Low-dimensional supramolecular architectures at metal surfaces

This thesis is concerned with supramolecular architectures assembled at metal surfaces. The investigations pursued two objectives. On the one hand, the focus was placed on the fabrication of low-dimensional surface-supported network structures by applying the concepts of conventional three-dimensional supramolecular chemistry at surfaces. Three main driving forces were explored for the construction of low-dimensional assemblies: hydrogen bonding, metal coordination and ionic interactions. The realized structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. In particular, the interplay between the adsorbate-substrate coupling and lateral adsorbate interactions was addressed in the experiments. Further, the electronic and magnetic properties of the fabricated coordination structures were investigated. This characterization was done by x-ray absorption experiments at the synchrotron facility in Grenoble. Moreover, the response of the metal units and open cavity structures to the adsorption of large organic and small gas molecules was studied by temperature controlled STM experiments. The first part of the thesis deals with hydrogen bonded supramolecular assemblies. Simple aromatic carboxylic acids were employed to study the assembly principles and in particular effects arising from the substrate and molecular structure. The investigation of TPA (1,4-benzenedicarboxylic acid) adlayers on Cu(100) at different substrate temperatures revealed how the protonation status of the carboxyl moieties affects the topology of the assemblies. Further, the influence of the molecular backbone functionality and symmetry is addressed in comparative investigations on TPA and PDA (2,5-pyrazinedicarboxylic acid) as well as BDA (4,4'-biphenyldicarboxylic acid) and SDA (4',4" trans-ethene-l,2-diyl-bisbenzoic acid) adlayers on Cu(100). In the second part of the thesis, metal-ligand interactions were employed for the construction of supramolecular architectures at a Cu(100) surface. The concepts of conventional coordination chemistry were successfully applied at the surface. Iron atoms in conjunction with rod-like aromatic molecular linkers comprising carboxyl, pyridyl and hydroxyl functional groups assemble into a rich variety of open network structures with distinctly arranged mono- and diiron coordination centers and well-defined cavities. The results show that the adsorbate-substrate coupling considerably affects the local coordination environment of the iron units when the molecular backbone length is varied. The occurrence of structural isomerism in open network structures is addressed in a comparative study of heterofunctional and homofunctional linker molecules. It is shown that the former ligand allows only one topologically pure structure while the other linker forms two phases with different topology. The hydroxyl ligand has been used to construct distorted hexagonal coordination networks containing threefold coordinated single iron atoms. The comparison between the networks on Cu(100) and Ag(111) surfaces shows marked differences, which are attributed to adsorbate-substrate interactions. Finally, a novel design strategy for the fabrication of two-dimensional coordination structures at surfaces is introduced by using alkali metal ions in conjunction with carboxylate ligands. The interaction between the components is dominated by electrostatic forces and is of intermediate bond strength as in the case of hydrogen bonding and transition metal coordination. It is expected, that substrate symmetry and registry plays a larger role in the network formation, because the ionic interaction is not directional. The chemical as well as magnetic properties of the fabricated coordination networks are examined in the last part of the thesis. The adsorption of C60 molecules on open network structures reveals how cavity size and chemical functionality affect the bond strength to the guest molecules as well as the number of accommodated C60 molecules. Since transition metal clusters, and in particular diiron units, are the catalytic active centers in various proteins, the reactivity of various mono- and diiron coordination structures to molecular oxygen was investigated in temperature controlled STM experiments. In particular, the focus was placed on the structural relaxation upon gas adsorption and chemical reaction. At last, the magnetic properties of the metal coordination centers are addressed in XMCD experiments. The iron units are found to be paramagnetic at temperatures down to 5 K and show distinct magnetic anisotropy and magnetization loops. These experiments demonstrate that surface-supported metal coordination structures are a promising class of materials for applications in the field of heterogenous catalysis and surface magnetism.

