π-Conjugated two-dimensional covalent organic frameworks (2D COFs) are emerging as a novel class of electroactive materials for (opto)electronic and chemiresistive sensing applications. However, understanding the intricate interplay between chemistry, structure, and conductivity in π-conjugated 2D COFs remains elusive. Here, we report a detailed characterization for the electronic properties of two novel samples consisting of Zn- and Cu-phthalocyanine-based pyrazine-linked 2D COFs. These 2D COFs are synthesized by condensation of metal-phthalocyanine (M = Zn and Cu) and pyrene derivatives. The obtained polycrystalline-layered COFs are p-type semiconductors both with a band gap of ∼1.2 eV. A record device-relevant mobility up to ∼5 cm2/(V s) is resolved in the dc limit, which represents a lower threshold induced by charge carrier localization at crystalline grain boundaries. Hall effect measurements (dc limit) and terahertz (THz) spectroscopy (ac limit) in combination with density functional theory (DFT) calculations demonstrate that varying metal center from Cu to Zn in the phthalocyanine moiety has a negligible effect in the conductivity (∼5 × 10-7 S/cm), charge carrier density (∼1012 cm-3), charge carrier scattering rate (∼3 × 1013 s-1), and effective mass (∼2.3m0) of majority carriers (holes). Notably, charge carrier transport is found to be anisotropic, with hole mobilities being practically null in-plane and finite out-of-plane for these 2D COFs.
Bibliographical noteFunding Information:
We thank financial support from EU Graphene Flagship, ERC Consolidator Grant (T2DCP), ERC Starting Grant (FC2DMOF, No. 852909), Coordination Networks: Building Blocks for Functional Systems (SPP 1928, COORNET) as well as the German Science Council, Center of Advancing Electronics Dresden, EXC1056, (cfaed), and OR 349/1. We acknowledge the Dresden Center for Nanoanalysis (DCN) at TUD; Dr. Philipp Schlender, Dr. Konrad Schneider (Leibniz Institute for Polymer Research, IPF, Dresden), Dr. Tilo Lübken, and Mr. Friedrich Schwotzer for the use of facilities. We also appreciate Mr. Ji Ma, Dr. Haixia Zhong, and Ms. Yu Zhang (UvA) for the MS, SEM, and XPS analysis, respectively. We thank Mr. Chi Xu (HZDR), Dr. Chongqing Yang, and Dr. Zhongquan Liao (IKTS) for the helpful discussions. Prof. T. Heine and Hung-Hsuan Lin acknowledge the Centre for Information Services and High-Performance Computing (ZIH) in Dresden, Germany, for the provided computational resources.
© 2019 American Chemical Society.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry