Date: Thursday, Apr 12th
Presenter: Prof. Xiaoqing Pan, Department of Materials Science and Engineering, University of Michigan
Ferroelectric materials are characterized by a spontaneous electric polarization that can be reoriented between different orientations by an applied electric field. The ability to form and manipulate domains with different polarization orientations at the nanometer scale is key to the utility of ferroelectric materials for devices such as nonvolatile memories. The ferroelectric switching occurs through the nucleation and growth of favorably oriented domains and is strongly mediated by defects and interfaces. Thus, it is critical to understand how the ferroelectric domain forms, grows, and interacts with defects. Here we show the nanoscale ferroelectric switching of a tetragonal PbZr0.2Ti0.8O3 thin film under an applied electric field using in situ transmission electron microscopy. We found that the intrinsic electric fields formed at ferroelectric/electrode interfaces determine the nucleation sites and growth rates of domains and the orientation and mobility of domain walls, while dislocations exert a weak pinning force on domain wall motion. We also show that localized 180° polarization switching initially form domain walls along unstable planes. After removal of the external field, they tend to relax to low energy orientations. In sufficiently small domains this process results in complete backswitching. Our results suggest that even thermodynamically favored domain orientations are still subject to retention loss, which must be mitigated by overcoming a critical domain size.
Xiaoqing Pan is the Richard F. and Eleanor A. Towner Professor of Engineering at the University of Michigan Ann Arbor. He received B.S. (1982) and M.S. (1985) degrees in Physics from Nanjing University, China, and Ph.D. degree (1991) in Physics from the University of Saarland, Germany. After a postdoctoral research fellow at the Max-Planck Institut für Metallforschung in Stuttgart, he joined the faculty of the University of Michigan Ann Arbor as an Associate Professor of Materials Science and Engineering in 1996. He was promoted to Professor of Materials Science and Engineering with tenure in 2004. Pan is Director (2009 present) of the Electron Microbeam Analysis Laboratory at the University of Michigan. He also served as the Director (Adjunct) of the Multifunctional Materials and Nano-device Division of the Ningbo Institute of Materials Technologies and Engineering, Chinese Academy of Science. He was the Chief Scientist of the CAS International Innovative Team (for Oversea Scientists) on Multifunctional Oxide Materials and Applications (2008-2011). Pan received the U.S. National Science Foundations CAREER Award and the Chinese Natural Science Foundations Outstanding Young Investigator Award. He was a named Cheung-Kong Distinguished Professor (visiting position at Nanjing University, 2008 - 2010), Chinese Ministry of Education. Pan was selected as a National Distinguished Professor (China 1000 Talent Program), as adjunct Professor at Nanjing University in 2009. He was selected as an oversea member of the Scientific Review Board, Chinese Academy of Science, 2005-2010. Pan was elected to be a Fellow of the American Ceramic Society.
Pans research focuses on the understanding of the atomic-scale structure-property relationships of advanced materials including transition metal compounds, ferroelectrics and multiferroics, oxide semiconductors, superconductors, and intelligent automotive catalysts. He is particularly interested in the atomic scale structure, chemistry, and functionalities of surfaces and interfaces in thin film heterostructures and nanostructured materials. He is a world-renowned electron microscopist, especially in the fields of ultra-high resolution transmission electron microscopy. His pioneering contributions to materials research and science include the development of in situ electron microscopy techniques and their innovative application to explore the mechanism of self-generation in intelligent automotive catalysts, to probe and understand the effects of boundary conditions on ferroelectricity, including polarization mapping, ferroelectric vortices, and domain dynamics, with atomic resolution. Pan has published over 200 peer-reviewed scientific papers in high impact scholarly journals including Nature, Science, Nature Mater., Nature Comm., Phys. Rev. Lett., Adv. Mater., and Nano Lett. His h-index is 38.