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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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.1c10266, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/D2NA00297C, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/d2na00297c, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.1c10266, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/chem.202101217, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.1c05360, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/chem.202004008, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/D0CS01541E, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.3762/bjnano.12.71, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.1c03134, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1088/2053-1583/abf234, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/jacs.0c00765, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.9b05055, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.9b00326, zuletzt geprüft am 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. Online verfügbar unter 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. Online verfügbar unter https://doi.org/10.1088/1367-2630/ab150e, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/c9nr07365e, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/chem.201806312, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.8b12260, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/smll.201603675, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1016/j.apsusc.2016.02.227, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/anie.201706104, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/c7nr06187k, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/acs.jpcc.6b05541, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1088/0953-8984/28/15/153003, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/chem.201503205, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/smll.201600915, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1002/chem.201504946, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1021/nl5036463, zuletzt geprüft am 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. Online verfügbar unter https://www.elettra.eu/science/highlights.html, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1038/ncomms7071, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/c5cc04940g, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1039/C5CC04940G, zuletzt geprüft am 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.
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(2014): Cyano-Functionalized Triarylamines on Au(111). Competing Intermolecular versus Molecule/Substrate Interactions. In: Advanced Materials Interfaces 1. Online verfügbar unter https://doi.org/10.1002/admi.201300025, zuletzt geprüft am 10.02.2023
Abstract: The self-assembly of cyano-substituted triarylamine derivatives on Au(111) is studied with scanning tunneling microscopy and density functional theory calculations. Two different phases, each stabilized by at least two different cyano bonding motifs are observed. In the fi rst phase, each molecule is involved in dipolar coupling and hydrogen bonding, while in the second phase, dipolar coupling, hydrogen bonding and metal-ligand interactions are present. Interestingly, the metal–ligand bond is already observed for deposition of the molecules with the sample kept at room temperature leaving the herringbone reconstruction unaffected. It is proposed that for establishing this bond, the Au atoms are slightly displaced out of the surface to bind to the cyano ligands. Despite the intact herringbone reconstruction, the Au substrate is found to considerably interact with the cyano ligands affecting the conformation and adsorption geometry, as well as leading to correlation effects on the molecular orientation.
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(2014): On-surface synthesis of a two-dimensional porous coordination network. Unraveling adsorbate interactions. In: Physical Review B: Condensed Matter and Materials Physics 90. DOI: 10.1103/PhysRevB.90.125408
DOI: https://doi.org/10.1103/PhysRevB.90.125408 Abstract: We present a detailed experimental and theoretical characterization of the adsorption of the perylene derivative 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) on Cu(111) and compare it to its threefold dehydrogenated derivative 3deh-DPDI, which forms in a surface reaction upon annealing. While DPDI itself does not give rise to long-range ordered structures due to lack of appropriate functional groups, 3deh-DPDI acts as an exoligand in a Cu-coordinated honeycomb network on Cu(111). The main focus of this work lies on the analysis of intermolecular and molecule-substrate interactions by combining results from scanning tunneling microscopy, x-ray photoelectron spectroscopy, x-ray standing wave measurements, and density functional theory. We show, in particular, that the interactions between metal atoms and organic ligands effectively weaken the molecule-surface interactions for 3deh-DPDI leading to an increase in molecule-substrate distances compared to the DPDI precursor. Our experimental findings also shed light on the applicability of current theories, namely van der Waals corrections to density functional theory.
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(2014): Supramolecular self-assembly of metal- free naphthalocyanine on Au(111). In: Physical Chemistry Chemical Physics 16, S. 8881-8885. Online verfügbar unter https://doi.org/10.1039/c4cp00634h, zuletzt geprüft am 16.12.2022
Abstract: The self-assembly of metal-free naphthalocyanine (H(2)Nc) on the Au(111) surface is studied under ultrahigh vacuum conditions at room temperature using a combination of scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The STM measurements reveal that the molecules form a well-ordered, defect-free structure with a square-like unit cell at monolayer coverage with their molecular plane parallel to the substrate plane. The molecular lattice direction is aligned along one of the principal directions of the underlying Au(111) substrate while the molecular orientation remains unchanged for different domains. XPS measurements show that there is no significant difference in the electronic structure of H2Nc between monolayer and multilayer coverage. Combining the information obtained from STM, LEED and XPS measurements demonstrates that the self-assembled structure of H2Nc on Au(111) is mainly stabilized by intermolecular interactions while the molecule-substrate interactions are responsible for the rotational alignment of the molecules with respect to the principal Au directions.
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(2014): Self-assembly of pyrene derivatives on Au(111). Substituent effects on intermolecular interactions. In: Chemical Communications 50, S. 14089-14092. Online verfügbar unter https://doi.org/10.1039/c4cc02753a, zuletzt geprüft am 02.12.2022
Abstract: The adsorption behaviour as well as the influence of bromine substituents on the formation of highly-ordered two-dimensional structures of pyrene derivatives on Au(111) are studied by a combination of scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations.
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(2014): Thermolubricity of gas monolayers on graphene. In: Nanoscale 6, S. 8062-8067. Online verfügbar unter https://doi.org/10.1039/c4nr01079e, zuletzt geprüft am 16.12.2022
Abstract: Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates.