Synthesis and Properties of Molecular Nanostructures on Surfaces

Nasiba Abdurakhmanova

One central aim of nanotechnology is the creation and exploitation of nanostructured materials with pre-programmed architecture and properties. The fabrication of functional nanostructures relies on versatile protocols of synthetic chemistry enabling the precise control of reactions at the molecular level. The surface-supported nanostructures investigated in this thesis have been synthesized using extended concepts of synthetic chemistry. To this end, mainly two chemical bonding types were employed: metal-organic coordination and covalent bonding. The resulting structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Furthermore, the electronic and magnetic properties of the coordination structures were investigated by x-ray absorption spectroscopy (XAS) at the synchrotron radiation facility in Grenoble, France. The thesis is organized into the following three topics: - Organizational chirality of metal-organic coordination networks on a metal surface. The metal coordination networks formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and Mn as well as Cs adatoms on Ag(100) are presented in chapter 3. The metal atoms form very similar chiral complexes with TCNQ. The complexes organize into densely packed and highly ordered domains whose organizational chirality depends on the metal center. This issue is addressed by theoretical modeling. It is shown that the exibility of the metal-organic coordination bond plays a key role in the supramolecular chirality. Mn forms rigid and directional bonds resulting in a heterochiral packing of the complexes. This organization avoids electrostatic repulsion between adjacent cyano groups of TCNQ. In the Cs-TCNQ4 structure this is achieved by an umbrella-like folding of four TCNQs around the central Cs atom, which yields a denser homochiral packing. This is enabled by the flexible nature of the ionic bonds. Moreover, the alkali atoms allow a mean to modify the electrostatic properties of the organic adlayer, which is important for the design of metal-organic interfaces in electronic devices. - Electronic and magnetic properties of the Me-TCNQ structures on metal surfaces. Previous investigations based on x-ray photoemission spectroscopy and density functional theory calculations for the Me-TCNQx systems on Cu(100) and Ag(100) surfaces show that two charge transfer (CT) channels are present, i.e. metal adatom-TCNQ and substrate-TCNQ. The choice of the substrate significantly influences CT, thereby controlling the electronic properties. In chapter 4 the magnetic properties of Ni-TCNQ and Mn-TCNQ structures on Ag(100) and Au(111) is presented. It is shown that the magnetic properties of Mn-TCNQ structures are essentially the same on Ag(100) and Au(111) surfaces, whereas for Ni-TCNQ the influence of the substrate is very pronounced. Both Ni-TCNQ/Au(111) and Ni-TCNQ/Ag(100) show ferromagnetic (FM) behavior, with stronger coupling observed on Au(111). This fact is especially interesting since the Ni impurities are found to be non-magnetic on both substrates. The observations are linked to the different CT channels and possible coupling mechanisms are identified. - The covalent bond formation reactions. Chapter 5 discusses the on-surface controlled synthesis of carbon-based nanostructures. The investigation of the properties of carbon-based nanostructures requires a better control over the synthesis process. To this end, one of the strategies proposed is the use of well-defined precursor molecules, which are deposited and activated on the surface to obtain the final structure. The surface catalyzed cyclodehydrogenation (SCDDH) reaction was used for the synthesis of fullerenes and nanotube caps. The selectivity of the C-C bond formation process is shown. Based on this result the isomeric pure C84 higher fullerene was synthesized. The same method was applied to the synthesis of nanotube caps. The precursor-based approach was also applied to the synthesis of N=9 and N=15 graphene nanoribbons (GNRs). Polyphenyl precursor molecules have been employed, which allow a synthetically simple mean to adjust the width of the ribbons. However, the resulting GNRs comprise many defects and the coupling mechanism is not very effective. The aspects of surface supported reactions are discussed and possible improvement strategies are proposed.

EPFL2012

Investigation of low-dimensional supramolecular architectures by low-temperature scanning tunneling microscopy

Sylvain Clair

In this thesis we report investigations on supramolecular architectures assembled at well-defined metal surfaces. Supramolecular chemistry is dedicated to the study and use of non-covalent interactions to build highly organized molecular arrangements, aiming ultimately at creating systems with tailored properties and useful functions. The adaptation of its powerful principles to fabricate molecular systems at surfaces has already been proven to be very promising; the extended possibilities for the choice of on the one hand molecules with defined size, shape, symmetry and function, and on the other hand substrates with controlled composition, symmetry and patterning allow for a quasi-infinite tuning of the structure and properties of the respective assemblies. The Scanning Tunneling Microscope (STM) is a tool particularly efficient for the nanoscale elucidation of supramolecular systems at surfaces, simultaneously providing topographic and spectroscopic information at the single-atom level. In this regard, a new Low-Temperature STM (LT-STM) operational at temperatures in the range 5 to 400 K has been constructed. A detailed description of the instrument is provided and the demonstration of its exquisite performance represents a major achievement. In addition to local STM studies, complementary experiments were performed with a synchrotron radiation source using integral techniques, namely X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD). We present results obtained with the linear organic linker 1,4-benzenedicarboxylic acid (terephthalic acid - TPA) on the Au(111), Cu(111) and Cu(100) surfaces. The three-dimensional design principles of supramolecular chemistry could be successfully adapted to these low-dimensional systems. TPA molecules adsorbed on Au(111) organize in two-dimensional molecular sheets, wherein the typical one-dimensional hydrogen-bonded chain motif is found. We show that the inhomogeneities induced by the surface reconstruction can be used to examine the response of the supramolecular self-assembly. In particular, variations in the hydrogen bond length of up to 20% are encountered. On Cu(111), a strict commensurability of the supramolecular sheet to the substrate is not possible because of the reduced lattice spacing of the latter, and the creation of defects is analyzed. Due to their peculiar reactivity, the elbow sites of the reconstructed Au(111) surface act as nucleation centers in the epitaxial growth of transition metals (Fe, Co, Ni), leading to the formation of regular arrays of nanoscale metallic islands. We take advantage of this patterning to steer the formation of metallosupramolecular nano-architectures. Co-deposited TPA molecules attack both Fe and Co clusters and metal-carboxylate linkages evolve. Depending on the deposition parameters, nanoporous grids, mesoscopically organized stripes or truly one-dimensional linkages are obtained. Furthermore, the dynamics of the formation of the nanogrids are monitored in situ. Finally, we report investigations of the magnetic and electronic properties of surface-supported coordination architectures by means of XAS and XMCD. We studied Fe-terephthalate systems engineered on a Cu(100) substrate. Both mononuclear Fe(TPA)4 compounds and diiron centers structures in 2D Fe-TPA lattices exhibit a paramagnetic behavior. Moreover, we demonstrate the decisive influence of the ligand rather than the substrate on the electronic ground state of the metal centers, thus illustrating new vistas to effectively tailor the valence state and magnetic moment of transition metal atoms at surfaces.

EPFL2004

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007