Overview
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Publications
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(2022): Induced Fit and Mobility of Cycloalkanes within Nanometer-Sized Confinements at 5 K. In: The Journal of Physical Chemistry Letters 13, S. 7504-7513. DOI: 10.1021/acs.jpclett.2c01592
DOI: https://doi.org/10.1021/acs.jpclett.2c01592 Abstract: Host-guest architectures provide ideal systems for investigating site-specific physical and chemical effects. Condensation events in nanometer-sized confinements are particularly interesting for the investigation of intermolecular and molecule-surface interactions. They may be accompanied by conformational adjustments representing induced fit packing patterns. Here, we report that the symmetry of small clusters formed upon condensation, their registry with the substrate, their lateral packing, and their adsorption height are characteristically modified by the packing of cycloalkanes in confinements. While cyclopentane and cycloheptane display cooperativity upon filling of the hosting pores, cyclooctane and to a lesser degree cyclohexane diffusively redistribute to more favored adsorption sites. The dynamic behavior of cyclooctane is surprising at 5 K given the cycloalkane melting point of >0 degrees C. The site-specific modification of the interaction and behavior of adsorbates in confinements plays a crucial role in many applications of three-dimensional porous materials as gas storage agents or catalysts/biocatalysts.
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(2022): Self-Assembly of a Triphenylene-Based Electron Donor Molecule on Graphene. Structural and Electronic Properties. In: The Journal of Physical Chemistry C 126, S. 9855-9861. Available online at https://doi.org/10.1021/acs.jpcc.1c10266, last checked on 16.12.2022
Abstract: In this study, we report on the self-assembly of the organic electron donor 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) on graphene grown epitaxially on Ir(111). Using scanning tunneling microscopy and low-energy electron diffraction, we find that a monolayer of HAT assembles in a commensurate close-packed hexagonal network on graphene/Ir(111). X-ray and ultraviolet photoelectron spectroscopy measurements indicate that no charge transfer between the HAT molecules and the graphene/Ir(111) substrate takes place, while the work function decreases slightly. This demonstrates that the HAT/graphene interface is weakly interacting. The fact that the molecules nonetheless form a commensurate network deviates from what is established for adsorption of organic molecules on metallic substrates where commensurate overlayers are mainly observed for strongly interacting systems.
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(2022): Length-dependent symmetry in narrow chevron-like graphene nanoribbons. In: Nanoscale Advances 4, S. 3531-3536. Available online at https://doi.org/10.1039/D2NA00297C, last checked on 16.09.2022
Abstract: We report the structural and electronic properties of narrow chevron-like graphene nanoribbons (GNRs), which depending on their length are either mirror or inversion symmetric. Additionally, GNRs of different length can form molecular heterojunctions based on an unusual binding motif.
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(2022): Length-dependent symmetry in narrow chevron-like graphene nanoribbons. In: Nanoscale Advances 4, S. 3531-3536. Available online at https://doi.org/10.1039/d2na00297c, last checked on 03.12.2022
Abstract: We report the structural and electronic properties of narrow chevron-like graphene nanoribbons (GNRs), which depending on their length are either mirror or inversion symmetric. Additionally, GNRs of different length can form molecular heterojunctions based on an unusual binding motif.
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(2022): Self-Assembly of a Triphenylene-Based Electron Donor Molecule on Graphene. Structural and Electronic Properties. In: The journal of physical chemistry. C, Nanomaterials and interfaces 126, S. 9855-9861. Available online at https://doi.org/10.1021/acs.jpcc.1c10266, last checked on 06.10.2022
Abstract: In this study, we report on the self-assembly of the organic electron donor 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) on graphene grown epitaxially on Ir(111). Using scanning tunneling microscopy and low-energy electron diffraction, we find that a monolayer of HAT assembles in a commensurate close-packed hexagonal network on graphene/Ir(111). X-ray and ultraviolet photoelectron spectroscopy measurements indicate that no charge transfer between the HAT molecules and the graphene/Ir(111) substrate takes place, while the work function decreases slightly. This demonstrates that the HAT/graphene interface is weakly interacting. The fact that the molecules nonetheless form a commensurate network deviates from what is established for adsorption of organic molecules on metallic substrates where commensurate overlayers are mainly observed for strongly interacting systems.
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(2021): Casimir and electrostatic forces from Bi2Se3 thin films of varying thickness. In: Physical Review B: Condensed Matter and Materials Physics 103. DOI: 10.1103/PhysRevB.103.L161102
DOI: http://www.scopus.com/inward/record.url?scp=85104421566&partnerID=8YFLogxK Abstract: The self-assembly of three porphyrin derivatives was studied in detail on a Cu(111) substrate by means of scanning tunneling microscopy (STM). All derivatives have two 4-cyanophenyl substituents in diagonally opposed meso-positions of the porphyrin core. but differ in the nature of the other two meso-alkoxyphenyl substituents. At coverages below 0.8 monolayers, two derivatives form molecular chains, which evolve into nanoporous networks at higher coverages. The third derivative self-assembles directly into a nanoporous network without showing a one-dimensional phase. The pore-to-pore distances for the three networks depend on the size and shape of the alkoxy substituents. All observed effects are explained by 1) different steric demands of the alkoxy residues, 2) apolar (mainly dispersion) interactions between the alkoxy chains, 3) polar bonding involving both cyanophenyl and alkoxyphenyl substituents, and 4) the entropy/enthalpy balance of the network formation.
