Ab initio study of organic molecules adsorbed on technologically relevant Surfaces

In this thesis we investigated materials of relevance to photovoltaics and organic electronics, and the studied systems involving surfaces with technological applications, such as graphene and TiO2 . We make use of the density functional theory (DFT) to calculate the structural and electronic properties of the system, and the Ab initio molecular dynamics to check the temperature dependent effects. Finally, we simulate the core excited spectroscopic measurements by the transition potential approach and use the Time-Dependent DFT to calculate the optical absorption coefficient. We first focused on covalent adsorption of aromatic radicals onto graphene.. Our results show that the adsorption of an aromatic radical generates two spin-dependent mid-gap states located around the Fermi energy which induce magnetic moments in graphene. This phenomenon can modify the band of pristine graphene and introduces a gap, but this effect is almost independent of the specific chemical functionalization by the aromatic radical. In this way it is possible to magnetize graphene just using s, p electrons without any d-metal impurities. Next we investigated the adsorption of prototypical dyes (catechol and isonicotinic acid) on the TiO2 anatase (101) and rutile (110) surfaces by DFT, Ab initio molecular dynamics and TDDFT calculations. We found that thermal fluctuation induce changes in the position of the molecular levels around the TiO2 valence band edge. For the anatase (101) surface, these fluctuations enhance significantly the low-energy tail of the absorption spectrum, and the sensitization can be improved by increasing the hybridization between the adsorbed dye and TiO2 states. But sensitization effects are less relevant for the rutile (110) surface. As an extension of this work towards more realistic materials for applications, we studied two more complex species, namely PTCDA and PTCDI adsorbed on the TiO2 rutile (110) surface. These molecules determine a more pronounced sensitization effect with a substantial red-shift of the first peak of the dye/TiO 2 absorption relative to the free dye. Finally, the unoccupied molecular orbitals of corannulene (C20H10 ) were studied by the transition-potential (TP-DFT) approach, and we collaborated with the experimental group of Dr. Andrea Goldoni at ELETTRA, Trieste, Italy who deposited a monolayer of such molecules on Ag (111), and measured X-ray photoemission spectroscopy (XPS) and Near-edge X-ray absorption fine structure (NEXAFS). From our calculation of the intrinsic dichroism resulting from the corannulene curvature and polarization dependent NEXAFS measurement of the unoccupied molecular orbitals, the molecules were found to be oriented at a small tilt angle of ~ 5 degrees from the surface. The molecular tilting results in different electron screening of the core hole in XPS. The corresponding core level shifts broaden the C 1s photoemission peak and produce a splitting of the NEXAFS LUMO resonance, which is strongly contributed by all the C atoms. Higher energy transitions involve different molecular orbitals (π and σ) depending on the excited C atom.