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TEM Studies of Domain Structure and Dynamic Behaviors of Ferroelectrics/Multiferroics with Atomic
Resolution in Real Time
Xiaoqing Pan
Department of Materials Science and Engineering, University of Michigan-Ann Arbor, Ann Arbor, MI, USA
National Laboratory for Microstructure Physics, Nanjing University, Nanjing, China
As advances in aberration-corrected transmiion electron microscopy(TEM)have enabled the determination of the three-dimensional structure of materials with sub-angstrom resolution, the recent development of in situ TEM in combination with scanning probe techniques allows us to study the dynamic behaviors of nanostructures under applied fields or stre while the atomic structure is imaged directly.We use these advanced TEM techniques to study the atomic structure and energies of domain walls in ferroelectric(PbZr0.2Ti0.8O3, PZT)and multiferroic BiFeO3.I will show that the atomic scale polarization map can be quantitatively determined using aberration-corrected TEM images owing to the large atomic displacements responsible for the dipole moment.Our studies revealed that interfaces in complex multidomain geometries lead to the formation of polarization vortices with electric flux closure domains.I will also show that the domain wall width and energy depend strongly on the tilt and rotation of the oxygen octahedra acro the domain walls.Using aberration-corrected transmiion electron microscopy in combination with in situ scanning probe, the kinetics and dynamics of ferroelectric switching are followed at millisecond temporal and sub-angstrom spatial resolution in ferroelectric thin films.We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of metastable ferroelectric states localized to the ferroelectric and ferromagnetic interface.These studies show how defects and interfaces impede full ferroelectric switching of a thin film.It was also found that even thermo-dynamically favored domain orientations are subject to retention lo, which must be mitigated by overcoming a critical domain size.Furthermore, a novel hindering effect was observed, which occurs via the formation of a transient layer with
a thickne of several unit cells at an otherwise charged interface between a ferroelastic domain and a switched domain.This transient layer poees a low magnitude polarization, with a dipole gla structure, resembling the “dead layer”.The present study provides an atomic level explanation of the hindering of ferroelectric domain motion by ferroelastic domains.Hindering can be overcome either by applying a higher bias or by removing the as-grown ferroelastic domains in fabricated nanostructures.Brief Biography:Xiaoqing Pan is an endowed Chair Profeor(Richard F.and Eleanor A.Towner Profeor of Engineering)in the University of Michigan's Department of Materials Science and Engineering.He is also Director of Electron Microbeam Analysis Laboratory at the University of Michigan, Ann Arbor.He has been an MSA member since 1998.He received his Bachelor's and Master's degrees in Physics from Nanjing University, and his Ph.D.degree in Physics(1991)from the University of Saarland, Germany.After postdoctoral research at the Max-Planck InstitutfürMetallforschung in Stuttgart, he joined the faculty of MS&E at Michigan as an Aociate Profeor without tenure in 1996, and was promoted to Profeor 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 awarded a named Cheung-Kong Distinguished Visiting Profeorship(Nanjing University 2008-2010), and was also awarded the National Distinguished Profeorship(China 1000 Talent Program), as Adjunct Profeor at Nanjing University in 2009.He was an overseas member of the Scientific Review Board, Chinese Academy of Science, 2005-2010.Pan was elected to be a Fellow of the American Ceramic Society in 2011.Pan's research interests center on understanding the atomic-scale structure-property relationships of advanced functional materials, including oxide electronics, nanostructured ferroelectrics and multiferroics, and catalysts.He is recognized internationally for his work in electron microscopy, that has led to the discovery of new properties and novel functionalities in these technologically important materials.His pioneering contributions include the development of quantitative TEM methods to map the electrical polarization in ferroelectrics at atomic resolution, and in situ TEM methods to uncover the effects of boundary conditions on ferroelectricity, ferroelectric vortices, and domain dynamics, and novel self-regenerating automotive catalysts, all with atomic resolution.He has published over 250 peer-reviewed scientific papers in scholarly high impact factor journals.His
work has been cited over 7000 times, and his publication h-factor is 46.He has given more than 150 invited talks or keynote presentations at national and international conferences, and more than 100 invited seminars.
