Speaker: Xiaoqing Pan (University of California - Irvine, Irvine, CA 92697, USA)

Time: May 18, 2017 16:00

Place: 郑裕彤讲堂

Abstract： Ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold promise for the design of high-density nonvolatile memories and multifunctional nano-devices. The applications of ferroelectric materials stem from the ordering of polarization and switching of the polarization states by applied bias. A fundamental understanding of the atomic scale mechanism underlying the domain formation and polarization switching, therefore, is critical for the design of devices.  In this work, I will present the emergent properties of polarization ordering and domain switching in multiferroic BiFeO₃ thin films, uncovered by in situ atomic resolution transmission electron microscopy (TEM).  We found that the charged domain walls can be created or erased by applying a bias, and the resistance of the local film strongly depends on the characteristics of these charged domain walls.  Furthermore, by mapping the polarization distribution, we found that a monolayer thick conducting oxide existing on the BiFeO₃ film surface causes a significant increase of local polarization and exotic high-density nano-domains with large strain variations emerge. Finally, I will show that small defects in ferroelectric thin films can act as nano-building-blocks for the emergence of novel topological states of polarization ordering, namely, hedgehog/antihedgehog nanodomain arrays in BiFeO₃.  The emergent polarization states such as hedgehog/antihedgehog and vortex/ antivortex topologies not only modify the local lattice symmetries and thus induce the coexistence of mixed-phases resembling the morphotropic phase boundary with high piezoelectricity, but also lead to flux-closure vortex structures.  Phase-field simulations suggest that the observed novel polarization states are formed due to the coupling between the polarization ordering and the charged defects existing in the films. Thus, engineering of defects may provide a new route for developing ferroelectic/multiferroic-based nanodevices.

Bio: Xiaoqing Pan is a Professor and Henry Samueli Endowed Chair in Engineering, in Department of Chemical Engineering & Materials Science and Department of Physics & Astronomy at UC Irvine.  He is also the inaugural Director of the Irvine Materials Research Institute (IMRI). Before moving to UC Irvine, Pan was the Richard F. and Eleanor A. Towner Professor of Engineering in Department of Materials Science and Engineering, and also Director of Electron Microbeam Analysis Laboratory at the University of Michigan, Ann Arbor. He received his Bachelor’s and Master’s degrees in Physics from Nanjing University, and his Ph.D. degree in Physics from the University of Saarland, Germany. After postdoctoral research at the Max-Planck Institut für Metallforschung in Stuttgart, he joined the faculty of MS&E at Michigan as an Associate Professor without tenure in 1996, and was promoted to Professor with tenure in 2004. Pan has received many awards, including the National Science Foundation’s CAREER Award and the Chinese NSF’s Outstanding Young Investigator Award. He was an overseas member of the Scientific Review Board, Chinese Academy of Science, 2005-2010.  He has been serving as a member on the Advisory Committee of the Overseas Chinese Affairs Office of the State Council, China, since 2011. He is also a member of the Physical Sciences Panel of the Hong Kong Research Grants Council (RGC) since 2013.  He was elected to be a Fellow of the American Ceramic Society in 2011, a Fellow of the American Physical Society in 2013, and a Fellow of the Microscopy Society of America.  

Prof. Pan has published over 300 peer-reviewed scientific papers in scholarly high impact factor journals, including Nature, Science, Nature Materials, Nature Communications, Physical Review Letters, Nano Letters, and Advanced Materials.  His work has been cited over 20000 times and his publication h-factor is >60. He has given more than 200 invited talks or keynote presentations at national/international conferences, and more than 150 invited seminars. He was the leading organizer of a number of national/international conferences.