Spherical aberration correction marks a milestone in the development of transmission electron microscopy (TEM) which allows the quantitative determination of 3D structure and composition of nanostructures with atomic resolution.
In combination with in-situ techniques, one can follow the phase transformations, chemical reactions, and dynamic behaviors of materials in response to applied fields and/or to the change of environment in real-time. The development of pixelated direct electron detectors and monochromators unlocked a door for a new era of discovery by electron microscopy. Today, beside the structure and composition, many properties of nanostructures and single defects can be determined by using scanning transmission electron microscopy (STEM).
In this talk, I will present a novel 4D STEM diffraction imaging technique that we developed for mapping the local electric field and charge density with sub-Å spatial resolution. Using this technique, we are able to directly measure the electrical charge density, dipole moment, valence electron distribution between atoms, and charge distribution at heterointerfaces.
Furthermore, taking advantage of recent instrumentation advances in electron energy-loss spectroscopy (EELS) in STEM that combine an energy resolution of about 5 meV (and an ability to detect energy shifts < 1 meV) with a high spatial resolution, we demonstrate that space- and angle-resolved vibrational spectroscopy in a TEM allows the study of the vibrational properties of nanostructures and individual crystal defects. We observed a red shift of several meV and major changes in the intensity of acoustic vibration modes of a single stacking fault in SiC, and demonstrate that the changes are confined to within a few nanometers of the stacking fault.
Our work opens the door to studying phonon propagation around defects, and to providing guidance to the engineering of specific thermal properties of materials.
Speaker: Xiaoqing Pan, UC Irvine
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