KSEA events High Symmetry Ordered Multigrain Nanocrystals to Reveal and Maximize the Effect of Grain Boundary Defects (181018 KSEA Berkeley Chapter Seminar)

Abstract: Grain boundary defectstopological defects around grain boundaries (GB)play an essential role in electrical, optical, magnetic, mechanical, and chemical properties of the polycrystalline materials. When the grain size of the materials decreases to the nanometer scale, the grain boundary defects predominantly determine those properties, opening up a variety of new applications. Although researchers have sought ways to study the contribution of specific grain boundary defects to the properties, non-uniformity in grain size and shape and random misorientation between the grains make these tasks challenging. This presentation will first discuss controlling 3D heteroepitaxy of colloidal faceted nanocrystals to grow uniform multiple grains organized into a well-defined geometry which gives a new opportunity for overcoming such limitations from the non-uniformity and randomness. Within the resulting nanocrystals, we will see that three-dimensionally patterned strain field exist in the form of disclinations and dislocations. We will then discuss how we can apply the 3D hetroepitaxy to investigate the effect of microscopic grain geometry on the macroscopic properties. For example, we can obtain a correlation between the GB defects and the catalytic property, because the patterned strain creates exact GB structure in a periodic fashion that serves as the active sites for catalysis. Lastly, we will highlight that the epitaxy-based design principles can significantly extend the library of grain structures in the nanocrystalline materials.  

Biography: Myounghwan Oh is a postdoc working in Paul Alivisatos group at UC Berkeley and is investigating the properties of the grain boundary defects in the nanocrystalline materials through high-precision control of 3D heteroepitaxy. He received his B.S. in Chemical Biological Engineering from Seoul National University (SNU), Korea, followed by a M.S. in Interdisciplinary of Nanoscience and Technology and a Ph.D. in Chemical Biological Engineering.