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(2014): Covalent assembly of a two-dimensional molecular ‘‘sponge’’ on a Cu(111) surface. Confined electronic surface states in open and closed pores. In: Chemical Communications 50, S. 7628-7631. Online verfügbar unter https://doi.org/10.1039/c4cc02463j, zuletzt geprüft am 03.12.2022
Abstract: We present a new class of on-surface covalent reactions, formed between diborylene-3,4,9,10-tetraaminoperylene and trimesic acid on Cu(111), which gives rise to a porous 2D-‘sponge’. This aperiodic network allowed the investigation of the dependence of electron confinement effects upon pore size, shape and even in partial confinement.
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(2014): Controlling the Dimensionality of On-Surface Coordination Polymers via Endo- or Exoligation. In: Journal of the American Chemical Society 136, S. 9355-9363. DOI: 10.1021/ja5020103
DOI: https://doi.org/10.1021/ja5020103 Abstract: The formation of on-surface coordination polymers is controlled by the interplay of chemical reactivity and structure of the building blocks, as well as by the orientating role of the substrate registry. Beyond the predetermined patterns of structural assembly, the chemical reactivity of the reactants involved may provide alternative pathways in their aggregation. Organic molecules, which are transformed in a surface reaction, may be subsequently trapped via coordination of homo- or heterometal adatoms, which may also play a role in the molecular transformation. The amino-functionalized perylene derivative, 4,9-diaminoperylene quinone-3,10-diimine (DPDI), undergoes specific levels of dehydrogenation (-1 H-2 or -3 H-2) depending on the nature of the present adatoms (Fe, Co, Ni or Cu). In this way, the molecule is converted to an endo- or an exoligand, possessing a concave or convex arrangement of ligating atoms, which is decisive for the formation of either ID or 2D coordination polymers.
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(2014) : From (supra)molecular assemblies to covalently coupled nanostructures on surfaces. Insight from scanning tunneling microscopy and photoemission experiments In: American Chemical Socienty: Abstracts of Papers: 247th National Meeting: Dallas, 16. - 20. März
Abstract: Supramolecular recognition is a useful tool to realize highly organized and periodic structures at the nanometer scale. The understanding of the interplay of the underlying intermolecular and molecule substrate interactions is highly important since the resulting molecular structures are determined by them. Since supramolecular recognition is based on non-covalent interactions the stability of the resulting structures is quite often weak. Thus, for the formation of molecular structures with improved stability and conductivity, the concept of on-surface polymerization [1] has been introduced recently.In my presentation, I will discuss different examples for the formation of supramolecular as well as polymeric structures on surfaces from specially designed molecular building blocks. For the supramolecular structures the self-assembly of an achiral triphenylamine derivative featuring cyano functionalities [2] as well as a binary H-bonded system will be discussed. For the two systems, the substrate influence both on the development of the intended intermolecular interactions and on the formation of periodic structures was studied. To obtain polymeric structures on surfaces, a biphenyl derivative featuring protecting groups was employed [3]. Control over the reactivity and the size of the polymeric structures was in the focus of these investigations.[1] G. Franc, A. Gourdon, Phys. Chem. Chem. Phys. 13 (2011) 14283[2] S. Gottardi et al., Ad. Mater. Interf. accepted[3] S. Boz et al., Angew. Chem. Int. Ed. 48 (2009) 3179
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(2013): Microscopic characterisation of suspended graphene grown by chemical vapour deposition. In: Nanoscale 5, S. 9057-9061. Online verfügbar unter https://doi.org/10.1039/c3nr02386a, zuletzt geprüft am 12.02.2023
Abstract: We present a multi-technique characterisation of graphene grown by chemical vapour deposition (CVD) and thereafter transferred to and suspended on a grid for transmission electron microscopy (TEM). The properties of the electronic band structure are investigated by angle-resolved photoelectron spectromicroscopy, while the structural and crystalline properties are studied by TEM and Raman spectroscopy. We demonstrate that the suspended graphene membrane locally shows electronic properties comparable with those of samples prepared by micromechanical cleaving of graphite. Measurements show that the area of high quality suspended graphene is limited by the folding of the graphene during the transfer.
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(2013): Chirality Transfer in 1D Self-Assemblies. Influence of H-Bonding vs Metal Coordination between Dicyano[7]helicene Enantiomers. In: Journal of the American Chemical Society 135, S. 15270-15273. DOI: 10.1021/ja407315f
DOI: https://doi.org/10.1021/ja407315f Abstract: Chiral recognition as well as chirality transfer in supramolecular self-assembly and on-surface coordination is studied for the enantiopure 6,13-dicyano[7]helicene building block. It is remarkable that, with this helical molecule, both H-bonded chains and metal-coordinated chains can be formed on the same substrate, thereby allowing for a direct comparison of the chain bonding motifs and their effects on the self-assembly in experiment and theory. Conformational flexure and both adsorbate/adsorbent and intermolecular interactions can be identified as factors influencing the chiral recognition at the binding site. The observed H-bonded chains are chiral, however, the overall appearance of Cu-coordinated chains is no longer chiral. The study was performed via scanning tunneling microscopy, X-ray-photoelectron spectroscopy and density functional theory calculations. We show a significant influence of the molecular flexibility and the type of bonding motif on the chirality transfer in the ID self-assembly. Chiral recognition as well as chirality transfer in supramolecular self-assembly and on-surface coordination is studied for the enantiopure 6,13-dicyano[7]helicene building block. It is remarkable that, with this helical molecule, both H-bonded chains and metal-coordinated chains can be formed on the same substrate, thereby allowing for a direct comparison of the chain bonding motifs and their effects on the self-assembly in experiment and theory. Conformational flexure and both adsorbate/adsorbent and intermolecular interactions can be identified as factors influencing the chiral recognition at the binding site. The observed H-bonded chains are chiral, however, the overall appearance of Cu-coordinated chains is no longer chiral. The study was performed via scanning tunneling microscopy, X-ray-photoelectron spectroscopy and density functional theory calculations. We show a significant influence of the molecular flexibility and the type of bonding motif on the chirality transfer in the ID self-assembly.
