Flatbands in 2D boroxine-linked covalent organic frameworks

Abstract

Density functional calculations have been performed to analyze the electronic and mechanical properties of a number of 2D boroxine-linked covalent organic frameworks (COFs), which are experimentally fabricated from di-borate aromatic molecules. Furthermore, the band structures are surprising and show flat-band characteristics which are mainly attributed to the delocalized π-conjugated electrons around the phenyl rings and can be better understood within aromaticity theories. Next, the effects of branch sizes and hydrostatic strains on their band structures are systematically considered within generalized gradient approximations. It is found that their band gaps will start to saturate when the branch size reaches 9. For boroxine-linked COFs with only one benzene ring in the branch, the band gap is robust under compressive strain while it decreases with the tensile strain increasing. When the branch size is equal or greater than 2, their band gaps will monotonously increase with the strain increasing in the range of [-1.0, 2.0] Å. All boroxine-linked COFs are semiconductors with controllable band gaps, depending on the branch length and the applied strain. In comparison with other 2D materials, such as graphene, hexagonal boron nitride, and even γ-graphyne, all boroxine-linked COFs are much softer and even more stable. That is, they can maintain the planar features under a larger compressive strain, which means that they are good candidates in flexible electronics.

Electron delocalization in single-layer phthalocyanine-based covalent organic frameworks: a first principle study

“…Importantly, there exist the at band edges for all of them, indicating the electron localized states. Our argument is consistent with the study by Wang et al 48 where they nd the at band edges in the boroxine-based COFs due to the poor aromaticity of boroxine unit. Our calculated band gap for boroxine-COF is 5.11 eV which is larger than their predicted value of 3.5 eV.…”

Design of multi-functional 2D open-shell organic networks with mechanically controllable properties

“…31–37 Although several theoretical works have demonstrated the potential of applying a bi-axial strain to tune fundamental characteristics of many 2D extended systems 79–81 (including graphene 82 and certain 2D-COFs 83 ) in experiments monolayered materials are often uniaxially stretched between two electrodes. 32 As mentioned above, we focus on uniaxial strain by increasing the a (or, equivalently, b ) cell parameter of our 2D-COFs as it was found to be the energetically most facile way of distorting the material and was that which induced a high degree of aryl ring twisting, which is the principal objective of our work.…”

Proximity Effect in Crystalline Framework Materials: Stacking‐Induced Functionality in MOFs and COFs

“…[61,78,[82][83][84][85][86][87] However, electronic structures are seldom discussed and, even then, mostly only single layers are considered. [60,[88][89][90] Thomas et al [60,90] investigated electronic structures of 2D π-conjugated polymers with three-and four-arm connections, which correspond to kagome and Lieb (Archimedean) lattices, and explained occurrence of flat and disperse bands. However, no stacking was taken into account.…”