Identifying electronic states of Covalent Organic Frameworks using STM

Two dimensional heterotriangulene-based Covalent Organic Frameworks (COFs) have been predicted to be semiconducting materials with both flat bands and Dirac-type bands [1]. We have synthesized nanometer-scale networks based on the dimethylmethylene-bridged triphenylamine (DTPA) units [2] on gold and silver surfaces using on-surface synthesis, and study them using Scanning Tunneling Microscopy and Spectroscopy (STM/STS). We attempt to match the features identified in the finite-sized network with the band structure obtained from first-principles calculations on the periodic lattice. For filled states, we find that the molecular bands appear similar to calculations of the freestanding COF electronic structure. For unfilled states, however, surface state electrons in the underlying metal contribute significantly to the STS spectra and spectral maps. We analyze the experimental data using a clustering algorithm to separate the contributions of molecular states and substrate states. Using tight-binding for molecular states and plane-waves for substrate states, we also explore how well the overlayer/substrate interaction, and boundary effects, can be modeled for finite-sized COF islands.

[1] Jing et. al, Two dimensional Kagome lattices made of heterotriangulenes are Dirac semimetals or single band semiconductors, J. Am. Chem. Soc. 2019, 141, 743−747

[2] Bieri et al., Surface supported 2D heterotriangulene polymers, Chem.Commun., 2011, 47, 10239–10241