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(2012): Controlling the Dimensionality and Structure of Supramolecular Porphyrin Assemblies by their Functional Substituents. Dimers, Chains, and Close-Packed 2D Assemblies. In: Chemistry - A European Journal 18, S. 14610-14613. Online verfügbar unter https://doi.org/10.1002/chem.201201037, zuletzt geprüft am 11.02.2023
Abstract: Repulsive interactions: A staging of supramolecular aggregation from (0D) clusters to (1D) chains and (2D) assemblies as a function of molecular coverage of dipolar porphyrins adsorbed on the Ag(111) surface is described. It displays a complex interplay of both attractive and repulsive molecule–molecule interactions, the emergence of chirality, and the registry of the substrate (see figure).
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(2012) : Scanning tunneling microscopy (STM) In: Gale, Philip A.; Steed, Jonathan W. (Hg.): Supramolecular Chemistry: From Molecules to Nanomaterials: Oxford: John Wiley & Sons, S. 647-658
Abstract: The scanning tunneling microscope (STM) offers the unique possibility to gain information on structural and electronic properties of a conducting surface down to the atomic scale. It was only a question of time until this potential was discovered for the investigation of molecules adsorbed on a substrate surface. Moreover, it was realized that invaluable information on surface-supported supramolecular architectures can be obtained which complements the knowledge on supramolecular systems in solution as well as in the solid state in a beneficial way. In addition, supramolecular engineering on solid substrates is expected to contribute to the development of functional nanomaterials, which boosted the research activities in this field substantially. In the beginning, the main focus of STM investigations was on the mere visualization of the adsorbed molecules, whereas now it addresses a variety of questions that comprises research on intermolecular and molecule–substrate interactions including electronic properties, the determination of the adsorption site, surface dynamics, and reactions. Especially, spectroscopy and manipulation techniques were important advancements for the development of the field.
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(2011): Visualizing the Product of a Formal Cycloaddition of 7,7,8,8-Tetracyano-p-quinodimethane (TCNQ) to an Acetylene-Appended Porphyrin by Scanning Tunneling Microscopy on Au(111). In: Chemistry - A European Journal 17, S. 5246-5250. Online verfügbar unter https://doi.org/10.1002/chem.201100733
Abstract: The applicability of a formal [2+2] cycloaddition between electron-rich alkynes and electron-deficient TCNQ on an atomically clean Au(111) surface was demonstrated by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). At low coverage, monomeric and self-assembled dimeric species of the initial compounds as well as of the reaction product, a TCNQ-conjugated porphyrin, could be visualized.
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(2011): Self-Assembly and Two-Dimensional Spontaneous Resolution of Cyano-Functionalized [7]Helicenes on Cu(111). In: Angewandte Chemie International Edition 50, S. 9982-9986. Online verfügbar unter https://doi.org/10.1002/anie.201102627, zuletzt geprüft am 25.11.2022
Abstract: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CN⋅⋅⋅HC(Ar) hydrogen bonding and CN⋅⋅⋅CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules.
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(2010): STM fingerprint of molecule–adatom interactions in a self-assembled metal–organic surface coordination network on Cu(111). In: Physical Chemistry Chemical Physics 12, S. 8815-8821. Online verfügbar unter https://doi.org/10.1039/c003660a, zuletzt geprüft am 12.02.2023
Abstract: A novel approach of identifying metal atoms within a metal–organic surface coordination network using scanning tunnelling microscopy (STM) is presented. The Cu adatoms coordinated in the porous surface network of 1,3,8,10-tetraazaperopyrene (TAPP) molecules on a Cu(111) surface give rise to a characteristic electronic resonance in STM experiments. Using density functional theory calculations, we provide strong evidence that this resonance is a fingerprint of the interaction between the molecules and the Cu adatoms. We also show that the bonding of the Cu adatoms to the organic exodentate ligands is characterised by both the mixing of the nitrogen lone-pair orbitals of TAPP with states on the Cu adatoms and the partial filling of the lowest unoccupied molecular orbital (LUMO) of the TAPP molecule. Furthermore, the key interactions determining the surface unit cell of the network are discussed.
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(2010): Aggregation and Contingent Metal/Surface Reactivity of 1,3,8,10-Tetraazaperopyrene (TAPP) on Cu(111). In: Chemistry - A European Journal 16, S. 2079-2091. Online verfügbar unter https://doi.org/10.1002/chem.200902596, zuletzt geprüft am 10.02.2023
Abstract: The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly Similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPP-Cu) chains.
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(2009): Protecting-Group-Controlled Surface Chemistry - Organization and Heat-Induced Coupling of 4,4 '-Di(tert-butoxycarbonylamino)biphenyl on Metal Surfaces. In: Angewandte Chemie International Edition 48, S. 3179-3183. Online verfügbar unter https://doi.org/10.1002/anie.200804845, zuletzt geprüft am 12.02.2023
Abstract: Like pearls on a string, molecular building blocks have been preorganized and then interlinked on a surface (see STM images). In this way both the supramolecular self-assembly of the reactants as well as the subsequent thermal activation to release the protecting group are controlled.
