Synchrotron radiation based studies of complex molecules on surfaces

Abstract

In this thesis two single molecule magnets based on the dodecamanganese (III, IV) cluster, with either benzoate or terphenyl-4-carboxylate ligands have been studied on the Au(111) and rutile TiO2(110) surfaces. We have used in situ electrospray deposition to produce a series of surface coverages from a fraction of a monolayer to multilayer films in both cases. X-ray absorption spectroscopy measured at the Mn L-edge (Mn 2p) has been used to study the effect of adsorption on the oxidation states of the manganese atoms in the core. In the case of the enzoate-functionalized complex, reduction of the manganese metal centres is observed due to the interaction of the manganese core with the underlying surface. In the case of terphenyl-4 carboxylate, the presence of this much larger ligand prevents the magnetic core from interacting with either the gold or the titanium dioxide surfaces and the characteristic Mn3+ and Mn4+ oxidation states necessary for magnetic behaviour are preserved. In contrast to the single molecule magnets where no charge transfer between the molecules and the substrates or within the molecules themselves were wanted, the molecules of bi isonicotinic acid and the giant zinc porphyrin nanorings have been studied on rutile TiO2(110) and Au(111) surfaces in the pursuit of charge transfer. In the case of the bi-isonicotinic acid it is studied on the rutile TiO2(110) where the technique of resonant inelastic X-ray scattering was been employed. Here we introduce the core-hole clock implementation to estimate the charge transfer from the molecule to the substrate. We verify previous results of ultrafast charge transfer in the sub-femtosecond regime (2.9 ± 0.3 femtoseconds) out of the LUMO+1 orbital. When the higher lying state of the LUMO+2 state is probed charge transfer out of this state and to the substrate is possibly there, but it is not possible to resolve it since it is masked by other effects originating from the inelastic scattering of the system. Furthermore, we see potential charge transfer within the molecule itself and new states observed in the inelastic scattering. Finally, zinc porphyrin nanorings were investigated on two surfaces of rutile TiO2(110) and Au(111). The techniques used here were X-ray photoemission spectroscopy and resonant photoemission spectroscopy. When the rutile TiO2(110) surface was employed hardly any participator decay was present suggesting charge transfer within the molecule itself or to the surface. This is further backed up by the fact that all of the core-excited unoccupied states are found to overlap energetically with the unoccupied states of the substrate, facilitating charge transfer out all the core-excited states. In the case of the Au(111) surface somewhat similar results are found, having all the core-excited states of the molecule located within the unoccupied states of the substrate, which again will facilitate charge transfer out all the core-excited states of the molecule. When the Au(111) substrate was employed the technique of near edge X-ray absorption fine structure was used to investigate the geometric orientation of the molecule on the surface. With the result of 86◦ ± 10◦ to the surface normal we verify previous scanning tunneling microscopy measurement that the zinc porphyrin nanorings will take a at lying orientation on the gold substrate