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(2021): Structural Transformation of Surface-Confined Porphyrin Networks by Addition of Co Atoms. In: Chemistry - A European Journal 27, S. 12430-12436. Available online at https://doi.org/10.1002/chem.202101217, last checked on 10.02.2023
Abstract: The self-assembly of a nickel-porphyrin derivative (Ni-DPPyP) containing two pyridyl coordinating sites and two pentyl chains at trans meso positions was studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) on Au(111). Deposition of Ni-DPPyP onto Au(111) gave rise to a close-packed network for coverages smaller or equal to one monolayer as revealed by STM and LEED. The molecular arrangement of this two-dimensional network is stabilized via hydrogen bonds formed between the pyridyl's nitrogen and hydrogen atoms from the pyrrole groups of neighboring molecules. Subsequent deposition of cobalt atoms onto the close-packed network and post-deposition annealing at 423 K led to the formation of a Co-coordinated hexagonal porous network. As confirmed by XPS measurements, the porous network is stabilized by metal-ligand interactions between one cobalt atom and three pyridyl ligands, each pyridyl ligand coming from a different Ni-DPPyP molecule.
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(2021): Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal. Organic Coordination Networks. In: The Journal of Physical Chemistry C 125, S. 24557-24567. Available online at https://doi.org/10.1021/acs.jpcc.1c05360, last checked on 10.02.2023
Abstract: In recent studies, porphyrin derivatives have been frequently used as building blocks for the fabrication of metal–organic coordination networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions (UHV). The porphyrin core can host a variety of 3d transition metals, which are usually incorporated in solution. However, the replacement of a pre-existing metal atom in the porphyrin core by a different metallic species has been rarely reported under UHV. Herein, we studied the influence of cyanophenyl and pyridyl functional endgroups in the self-assembly of structurally different porphyrin-based MOCNs by the deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized (Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy (STM). A comparative analysis of the influence of the cyano and pyridyl endgroups on the formation of different in-plane coordination motifs is performed. Each porphyrin derivative formed two structurally different Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination nodes, respectively. Interestingly, the codeposited Fe atoms did not only bind to the functional endgroups but also reacted with the porphyrin core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange reaction took place in the porphyrin core where the codeposited Fe atoms replaced the Zn atoms. This was evidenced by the appearance of molecules with an enhanced (centered) STM contrast compared with the appearance of Zn-TPyP, which suggested the formation of a new molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered STM contrast after the deposition of Fe atoms, which was attributed to the binding of the Fe atoms on the top site of the Co-substituted porphyrin core. In summary, the deposition of metal atoms onto organic layers can steer the formation of structurally different MOCNs and may replace pre-existing metal atoms contained in the porphyrin core.
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(2021): Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar, Non-Porous Naphthalenediimide-Template on HOPG. In: Chemistry - A European Journal 27, S. 207-211. Available online at https://doi.org/10.1002/chem.202004008, last checked on 02.02.2023
Abstract: The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC33-NDI-uC33, a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC33-NDI-uC33 template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC33-NDI-uC33 monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces. The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC33-NDI-uC33, a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC33-NDI-uC33 template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC33-NDI-uC33 monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.
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(2021): Atomically precise graphene nanoribbons. Interplay of structural and electronic properties. In: Chemical Society Reviews 50, S. 6541-6568. Available online at https://doi.org/10.1039/D0CS01541E, last checked on 16.12.2022
Abstract: Graphene nanoribbons hold great promise for future applications in nanoelectronic devices, as they may combine the excellent electronic properties of graphene with the opening of an electronic band gap – not present in graphene but required for transistor applications. With a two-step on-surface synthesis process, graphene nanoribbons can be fabricated with atomic precision, allowing precise control over width and edge structure. Meanwhile, a decade of research has resulted in a plethora of graphene nanoribbons having various structural and electronic properties. This article reviews not only the on-surface synthesis of atomically precise graphene nanoribbons but also how their electronic properties are ultimately linked to their structure. Current knowledge and considerations with respect to precursor design, which eventually determines the final (electronic) structure, are summarized. Special attention is dedicated to the electronic properties of graphene nanoribbons, also in dependence on their width and edge structure. It is exactly this possibility of precisely changing their properties by fine-tuning the precursor design – offering tunability over a wide range – which has generated this vast research interest, also in view of future applications. Thus, selected device prototypes are presented as well.
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(2021): Molecular assemblies on surfaces. Towards physical and electronic decoupling of organic molecules. In: Beilstein Journal of Nanotechnology, S. 950-956. Available online at https://doi.org/10.3762/bjnano.12.71, last checked on 03.12.2022
Abstract: Over the past two decades, organic molecules adsorbed on atomically defined metal surfaces have been intensively studied to obtain an in-depth understanding of their self-assembly behavior, on-surface reactivity, as well as their structural and electronic properties [1-6]. An important aspect to unravel their potential use in electronic and optoelectronic devices is how their functionality can be preserved when adsorbed on surfaces. Unfortunately, the (strong) interaction of the molecules with the metallic surface, for example, due to hybridization of molecular states with electronic bands from the metallic substrate, often alters the electronic properties of the molecules and, moreover, can even turn off their sought-after functionality. As a result of the (strong) interaction, the molecular scaffolds can also become distorted, electronic states may be significantly broadened and shifted, and vibronic states may even be quenched. Decoupling strategies offer unique opportunities to reduce these (strong) interactions. In the following, recent progress to decouple both single molecules and molecular assemblies physically and electronically from a (strongly) interacting support is briefly reviewed.
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(2021): Unveiling Adatoms in On-Surface Reactions. Combining Scanning Probe Microscopy with van’t Hoff Plots. In: The Journal of Physical Chemistry C 125, S. 9847-9854. Available online at https://doi.org/10.1021/acs.jpcc.1c03134, last checked on 16.12.2022
Abstract: Scanning probe microscopy has become an essential tool to not only study pristine surfaces but also on-surface reactions and molecular self-assembly. Nonetheless, due to inherent limitations, some atoms or (parts of) molecules are either not imaged or cannot be unambiguously identified. Herein, we discuss the arrangement of two different nonplanar molecular assemblies of para-hexaphenyl-dicarbonitrile (Ph6(CN)2) on Au(111) based on a combined theoretical and experimental approach. For deposition of Ph6(CN)2 on Au(111) kept at room temperature, a rhombic nanoporous network stabilized by a combination of hydrogen bonding and antiparallel dipolar coupling is formed. Annealing at 575 K resulted in an irreversible thermal transformation into a hexagonal nanoporous network stabilized by native gold adatoms. However, the Au adatoms could neither be unequivocally identified by scanning tunneling microscopy nor by noncontact atomic force microscopy. By combining van’t Hoff plots derived from our scanning probe images with our density functional theory calculations, we were able to confirm the presence of the elusive Au adatoms in the hexagonal molecular network.
