Mapping Grains, Boundaries, and Defects in 2D Covalent Organic Framework Thin Films

Area of Science:

• Materials Science
• Nanotechnology
• Chemistry

Background:

• Two-dimensional covalent organic frameworks (2D COFs) hold significant promise for various applications.
• Characterizing the nanoscale structure of these beam-sensitive materials using high-resolution transmission electron microscopy (HR-TEM) presents a significant challenge.

Purpose of the Study:

• To develop a robust understanding of 2D COF structure for improved synthesis and device realization.
• To overcome HR-TEM imaging challenges for beam-sensitive 2D COF films.
• To establish a method for rapid nanoscale structural information extraction.

Main Methods:

• Utilized low electron flux imaging conditions and high-speed direct electron counting detectors for HR-TEM.
• Developed a Fourier mapping postprocessing script to analyze TEM images.
• The script analyzes 2D Fourier transforms to extract information on domain orientations, sizes, and defects.

Main Results:

• Successfully acquired high-resolution TEM images of 2D COF films under challenging conditions.
• The Fourier mapping technique rapidly extracts nanoscale structural information, including polycrystalline domain orientations and average domain sizes.
• Identified defects such as overlapping grains and grain boundaries, pinpointing broken B-O bonds in COF-5 as a primary defect origin.

Conclusions:

• The developed Fourier mapping approach provides fundamental insights into the microstructure and crystallographic properties of 2D COF films.
• Observed nanoscale features are directly linked to chemical phenomena during polymerization.
• This methodology will guide future 2D polymerization strategies and the development of 2D COF-based thin-film devices.