Some materials are like ham and cheese—two separate layers that don’t intermingle. Other materials are like peanut butter and jelly—mixing and swirling together at their interface until it becomes impossible to distinguish where one spread ends and the other begins.
Combining peanut-butter-and-jelly-like materials in a manner more akin to ham and cheese could help pave the way toward quantum computers or spintronic devices.
“Many exotic and useful properties arise at the interface between two crystalline materials,” says Jason Kawasaki, an assistant professor of materials science and engineering at the University of Wisconsin-Madison.
With support from a prestigious National Science Foundation CAREER award, Kawasaki is devising new strategies to sandwich together two materials while maintaining atomically sharp boundaries between the layers.
Most modern electronic devices already take advantage of material “sandwiches” with clean interfaces—as distinct and separate as ham and cheese. Transistors and diodes, for example, contain two layers of semiconductors that pass electrons between each other without themselves becoming intermingled.
Unfortunately, few materials are so well-behaved.
“You can’t just grow one material on top of another and expect it to have a clean interface,” says Kawasaki. “In the past we have had to rely on serendipity: specific material combinations provided by nature, that play nicely together.”
Many materials are more like peanut butter, tending to ooze into whatever’s below. In particular, a potentially useful group of substances named magnetic Hesulers are especially prone to mixing and reacting with other materials, hampering the performance of real devices
Heusler compounds have special magnetic, electronic, and phase change properties that are not found in conventional semiconductors like silicon. Making layered sandwiches of Heuslers with other materials could lead to advanced electronics like spintronic devices that store information in different ways from conventional circuits.
Enter the ideal intermediary. Kawasaki realized that a perfect junction in the magnet-semiconductor sandwich could be achieved by placing a selectively porous barrier between the layers—a material that allows electrons to flow but prevents larger atomic species from mixing.
Laying down such a partition isn’t as simple as spreading peanut butter onto a slice of bread, though. The separator must block mingling between layers, while allowing the two materials to influence one another. Specifically, the bottom and top layers must adopt the same spacing and organizations of their atoms to achieve matching crystal structures. This process is called epitaxy, in which the bottom layer serves as a crystalline template for the top layer.
“You want the interaction so that the crystal lattices match, but you don’t want the atoms between layers to intermix,” says Kawasaki.
What wonder substance allows materials to pass electrons and interact, but prevents mixing?
“The answer is graphene,” says Kawasaki.
Graphene is a one-atom thick layer of the element carbon arranged in a honeycomb pattern. The delicate sheets contain tiny hexagonal pores that allow electrons to pass through unaltered yet stop larger atomic nuclei.
Kawasaki plans to slowly grow layers of semiconductors and magnets separated by graphene, closely watching the crystal structures for evidence of defects or mixing during every step of the way. His lab contains a specialized setup for the atom-by-atom material synthesis technique called molecular beam epitaxy, combined with advanced atomic-resolution imaging equipment including a scanning tunneling electron microscope. These experiments allow his group to watch the crystals as they grow, one atomic layer at a time.
“Atomic-scale control of interfaces is a longstanding challenge, but we are excited that we now have the potential tools to control it and design new material sandwiches not previously possible,” says Kawasaki.
He will also investigate the theoretical basis for the project in collaboration with Izabela Szlufarska, the Harvey D. Spangler Professor in materials science and engineering at UW-Madison.
Because most people don’t think about the advances in materials science that helped bring about modern conveniences such as microchips, Kawasaki is planning an outreach activity called “Atoms to iPhones” for middle school students in collaboration with UW-Madison Institute for Chemical Education and the Boys and Girls Club of Dane County.
The grant provides $700,000 of support over five years.