Seed

The Wisconsin MRSEC has shown that molecules inside in a type of topological photonic material called a Weyl crystal can exchange energy over much larger distances. The intricate twisting structure of the material uses light to connect one molecule to others much farther away. Developing photonic Weyl crystals may contribute to more efficient LEDs and solar cells and improve molecular sensors.

The Wisconsin MRSEC has developed machine learning techniques that enable the design of new toxin sensors using liquid crystal droplets that respond to the presence of different bacterial toxins and at extremely low concentrations by changing shape and appearance. Machine learning enables computers to automatically analyze the droplet responses to measure toxin concentration and type automatically at high accuracy. More generally, these results demonstrate that the machine learning approach can quickly extract valuable information from complex datasets.

Bacteria communicate via molecular signals that they produce in high concentrations. Bacterial communication promotes the formation of biofilms that can be harmful to humans and costly to industry. We have shown that collections of individual bacterial signaling molecules interact in water to form soft materials (“self-assemble”) with spherical, layered, or cylindrical structures. Simulation images showing the formation of a spherical structure (“micelle”) are shown with corresponding experimental images.

Heusler compounds are promising materials for next generation devices for direct conversion of heat to electricity (thermoelectricity) and for magnetic computer memory. Performance in these applications depends sensitively on the arrangement of the atoms and the behavior of electrons, both of which are hard to predict and harder to control for Heuslers. We have grown thin films of FeVSb, a new Heusler compound, using molecular beam epitaxy, a kind of spray painting with “cans” of different atoms. The top picture is an electron microscope image showing the arrangement of the Fe, V, and Sb as different size dots. On the right, the image shows the material we want, FeVSb. On the left, there is a completely new, unexpected material, Fe2VSb, which is a new kind of magnet.

This Seed project engaged underrepresented minority students in STEM through the MRSEC-sponsored summer REU program at UW-Madison. Doris A. Vargas Valentin, an undergraduate student from the University of Puerto Rico—Mayaguez, learned how to use dynamic light scattering and surface and surface tensiometry to characterize the self-assembly of smallmolecule amphiphiles in solution, analyze her experimental results, and present the results of her work in a formal setting during an eight-week stay in Madison. This experience also provided opportunities for Benjamin J. Ortiz, a senior graduate student who is also an underrepresented minority student in the Wisconsin MRSEC, to develop and hone his mentoring skills.

Many bacteria have evolved dynamic networks of amphiphilic molecules that form a chemical “language” that they use to communicate and regulate group behaviors. This communication, in turn, governs the synthesis of bacterial biofilms and the production of other chemical goods, including other amphiphilic or redoxactive species, that are unique to large groups or communities of bacteria typically associated with bacterial infections. Researchers at the Wisconsin MRSEC are investigating the self-assembly of this chemical alphabet, and the properties of the nanostructures that form in solution and at interfaces, to design new types of synthetic and responsive soft materials that can respond to or “communicate” selectively with bacterial communities in ways that are distinct from those of existing materials, which are generally designed to interact with or kill individual bacterial cells.

SEED-funded graduate students led scientific demonstrations for the general public during the 2018 Engineering Expo, hosted at the University of Wisconsin-Madison. The Kawasaki group also hosted a local middle school teacher as part of the Research Experiences for Teachers (RET) program of the Wisconsin
MRSEC.

Heusler compounds are a promising class of thermoelectric materials that can convert waste heat into electricity. Importantly, they are composed of Earth-abundant elements. Their efficiency depends sensitively on electronic structure, however, challenges in preparing high quality single crystalline samples have inhibited such measurements. Now, as part of a SEED project within the Wisconsin MRSEC, scientists have directly measured the electronic structure of high electron mobility (500 cm2/Vs) FeVSb thin films, using angle-resolved photoemission spectroscopy (ARPES). Surprisingly, the valence band of this
material is narrower and the effective mass is higher than predicted by density functional theory calculations. These results call for a re-examination of our understanding of the electronic structure in these materials, and in particular, the potential role of electron-electron correlations.