Engineering the Electronic and Thermal Properties of Two-Dimensional Covalent Organic Frameworks

Abstract

Two-dimensional covalent organic frameworks (2D COFs) are a class of modular polymeric crystals with high porosities and large surface areas. Their tunable microstructure (with a wide array of choices for the molecular building block) provides the opportunity for their bottom-up design and potentially tailorable physical properties. In this work, through combined density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we study the influence of different molecular functional groups and varying porosities on the electronic and thermal properties of 2D COFs. More specifically, by performing DFT calculations on 24 different 2D COFs, we demonstrate that one of the main descriptors dictating their band gaps are their mass densities or network porosities. Furthermore, we also find that specific functional groups forming the nodes can lead to larger localization of charge densities resulting in wider band gaps. By performing MD simulations to investigate their thermal properties, we show that (similar to their electronic properties) mass density is also one of the main factors dictating heat conduction, where higher densities are associated with relatively higher thermal conductivities along the 2D sheets. Our spectral energy density calculations provide insights into the highly anharmonic nature of these materials. We find that increasing porosities lead to larger anharmonic interactions and thus reduced thermal conductivities in these materials. Similar to their electronic band gaps, the nodes forming the 2D COFs also have a significant contribution in dictating their thermal conductivities with bigger nodes (accompanied by higher densities) generally resulting in relatively higher thermal conductivities in 2D COFs. Taken together, the resulting changes in the electronic and thermal properties from variations in the building blocks in 2D COFs lend insights into fundamental changes in the microscopic thermodynamics that arise from systematically changing their molecular structure. Therefore, our study provides a blueprint for the strategic syntheses of 2D COFs with “user-defined” electronic and thermal properties that will ultimately aide in their incorporation into various applications.