Rationally Designed Semiconducting 2D Surface-Confined Metal-Organic Network.

Two-dimensional (2D) surface-confined metal-organic networks (SMONs) are metal-doped self-assembled monolayers of molecules on solid surfaces. We report the formation of uniform large-area solution-processed semiconducting SMONs of Pd and Zn with mellitic acid (MA) on a highly oriented pyrolytic graphite (HOPG) surface under ambient conditions. The microscopic structure is determined using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Using tunneling spectroscopy, we observed a reduction in the band gap of ≈900 and ≈300 meV for MA-Pd and MA-Zn SMONs, respectively, compared to the pure MA assembly. Concomitant density functional theory (DFT) calculations reveal that the coordination geometry and microscopic arrangement give rise to the observed reduction in the band gap. The dispersion of the frontier bands and their delocalization due to strong electronic coupling (between MA and metal) suggest that the MA-Pd SMON could potentially be a 2D electronic material.