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(2009): Modification of Supramolecular Binding Motifs Induced By Substrate Registry. Formation of Self-Assembled Macrocycles and Chain-Like Patterns. In: Chemistry - A European Journal 15, S. 11139-11150. Online verfügbar unter https://doi.org/10.1002/chem.200901502, zuletzt geprüft am 12.02.2023
Abstract: The self-assembly properties of two Zn(II) porphyrin isomers on Cu(111) are studied at different coverage by means of scanning tunneling microscopy (STM). Both isomers are substituted in their meso-positions by two voluminous 3,5-di(tert-butyl)phenyl and two rod-like 4'-cyanobiphenyl groups, respectively. In the trans-isomer, the two 4'-cyanobiphenyl groups are opposite to each other, whereas they are located at right angle in the cis-isomer. For coverage up to one monolayer, the cis-substituted porphyrins self-assemble to form oligomeric macrocycles held together by antiparallel CN center dot center dot center dot CN dipolar interactions and CN center dot center dot center dot H-C(sp(2)) hydrogen bonding. Cyclic trimers and tetramers occur most frequently but everything from cyclic dimers to hexamers can be observed. Upon annealing of the samples at temperatures >150 degrees C, dimeric macrocyclic structures are observed, in which the two porphyrins are bridged by Cu atoms, originating from the surface, under formation of two CN center dot center dot center dot Cu center dot center dot center dot NC coordination bonds. The trans-isomer builds up linear chains on Cu(111) at low coverage, whereas for higher coverage the molecules assemble in a periodic, densely packed structure. Both cis- and trtins-bis(4'-cyanobiphenyl)-substituted Zn(II) porphyrins behave very differently on Cu(111) compared to similar porphyrins in literature on less reactive surfaces such as Au(111) and Ag(111). On the latter surfaces, there is no signal visible between molecular orientation and the crystal directions of the substrate, whereas on Cu(I 11), very strong adsorbate-substrate interactions have a dominating influence on all observed structures. This strong porphyrin-substrate interaction enables a much broader variety of structures, including also less favorable intermolecular bonding motifs and geometries.
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(2009): Band Formation from Coupled Quantum Dots Formed by a Nanoporous Network on a Copper Surface. In: Science 325, S. 300-303. Online verfügbar unter https://doi.org/10.1126/science.1175141, zuletzt geprüft am 12.02.2023
Abstract: The properties of crystalline solids can to a large extent be derived from the scale and dimensionality of periodic arrays of coupled quantum systems such as atoms and molecules. Periodic quantum confinement in two dimensions has been elusive on surfaces, mainly because of the challenge to produce regular nanopatterned structures that can trap electronic states. We report that the two-dimensional free electron gas of the Cu(111) surface state can be trapped within the pores of an organic nanoporous network, which can be regarded as a regular array of quantum dots. Moreover, a shallow dispersive electronic band structure is formed, which is indicative of electronic coupling between neighboring pore states.
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(2009): Conformation-controlled networking of H-bonded assemblies on surfaces. In: Chemical Communications 45, S. 3525-3527. Online verfügbar unter https://doi.org/10.1039/B902120E, zuletzt geprüft am 11.02.2023
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(2009): Self-Assembly of Individually Addressable Complexes of C-60 and Phthalocyanines on a Metal Surface. Structural and Electronic Investigations. In: The Journal of Physical Chemistry C 113, S. 19373-19375. DOI: 10.1021/jp908086p
DOI: https://doi.org/10.1021/jp908086p Abstract: The hosting properties of a close-packed layer of phenoxy-substituted phthalocyanine derivatives adsorbed on Ag(III) were investigated for the adsorption of C-60 molecules. The C-60 molecules bind to two clearly distinguishable sites, namely, to the underlying metal substrate in between two adjacent phthalocyanine derivatives, leading to structural and electronic properties analogous to those of C-60 adsorbed on Ag(III), and to the core of the underlying phthalocyanine derivative, indicative of the formation of 1:1 donor-acceptor (D-A) dyads through a true host-guest interaction. The electronic properties have been determined by scanning tunneling spectroscopy measurements on individual D-A dyads.
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(2008): Trimodular engineering of linear supramolecular miniatures on Ag(111) surfaces controlled by complementary triple hydrogen bonds. In: Angewandte Chemie International Edition 47, S. 7726-7730. Online verfügbar unter https://doi.org/10.1002/anie.200802325, zuletzt geprüft am 12.02.2023
Abstract: Simultaneous three-component assembly on surfaces mediated by triple H-bonding interactions leading to the formation of linear supramolecular miniatures has been studied on Ag(111) surfaces by STM. In particular, the complementary assembly of two linear modules (see picture, blue and green) and an anthracenyl-capped molecular stopper (red) led to the formation of discrete linear oligomeric, pentameric, and trimeric nanoassemblies.
