报告题目：Probing the Structure and Dynamic Behaviors of Ferroelectrics by Electron Microscopy with Atomic Resolution in Real Time

报 告 人：Xiaoqing Pan，Irvine Materials Research Institute (IMRI)

报告时间：2017-05-18 16:00

报告地点：郑裕彤讲堂

报告摘要：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 BiFeO₃thin 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.