This thesis is concerned with the structural and chemical control of two-dimensional (2D) supramolecular self-assemblies through judiciously manipulating bonding motifs at various intrinsic and external conditions. The self-selection and the self-recognition of the non-covalent interactions among organic and/or metallic building blocks govern the structural and chemical properties of the resultant self-assembled two-dimensional nanostructures, accompanying with the thermodynamic and kinetic process as well. In this thesis, we have investigated the supramolecular self-assembly achieved via coordination bonds assisted by transition metals and functional ligands on metal surfaces. The self-assembled nanostructures were studied by ultra-high vacuum scanning tunneling microscopy working at room temperatures. The structural transition processes were also inspected via the low energy electron diffraction. Further, artificial “quantum dots” represented by the cavities of the self-assembled networks were investigated. The modulation of surface electrons by these “quantum dots” was characterized by the local density of states detected by low-temperature scanning tunneling spectroscopy.
The major contributions of this thesis are outlined as below:
(1) Through modifying the chemical states of organic ligands, a unique coordination Kagome network structure was obtained for the first time by two distinct methods. TPyP (5, 10, 15, 20-tetra(4-pyridyl)porphyrin) species on Au(111) surfaces form the TPyP-Au coordination Kagome network achieved by a novel treatment that was suggested to modify the chemical state of the TPyP. In a condition that the TPyP coexists with Cu on a Au(111), Cu adatoms play two roles in the self-assembly – the coordination with pyridyl end-groups and the reaction with TPyP macrocycles, which control the chemical and structural phase of the self-assembly. Following a high temperature annealing, the same Kagome structure emerged from a precursor rhombus network structure. We proposed a new mechanism which provides a consistent explanation to both assembly methods. ZnTPyP (zinc 5, 10, 15, 20-tetra(4-pyridyl)porphyrin) molecules show chemical stability at high annealing temperature, which allows for preparing chemically pure ZnTPyP-Cu bimetallic networks. Furthermore, a reversible structural transformation between a hydrogen-bonded network and a coordination network was realized by either adding Cu atoms or annealing samples at certain temperatures.
(2) The influence of the thermodynamic and kinetic effects on the selection of binding modes was studied by a combined STM and LEED investigation which offered spatial as well as temporal insights. The molecules of TPyB (1,3,5-trispyridylbenzene) coordinate with Cu or Fe respectively, forming two distinct polymorphism network structures. Two coordination binding modes show different binding energies. By the kinetic and thermodynamic control, either of binding modes was selected. LEED patterns revealed the dynamic process of structural transition from that of low binding-energy mode to that of high binding-energy mode. In the latter section the structural phase transition induced by two-dimensional compression is introduced. Pyridyl-Cu coordination bond is of certain liability, allowing for the alternation of bonds under various environments. Through increasing the coverage of molecules, distinctive polygraphic networks presented via different pyridyl-Cu binding modes.
(3) The self-assembly of multiple components represent a much more complicated assembly system, where the elaborate balance of interactions among all components and substrates comprises greater challenging. To study such a system, the third part deals with the multiple-ligand self-assembly. Achieved by TPyP and PBTP (4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine)) molecules and Fe, two types of network structure formed on Au(111) surfaces. Both networks coexisted due to similar bonding configurations and free energies. However, the entropy is presumably suggested to promote the selection of the lower-complexity network structure, and the guest-inclusion effect dominates the exclusive emergence of the other network structure. Again, a chemically pure network was fabricated by displacing TPyP to chemical stable ZnTPyP species. The co-evaporation of TPyP, TPyB and Cu led to the coexistence of three structural phases – one of them was a hybrid phase involving both organic ligands. Tuning the ratio of organic reactants resulted to the hybrid network emerging exclusively. The mechanism of the amplification was still unknown, while some assumptions based on the experimental observations were proposed.
(4) We have examined the modulation of the surface-state electrons by the artificial “quantum dots” provided by the self-assembled porous networks on a surface. The technique of low temperature tunneling spectroscopy was employed to detect the local density of states inside a cavity of the self-assembled coordination network. The experimental results were compared and discussed in comparison with calculations. The distinctive effective potentials of molecules and metal centers were discovered to modulate the local density of states at the center of the cavities.
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