Xiaoqing Pan
University of California, Irvine
Probing the Emergent Properties and Dynamics of Interfaces by Electron Microscopy
The advents of aberration correctors, pixelated direct electron detectors, and monochromators mark major milestones in the development of transmission electron microscopy (TEM). Today, one can image the structure, composition, and functional properties of materials with the atomic resolution. In this talk, I will first present a novel four-dimensional scanning transmission electron microscopy (4D STEM) method that enables the imaging of local polarization, electric field, and charge density in functional nanostructures or interfaces in real space with the sub-angstrom resolution. With in-situ biasing TEM holders, we can directly image the dynamics of ferroelectric switching under applied electric field in TEM. I will show how the polarization at ferroelectric/insulator interfaces controls the properties of the interfaces such as the formation of two-dimensional electron/hole gases at the interfaces. By combining 4D STEM with scanning probe microscopy, we have recently observed novel skyrmion-like polar nanodomains in freestanding lead titanate/strontium titanate bilayers transferred onto silicon. These nanodomains can be switched from one type to another by an applied electric field, which substantially modifies their resistive behaviors. In the second part of this talk, I will introduce a novel space- and angle-resolved electron energy-loss spectroscopy (EELS) method that reveals the occurrence of novel vibrational modes and phonons emergent at defects or interfaces. We found that an abrupt boundary between distinct materials tends to reflect heat much more efficiently that a gradual, diffuse one. With the vibrational EELS techniques we can also map the momenta change of phonons, that reveals the direction of phonon propagation, thus the direction of heat flow, on the nanoscale. These techniques can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.
Xiaoqing Pan is the Henry Samueli Endowed Chair in Engineering, professor of materials science and engineering, and professor of physics and astronomy. He is also the inaugural director of the Irvine Materials Research Institute (IMRI), and founding director of the Center for Complex Active Materials – an NSF MRSEC. Pan is an internationally recognized materials scientist and electron microscopy expert due to his pioneering development and applications of novel transmission electron microscopy (TEM) methods for probing the atomic scale structure, properties and dynamic behaviors of materials. His work has led to the discoveries of new materials and novel functionalities. Pan has received the NSF CAREER Award and the Chinese NSF’s Outstanding Young Investigator Award. He is an elected fellow of the American Ceramic Society, American Physical Society, Microscopy Society of America, and the Materials Research Society. He has published over 400 peer-reviewed scientific papers in high impact journals.