Search

Two-dimensional metal-organic frameworks (2D-MOFs) represent a category of atomically thin materials that combine the structural tunability of molecular systems with the crystalline structure characteristic of solids. The strong bonding between the organic linkers and transition metal centers is expected to result in delocalized electronic states. However, it remains largely unknown how the band structure in 2D-MOFs emerges through the coupling of electronic states in the building blocks. Here, we demonstrate the on-surface synthesis of a 2D-MOF exhibiting prominent π-conjugation. Through a combined experimental and theoretical approach, we provide direct evidence of band structure formation upon hierarchical self-assembly, going from metal-organic complexes to a conjugated two-dimensional framework. Additionally, we identify the robustly dispersive nature of the emerging hybrid states, irrespective of the metallic support type, highlighting the tunability of the band structure through charge transfer from the substrate. Our findings encourage the exploration of band-structure engineering in 2D-MOFs for potential applications in electronics and photonics.

Employing Pt(111) supported 2D Pt-core Au-shell model catalysts, we demonstrate that 2D core-shell surfaces prepared under ultrahigh vacuum (UHV) conditions constitute excellent model systems to determine the activity of step sites in electrocatalysis, especially because UHV-scanning tunneling microscopy (STM) enables control of the quality of narrow step modifications with high accuracy on such systems. As verified with STM, cyclic voltammetry (CV), and temperature-programmed desorption (TPD) measurements, this approach allows us (i) to increase the step density by homoepitaxial growth of monolayer high islands on the respective single crystal and (ii) to modify the step sites for adsorption of reactants by selective deposition of a guest metal. Herein, STM imaging in combination with electrochemical characterization provides a direct control to ascertain a selective modification of the entire steps. Comparing the electrocatalytic activity of 2D core-shell systems with and without the shell enables us to identify the activity of step sites for electrocatalytic reactions, as demonstrated for the bulk CO electro-oxidation.