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(2008): Transforming Surface Coordination Polymers into Covalent Surface Polymers. Linked Polycondensed Aromatics through Oligomerization of N-Heterocyclic Carbene Intermediates. In: Angewandte Chemie International Edition 47, S. 2414-2417. Online verfügbar unter https://doi.org/10.1002/anie.200704072, zuletzt geprüft am 11.02.2023
Abstract: Isomerize and polymerize! Thermally induced tautomerization of the N-heteropolycyclic 1,3,8,10-tetraazaperopyrene to a Wanzlick-type carbene intermediate on a Cu(111) surface leads to covalently linked polyaromatic chains, which can be mechanically manipulated.
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(2008): Two-dimensional multiphase behavior induced by sterically hindered conformational optimization of phenoxy-substituted phthalocyanines. In: The Journal of Physical Chemistry C 112, S. 6139-6144. DOI: 10.1021/jp710887g
DOI: https://doi.org/10.1021/jp710887g Abstract: Symmetrically substituted phthalocyanines (Pcs) with eight peripheral di-(tert-butyl)phenoxy (DTPO) groups self-organize on Ag(111) and Au(111) substrates into various assembly structures. These different structural phases were studied by scanning tunneling microscopy (STM). On the basis of high-resolution STM images, molecular models are provided for each phase that account for the observed unequal surface densities. Notably, the specificity of the studied Pc derivative featuring the peripheral phenoxy groups remarkably increases its conformational possibilities. Particularly, the rotational degrees of freedom allow all the DTPO substituents to be arranged above the plane of the Pc core, forming a bowl-like structure, which in turn enables the interaction of the Pc core with the metal substrate. The proximity of the Pc core to the metal substrate together with the steric entanglement between neighboring DTPO substituents causes significant retardation of the thermodynamic optimization of the conformations.
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(2008): Supramolecular synthons on surfaces. Controlling Dimensionality and Periodicity of Tetraarylporphyrin Assemblies by the Interplay of Cyano and Alkoxy Substituents. In: Chemistry - A European Journal 14, S. 5794-5802. Online verfügbar unter https://doi.org/10.1002/chem.200800746, zuletzt geprüft am 19.11.2022
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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(2007): Supramolecular nanostructuring of silver surfaces via self-assembly of [60]fullerene and porphyrin modules. In: Advanced Functional Materials 17, S. 1051-1062. Online verfügbar unter https://doi.org/10.1002/adfm.200600586, zuletzt geprüft am 12.02.2023
Abstract: Recent achievements in our laboratory toward the "bottom-up" fabrication of addressable multicomponent molecular entities obtained by self-assembly of C-60 and porphyrins on Ag(100) and Ag(111) surfaces are described.. Scanning tunneling microscopy (STM) studies on ad-layers constituting monomeric and triply linked porphyrin modules showed that the molecule:, self-organize into ordered supramolecular assemblies, the ordering of which is controlled by the porphyrin chemical structure, the metal substrate, and the surface coverage. Specifically, the successful preparation of unprecedented two-dimensional porphyrin-based assemblies featuring regular pores on Ag(111) surfaces has been achieved. Subsequent co-deposition of C-60 molecules on top of the porphyrin monolayers results in selective self-organization into ordered molecular hybrid bilayers, the organization of which is driven by both fullerene coverage and porphyrin structure. In all-ordered fullerene-porphyrin assemblies, the C-60 guests organize, unusually, into long chains and/or two-dimensional arrays. Furthermore, sublimation of C-60 on top of the porous porphyrin network reveals the selective long-range inclusion of the fullerene guests within the hosting cavities. The observed mode of the C-60 self-assembly originates from a delicate equilibrium between substrate-molecule and molecule-molecule interactions involving charge-transfer processes and conformational reorganizations as a consequence of the structural adaptation, of the fullerene-porphyrin bilayer.
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(2007): Self-assembly, DNA complexation, and pH response of amphiphilic dendrimers for gene transfection. In: Langmuir 23, S. 737-746. DOI: 10.1021/la0624891
DOI: https://doi.org/10.1021/la0624891 Abstract: Cationic lipids and polymers are routinely used for cell transfection, and a variety of structure-activity relation data have been collected. Few studies, however, focus on the structural aspects of self-assembly as a crucial control parameter for gene delivery. We present here the observations collected for a set of cationic dendritic amphiphiles based on a stiff tolane core (1-4) that are built from identical subunits but differ in the number and balance of their hydrophobic and cationic hydrophilic moieties. We established elsewhere that vectors 3 and 4 have promising transfection properties. Scanning probe microscopy (AFM, STM), cryo-transmission electron microscopy (cryo-TEM), and Langmuir techniques provide insight into the self-assembly properties of the molecules under physiological conditions. Furthermore, we present DNA and pH "jump" experiments where we study the response of Langmuir films to a sudden increase in DNA concentration or a drop in pH. We find that the primary self-assembly of the amphiphile is of paramount importance and influences DNA binding, serum sensitivity, and pH response of the vector system.
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(2007): Lateral manipulation for the positioning of molecular guests within the confinements of a highly stable self-assembled organic surface network. In: Small 3, S. 1336-1340. DOI: 10.1002/smll.200700099
DOI: https://doi.org/10.1002/smll.200700099 Abstract: C60 complexes with zinc octaethylporphyrin (ZnOEP) were deposited in the hexagonal pores of the highly stable honeycomb network generated by thermal dehydrogenation of 4,9-diaminoperylene-quinone-3,10-diimine on a Cu(111) surface (see picture). The piece-by-piece assembly of the weakly aggregated ZnOEP–C60 complex, which may be viewed as a “supramolecular bearing”, was achieved with the aid of an STM tip.
