From building blocks to 2D networks
The aim of this work is to further the understanding of the important parameters in the\nformation process of 2D nanostructures and therewith pioneer for novel applications. Such 2D\nnanostructures can be composed of specially designed organic molecules, which are adsorbed\non various surfaces. In order to study true 2D structures, monolayers were deposited. Their\nproperties have been investigated by scanning tunneling microscopy (STM) under ultra-high\nvacuum (UHV) conditions as well as under ambient conditions. The latter is a highly\ndynamic environment, where several parameters come into play. Complementary surface\nanalysis techniques such as low-energy electron diffraction (LEED), X-Ray photo-emission\nspectroscopy (XPS), and Raman spectroscopy were used when necessary to characterize\nthese novel molecular networks.\nIn order to conduct this type of experiments, high technical requirements have to be\nfulfilled, in particular for UHV experiments. Thus, the focus is on a drift-stable STM, which\nlays the foundation for high resolution STM topographs. Under ambient conditions, the\nliquid-solid STM can be easily upgraded by an injection add-on due to the highly flexible\ndesign. This special extension allows for adding extra solvent without impairing the high\nresolution of the STM data. Besides the device, also the quality of the tip is of pivotal\nimportance. In order to meet the high requirements for STM tips, an in vacuo ion-sputtering\nand electron-beam annealing device was realized for the post-preparation of scanning probes\nwithin one device. This two-step cleaning process consists of an ion-sputtering step and\nsubsequent thermal annealing of the probe.\nOne study using this STM setup concerned the incorporation dynamics of coronene (COR)\nguest molecules into pre-existent pores of a rigid 2D supramolecular host networks of trimesic\nacid (TMA) as well as the larger analogous benzenetribenzoic acid (BTB) at the liquid-solid\ninterface. By means of the injection add-on the additional solution containing the guest\nmolecules was applied to the surface. At the same time the incorporation process was\nmonitored by the STM. The incorporation dynamics into geometrically perfectly matched\npores of trimesic acid as well as into the substantially larger pores of benzentribenzoic acid\nexhibit a clearly different behavior. For the BTB network instantaneous incorporation within\nthe temporal resolution of the experiment was observed; for the TMA network, however,\nintermediate adsorption states of COR could be visualized before the final adsorption state\nwas reached.\nA further issue addressed in this work is the generation of metal-organic frameworks (MOFs)\nunder ultra-high vacuum conditions. A suitable building block therefore is an aromatic\ntrithiol, i.e. 1,3,5-tris(4-mercaptophenyl)benzene (TMB). To understand the specific role\nof the substrate, the surface-mediated reaction has been studied on Cu(111) as well as on\nAg(111). Room temperature deposition on both substrates results in densely packed trigonal\nstructures. Yet, heating the Cu(111) with the TMB molecules to moderate temperature\n(150 \xb0C) yields two different porous metal coordinated networks, depending on the initial\nsurface coverage. For Ag(111) the first structural change occurs after annealing the sample\nat 300 \xb0C. Here, several disordered structures with partially covalent disulfur bridges were\nidentified.\nProceeding further in the scope of increasing interaction strength between the building\nblocks, covalent organic frameworks (COFs) were studied under ultra-high vacuum conditions\nas well as under ambient conditions. For this purpose, a promising strategy is covalent\ncoupling through radical addition reactions of appropriate monomers, i.e. halogenated\naromatic molecules such as 1,3,5-tris(4-bromophenyl)benzene (TBPB) and 1,3,5-tris(4-\niodophenyl)benzene (TIPB). Besides the correct choice of a catalytic surface, the activation\nenergy for the scission of the carbon-halogen bonds is an essential parameter. In the case\nof ultra-high vacuum experiments, the influence of substrate temperature, material, and\ncrystallographic orientation on the coupling reaction was studied. For reactive Cu(111) and\nAg(110) surfaces room temperature deposition of TBPB already leads to a homolysis of the\nC-Br bond and subsequent formation of proto-polymers. Applying additional heat facilitates\nthe transformation of proto-polymers into 2D covalent networks. In contrast, for Ag(111)\njust a variety of self-assembled and rather poorly ordered structures composed of intact\nmolecules has emerged. The deposition onto substrates held at 80 K has never resulted in\nproto-polymers.\nFor ambient conditions, the polymerization reaction of 1,3,5-tri(4-iodophenyl)benzene\n(TIPB) on Au(111) was studied by STM after drop-casting the monomer onto the substrate\nheld either at room temperature or at 100 \xb0C. For room temperature deposition only poorly\nordered non-covalent arrangements were observed. In accordance with the established UHV\nprotocol for halogenated coupling reaction, a covalent aryl-aryl coupling was accomplished\nfor high temperature deposition. Interestingly, these covalent aggregates were not directly\nadsorbed on the Au(111) surface, but attached on top of a chemisorbed monolayer comprised\nof iodine and partially dehalogenated TIPB molecules. For a detailed analysis of the processes,\nthe temperature dependent dehalogenation reaction was monitored by X-ray photoelectron\nspectroscopy under ultra-high vacuum conditions.