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(2021): Transfer of Large-Scale Two-Dimensional Semiconductors. Challenges and Developments. In: 2D Materials 8. Available online at https://doi.org/10.1088/2053-1583/abf234, last checked on 20.11.2022
Abstract: Two-dimensional (2D) materials offer opportunities to explore both fundamental science and applications in the limit of atomic thickness. Beyond the prototypical case of graphene, other 2D materials have recently come to the fore. Of particular technological interest are 2D semiconductors, of which the family of materials known as the group-VI transition metal dichalcogenides (TMDs) has attracted much attention. The presence of a bandgap allows for the fabrication of high on-off ratio transistors and optoelectronic devices, as well as valley/spin polarized transport. The technique of chemical vapour deposition (CVD) has produced high-quality and contiguous wafer-scale 2D films, however, they often need to be transferred to arbitrary substrates for further investigation. In this Review, the various transfer techniques developed for transferring 2D films will be outlined and compared, with particular emphasis given to CVD-grown TMDs. Each technique suffers undesirable process-related drawbacks such as bubbles, residue or wrinkles, which can degrade device performance by for instance reducing electron mobility. This Review aims to address these problems and provide a systematic overview of key methods to characterize and improve the quality of the transferred films and heterostructures. With the maturing technological status of CVD-grown 2D materials, a robust transfer toolbox is vital.
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(2020): Engineering Long-Range Order in Supramolecular Assemblies on Surfaces. The Paramount Role of Internal Double Bonds in Discrete Long-Chain Naphthalenediimides. In: Journal of the American Chemical Society 142, S. 4070-4078. Available online at https://doi.org/10.1021/jacs.0c00765, last checked on 10.02.2023
Abstract: Achieving long-range order with surface-supported supramolecular assemblies is one of the pressing challenges in the prospering field of non-covalent surface functionalization. Having access to defect-free on-surface molecular assemblies will pave the way for various nanotechnology applications. Here we report the synthesis of two libraries of naphthalenediimides (NDIs) symmetrically functionalized with long aliphatic chains (C28 and C33) and their self-assembly at the 1-phenyloctane/highly oriented pyrolytic graphite (1-PO/HOPG) interface. The two NDI libraries differ by the presence/absence of an internal double bond in each aliphatic chain (unsaturated and saturated compounds, respectively). All molecules assemble into lamellar arrangements, with the NDI cores lying flat and forming 1D rows on the surface, while the carbon chains separate the 1D rows from each other. Importantly, the presence of the unsaturation plays a dominant role in the arrangement of the aliphatic chains, as it exclusively favors interdigitation. The fully saturated tails, instead, self-assemble into a combination of either interdigitated or non-interdigitated diagonal arrangements. This difference in packing is spectacularly amplified at the whole surface level and results in almost defect-free self-assembled monolayers for the unsaturated compounds. In contrast, the monolayers of the saturated counterparts are globally disordered, even though they locally preserve the lamellar arrangements. The experimental observations are supported by computational studies and are rationalized in terms of stronger van der Waals interactions in the case of the unsaturated compounds. Our investigation reveals the paramount role played by internal double bonds on the self-assembly of discrete large molecules at the liquid/solid interface.
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(2020): Thiol-free self-assembled oligoethylene glycols enable robust air-stable molecular electronics. In: Nature Materials 19, S. 330-337. DOI: 10.1038/s41563-019-0587-x
DOI: https://doi.org/10.1038/s41563-019-0587-x Abstract: Self-assembled monolayers (SAMs) are widely used to engineer the surface properties of metals. The relatively simple and versatile chemistry of metal–thiolate bonds makes thiolate SAMs the preferred option in a range of applications, yet fragility and a tendency to oxidize in air limit their long-term use. Here, we report the formation of thiol-free self-assembled mono- and bilayers of glycol ethers, which bind to the surface of coinage metals through the spontaneous chemisorption of glycol ether-functionalized fullerenes. As-prepared assemblies are bilayers presenting fullerene cages at both the substrate and ambient interface. Subsequent exposure to functionalized glycol ethers displaces the topmost layer of glycol ether-functionalized fullerenes, and the resulting assemblies expose functional groups to the ambient interface. These layers exhibit the key properties of thiolate SAMs, yet they are stable to ambient conditions for several weeks, as shown by the performance of tunnelling junctions formed from SAMs of alkyl-functionalized glycol ethers. Glycol ether-functionalized spiropyrans incorporated into mixed monolayers lead to reversible, light-driven conductance switching. Self-assemblies of glycol ethers are drop-in replacements for thiolate SAMs that retain all of their useful properties while avoiding the drawbacks of metal–thiolate bonds.
