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

In this work, we first investigate the localized electronic states in the band structures of three single-layer COFs based on typical building units of COFs chemistry. Our results confirm that the polar nature of strong bonds in these building units is a hindrance to a fully delocalized structure and disfavors the band-like mechanism of transport. We then show that a rational design of the building units can lead to dispersive band states in the electronic structure and results in conducting single-layer COFs. We demonstrate this strategy by investigating the charge carrier transport in a series of single-layer Ni-phthalocyanine (NiPc) covalent organic frameworks (COFs), namely, NiPc-P, NiPc-2P, and NiPc-3P. Three proposed COFs exhibit semiconducting band gaps ranging from 0.55 to 0.91 eV. Their room-temperature intrinsic mobility is predicted to be in range of 200-600 cm² V^-1 s^-1 and 20 000-60 000 cm² V^-1 s^-1 for electrons and holes, respectively, which are comparable to those of phosphorene and higher than those of the trigonal prismatic molybdenum disulfide. NiPc are dynamically and mechanically stable and can be synthesized via the co-evaporation between Ni and corresponding tetracyano linkers. Importantly, we demonstrate that the properties of the single-layer COFs can be tuned by engineering the organic building blocks. Our theoretical study not only provides insight into the design principles for semiconducting single-layer COFs but also highlights the significance of reticular chemistry in the development of a new generation of two-dimensional materials for optoelectronic applications.