3D Imaging Reveals Widespread Stacking Disorder in Single Crystal 2D Covalent Organic Frameworks
Priti Kharel, Patrick Carmichael, Anusree Natraj, Chloe E. Pelkowski, Sang Hyun Bae, William R. Dichtel, Pinshane Y. Huang
Abstract
Although tailored porosity is a defining feature of layered, two-dimensional (2D) polymers known as 2D covalent organic frameworks (COFs), understanding the interplanar stacking of 2D COFs and their resulting three-dimensional (3D) pore structure remains challenging. Here, we use scanning transmission electron microscopy and ptychography, an emerging 3D angstrom-scale imaging method, to study single-crystalline particles of the imine-linked 2D COF TAPB-DMPDA. Previously assumed to adopt an average-eclipsed structure with only angstrom-level stacking disorder, we find the crystals contain widespread stacking disorder of larger magnitudes, including interplanar shifts up to a half unit cell and nanoscale inhomogeneities in stacking and tilt. 3D visualizations show pore channels are distorted by this stacking disorder. The extensive stacking disorder found in even high-quality 2D COFs has profound implications for envisioned applications and should motivate the development of design strategies to control their 3D structures.