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(2019): Comparison of Casimir forces and electrostatics from conductive SiC-Si/C and Ru surfaces. In: Physical Review B: Condensed Matter and Materials Physics 100. DOI: 10.1103/PhysRevB.100.245422
DOI: https://doi.org/10.1103/PhysRevB.100.245422 Abstract: Comprehensive knowledge of Casimir forces and associated electrostatics from conductive SiC and Ru surfaces can be essential in diverse areas ranging from micro/nanodevice operation in harsh environments to multilayer coatings in advanced lithography technologies. Hence, the Casimir force was measured between an Au-coated microsphere and N-doped SiC samples with Si- and C-terminated faces, and the results were compared with the measurements using the same microsphere and a metallic Ruthenium surface. Electrostatic calibration showed that the Si- and C-faces behave differently with a nearly similar to 0.6-0.7 V difference in the contact potentials V-0(Si/C). We attribute this to a higher incorporation of N on the C-terminated face in the near surface region resulting in the formation of NOx and an increased work function compared to the Si-terminated surface, which is in agreement with x-ray photoelectron spectroscopy data. Notably, the contact potential of the SiC-C face (V-0(C) similar to 0.1 V) was closer to the metallic Ru-Au system (V-0(Ru) similar to 0.05 V). However, the measured optical properties of the SiC-Si/C terminated surfaces with ellipsometry did not show any substantial differences indicating that the effective depth of the Si/C terminating surface layers are significantly smaller than the photon penetration depth not leading to any differences in the calculated forces via Lifshitz theory. Nonetheless, the measured Casimir forces, after compensation of the electrostatics contributions, showed differences between the Si/C faces, whereas the comparison with the Lifshitz theory prediction shows better agreement for the SiC-Si face. Finally, comparison of the Casimir forces below 40 nm separations between the SiC-Si/C and Ru surfaces indicated that the short-range roughness effects on the Casimir force increase in magnitude with increasing metallic behavior of the plate surface. Therefore, not only the material optical properties but also the conductive state and roughness of the surface layers must be carefully taken into account in short range Casimir interactions between more complex dielectric materials.
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(2019): Coverage-Dependent Structural Transformation of Cyano-Functionalized Porphyrin Networks on Au(111) via Addition of Cobalt Atoms. In: The Journal of Physical Chemistry C 123, S. 19681-19687. Available online at https://doi.org/10.1021/acs.jpcc.9b05055, last checked on 10.02.2023
Abstract: The self-assembly process of a cobalt-porphyrin derivative (Co-TCNPP) containing cyanophenyl substituents at all four meso positions on Au(111) was studied by means of scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) under ultrahigh vacuum conditions. Deposition of Co-TCNPP onto Au(111) gave rise to the formation of a close-packed H-bonded network, which was independent of coverage as revealed by STM and LEED. However, a coverage-dependent structural transformation took place upon the deposition of Co atoms. At monolayer coverage, a reticulated long-range ordered network exhibiting a distinct fourfold Co coordination was observed. By reduction of the molecular coverage, a second metal-organic coordination network (MOCN) was formed in coexistence with the fourfold Co-coordinated network, that is, a chevron structure stabilized by a simultaneous expression of H-bonding and threefold Co coordination. We attribute the coverage-dependent structural transformation to the in-plane compression pressure exerted by the molecules deposited on the surface. Our study shows that a subtle interplay between the chemical nature of the building blocks (molecules and metallic atoms) and molecular coverage can steer the formation of structurally different porphyrin-based MOCNs.
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(2019): Edge Phonon Excitations in a Chiral Self-Assembled Supramolecular Nanoribbon. In: The Journal of Physical Chemistry Letters 10, S. 5830-5835. DOI: 10.1021/acs.jpclett.9b02001
DOI: https://doi.org/10.1021/acs.jpclett.9b02001 Abstract: By design, coupled mechanical oscillators offer a playground for the study of crystalline topology and related properties. Particularly, non-centrosymmetric, supramolecular nanocrystals feature a complex phonon spectrum where edge modes may evolve. Here we show, employing classical atomistic calculations, that the edges of a chiral supramolecular nanoribbon can host defined edge phonon states. We suggest that the topology of several edge modes in the phonon spectrum is nontrivial and thermally insulated from bulk states. By means of molecular dynamics, we excite a supramolecular bond to launch a directional excitation along the edge without considerable bulk or back-propagation. Our results suggest that supramolecular monolayers can be employed to engineer phonon states that are robust against backscattering, toward supramolecular thermal waveguides, diodes, and logics.
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(2019): Low-Dimensional Metal−Organic Coordination Structures on Graphene. In: The Journal of Physical Chemistry C 123, S. 12730-12735. Available online at https://doi.org/10.1021/acs.jpcc.9b00326, last checked on 16.12.2022
Abstract: We report the formation of one- and two-dimensional metal-organic coordination structures from para-hexaphenyl-dicarbonitrile (NC-Ph-6-CN) molecules and Cu atoms on graphene epitaxially grown on Ir(111). By varying the stoichiometry between the NC-Ph-6-CN molecules and Cu atoms, the dimensionality of the metal-organic coordination structures could be tuned: for a 3:2 ratio, a two-dimensional hexagonal porous network based on threefold Cu coordination was observed, while for a 1:1 ratio, one-dimensional chains based on twofold Cu coordination were formed. The formation of metal-ligand bonds was supported by imaging the Cu atoms within the metal-organic coordination structures with scanning tunneling microscopy. Scanning tunneling spectroscopy measurements demonstrated that the electronic properties of NC-Ph-6-CN molecules and Cu atoms were different between the two-dimensional porous network and one-dimensional molecular chains.
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(2019): Triphenylene-Derived Electron Acceptors and Donors on Ag(111). Formation of Intermolecular Charge-Transfer Complexes with Common Unoccupied Molecular States. In: Small 15. Available online at https://doi.org/10.1002/smll.201901741
Abstract: Over the past years, ultrathin films consisting of electron donating and accepting molecules have attracted increasing attention due to their potential usage in optoelectronic devices. Key parameters for understanding and tuning their performance are intermolecular and molecule–substrate interactions. Here, the formation of a monolayer thick blend of triphenylene‐based organic donor and acceptor molecules from 2,3,6,7,10,11‐hexamethoxytriphenylene (HAT) and 1,4,5,8,9,12‐hexaazatriphenylenehexacarbonitrile (HATCN), respectively, on a silver (111) surface is reported. Scanning tunneling microscopy and spectroscopy, valence and core level photoelectron spectroscopy, as well as low‐energy electron diffraction measurements are used, complemented by density functional theory calculations, to investigate both the electronic and structural properties of the homomolecular as well as the intermixed layers. The donor molecules are weakly interacting with the Ag(111) surface, while the acceptor molecules show a strong interaction with the substrate leading to charge transfer and substantial buckling of the top silver layer and of the adsorbates. Upon mixing acceptor and donor molecules, strong hybridization occurs between the two different molecules leading to the emergence of a common unoccupied molecular orbital located at both the donor and acceptor molecules. The donor acceptor blend studied here is, therefore, a compelling candidate for organic electronics based on self‐assembled charge‐transfer complexes.
