Self-organized growth of ordered functional nanostructures on templates
In this thesis, the use of nanotemplates for steering the molecular self-organization at metallic surfaces is investigated. Nanotemplates are surfaces that exhibit a periodic variation of the adhesion energy on the nanometer scale. Preferential adsorption on low energy sites is utilized to effectively transfer the template structure to the overlayer of the deposited adsorbates. Surfaces with an anisotropic lattice geometry and self-organized alternating oxide and metallic stripes are exemplarily investigated as model for one-dimensional nanotemplates of different periodicities. Their effects on the adsorption of deposited organic molecules and on the formation of extended supramolecular architectures are studied by scanning tunneling microscopy (STM) and complementary experimental and theoretical techniques. The first part of the thesis deals with the growth of metal-organic coordination networks (MOCNs), a concept which has recently been transferred from 3D supramolecular chemistry to 2D surface grids. It is shown here for the first time, that it is possible to create surface supported 1D metal-organic coordination chains (MOCCs) using the Cu(110) surface as a nanotemplate, along with the same methods which are used to create 2D MOCNs. 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA) is employed as organic linker molecule forming monodisperse and highly regular chains with Cu adatoms. The atomistic structure of the MOCCs is investigated in a combined experimental and theoretical approach, using STM, low energy electron diffraction (LEED), reflection-absorption infrared spectroscopy (RAIRS) and density functional theory (DFT). A detailed understanding of the MOCCs is reached and the possibility to generalize these results to other metal-carboxylate systems is explored by studying a different linker molecule, 1,4-benzenedicarboxylic acid (terephthalic acid, TPA). MOCCs are potentially relevant for catalytic or magnetic applications as they offer lowly coordinated but thermodynamically stable 1D arrangements of metallic centers. Functional MOCCs are created on Cu(110) by co-deposition of TMA molecules and Fe atoms. These structures are thoroughly analyzed by STM and rationalized by means of DFT calculations. A net spin-moment is predicted for the Fe centers inside the chain which is comparable to that of free Fe adatoms on the surface. The adhesion of a dipeptide, Phe-Phe, on Cu(110) is investigated in order to further analyze the templating role of this substrate for the structure formation of larger organic molecules. Phe-Phe forms homochiral chains showing that the molecular chirality is effectively transferred to the supramolecular level. The comparison with the Phe-Phechains obtained on the chemically similar but structurally different Cu(100) substrate, demonstrates that commensurability to the surface lattice is one of the most important parameters governing the supramolecular growth modes. Furthermore, the possibilities to create mixed two-dimensional surface crystals using two different molecular adsorbates is investigated. The metal-organic chain structures based on TMA are combined with the chiral Phe-Phe molecules. A single mixed-phase with a hierarchical substructure is obtained, which unites the properties of the individual structures, namely metal-organic coordination and enantiopure chirality. This result represents a further step towards transferring the supramolecular design concepts known in 3D supramolecular chemistry to surfaces. Finally, the Cu(110)-(2x1)0-striped phase is used as a tunable, self-organized template for the molecular structures reported above. MOCCs are created in the bare Cu surface areas that define the length and position of the chains. The ordering is limited, though, by interactions of the MOCCs with the Cu-O stripes which are traced back to a competition for Cu adatoms necessary for the formation of both structures. No such concurrence exists for Phe-Phe which is correspondingly found to be efficiently contained to the bare Cu ares and well ordered by the Cu-O-stripes even for the second layer of Phe-Phe. These results are discussed in comparison to literature data and with respect to a wider use of this nanotemplate for the adsorption of larger organic molecules.