Thesis Defense: Xi Chen

Title

Characterizing thermal vibrations at the atomic scale with electron microscopy

Abstract

Thermal vibrations play a crucial role in determining the behavior of materials, impacting thermal conductivity, traditional superconductivity, and ferroelectric material stability. In particular, defects have been employed to adjust thermal conductivity, aiming for improved thermoelectric performance. Studying these vibrations can thus lead to new ways to optimize functional material behavior. It is thus imperative to recognize how defects, microstructure, and alloying affect thermal vibrations, necessitating a close examination at the atomic level. However, many conventional techniques fall short of providing the necessary spatial resolution. Scanning transmission electron microscopy (STEM) offers a promising solution with resolution down to 10’s of pm, shedding light on the intricate relationships between atomic structures, thermal vibrations, and their broader material implications. Despite this, most electron microscopy studies on thermal vibrations primarily rely on electron energy loss spectroscopy (EELS). This method faces limitations: the meV energy resolution is not widely available. Furthermore, its efficiency in measuring phonon modes remains relatively low. This thesis attempts to address these constraints, by developing alternative approaches to characterize thermal behaviors with thermal diffuse scattering (TDS) and electron ptychography. This work overcomes challenges related to existing methods (spatial resolution and energy resolution), facilitating comprehensive investigations of thermal vibrations at the atomic level and deepening our understanding of the phonon scattering processes of electrons.

Thesis Advisor

James M. LeBeau: Associate Professor of Materials Science and Engineering, MIT