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(2019): Effective determination of surface potential landscapes from metal-organic nanoporous network overlayers. In: New Journal of Physics 21. Available online at https://doi.org/10.1088/1367-2630/ab150e, last checked on 03.12.2022
Abstract: Determining the scattering potential landscape for two-dimensional superlattices provides key insight into fundamental quantum electron phenomena. Theoretical and semiempirical methods have been extensively used to simulate confinement effects of the two-dimensional electron gas (2DEG) on superlattices with a single scatterer in the form of vicinal surfaces and dislocation networks or isolated structures such as quantum corrals and vacancy islands. However, the complexity of the problem increases when the building blocks (or scatterers) are heterogeneous, as in metal-organic nanoporous networks (MONNs), since additional potentials may come into play. Therefore, the parametrization of the surface potential landscape is often inaccurate, leading to incorrect scattering potentials. Here, we address this issue with a combination of scanning tunneling microscopy/spectroscopy, angle resolved photoemission spectroscopy and Kelvin probe force microscopy measurements together with electron plane-wave expansion simulations on a MONN grown on Cu(111). This experimental-theory approach, enables us to capture the 2DEG response to the intricate scattering potential landscape, and reveals systematic modeling procedures. Starting from a realistic geometry of the system, we determine the repulsive scattering potentials for both molecules and coordinated metal adatoms, the latter contradicting the established simulation framework. Moreover, we reveal local asymmetries and subtle renormalization effects of the 2DEG that relate to the interaction of the MONN and the underlying substrate.
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(2019): Surface state tunable energy and mass renormalization from homothetic quantum dot arrays. In: Nanoscale 11, S. 23132-23138. Available online at https://doi.org/10.1039/c9nr07365e, last checked on 03.12.2022
Abstract: Quantum dot arrays in the form of molecular nanoporous networks are renowned for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the band bottom of the confined states can exhibit downward shifts accompanied by a lowering of the effective masses simultaneous to the appearance of tiny gaps at the Brillouin zone boundaries. We observed these effects by angle resolved photoemission for two self-assembled homothetic (scalable) Co-coordinated metal-organic networks. Complementary scanning tunneling spectroscopy measurements confirmed these findings. Electron plane wave expansion simulations and density functional theory calculations provide insight into the nature of this phenomenon, which we assign to metal-organic overlayer-substrate interactions in the form of adatom-substrate hybridization. To date, the absence of the experimental band structure resulting from single metal adatom coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer to be general for low interacting and well-defined adatom arrays.
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(2019): Comparing the Self-Assembly of Sexiphenyl-Dicarbonitrile on Graphite and Graphene on Cu(111). In: Chemistry - A European Journal 25, S. 5065-5070. Available online at https://doi.org/10.1002/chem.201806312, last checked on 26.11.2022
Abstract: A comparative study on the self-assembly of sexiphenyl-dicarbonitrile on highly oriented pyrolytic graphite and single-layer graphene on Cu(111) is presented. Despite an overall low molecule-substrate interaction, the close-packed structures exhibit a peculiar shift repeating every four to five molecules. This shift has hitherto not been reported for similar systems and is hence a unique feature induced by the graphitic substrates.
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(2019): Coverage-Controlled Polymorphism of H-Bonded Networks on Au(111). In: The Journal of Physical Chemistry C 123, S. 7151-7157. Available online at https://doi.org/10.1021/acs.jpcc.8b12260, last checked on 26.11.2022
Abstract: We report on the self-assembly of a conformational flexible organic compound on Au(111) using scanning tunneling microscopy and low-energy electron diffraction measurements. We observed different conformers of the compound upon adsorption on the reconstructed Au(111) surface. Increasing the molecular coverage enhanced the lateral pressure, that is, parallel to the surface, favoring a coverage-controlled transition from a supramolecular network displaying only one molecular organization, into a polymorphic array with two coexisting arrangements. Our results give insights into the role of substrate-induced conformational changes on the formation of polymorphic supramolecular networks.
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(2018) : Role of Cyano Groups in the Self-Assembly of Organic Molecules on Metal Surfaces In: Wandelt, Klaus: Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry: Amsterdam: Elsevier, S. 153-165
DOI: https://doi.org/10.1016/B978-0-12-409547-2.13540-1 Abstract: Molecular building blocks substituted with cyano groups have shown to form a myriad of supramolecular architectures on coinage metal substrates upon self-assembly. The flexibility of the polar cyano group to be involved in various bonding motifs such as H-bonding, metal–ligand bonding, or dipolar coupling is essential for the formation of these structures. In addition to the intermolecular interactions, the molecule–substrate interactions depended on the structure and charge distribution within the molecular building blocks have a direct impact on the structure of the organic networks. Here, we discuss the influence of cyano endgroups in the self-assembly of organic molecules on metal surfaces by means of their bonding motifs. Especially, porphyrin and polyphenyl derivatives are excellent candidates for cyano substitution since the number as well as the position of the cyano substituents can be precisely adjusted offering a high degree of both control and variability over the outcome of the self-assembly process.
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(2018): Adsorbate-Induced Modification of the Confining Barriers in a Quantum Box Array. In: ACS nano 12, S. 768-778. DOI: 10.1021/acsnano.7b07989
DOI: https://doi.org/10.1021/acsnano.7b07989 Abstract: Quantum devices depend on addressable elements, which can be modified separately and in their mutual interaction. Self-assembly at surfaces, for example, formation of a porous (metal-) organic network, provides an ideal way to manufacture arrays of identical quantum boxes, arising in this case from the confinement of the electronic (Shockley) surface state within the pores. We show that the electronic quantum box state as well as the interbox coupling can be modified locally to a varying extent by a selective choice of adsorbates, here C60, interacting with the barrier. In view of the wealth of differently acting adsorbates, this approach allows for engineering quantum states in on-surface network architectures.
