This dissertation describes a set of experiments loosely unified under the general theme of 3D electron crystallography.
Chapter 1 provides a comprehensive overview of the scientific literature in the field, with particular emphasis on experiments aimed at using electron diffraction to elucidate the atomic structure of three-dimensional molecular crystals.
Chapter 2 focuses on the development of a publicly accessible web server, FAES (Factors of Atomic Electron Scattering, https://srv.mbi.ucla.edu/faes), containing a database of electron scattering factors parameterized into Gaussian approximations compatible with widely used least-squares refinement programs. These include all neutral and ionic species tabulated in the International Tables for Crystallography, as well as fractionally charged scattering factors calculated by computing linearly weighted sums of adjacent integral neighbors. FAES provides numerical fitting coefficients, statistical goodness-of-fit values, and elastic and estimated inelastic cross-sections at a range of accelerating voltages relevant to transmission electron microscopy.
Chapter 3 details rigorous studies involving the mapping of electron beam-induced radiolytic damage in molecular crystals using 4D scanning transmission electron microscopy (4D-STEM), conducted on a variety of organic and organometallic species spanning a wide gamut of chemical space. By acquiring a series of consecutive 4D-STEM scans on the same crystal, we explicitly visualize the spatial evolution of coherently diffracting zones (CDZs) as a function of accumulating electron fluence, providing a detailed, time-resolved map of the internal lattice reorientation induced by radiolysis. These experiments also unveil the resolution-dependent propagation of tides of amorphization from impact craters created by asymmetric, localized delivery of incident electrons.
Chapter 4 relates the development of the 4D-STEM method nanobeam electron diffraction tomography into a technique capable of overcoming obstacles that thwart structural elucidation by conventional microcrystal electron diffraction (microED). 4D-STEM's unique ability to pinpoint a specific nanoscale volume for data analysis enables pixel-by-pixel spatial exclusion of unwanted signal from disordered or Bragg-silent regions, empowering us to simply pick and choose whichever CDZs generate the highest-quality diffraction patterns. These experiments represent the first 4D-STEM structures phased ab initio by direct methods.
