Interaction of metal and oxide surfaces with thin films of polar phthalocyanines

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Dokumentart: Dissertation
Date: 2022-04-01
Language: English
Faculty: 7 Mathematisch-Naturwissenschaftliche Fakultät
Department: Chemie
Advisor: Chassé, Thomas (Prof. Dr.)
Day of Oral Examination: 2022-02-28
DDC Classifikation: 500 - Natural sciences and mathematics
540 - Chemistry and allied sciences
Other Keywords: Phthalocyanine
titanium dioxide
polar phthalocyanines
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In the first part of this thesis, interface properties of polar phthalocyanines (Pc’s): chloroaluminum(III) phthalocyanine (AlClPc) and fluoroaluminum(III) phthalocyanine (AlFPc) are investigated in form of thin films deposited on two types of silver surfaces: a single crystal and a foil. The samples were analyzed with X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). A strong interaction between the organic material and silver surface has been found, which is independent of silver substrate type used. The molecules undergo a loss of their central atom ligand, with the molecule ring staying intact. After de-attachment, the central atom ligand forms a bond with silver surface leading to the formation of Ag-Cl and Ag-F bonds on the surface of silver. What is more, roughness of the substrate surface influences the interface properties. An additional intensity is formed close to the Fermi edge for monolayer thick films. It is present only on substrates with high roughness and is an indication of gap states formation. In the second part of thesis, interface properties of chloroaluminum(III) phthalocyanine (AlClPc) at the interface to two rutile titanium dioxide single crystal surfaces: (001) and (100) surface, have been studied with XPS and UPS. Additionally, the substrates were characterized with low energy electron diffraction (LEED) to provide information on surface structure and reconstruction. A strong interaction has been found between nitrogen atoms of phthalocyanine ring and titanium dioxide surface. The interaction is stronger on the (001) surface than the (100) surface due to their different surface structure, leading to the lower reactivity of the latter one. The reactivity can be varied with number of defects in the crystal structure. The interaction was found to depend on the amount of defects present. As a consequence, using oxygen during preparation steps decreases the amount of defects and allows to quench the interaction. Furthermore, investigation of film growth on TiO2 substrates was performed using mainly microscopy methods: atomic force microscopy and scanning electron microscopy (AFM, SEM). Decrease of substrate signal intensity observed in XPS provided complementary information on film growth. It has been found that the surface type has a strong influence on the AlClPc growth mode since the organic films have a layer-island growth on the (001) surface and possible island-only growth on the (100) surface. Introduction of defects influences the film morphology since the islands are more dense and smaller in size on oxygen treated (“defect-free”) surface, while the island distribution on a reduced (defect-rich) (100) surface is scarce, and the islands are bigger in size. Based on the results of experiments presented in this thesis, it was found that the interaction of phthalocyanines can be controlled with the substrate surface type: although charge transfer from the molecule to the substrate is observed on both surfaces, the interaction with silver influences the phthalocyanine center, while interaction with titanium dioxide influences the phthalocyanine ring. Additionally, the reactivity of phthalocyanine molecules towards the titanium dioxide single crystals is dependent on the amount of defects present, thus can be controlled through adjustment of sample preparation conditions. The type and preparation of TiO2 surface has influence on film growth mode as well.

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