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Nanostructured Surfaces: Molecular Functionality on advanced sp2-bonded substrates

Final Report Summary - NANOSURFS (Nanostructured Surfaces: Molecular Functionality on advanced sp2-bonded substrates)

Artificial structures comprising functional molecules and nanoscale interfaces bear great promise for applications in diverse fields including energy conversion, heterogeneous catalysis, optoelectronics, and quantum technologies. Accordingly, they are central to many current scientific and technological challenges - and thus of societal relevance.
The primary goal of the NanoSurfs project was to engineer molecular properties by combining suitable molecular units with atomically thin two-dimensional (2D) materials, including sp2-bonded substrates. Hereby, we focused on molecular functionalities as reactivity, charge transfer, switching, ligation of gases, and self-assembly into supramolecular architectures. On the one hand, we grew well-defined, nanostructured sp2 monolayers as hexagonal boron nitride (BN) or graphene on suitable supports by scalable processes such as chemical vapour deposition (CVD). On the other hand, we studied and controlled individual functional molecules and their assembly on these advanced supports in exquisite detail by molecular-level scanning tunnelling microscopy, spectroscopy, as well as frequency-modulated atomic force microscopy. These studies provided a comprehensive characterization of molecular nanostructures on the atomic level. The insight gained was used to deliberately tailor the molecule / 2D material / support interfaces to achieve heterostructures with targeted properties. In addition, space-averaging methods as lab- or synchrotron-based x-ray spectroscopy techniques yielded complementary information on electronic, chemical and structural aspects of our systems, which provided important benchmarks for theoretical modelling.
We achieved the formation of atomically-defined boron nitride and graphene layers on rather inert metallic supports like silver single crystals or films, thus systematically extending the library of known sp2 / metal systems. To this end, we developed and refined preparation protocols covering atomic deposition, ion-gun assisted CVD, and intercalation of metals. The resulting interfaces were comprehensively characterized both by complementary experimental techniques and computational modelling. These comparative studies revealed for example how the work function and electronic corrugation of a BN terminated surface can be controlled, thus providing tailored supports for molecular adsorbates.
We made significant progress regarding the self-assembly of molecular nanostructures on BN on copper platforms. Two-dimensional nanoporous metal-organic networks featuring Co centres in distinct environments were assembled from novel carbonitrile-functionalized porphyrin molecules. Furthermore, we demonstrated for the first time an in situ metallation of tetrypyrrole macrocycles with deposited metal atoms directly on an sp2 sheet. Highly ordered oligophenylene monolayers reveal a spatially modulated conductance at the nanometre-scale. Moreover, conductance variations at the molecular level are resolved and assigned to the excitation of vibrational modes of the oligophenylene. The functionalization of pyrenes – photoactive polycyclic aromatic hydrocarbons – with pyridyl moieties in distinct positions was used to assemble supramolecular arrays electronically decoupled from the underlying copper support by the BN spacer. These examples highlight properties that could not be achieved by the respective molecular systems coupled directly to a metallic surface. Furthermore, interdisciplinary collaborative research efforts allowed us to introduce a versatile synthetic route for the formation of stable polycyclic aromatic nanostructures on BN and other supports.
Novel, stable sp2 / tetrapyrrole hybrid structures could be achieved by covalently attaching porphines to the edges of graphene flakes grown on a silver platform employing an on-surface reaction only yielding hydrogen as byproduct. The resulting interfacial bonding motifs – featuring up to four additional C-C bonds – were resolved by frequency-modulated atomic force microscopy, whereas scanning tunnelling microscopy and spectroscopy was employed to characterize the electronic structure near the Fermi level. We notably demonstrated that metallation and ligation reactions characteristic for macrocyclic compounds persist in the hybrid structure, thus opening up pathways to engineer properties of the system.
In summary, the NanoSurfs project provided new insights into physical and chemical processes at interfaces of sp2-bonded two-dimensional materials and functional molecules. The atomically precise characterization and control of interfacial properties in such systems yield prospects for the engineering of novel hybrid materials.