Probing small organic molecules at the angstrom-scale in the scanning transmission electron microscope

Janicek, Blanka E

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https://hdl.handle.net/2142/113946 Copy

Description

Title

Probing small organic molecules at the angstrom-scale in the scanning transmission electron microscope

Author(s)

Janicek, Blanka E

Issue Date

2021-08-25

Director of Research (if dissertation) or Advisor (if thesis)

Huang, Pinshane Y

Doctoral Committee Chair(s)

Huang, Pinshane Y

Committee Member(s)

• Zuo, Jian-Min
• Schleife, Andre
• Murphy, Catherine J

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• small organic molecules
• electron microscopy
• scanning transmission electron microscopy
• atomic-resolution imaging
• electron beam damage

Abstract

Small organic molecules are the foundation of chemistry, and correspondingly, unambiguous determination of their structure is essential. While scanning transmission electron microscopy (STEM) is a powerful tool for atomic-resolution structural determination, applying STEM to small organic molecules has proved challenging due to their weak electron scattering and extreme sensitivity to electron beam damage. In this thesis, we develop sample preparation, STEM imaging, and data processing techniques that enable us to directly probe the structure and arrangements of small organic molecules at the angstrom scale. We develop methods for large-scale production and deposition of samples onto ultra-low background graphene substrates, enabling the detection of the weak molecular signal. Additionally, our studies benefit from the stability provided by the graphene substrates, which increase the dose resistance of the molecules 2-6x due to their impermeability and high electrical and thermal conductivity. We then combine graphene substrates and STEM to study two technologically important small organic molecule systems: ensembles of molecules at the surface of inorganic nanoparticles and 2-dimensional (2D) molecular crystals. A longstanding challenge in nanoparticle characterization is understanding local variations in the distributions of surface molecules. We use STEM and electron energy loss spectroscopy to quantify the molecular distribution on the nanoscale, revealing up to 30% lower binding density on the ends of the particles. This anisotropy has important implications for nanoparticle synthesis and interactions, directing their growth, chemical functionalization, and colloidal assembly. Importantly, these methods enable the study of surface chemistry with site-specificity and are broadly applicable to a range of nanomaterials. An ultimate challenge in electron microscopy is to image the structure of an individual small organic molecule. We combine the protection from graphene substrates, low-dose STEM, and advanced averaging techniques to enable direct, atomic-scale imaging of 2D small organic molecule crystals. We achieve 1.3 angstrom resolution imaging a single molecule, sufficient to distinguish individual nitrogen and carbon atoms. Importantly, our methods are well-suited to analyzing nanogram quantities of material and systems for which large crystals cannot easily be grown. Our results demonstrate the potential of atomic-scale STEM as a powerful new method for structural solution of small organic molecules. Overall, this thesis demonstrates advances in sample preparation and data analysis techniques that enable nano- and atomic-scale STEM imaging of small organic molecules.

Graduation Semester

2021-12

Type of Resource

Thesis

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Copyright and License Information

Copyright 2021 Blanka E. Janicek

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