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(2018) : Molecular Self-Assembly on Graphene. The Role of the Substrate In: Wandelt, Klaus: Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry: Amsterdam: Elsevier, S. 110-119
DOI: https://doi.org/10.1016/B978-0-12-409547-2.14162-9 Abstract: Graphene is a single layer of carbon atoms arranged in a 2D honeycomb structure. It exhibits countless outstanding properties, which make it the candidate of choice for usage in future applications ranging from electronics to sensing and coatings. However, despite the fact that graphene is considered a miracle material, there are also properties which are not inherently present in graphene such as an electronic band gap and magnetic and catalytic properties. Chemical functionalization of graphene could be utilized to introduce some of the missing properties. Molecular self-assembly based on noncovalent interactions is one way to functionalize graphene, and charge transfer from the adsorbed molecular networks could result in breaking graphene’s sublattice symmetry and introduction of the missing band gap. However, the underlying substrate, on which graphene is grown, can affect the formation of the molecular adlayer as well as the molecule–substrate interactions to a varying degree. In this article, recent progress on molecular self-assembly on epitaxial graphene is reviewed. In particular, we discuss the influence of the substrate underlying graphene on the self-assembly on top of graphene.
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(2018): Bias-induced conformational switching of supramolecular networks of trimesic acid at the solid-liquid interface. In: The Journal of Chemical Physics 148. DOI: 10.1063/1.5017930
DOI: https://doi.org/10.1063/1.5017930 Abstract: Using the tip of a scanning tunneling microscope, an electric field-induced reversible phase transition between two planar porous structures ("chickenwire" and "flower") of trimesic acid was accomplished at the nonanoic acid/highly oriented pyrolytic graphite interface. The chicken wire structure was exclusively observed for negative sample bias, while for positive sample bias only the more densely packed flower structure was found. We suggest that the slightly negatively charged carboxyl groups of the trimesic acid molecule are the determining factor for this observation: their adsorption behavior varies with the sample bias and is thus responsible for the switching behavior.
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(2017) : Hierarchical assembly of Xe atoms in an atomically precise array of quantum boxes In: American Chemical Socienty: Abstracts of Papers: 253rd National Meeting: San Francisco, 2. - 6. April
Abstract: Quantum boxes (QBs) have been arranged in extended 2D arrays by the self-assembled formation of a porous on-surface coordinated network. These boxes provide a particular environment to study the condensation of atoms and small molecules. The electronic states contained in these arrays can be configured in an atom-by-atom manner. The localized perturbation is controlled by the targeted filling level of the individual QBs with Xe atoms after Xe repositioning. It is shown that specific filling patterns of the network of QBs which are coupled in an inherently precise way by self assembly specifically perturb, and thus modify the localized and delocalized quantum box states (QBSs). In particular the energy levels of the QBSs and their coupling across the 2D QBarray is modulated which provides an analogy to a breadboard as it is used in the design and testing of electronic circuitry.
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(2017): Chiral-Selective Formation of 1D Polymers Based on Ullmann-Type Coupling. The Role of the Metallic Substrate. In: Small 13. Available online at https://doi.org/10.1002/smll.201603675, last checked on 02.12.2022
Abstract: The chiral-selective formation of 1D polymers from a prochiral molecule, namely, 6,12-dibromochrysene in dependence of the type of metal surface is demonstrated by a combined scanning tunneling microscopy and density functional theory study. Deposition of the chosen molecule on Au(111) held at room temperature leads to the formation of a 2D porous molecular network. Upon annealing at 200 °C, an achiral covalently linked polymer is formed on Au(111). On the other hand, a chiral Cu-coordinated polymer is spontaneously formed upon deposition of the molecules on Cu(111) held at room temperature. Importantly, it is found that the chiral-selectivity determines the possibility of obtaining graphene nanoribbons (GNRs). On Au(111), upon annealing at 350 °C or higher cyclo-dehydrogenation occurs transforming the achiral polymer into a GNR. In contrast, the chiral coordination polymer on Cu(111) cannot be converted into a GNR.
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(2017): Temperature dependence of the partially localized state in a 2D molecular nanoporous network. In: Applied Surface Science 391, S. 39-43. Available online at https://doi.org/10.1016/j.apsusc.2016.02.227, last checked on 02.12.2022
Abstract: Two-dimensional organic and metal-organic nanoporous networks can scatter surface electrons, leading to their partial localization. Such quantum states are related to intrinsic surface states of the substrate material. We further corroborate this relation by studying the thermally induced energy shifts of the electronic band stemming from coupled quantum states hosted in a metal-organic array formed by a perylene derivative on Cu(111). We observe by angle-resolved photoemission spectroscopy (ARPES), that both, the Shockley and the partially localized states, shift by the same amount to higher binding energies upon decreasing the sample temperature, providing evidence of their common origin. Our experimental approach and results further support the use of surface states for modelling these systems, which are expected to provide new insight into the physics concerning partially confined electronic states: scattering processes, potential barrier strengths, excited state lifetimes or the influence of guest molecules.
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(2017): On-Surface Formation of Cumulene by Dehalogenative Homocoupling of Alkenyl gem-Dibromides. In: Angewandte Chemie International Edition 56, S. 12165-12169. Available online at https://doi.org/10.1002/anie.201706104, last checked on 20.11.2022
Abstract: The on-surface activation of carbon-halogen groups is an efficient route to produce radicals for constructing various hydrocarbons and carbon nanostructures. To date, the employed halide precursors have only one halogen attached to a carbon atom. It is thus of interest to study the effect of attaching more than one halogen atom to a carbon atom with the aim of producing multiple unpaired electrons. By introducing an alkenyl gem-dibromide, cumulene products were fabricated on a Au(111) surface by dehalogenative homocoupling reactions. The reaction products and pathways were unambiguously characterized by a combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy measurements together with density functional calculations. This study further supplements the database of on-surface synthesis strategies and provides a facile manner for incorporation of more complicated carbon scaffolds into surface nanostructures.