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(2007): Rotation-libration in a hierarchic supramolecular rotor-stator system. Arrhenius activation and retardation by local interaction. In: Chemical Communications 43, S. 1349-1351. Online verfügbar unter https://doi.org/10.1039/b700909g, zuletzt geprüft am 20.11.2022
Abstract: Fourfold symmetric zinc-octaethylporphyrin (OEP) has been incorporated in the holes of the hexagonal molecular network generated by thermal dehydrogenation of 4,9-diaminoperylenequinone3,10- diimine (DPDI) on a Cu(111) surface and displayed hindered rotation; the reorganization between the potential minima, a rotation-libration, which is characterized by an activation energy of E-D=0.17 +/- 0.03 eV, has been monitored in the STM tunnelling currents as a bi-state ''switching''.
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(2007): A supramolecular multiposition rotary device. In: Angewandte Chemie International Edition 46, S. 4089-4092. Online verfügbar unter https://doi.org/10.1002/anie.200700285, zuletzt geprüft am 19.11.2022
Abstract: A supramolecular rotary device, reminiscent of a mechanical rotary switch, was engineered by a bottom-up approach. Self-assembly of a functionalized porphyrin molecule leads to the formation of a porous network that features chiral cavities. These serve as hosts for molecular guests, which can be induced to rotate either thermally or by using the scanning tunneling microscopy tip.
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(2006): Hindered rotation of a copper phthalocyanine molecule on C60. Experiments and molecular mechanics calculations. In: Physical Review B: Condensed Matter and Materials Physics 73. DOI: 10.1103/PhysRevB.73.115433
Abstract: If copper phthalocyanine (CuPc) molecules are deposited on a Au(111) surface covered with a monolayer of C60, the molecules are found to adsorb individually onto the close-packed layer of C60. As the adsorption site of the CuPc is not symmetric with respect to the underlying C60 layer, the CuPc molecule has six equivalent orientations according to the hexagonal packing of the C60. Scanning tunneling microscopy (STM) measurements reveal that at room temperature, the molecules may hop between these adsorption sites due to thermal activation and therefore appear as a ring with six maxima. This paper reviews the STM measurements already carried out in the light of molecular force field calculations. The potential energy for an individual CuPc molecule has been calculated as a function of the lateral position on the C60 layer based on the van der Waals interactions between molecules. For each position, the minimal energy has been evaluated, taking the vertical and the rotational degrees of freedom into account. A map of the potential energy not only reveals the preferred adsorption sites, it also indicates several favorable paths between these minima. The calculations consistently explain the experimental observations at low and ambient temperature.
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(2006): Adsorption and dynamics of long-range interacting fullerenes in a flexible, two-dimensional, nanoporous porphyrin network. In: ChemPhysChem 7, S. 1462-1470. Online verfügbar unter https://doi.org/10.1002/cphc.200600186, zuletzt geprüft am 11.02.2023
Abstract: Herein, a detailed investigation of the adsorption and dynamics of C-60 and C-70 fullerenes hosted in a self-assembled, two-dimensional, nanoporous porphyrin network on a solid Ag surface is presented. Time-resolved scanning tunneling microscopy (STM) studies of these supramolecular systems at the molecular scale reveal distinct host-guest interactions giving rise to a pronounced dissimilar mobility of the two fullerenes within the porphyrin network. Furthermore, long range coverage-dependant interactions between the all-carbon guests, which clearly affect their mobility and are likely mediated by a complex mechanism involving the Ag substrate and the flexible porphyrin host network, are observed. At increased fullerene coverage, this unprecedented interplay results in the formation of large fullerene chains and islands. By applying a lattice gas model with nearest-neighbour interactions and by evaluating the fullerence-pair distribution functions, the respective coverage-dependant guest-guest interaction energies are estimated.
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(2006): A two-dimensional porphyrin-based porous network featuring communicating cavities for the templated complexation of fullerenes. In: Advanced Materials 18, S. 275-279. Online verfügbar unter https://doi.org/10.1002/adma.200501734, zuletzt geprüft am 25.11.2022
Abstract: An unprecedented two-dimensional porphyrin network featuring dynamic pores capable of hosting fullerenes is realized following a bottom-up approach at a single-crystal silver surface. Surface- and porphyrin-driven long-range interactions between the C-60 guest molecules and porphyrin layer result in the formation of exceptionally large supramolecular hybrid chains and islands (see Figure and Cover).
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(2005): Immobilization of Rhodium Complexes at Thiolate Monolayers on Gold Surfaces. Catalytic and Structural Studies. In: Journal of the American Chemical Society 127, S. 8720-8731. DOI: 10.1021/ja0500714
DOI: https://doi.org/10.1021/ja0500714 Abstract: Chiral rhodium-diphosphine complexes have been incorporated into self-assembled thiolate monolayers (SAMS) on gold colloids. Catalysts of this type are of interest because they combine properties of homogeneous and heterogeneous systems. In addition, it should be possible to influence the catalytic properties of the metal center by the neighboring thiolate molecules. Colloids with a diameter of ca. 3 nm, coated with a mixed monolayer of n-octanethiolates and thiolates with chiral rhodium-PYRPHOS end groups, were studied as hydrogenation catalysts. With methyl a-acetamido-cinnamate as substrate, virtually the same enantioselectivities (up to 93% ee) and full conversion were obtained as with the corresponding homogeneous [Rh(COD)(PYRPHOS)]BArF catalyst. The colloids were easily recovered by filtration and reused as catalysts three times without loss of enantioselectivity. STM studies of analogous SAMS on Au(111) gave a detailed picture of the structure and dynamics of mixed monolayers of this type. The STM images showed that the catalyst-bearing thiolates are distributed statistically on the surface and that the ordered structure of the n-octanethiolate SAM can be retained during incorporation of the catalyst-bearing thiols using the place-exchange methodology.
