Angstrom-scale imaging of 2D molecular materials via scanning transmission electron microscopy and ptychography

Accurate structural determination of 2D molecular materials is crucial for their development and application across diverse fields, including biomedicine, energy storage, and electronics. However, conventional diffraction-based techniques often provide limited structural information, particularly for systems such as thin films, polycrystalline particles, and nanopowders— forms in which many 2D molecular materials are commonly synthesized. This thesis presents a direct approach for resolving structures of such 2D molecular materials using scanning transmission electron microscopy (STEM) to image atomic connectivity and higher-order 3D structures. We address the longstanding challenges of radiation damage and weak electron scattering through a multi-pronged strategy that includes using protective graphene substrates, low-dose imaging, maximizing signal to noise ratio in the images, and advanced data processing. In atomically thin 2D small molecular crystals, our methods yield elementally sensitive images with a resolution of up to 1.3 Å, enabling the distinction of light elements such as carbon and nitrogen. When applied to 2D covalent organic frameworks (2D COFs), we uncover significant disorder in inter-layer stacking with nanometer-scale offsets, challenging their previous depictions as average eclipsed structure. Using multi-slice electron ptychography, we extend our imaging to three dimensions, revealing the 3D arrangement of COF layers and distortions in the shape of the pore channels that are critical for their charge and mass transport applications. Overall, this thesis introduces powerful new direct techniques for the structural characterization of 2D molecular materials, providing synthetic chemists and materials scientists with an invaluable tool for ab initio structure determination and accurate exploration of structure-property relationships.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Kharel, Priti

Contributors dc:contributor

• Huang, Pinshane
• Murphy, Catherine Jones
• Lopez, Joaquin Rodriguez
• Zuo, Jian-Min

• Structure Determination
• Molecular Crystals
• Covalent Organic Framework
• 3d Structure
• Scanning Transmission Electron Microscopy
• Atomic Resolution
• Stacking Disorder

Statement dc:rights

• Copyright 2024 Priti Kharel

Language dc:language

eng, en

Handle dc:identifier

https://hdl.handle.net/2142/127332

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127332