Synthesis and Properties of Molecular Nanostructures on Surfaces

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.


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