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Growth and Characterization of Single Molecular Wires on Metal Surfaces


Koch,  Matthias
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Koch, M. (2013). Growth and Characterization of Single Molecular Wires on Metal Surfaces. PhD Thesis, Freie Universität, Berlin.

Molecular wires will be an essential part in future nanotechnology as they are necessary to connect the functional molecules. In this thesis the growth and characterization of molecular wires are investigated by scanning tunneling microscopy. The bottom-up approach "on-surface polymerization" was used to fabricate the moleculare wires. Two types of molecular wires are investigated in this thesis: porphyrin tapes and graphene nanoribbons. Steric hindrance limits the polymerization of the porphyrin-based molecules and the trapezoid-shaped molecules so that only very short or fragmented chains are observed. Furthermore, the influence of the surface was studied in regard to the polymerization process. In contrast to Au(111), on Cu(111) 2,11-dibromohexabenzocoronene dehalogenate at room temperature, and molecular chains emerge. The higher catalytic activity of Cu(111) reduces the dehalogenation temperature. These chains are linked by a metal-ligand bond and are not covalently bound as it is observed on Au(111). On Cu(111) copper adatoms are available at room temperature. The surface can also be used to suppress the growth of polymerized species. On Au(111) the polymerization of 2,3-dibromoanthracene leads to dimers and trimers, whereas on Au(100) only dimers are produced. One aspect of this thesis is the study of the conductance and the electronic properties of graphene nanoribbons, which are polymerized from 10,10'-dibromo-9,9'-bianthryl building blocks. Scanning tunneling spectroscopy is used to investigate the electronic structure, and the highest occupied (HOMO) as well as the lowest unoccupied molecular orbital (LUMO) are found to be delocalized along the armchair edges of the ribbon. The HOMO-LUMO gap amounts to 2.8 eV in agreement with expectations. At the short zigzag edges a localized Tamm state is detected, which is only observed for defect-free termini. To measure the conductance the molecule is lifted from the surface by the tip while the current is recorded. The role of the molecular orbitals on the charge transport is investigated by recording length-dependent conductance spectra for electron energies in the range of +1.8 and -2.4 eV. Independent of the electron energies the measured current decays exponentially with the tip height. If the electron energy coincides with the molecular orbitals the inverse decay length of the ribbon drops from ~0.45 A^(-1) in the HOMO-LUMO gap to ~0.10^(-1) which points to an improved conductance. Furthermore, molecular vibrations of a single graphene nanoribbon in pulling geometry are observed by inelastic electron tunneling spectroscopy, and, amongst others, the D mode and the G mode are detected. When the ribbon geometry is changed from planar to bent no shift in the energy of the molecular vibrations is detected. In the last part of this thesis the required force is studied to lift a molecular chain from the surface. Long polyfluorene chains are fabricated and pulled from the surface with an atomic force microscope tip while the frequency shift signal is recorded. In contrast to the conductance experiments, the frequency shift does not decay as a function of the tip height. The polyfluorene chains are lifted stepwise from the surface, and the main contribution in the frequency shift signal results from the fluorene unit lifted up next. In agreement with expectations a maximum attractive force between the molecule and the surface of -0.5 nN is determined.