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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - SURFLINK (MOLECULAR CARPETS ON INSULATING SURFACES: RATIONAL DESIGN OF COVALENT NETWORKS)

Teaser

Summary

Covalent organic frameworks (COFs) are organic solids with extended crystalline porous structures in which the molecular building blocks are linked by robust covalent bonds. The polymeric scaffolds have unique properties suitable for many scientific and technological applications including nano-electronic and sensor devices as well as catalysis. The bottom-up fabrication via on-surface synthesis provides for atomically-precise surface-supported 1D and 2D nanostructures. On-surface synthesis of covalent structures is mainly limited to metal surfaces so far because controlled growth procedures of molecules on insulators are often hindered by the weak, unspecific interaction with the substrate. The overall aim of the SURFLINK project is to construct and understand the properties of novel covalently-linked, organic networks in a bottom-up approach with a focus on 2D networks on insulating surfaces. We will establish suitable concepts for the covalent linking of molecules on insulators, which will significantly advance the atomic-scale understanding of molecular structures on insulators. Specially designed molecular building blocks will be used to create functional 2D networks with tunable electronic properties and nanometer-sized pores.
The SURFLINK project uses a surface science approach in ultra-high vacuum to understand the fundamental mechanisms and properties of covalently-linked networks at the atomic level. The covalent networks will be studied by high-resolution scanning probe microscopy and spectroscopy at the atomic-scale. We will determine the electronic properties of the novel nano-porous networks that can be tailored by their geometry. The rational design of the networks proposed in the SURFLINK project has great potential for materials research and will ultimately result in the development of new materials with adjustable electronic properties.

Work performed

We successfully fabricated porous surface-supported nanostructures including macrocycles, 1D nanoribbons, and long-range ordered 2D networks on metal surfaces via Ullmann-type coupling reactions. A hierarchical synthesis has been applied to fabricate porous 2D networks, where hexagonal macrocycles and chains were assembled in a first reaction step and connected to extended porous networks in a second reaction step. The growth of porous 1D nanoribbons was achieved via the isomerization of conformationally flexible polymer chains followed by dehydrogenation reactions using thermal annealing.
The electronic properties of bottom-up fabricated 2D networks remained until now experimentally widely unexplored, despite the impressive physical properties that were predicted by density functional theory (DFT). We demonstrated for the first time a narrowing of the electronic band gap in the covalent structures when going from the monomer to one-dimensional chains and two-dimensional networks using scanning tunneling spectroscopy (STS), thus corroborating the extension of the effective Ï€-system. Organometallic networks use the advantage of a reversible structure formation in contrast to C-C coupling reactions. We studied the electronic structure and the covalent bond character of surface-supported organometallic networks with Ag-bis-acetylide bonds. STS revealed a frontier, unoccupied electronic state that is delocalized along the entire organometallic network and that proves the covalent nature of the Ag-bis-acetylide bonds.
Concerning the bulk insulating surfaces, an in-situ cleaver was built, and suitable preparation procedures were developed for several salt and metal oxide surfaces. We achieved atomically-resolved imaging on those surfaces using non-contact atomic force microscopy (nc-AFM) and have characterized common surface defects, which might act as reactive centers to initiate surface reactions. We showed that the self-assembly of polycyclic aromatic hydrocarbons could be steered on insulating materials by balancing the strength of the intermolecular interactions. Controlled structure formation of one-, two, and three-dimensional triphenylamine derivatives was demonstrated on KBr and presented by means of nc-AFM measurements in combination with DFT calculations. The molecular self-assemblies upon adsorption at room temperature provide a structural template for the polymerized nanostructures. In order to establish on-surface reactions on insulators, we first studied several coupling reaction mechanisms on metals; and test them now on bulk insulating surfaces.

Final results

We developed novel concepts using a preprogrammed hierarchical on-surface synthesis to fabricate high-quality 2D porous networks with long-range order, which is one of the challenges in all covalently-linked surface-supported structures in ultra-high vacuum. The high structural quality of the fabricated 2D covalent networks, allowed us for the first time to obtain insights into the electronic properties of surface-supported covalent networks. Moreover, the controlled structure formation of molecular assemblies on bulk insulators is prerequisite for the growth of ordered covalently-linked structures. In future, we will explore in more detail how to tune on-surface reactions on bulk insulators.