fall. Prior to joining the department, he served as an Argonne Scholar and Joseph Katz Distinguished Fellow in the X-ray Science Division at Argonne National Laboratory. He received a Ph.D. in 2010 in Materials Science from the University of California and a B.S. in Materials Science and Engineering, cum laude, from Northwestern University in 2006.