Surface-Assisted Chemistry at Interfaces between Metals and Organic Thin Films
Interfaces between metals and organic thin films are of paramount importance for organic electronic devices. By photoemission spectroscopy (XPS), scanning tunneling microscopy/spectroscopy (STM/STS), temperature programmed desorption/reaction (TPD/TPR) and density functional theory calculations (DFT...
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|Zusammenfassung:||Interfaces between metals and organic thin films are of paramount importance for organic electronic devices. By photoemission spectroscopy (XPS), scanning tunneling microscopy/spectroscopy (STM/STS), temperature programmed desorption/reaction (TPD/TPR) and density functional theory calculations (DFT), chemical reactions and diffusion processes during the formation of interfaces/interphases, and their electronic structures are investigated. The presented thesis focuses on two distinct substance classes, arranged by the reacting functional groups of the involved organic phases. Various reaction systems including organic-on-metal and metal-on-organic interfaces are visited. In the first major topic, the surface coordination chemistry of thin films formed by porphyrinoid molecules with coinage metals - the latter either as crystalline substrate surface or dosed onto the films as atoms - is characterized. Molecular film thicknesses extend from incomplete layers, i.e., submonolayer coverages, to a few ten nanometers, i.e., multilayers. Metalation of 2H-tetraphenylporphyrin (2HTPP) at submonolayer coverages and 2H-phthalocyanine (2HPc) for up to multilayer coverages is found to be possible at elevated temperatures even from an atomically flat copper substrate. TPD/TPR studies reveal that the presence of an exchange mechanism could be responsible for the observed substrate based multilayer metalation. Besides the interfacial chemical bond, i.e., between the surface and the metal complex, secondary effects - such as charge transfer and weak band bending - are characterized in detail for cobalt phthalocyanine adsorbed on Cu(111). Individual molecules in porphyrin or phthalocyanine thin films are able to oxidize cobalt atoms into formal Co(II) species. By modification of the reaction center from porphin to corrole, i.e., essentially by the replacement of an iminic by a pyrrolic nitrogen in the macrocycle, the formation of Co(III) complexes is rendered possible. Moreover, multilayer metalation of 2HTPP molecules with deposited cobalt atoms is investigated in detail by both laboratory X-ray source (XPS) and synchrotronbased hard X-rays (HAXPES). Since the latter is a nondestructive method for bulk composition probing, chemical and physical properties can be investigated. This technique is also applied to obtain a depth profile of a layered battery cathode - initially comprising lithium-nickel-manganese-oxide and lithium-titanium-oxide - after ex situ electrochemical cycling. The second major topic is dedicated to the behavior of purposively designed terminal oligophenylene dibromides on the flat hexagonal single crystal surfaces of copper and silver. Submonolayer amounts of oligophenylene dibromides deposited at cryogenic temperatures on the copper substrate show bromine detachment upon moderate temperature increase along with an intermediate formation of metal-organic oligomers. Upon further annealing, the metal is eliminated and entirely organic covalent structures are formed on the surface in analogy to the Ullmann coupling. The here presented surface reaction allows in situ synthesis of giant hexagonal macrocycles consisting of thirty phenylene units. Macrocycles with altered numbers of members are also accessible with reduced yield. Since these square, pentagonal and heptagonal shapes are not fully compatible with the hexagonal motive of the oligophenylene monomers, they exhibit strained geometries leading to modified electronic structures. These huge molecules are further used as organic quantum corrals to achieve and analyze the induced surface state confinements. Similar precursor compounds are deposited on a silver substrate in order to increase the mobility of the organic molecules as well as to suppress the formation of covalent bonds with the metal. This enables the generation of equilibrium based, self-assembling structures. Utilizing building blocks with three-fold symmetry, defect-free molecular fractal-like patterns - resembling Sierpinski triangles - are obtained. Revealing the chemical and physical processes at the interfaces important for device performance is the intent of this thesis. By fine tuning various intrinsic and external conditions, structural and chemical control of two-dimensional supramolecular phases is achieved. Surface-assisted chemistry - here in situ metalation of porphyrinoid molecules, synthesis of giant honeycombene macrocycles, and two-dimensional molecular selfassembled networks - as well as the properties, e.g., adsorbate-substrate interaction, of formed, and in some cases buried, interfaces/interphases between metals and organic thin films are comprehensively studied with a wide range of complementary ultrahigh vacuum based surface science techniques. The results and conclusions of the therefrom emerged publications are summarized in this work.|