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(2005): Controlling molecular assembly in two dimensions. The concentration dependence of thermally induced 2D aggregation of molecules on a metal surface. In: Angewandte Chemie International Edition 44, S. 7394-7398. Online verfügbar unter https://doi.org/10.1002/anie.200502316, zuletzt geprüft am 25.11.2022
Abstract: One building block yields three forms of aggregation on variation of the surface concentration of the molecular precursor (see picture; ML=monolayer coverage). Upon annealing at 300 °C, highly robust 2D aggregates are obtained on metal surfaces which lend themselves to the construction of hierarchic systems of higher complexity.
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(2004): Growth of 3,4,9,10-perylenetetracarboxylic-dianhydride crystallites on noble metal surfaces. In: Organic Electronics 5, S. 35-43. Online verfügbar unter https://doi.org/10.1016/j.orgel.2003.12.001, zuletzt geprüft am 20.11.2022
Abstract: The growth of organic crystallites formed by 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) has been investigated by scanning tunneling microscopy on Cu(110), Cu(111) and Au(111). For the chosen parameters PTCDA exhibits Stranski-Krastanov growth. If the average coverage exceeds about 3-5 monolayers the formation of crystalline islands begins. Working at very low tunneling currents it has been possible to obtain molecular resolved images of crystallites formed by up to 100 molecular layers, while the overall coverage has a value of about 10-20 monolayers. Since the side walls typically exhibit a moderate slope in the range of 14-20degrees their structure may be as well analyzed as the flat top layer. As will be presented it is not only possible to determine the unit cell within the molecular layer but also their relative displacement between subsequent layers. A comparison between the 3D structures of several crystals will be given.
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(2002): Growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110) studied by STM. In: Applied Physics A 74, S. 303-305. Online verfügbar unter https://doi.org/10.1007/s003390101039, zuletzt geprüft am 11.02.2023
Abstract: The structure of thin 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films on Cu(110) was studied by scanning tunnelling microscopy (STM) from submonolayer to monolayer coverage. While no long-range ordering was found after deposition at room temperature, the formation of a well-defined superstructure is observed after thermal annealing. It appears that the formation of the superstructure is driven by the interaction between the oxygen atoms of the PTCDA and the copper atoms of the substrate. While the distance between the molecules fits well to the atomic lattice of the Cu(110) surface along the [110] direction, the mismatch along the [001] direction leads to a periodic buckling normal to the surface accompanied by a restructuring of the substrate.
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(2002): STM investigation on single, physisorbed dendrimers. In: Single Molecules 3, S. 295-299. Online verfügbar unter https://doi.org/10.1002/1438-5171(200211)3:5/6<295::AID-SIMO295>3.0.CO;2-T, zuletzt geprüft am 16.02.2023
Abstract: Porphyrin dendrimers were synthesized to mimic naturally occurring proteins, which catalyze a number of biochemically important reactions. In addition, chiral dendrimers were prepared as model compounds for the study of nanoscopic chirality. The structures of these dendrimers cannot be characterized by x-ray, as most dendrimers do not crystallize. We succeeded to image single, physisorbed dendrimers on noble metal surfaces with STM. All examined dendrimers can easily be (re)moved with the STM tip, even at low scanning currents (low pA range). One possibility to avoid this, is to change the peripheral groups of the dendrimer, so that they chemisorb on the surface. We decided to study physisorbed macromolecules with unchanged molecular properties, and investigated how an other approach for chemisorbed polyamidoamin (PAMAM) dendrimers, could be adapted to our molecules. Indeed, even physisorbed molecules are stabilized by embedding them in a self-assembled monolayer of covalently bonded 1-hexadecanethiol. The so embedded dendrimers show a reduced mobility on the substrate with a conformation closer to the one in solution, permitting STM imaging at various temperatures.
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(2002): Direct observation of hindered eccentric rotation of an individual molecule. Cu-phthalocyanine on C60. In: Physical Review B: Condensed Matter and Materials Physics 65. DOI: 10.1103/PhysRevB.65.033404
DOI: https://doi.org/10.1103/PhysRevB.65.033404 Abstract: Individual Cu-phthalocyanine molecules have been investigated by scanning tunnel microscopy on a closed packed film of C-60 at various temperatures. The molecules are found to bind asymmetrically to one C-60. While they remain in one position at low temperature, they can hop between six equivalent positions at higher temperatures performing a hindered eccentric rotation around the binding C-60 molecule. At room temperature only the time average over different positions is observed.
