Thesis defense of Henning Sturmeit

Metal-organic interfaces are key constituents of the various functional building blocks that can be found in molecular electronics and spintronics. Long electron and spin relaxation times of organic molecules make them superior to inorganic materials for many technological devices. When put into contact with a metal electrode, the hybridization of molecular orbitals and metallic states can lead to several intriguing effects, which strongly affect the electronic and magnetic properties of the system. In this regard, the interface obtained by depositing nickel tetraphenyl porphyrin onto the copper (100) surface (NiTPP/Cu(100)) can be seen as an interesting model system. Previous experiments reported an unexpected high charge transfer leading to a partial filling of the molecular orbitals up to the LUMO+3 and a reduction of the central nickel atom. Considering this observation as the point of departure, this thesis aims to develop different approaches to alter the hybridization at this interface.

The results of this thesis can be divided into three main topics. First, it is shown that a pre-oxidation of the copper substrate leads to a substantial quenching of the charge transfer from the metal to the molecule and, thereby, weakens the interaction. Crucially, the molecules adsorb in a well-defined geometry, thus allowing for the determination of the molecular orbital symmetry. This is a major advantage over gas-phase and multilayer measurements, as in such cases, a random molecular orientation prevents an evaluation of the orbital symmetry via near-edge x-ray absorption fine structure spectroscopy. Moreover, the so-obtained NiTPP/O-Cu(100) interface can be used as a reference system, which enables a direct comparison of the weakly and strongly hybridized NiTPP molecules.

In a second approach, the NiTPP/Cu(100) interface was exposed to a few Langmuir of nitric dioxide (NO2) gas. The NO2 molecules are found to bind to the central nickel atom of the NiTPP molecules. A comparison of the d  shell configuration of the pristine and NO2-modified system reveals that the on-top ligation leads to a reduction of the nickel center. Remarkably, the nickel center in NO2-NiTPP/Cu(100) has a d 8 configuration with two unpaired electrons, which is in contrast to the situation in gas-phase molecules where the central ion is in a d 8 configuration without unpaired electrons. Thus, NO2 adsorption enables a spin switching from d 9 to d 8 with two unpaired electrons. Interestingly, the electronic structure of the molecular macrocycle is only weakly perturbed by the NO2 adsorption. Moreover, annealing up to 390 K suffices to remove the adsorbed NO2 molecules and restore the original electronic configuration of the central nickel atom.

When annealed to high temperatures, substrate-supported porphyrins often undergo structural changes, which is disadvantageous for many applications. The third part of the results addresses the temperature-induced changes at the NiTPP/Cu(100) interface upon annealing. Up to the limit of thermal decomposition, the NiTPP molecules do not undergo chemical changes but only conformational modifications. This is in contrast to what has been observed for similar metal-molecule combinations. Generally, porphyrins are prone to dehydrogenation reactions at the periphery of the molecule, which is usually connected to a ring-closing reaction in which peripheral substituents bind to the macrocycle. A rotation of the phenyls to a more coplanar orientation and an enhanced charge transfer from the metal to the molecule result in a strengthened molecule-substrate interaction. All experimental evidence points toward molecular pinning as the underlying reason that prevents the molecule from a complete flattening.

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