Band gap opening from displacive instabilities in layered covalent-organic frameworks $^\dag$

TL;DR

This study computationally investigates layer stacking in covalent organic frameworks, revealing that slipped stacking configurations induce significant band gap openings, impacting their electronic properties and suitability for energy applications.

Contribution

It demonstrates that non-eclipsed stacking sequences in COFs lead to band gap openings, highlighting the importance of stacking order in their electronic structure and energy application potential.

Findings

Slipped stacking structures are energetically favored in studied COFs.

Band gaps of 0.8 - 1.4 eV are induced by stacking modifications.

Stacking order significantly alters electronic properties relevant to energy storage.

Abstract

Covalent organic frameworks (COFs) offer a high degree of chemical and structural flexibility. There is a large family of COFs built from 2D sheets that are stacked to form extended crystals. While it has been common to represent the stacking as eclipsed with one repeating layer ("AA"), there is growing evidence that a more diverse range of stacking sequences is accessible. Herein, we report a computational study of layer stacking in two prototypical COFs, Tp-Azo and DAAQ-TFP, which have shown high performance as Li-ion battery electrodes. We find a striking preference for slipped structures with horizontal offsets between layers ranging from 1.7 \r{A} to 3.5 \r{A} in a potential energy minimum that forms a low energy ring. The associated symmetry breaking results in a pronounced change in the underlying electronic structure. A band gap opening of 0.8 - 1.4 eV is found due to modifications of the underlying valence and conduction band dispersion as explained from changes in the $\pi$ orbital overlap. The implications for the screening and selection of COF for energy applications are discussed.