Symposium X—MRS/The Kavli Foundation Frontiers of Materials

Xiaoqing Pan, University of California, Irvine

Probing the emergent properties and dynamics of interfaces by electron microscopy

Written by Vineeth Venugopal

In his Symposium X presentation, Xiaoqing Pan of the University of California, Irvine, talked about his extensive research on the study of functional materials systems using the electron microscope. The electron microscope as a characterization tool has undergone rapid development in the last two decades with specialized aberration correction devices approaching sub-angstrom resolutions. It is being used to study the dynamic behavior of materials, their electronic properties, and phonon characteristics, among other modalities. 

Pan spoke at length about his work on using the electron microscope to study the properties of ferroelectric devices. “Ferroelectrics are materials with spontaneous polarization,” he said. A good example is bismuth ferrite or BiFeO₃, which, like all ferroelectrics, has multiple microscopic regions of polarization called domains. The polarization in each domain is in the same direction but that of adjacent domains can be at an angle to each other. In the case of BiFeO₃, adjacent domains can be at 71, 180, or 109 degrees and they appear as neatly demarcated stripes in a microscopic image. The surfaces of these domains attract charge which is retained as the so-called “bound charge.” Different functionalities of the electron microscope have been used to extract complementary information about ferroelectric domains and thus have helped in our understanding of the origin of ferroelectricity. 

Using the electron microscope, Pan and his colleagues have been able to measure ferroelectric polarization and map its spatial distribution with atomic resolution. This has allowed correlating the dipole moment of BiFeO₃ with the real time atomic displacements of oxygen in the material. They have also been able to probe the dynamics of domain nucleation and polarization switching at the atomic scale as well as to image the local charge density in real space. This has led to the real time observation of the pinning of ferroelectric domains at dislocations in the crystal lattice.

In other systems such as strontium titanate, the electron microscope has revealed the surface is conductive at microscopic scales even though there is no overall macroscopic conduction. By using charge density imaging techniques, the momentum space can be mapped by an electron probe which shows the presence of polarization vortices in several of these materials due to local heterogeneity and bound charges. The probe in a four-dimensional scanning transmission electron microscope interacts with the local charge directly which enables the quantitative analysis of charge transport and local dipole moment. 

In addition, Pan has used the electron microscope to study long and medium wave phonons in the crystal system which is useful in the study of thermal conductivity of materials. In SiGe quantum dot superlattices, the electron beam has been used as a source of phonon that can be used to study the stepwell phonon emission spectra. He pointed out that a more stable low temperature sample holder will be very useful for the future of electron microscopy studies by allowing far more measurements than we are able to do today. 

Symposium X—MRS/The Kavli Foundation Frontiers of Materials features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.