Periodic Reporting for period 4 - SURFLINK (MOLECULAR CARPETS ON INSULATING SURFACES: RATIONAL DESIGN OF COVALENT NETWORKS)
Reporting period: 2019-11-01 to 2021-04-30
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 carbon-based structures. 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 obtain fundamental insights into reaction mechanisms and properties of covalently-linked networks at the atomic level. The structure of the covalent networks will be studied by high-resolution scanning probe microscopy while the electronic properties will be investigated by tunneling spectroscopy. 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.
The SURFLINK project uses a surface science approach in ultra-high vacuum to obtain fundamental insights into reaction mechanisms and properties of covalently-linked networks at the atomic level. The structure of the covalent networks will be studied by high-resolution scanning probe microscopy while the electronic properties will be investigated by tunneling spectroscopy. 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.
The ERC project SURFLINK aims to fabricate surface-supported covalently-linked molecular networks in a bottom-up approach, which are characterized by scanning probe microscopy techniques. Specially designed molecular building blocks are used to create low-dimensional carbon-based materials, in particular, functionalized 2D networks with tunable electronic properties and nanometer-sized pores. We successfully fabricated porous surface-supported nanostructures including macrocycles, 1D nanoribbons, and long-range ordered 2D networks on metal surfaces via Ullmann-type coupling and other coupling reactions reactions. Some of the main achievements of SURFLINK include:
I. We achieved to grow high-quality covalently-linked 2D networks using a hierarchical synthesis, where hexagonal macrocycles and chains were assembled in a first reaction step and connected to extended porous networks in a second reaction step. We used thereby N-heterotriangulene as precursors. We demonstrated experimentally for the first time in carbon structures produced by on-surface synthesis the reduction of the electronic bandgap going from the monomer to the one-dimensional chains and the two-dimensional networks using scanning tunneling spectroscopy (STS), thus corroborating the extension of the effective pi-system (Nat. Comm. 2017).
II. We successfully demonstrated the on-surface synthesis of 1D porous carbon nanoribbons via a preprogrammed isomerization of conformationally flexible polymer chains (JACS 2017).
III. We unraveled the properties of metallated graphyne-like networks as 2D materials. 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. (Nanoscale 2018, ACS Nano 2020)
IV. We investigated the host-guest chemistry in triphenylamine-based COFs and identified strategies for supermolecular doping in COFs. (Nanosclae 2021)
V. 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. 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.
I. We achieved to grow high-quality covalently-linked 2D networks using a hierarchical synthesis, where hexagonal macrocycles and chains were assembled in a first reaction step and connected to extended porous networks in a second reaction step. We used thereby N-heterotriangulene as precursors. We demonstrated experimentally for the first time in carbon structures produced by on-surface synthesis the reduction of the electronic bandgap going from the monomer to the one-dimensional chains and the two-dimensional networks using scanning tunneling spectroscopy (STS), thus corroborating the extension of the effective pi-system (Nat. Comm. 2017).
II. We successfully demonstrated the on-surface synthesis of 1D porous carbon nanoribbons via a preprogrammed isomerization of conformationally flexible polymer chains (JACS 2017).
III. We unraveled the properties of metallated graphyne-like networks as 2D materials. 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. (Nanoscale 2018, ACS Nano 2020)
IV. We investigated the host-guest chemistry in triphenylamine-based COFs and identified strategies for supermolecular doping in COFs. (Nanosclae 2021)
V. 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. 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.
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 a prerequisite for the growth of ordered covalently-linked structures. In the future, we will explore in more detail how to tune on-surface reactions on bulk insulators.