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(2002): Investigation of the growth of PTCDA on Cu(110). An STM study. In: Surface Science, S. 330-334. Online verfügbar unter https://doi.org/10.1016/S0039-6028(02)01266-9, zuletzt geprüft am 25.11.2022
Abstract: The growth of the organic semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Cu(1 1 0) was studied by STM. The PTCDA coverage was varied from submonolayer to multilayer. For submonolayer and monolayer coverage the PTCDA molecules induce a restructuring of the underlying Cu(1 1 0) Surface by addition or removal of Cu-rows in the [1 1 0]-direction. The rectangular PTCDA superstructure indicates a strong interaction between the substrate and PTCDA. Investigation of higher coverage of PTCDA shows that it g-rows in the Stranski-Krastanov-mode. Well ordered mesatype three-dimensional-crystallites are formed. Both the top layer and the flanks were molecularly resolved allowing to analyze the layer by layer growth and the crystal structure.
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(2002): Analysis of the three-dimensional structure of a small crystallite by scanning tunneling microscopy. Multilayer films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110). In: Europhysics Letters 59, S. 423-429. Online verfügbar unter https://doi.org/10.1209/epl/i2002-00212-8, zuletzt geprüft am 25.11.2022
Abstract: The growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110) has been investigated by STM in the range of a few up to more than 20 molecular layers. It is found to grow in the Stranski-Krastanov mode leading to well-ordered mesa-type crystalline grains of different structure and orientation relative to the substrate. By operating a scanning tunneling microscope at a low tunneling current of 2 pA, we were able to resolve the individual molecules on the at top of the crystallites as well as on the facets descending to the substrate. As will be shown, this offers the unique possibility to determine unambiguously the three-dimensional structure. For a given grain a detailed analysis will be presented yielding the unit cell and the relative displacement between subsequent layers.
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(2002): Hourglass-shaped dendrimers on surfaces. A comparison of different scanning-tunneling-microscopy approaches. In: Helvetica Chimica Acta 85, S. 4255-4263. Online verfügbar unter https://doi.org/10.1002/hlca.200290010, zuletzt geprüft am 20.11.2022
Abstract: Large molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) under various environmental conditions: on a dry surface in air or vacuum, and at the solid-liquid interface. However, can measurements under dissimilar conditions be compared, e.g., when sample A was studied at the solid-liquid interface and sample B in a dry environment? Only rarely can the same substance be examined with more than one approach. since completely different set-up and preparations are necessary. Furthermore, few substances are suitable for several methods of sample preparation and characterization. We have chosen a large, flexible. nonplanar molecule. namely an alkoxy-substituted second-generation dendritic compound with a chiral core unit, which is peculiar for its 'hourglass' conformation, The assembly properties have been explored by STM both in solution-cast self-organized monolayers (SOMs) and multilayer films, as well as at the solid-liquid interface, The complexity and limits of the three approaches applied to our hourglass-shaped dendritic compound are discussed. Depending on the approach and environmental conditions, several quality levels of image resolution could be achieved measurements carried out at low temperatures led to highest resolution on the aromatic parts of the molecule. A comparison of equally sized images obtained under these varying conditions reveals not only different packing arrangements. but also spots of unlike shape. Therefore, when the approach, preparation, and/or environmental conditions are not the same, STM measurements of different compounds have to he compared with greatest care. Large molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) under various environmental conditions: on a dry surface in air or vacuum, and at the solid-liquid interface. However, can measurements under dissimilar conditions be compared, e.g., when sample A was studied at the solid-liquid interface and sample B in a dry environment? Only rarely can the same substance be examined with more than one approach. since completely different set-up and preparations are necessary. Furthermore, few substances are suitable for several methods of sample preparation and characterization. We have chosen a large, flexible. nonplanar molecule. namely an alkoxy-substituted second-generation dendritic compound with a chiral core unit, which is peculiar for its 'hourglass' conformation, The assembly properties have been explored by STM both in solution-cast self-organized monolayers (SOMs) and multilayer films, as well as at the solid-liquid interface, The complexity and limits of the three approaches applied to our hourglass-shaped dendritic compound are discussed. Depending on the approach and environmental conditions, several quality levels of image resolution could be achieved measurements carried out at low temperatures led to highest resolution on the aromatic parts of the molecule. A comparison of equally sized images obtained under these varying conditions reveals not only different packing arrangements. but also spots of unlike shape. Therefore, when the approach, preparation, and/or environmental conditions are not the same, STM measurements of different compounds have to he compared with greatest care.
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(2001): Binary molecular layers of C-60 and copper phthalocyanine on Au(111). Self-Organized Nanostructuring. In: Advanced Functional Materials 11, S. 175-178. Online verfügbar unter https://doi.org/10.1002/1616-3028(200106)11:3<175::AID-ADFM175>3.0.CO;2-L, zuletzt geprüft am 25.11.2022
Abstract: The binary molecular system of C-60 and copper phthalocyanine(CuPc) molecules has been investigated by scanning tunneling microscopy (STM) at room temperature and at 50 K. As substrate Au(111) was chosen. When C-60 and CuPc molecules are sequentially deposited, it is found that well-ordered domains of both molecules may coexist simultaneously Hence hexagonal ordering of C-60 and quadratic ordering of CuPc is observed side by side but no ordered mixed layer of both molecules or heteroepitaxy from one molecule on the other is found. Instead the boundaries of the CuPc domains are often decorated by C-60 molecules and for a particular choice of parameters, with regard to the film preparation, individual CuPc molecules may adsorb on top of a C-60 layer. The interaction with the underlying C-60 layer permits the molecules to perform a localized, hindered rotation. At room temperature the hopping frequency is so high that only the time average of the rotation is seen by STM while at 50 K the rotation is frozen and the CuPc molecule is trapped in one definite position.