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(2017): Surface-confined [2+2] cycloaddition towards one-dimensional polymers featuring cyclobutadiene units. In: Nanoscale 9, S. 18305-18310. Available online at https://doi.org/10.1039/c7nr06187k, last checked on 20.11.2022
Abstract: Surface-confined synthesis has been offering a wide range of opportunities for the construction of novel molecular nanostructures. Exploring new types of on-surface coupling reactions is considered essential for being able to deliberately tune the materials properties. Here, we report on the formation of a covalent C-C bonding motif, namely 1,3-cyclobutadiene, via surface-confined [2 + 2] cycloaddition between pyrene moieties using low temperature scanning tunneling microscopy (LT-STM) and X-ray photo-emission spectroscopy (XPS) measurements. By employing a hydrogen dosing treatment together with low-temperature activation, we were able to both eliminate residual byproducts and obtain covalent 1D polymers through the formation of 1,3-cyclobutadiene units. The resulting C-C bonding motif has so far hardly been explored in surface chemistry and substantial evidence is provided that the hydrogen treatment is crucial towards the removal of byproducts in surface-confined polymerization.
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(2016): 1,3,5-Benzenetribenzoic Acid on Cu(111) and Graphene/Cu(111). A Comparative STM Study. In: The Journal of Physical Chemistry C 120, S. 18093-18098. Available online at https://doi.org/10.1021/acs.jpcc.6b05541, last checked on 16.12.2022
Abstract: The self-assembly of 1,3,5-benzenetribenzoic acid (BTB) molecules on both Cu(111) and epitaxial graphene grown on Cu(111) were studied by scanning tunneling microscopy (STM) and, low-energy electron diffraction (LEED) under ultrahigh vacuum conditions. On Cu(111), the BTB molecules were found to mainly arrange in close-packed structures through H-bonding between the (partially) deprotonated carboxylic acid groups. In addition, porous structures formed by intact BTB molecules-and also based on H-bonding-were observed. On graphene grown on Cu(111) the BTB molecules mainly form porous structures accompanied by small patches of disordered close-packed structures. Upon annealing, BTB adsorbed on Cu(111) is fully deprotonated and arranges in the close-packed structure while in contrast on graphene/Cu(111) the porous network is exclusively formed. This shows that the molecular self-assembly behavior is highly dependent on the first substrate layer: one graphene layer is sufficient to considerably alter the interplay of molecule substrate and intermolecular interactions in favor of the latter interactions.
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(2016): Confinement properties of 2D porous molecular networks on metal surfaces. In: Journal of Physics: Condensed Matter 28. Available online at https://doi.org/10.1088/0953-8984/28/15/153003, last checked on 16.12.2022
Abstract: Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article.
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(2016): Cyano-Functionalized Triarylamines on Coinage Metal Surfaces. Interplay of Intermolecular and Molecule–Substrate Interactions. In: Chemistry - A European Journal 22, S. 581-589. Available online at https://doi.org/10.1002/chem.201503205, last checked on 16.12.2022
Abstract: The self-assembly of cyano-functionalized triarylamine derivatives on Cu(111), Ag(111) and Au(111) was studied by means of scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy and density functional theory calculations. Different bonding motifs, such as antiparallel dipolar coupling, hydrogen bonding and metal coordination, were observed. Whereas on Ag(111) only one hexagonally close-packed pattern stabilized by hydrogen bonding is observed, on Au(111) two different partially porous phases are present at submonolayer coverage, stabilized by dipolar coupling, hydrogen bonding and metal coordination. In contrast to the self-assembly on Ag(111) and Au(111), for which large islands are formed, on Cu(111), only small patches of hexagonally close-packed networks stabilized by metal coordination and areas of disordered molecules are found. The significant variety in the molecular self-assembly of the cyano-functionalized triarylamine derivatives on these coinage metal surfaces is explained by differences in molecular mobility and the subtle interplay between intermolecular and molecule–substrate interactions.
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(2016): Configuring Electronic States in an Atomically Precise Array of Quantum Boxes. In: Small 12, S. 3757-3763. Available online at https://doi.org/10.1002/smll.201600915, last checked on 02.12.2022
Abstract: A 2D array of electronically coupled quantum boxes is fabricated by means of on-surface self-assembly assuring ultimate precision of each box. The quantum states embedded in the boxes are configured by adsorbates, whose occupancy is controlled with atomic precision. The electronic interbox coupling can be maintained or significantly reduced by proper arrangement of empty and filled boxes.
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(2016): Comparing Ullmann Coupling on Noble Metal Surfaces. On-Surface Polymerization of 1,3,6,8-Tetrabromopyrene on Cu(111) and Au(111). In: Chemistry - A European Journal 22, S. 5937-5944. Available online at https://doi.org/10.1002/chem.201504946, last checked on 03.12.2022
Abstract: The on-surface polymerization of 1,3,6,8-tetrabromopyrene (Br4Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br4Py on Cu(111) held at 300K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C-Cu-C bonds. After annealing at 473K, the C-Cu-C bonds were converted to covalent C-C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self-assembled two-dimensional (2D) patterns stabilized by both Br-Br halogen and Br-H hydrogen bonds were observed upon deposition of Br4Py on Au(111) held at 300K. Subsequent annealing of the sample at 473K led to a dissociation of the C-Br bonds and the formation of disordered metal-coordinated molecular networks. Further annealing at 573K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br4Py on the different substrates.
