Prof. Paul Voyles
Solving Structurally Complex Materials using Electron Microscopy
Crystallography offers tremendously powerful approaches based on diffraction for solving structures with translation symmetry and a limited number of degrees of freedom. Nanostructures with a large fraction of surface atoms, interfaces, and glasses have much higher structural complexity – up to 3N degrees of freedom for a glass of N atoms – and require different approaches, often including experimental data from microscopy. This seminar will discuss our recent efforts combining electron microscopy data with tools from data science to solve the structure of complex materials. We have used non-rigid image registration to create highly-quantitative, sub-picometer precision high-resolution STEM images . We have used those images to study surface distortion on Pt nanocatalysts  and combined them with structural modeling by DFT to image single La vacancies in LaMnO3 with sub-unit cell resolution in three dimensions. Chemically sensitive imaging using STEM EELS, combined with DFT calculations, has been used to solve Co2MnSi / GaAs interfaces designed with different interface terminations. For more complex structures not uniquely constrained by experimental data alone, we have developed structure refinement methods that combine microscopy data with simulated system energies and applied them to Au nanoparticles  and metallic glasses . Structural models derived from automatic optimization or by-hand modeling can be interrogated to develop abstract structural characteristics of the material or used as the starting point for additional simulations to uncover structure-property relationships, such as the spin transport properties of Co2MnSi / GaAs interfaces.
Bio: Prof. Voyles earned a B.A. in physics from Oberlin College, and a M.S. and a Ph.D. in physics from the University of Illinois at Urbana-Champaign. After a post-doc at Bells Lab in Murray Hill, NJ, he joined the faculty in the Materials Science and Engineering Department of the University of Wisconsin-Madison in 2002. His research focus is the structure of materials determined using electron microscopy. He specializes in the structure of glasses studied using fluctuation electron microscopy and the structure of crystal defects and interfaces studied using extreme high-resolution Z-contrast STEM imaging. He has published over 150 papers and been awarded an NSF CAREER award, the Harvey Spangler Award for Technology Enhanced Instruction, and numerous journal best paper awards. In 2012 he held visiting appointments at RWTH Aachen University and the Forschungzentrum Jülich, and in 2016 he was appointed Beckwith-Bascom Professor and Chair of the Department of Materials Science and Engineering at the University of Wisconsin – Madison.