Center for Functional Nanomaterials Seminar

Crystalline imperfections such as stacking faults and interfaces are recognized as controlling factors in modifying thermal properties and heat transport by scattering phonons and changing vibrational spectra. They play a more pivotal role when the material's dimension is reduced to the nanoscale. However, it is conventionally challenging to achieve experimental evidence of defect-induced phonon modes and phonon dynamics due to the lack of effective tools with sufficient spatial resolution. The state-of-the-art monochromated electron energy-loss spectroscopy (EELS) in the aberration-corrected scanning transmission electron microscope (AC-STEM) reaches an unprecedented energy resolution of a few millielectronvolts (meV), enabling the detection of electronic and vibrational states with nanoscale and even atomic resolution. In this burgeoning field, several novel methodologies have been developed with controllable spatial, momentum, and energy resolutions, and implemented to reveal exotic defect-induced phonon modes, vibrational anisotropies, and their dynamics near a variety of crystalline imperfections. In this talk, I will first give a summary of my work using advanced STEM-EELS methods to understand the structure-property relationships in various cutting-edge materials. Low-voltage AC-STEM imaging is used to investigate the atomic structures of several intriguing two-dimensional materials, catalysts, and complex heterostructures. Meanwhile, their electronic structures and oxidation states are spatially revealed using monochromated EELS. Then, I will show a few case studies that develop and implement a series of space- and angle-resolved vibrational EELS methods to reveal exotic vibrational states and phonon dynamics in diverse scenarios. (1) The acoustic phonons of SiC undergo an energy red shift at stacking faults. Similarly, exotic interfacial phonon modes are detected at several heterointerfaces. (2) The phonon flux and specular phonon reflection can be visualized due to the propagation of nonequilibrium phonons near compositionally abrupt interfaces. (3) A novel momentum-selective dark-field EELS method is developed to observe frequency-linked and polarization-induced vibrational anisotropies of individual oxygen atoms in perovskites. These observed phonon behaviours could significantly modify the dielectric, optical, thermal, and superconducting properties in bulks and across solid-solid interfaces. My work charts a definitive course for investigating phonon propagation in perfect crystals and near defects and provides guidance for the thermal nanoengineering of nanotransistors, power electronics, and thermoelectric devices. Biography: Xingxu Yan received his B.S. (2010) and Ph.D. (2015) in Materials Science and Engineering at Tsinghua University. He is currently an Assistant Project Scientist in the Department of Materials Science and Engineering at the University of California, Irvine after finishing a postdoctoral period in Prof. Xiaoqing Pan's group. His research interest focuses on monochromated electron energy-loss spectroscopy for revealing vibrational and electronic structures of emerging materials.