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(2015): Comparing Graphene Growth on Cu(111) versus Oxidized Cu(111). In: Nano Letters 15, S. 917-922. Available online at https://doi.org/10.1021/nl5036463, last checked on 03.12.2022
Abstract: The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.
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(2015) : Comparing graphene growth on Cu(111) versus oxidized Cu(111) In: Bais, Giorgio; Callegari, Carlo; Cucini, Riccardo; Dreossi, Diego; Ferrari, Eugenio; Feyer, Vitaliy; Goldoni, Andrea; Pertosi, Adriana; Petaccia, Luca (Hg.): Elettra Highlights 2014-2015. Elletra Sincrotrone Trieste: Trieste, S. 64-65. Available online at https://www.elettra.eu/science/highlights.html, last checked on 16.02.2023
Abstract: The epitaxial growth of graphene – a single layer of carbon atoms – on metallic and oxidized Cu(111) surfaces was investigated by angle-resolved photoelectron spectroscopy and scanning tunnelling microscopy. While graphene grown via chemical vapour deposition on metallic Cu(111) is n-type doped, the pristine properties of freestanding graphene – no doping, Fermi velocity of 1.4x106 m/s – are preserved on the oxidized Cu(111) surface. Here, we report on a new route to prepare high quality quasi-freestanding graphene on a high-k dielectric surface –oxidized copper – in a single growth step.
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(2015): Interplay of weak interactions in the atom-by-atom condensation of xenon within quantum boxes. In: Nature Communications. Available online at https://doi.org/10.1038/ncomms7071, last checked on 03.12.2022
Abstract: Condensation processes are of key importance in nature and play a fundamental role in chemistry and physics. Owing to size effects at the nanoscale, it is conceptually desired to experimentally probe the dependence of condensate structure on the number of constituents one by one. Here we present an approach to study a condensation process atom-by-atom with the scanning tunnelling microscope, which provides a direct real-space access with atomic precision to the aggregates formed in atomically defined ‘quantum boxes’. Ouranalysis reveals the subtle interplay of competing directional and nondirectional interactions in the emergence of structure and provides unprecedented input for the structural comparison with quantum mechanical models. This approach focuses on—but is not limited to—the model case of xenon condensation and goes significantly beyond the well-established statistical size analysis of clusters in atomic or molecular beams by mass spectrometry.
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(2015): Heat-induced formation of one-dimensional coordination polymers on Au(111). An STM study 51, S. 14473-14476. Available online at https://doi.org/10.1039/c5cc04940g, last checked on 20.11.2022
Abstract: The formation of one-dimensional coordination polymers of cyano-substituted porphyrin derivatives on Au(111) induced by thermal annealing is demonstrated by means of scanning tunnelling microscopy. The polymer is stabilised by an unusual threefold coordination motif mediated between an Au atom and the cyano groups of the porphyrin derivatives.
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(2015): Heat-induced formation of one-dimensional coordination polymers on Au(111). An STM study. In: Chemical Communications 51, S. 14473-14476. Available online at https://doi.org/10.1039/C5CC04940G, last checked on 16.02.2023
Abstract: The formation of one-dimensional coordination polymers of cyano-substituted porphyrin derivatives on Au(111) induced by thermal annealing is demonstrated by means of scanning tunnelling microscopy. The polymer is stabilised by an unusual threefold coordination motif mediated between an Au atom and the cyano groups of the porphyrin derivatives.
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(2015): From hydrogen bonding to metal coordination and back. Porphyrin-based networks on Ag(111). In: The Journal of Chemical Physics 142. DOI: 10.1063/1.4908535
DOI: https://doi.org/10.1063/1.4908535 Abstract: The self-assembly of a metal-free porphyrin bearing two pyridyl coordinating sites and two pentyl chains at trans meso positions was investigated under ultrahigh vacuum on a Ag(111) surface by scanning tunneling microscopy (STM). The STM measurements revealed a well-ordered close-packed structure with a rhombic unit cell for coverages ≤1 monolayer with their molecular plane parallel to the surface. The growth direction of the molecular islands is aligned along the step edges, which are restructured due to molecule-substrate interactions. The shorter unit cell vector of the molecular superstructure follows the 〈1-10〉 direction of the Ag(111) substrate. Hydrogen bondsbetween pyridyl and pyrrole groups of neighboring molecules as well as weak van der Waals forcesbetween the pentyl chains stabilize the superstructure. Deposition of cobalt atoms onto the close-packed structure at room temperature leads to the formation of a hexagonal porous network stabilized by metal-ligand bonding between the pyridyl ligands and the cobalt atoms. Thermal annealing of the Co-coordination network at temperatures >450 K results in the transformation of the hexagonal network into a second close-packed structure. Changes in the molecule-substrate interactions due to metalation of the porphyrin core with Co as well as intermolecular interactions can explain the observed structural transformations.
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(2014): Coverage-Dependent Disorder-to-Order Phase Transformation of a Uracil Derivative on Ag(111). In: The Journal of Physical Chemistry C 118, S. 15286-15291. DOI: 10.1021/jp503188m
DOI: https://doi.org/10.1021/jp503188m Abstract: The self-organization of an angular bis(uracil-ethynyl) benzene derivative is investigated on Ag(111) by means of scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. It is found—starting at low submonolayer coverage—that upon increasing the molecular coverage a disorder-to-order phase transformation occurs. Specifically, at low and intermediate molecular coverage a glassy phase consisting of one-dimensional (1D) chains and 2D aggregates is observed, while close to a first complete molecular layer, a well-ordered 2D close-packed phase is revealed. The main driving forces responsible for the structure formation are (i) the high self-complementarity of the uracil (U) moiety, resulting in U–U homopairs through 2-fold C═O···H–N H-bonds and (ii) the steric hindrance induced in the system by the alkyl chains. The delicate balance between the molecule–molecule and the molecule–substrate interactions leads to a complex phase behavior of the uracil derivative at the solid–vacuum interface. On the basis of this detailed study, we present a qualitative understanding of the peculiar phase behavior of the system.