Studying magnetic and 3D atomic structure using 4D scanning transmission electron microscopy

Transmission electron microscopy (TEM) has long been a powerful tool for materials characterization, capable of extracting structural and chemical information over length scales of microns to angstroms. More recently, advances in electron detectors facilitated four-dimensional scanning transmission electron microscopy (4D-STEM), where a convergent electron beam is scanned over the sample and a diffraction pattern is collected at each scan position. A 4D-STEM dataset offers more information than conventional imaging modes like annular dark field STEM, but the interpretation of electron diffraction patterns is often not straightforward. In this dissertation, I present 4D-STEM experiments for studying magnetic and 3D atomic structure, complemented with simulations to validate those results. First, we explore 4D-STEM techniques for imaging the magnetic structure of antiferromagnets. Antiferromagnets can potentially be used to create faster, more robust memory devices. While TEM is indeed sensitive to the magnetic fields in a sample and has been used to study the magnetic structure of ferromagnetic materials over length scales of nanometers to microns, TEM has seldom been applied to antiferromagnets, where it is necessary to reach nearly atomic resolution. A major challenge is to separate the relatively weak signals of a sample's magnetic structure from the much stronger signals of the atomic structure. We demonstrate experimentally imaging the local magnetic moment of antiferromagnetic Fe2As with a spatial resolution of 6 angstroms by performing 4D-STEM with an angstrom-scale probe and using center-of-mass imaging. We also develop simulations of electron scattering from magnetic materials, through which we show that our our process for magnetic imaging of Fe2As takes advantage of how the magnetic unit cell of Fe2As is twice as large as the crystallographic unit cell. We use our simulations to identify optimal experimental conditions for this mode of imaging. Furthermore, we use simulations to study the feasibility of using 4D-STEM to image magnetic structure with atomic resolution. Is it possible to resolve individual atomic magnetic moments? We show that this is technically possible using electron ptychography under ideal circumstances, but practical considerations like electron dose and position errors will make it extremely challenging to achieve in reality. Next, we investigate the limits of depth resolution in multislice electron ptychography (MEP), a computational technique that, given a 4D-STEM dataset, creates a 3D reconstruction of a sample. MEP has been shown to have excellent in-plane resolution of as high as 0.2 angstroms, while depth resolution is limited to 2-4 nm. However, we show through simulations that MEP of very thin samples (thinner than 2 nm) can straightforwardly achieve sub-nanometer depth resolution. In light of those predictions, we experimentally apply MEP to study the 3D atomic structure of 2D moire materials. Such materials undergo structural reconstruction which helps produce exotic electronic phases such as superconductivity, but the out-of-plane component of this reconstruction has been difficult to measure experimentally. We achieve MEP reconstructions of twisted bilayer WSe2 with sub-nanometer depth resolution, which allowed us to extract 3D atomic coordinates. Furthermore, by incorporating prior knowledge about the structure of WSe2, we were able to refine the coordinates from ptychography to reach picometer-scale accuracy in all three dimensions, allowing us to map the subtle out-of-plane deformations of WSe2 resulting from the moire lattice. While theory generally predicts corrugations that are periodic with the moire lattice, we observed that the type of out-of-plane corrugation can vary from one moire unit cell to the next. These results demonstrate that 4D-STEM data acquired from just a single sample orientation can potentially be used to characterize the 3D atomic structure of 2D materials and other thin samples.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Huang, Jeffrey

Contributors dc:contributor

• Huang, Pinshane Y
• Shoemaker, Daniel P
• Wagner, Lucas K
• van der Zande, Arend M

• electron microscopy
• 4D-STEM
• electron ptychography
• 2D materials

Statement dc:rights

• Copyright 2025 Jeffrey Huang

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132777

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132777