How the Layer Alignment in Two-Dimensional Nanoporous Covalent Organic Frameworks Impacts Its Electronic Properties

Two-dimensional nanoporous covalent organic frameworks
(2D COFs)
have gathered significant interest due to their wide range of applications.
Due to the lack of strong covalent interlayer interactions, their
layers can be stacked in countless ways, each resulting in unique
nanoscale characteristics impacting the structural, chemical, and
electronic properties. To characterize and understand the layer stacking
in 2D COFs and its effect on the structural and electronic properties,
we carried out a detailed density functional theory investigation
on four materials, CTF-1, COF-1, COF-5, and Pc-PBBA.
This entailed an in-depth evaluation of the potential energy as a
function of the interlayer distance and offset, the powder X-ray diffraction
(PXRD) pattern, and the electronic properties. From the potential
energy surfaces, the typical slipped AA-stacking configuration was
confirmed as optimal for each of the 2D COFs, with a slight offset
from a perfect alignment of the layers. The statically calculated
PXRD patterns based on these optimized stacking configurations showed
discrepancies when compared to experimental data. Instead, when properly
accounting for dynamic fluctuations by calculating the average diffraction
pattern over the course of a molecular dynamics simulation, a better
agreement with the experiment is obtained. Different stacking configurations
also profoundly affected the electronic band structure of COFs as
the interlayer π–π interactions are significantly
impacted by the layer offset. Evidently, with decreasing layer offsets,
the π–π interactions increase due to the layer
alignment, leading to a decrease in the band gap and an increase in
interlayer charge mobility. Our study highlights the need for accurate
modeling of the stacking configuration in 2D COFs as a small-scale
deviation in the adjacent layer position can significantly affect
the structural and electronic properties.