Study of molecule metal interfaces by means of the normal incidence x-ray standing wave technique

Functional surfaces based on monolayers of organic molecules are currently subject of an intense research effort due to their applications in molecular electronics, sensing and catalysis. Because of the strong dependence of organic based devices on the local properties of the molecule-metal interface, a direct investigation of the interface chemistry is of paramount importance. In this context, the bonding distance, measured by means of the normal incidence x-ray standing wave technique (NIXSW), provides a direct access to the molecule-metal interactions. At the same time, NIXSW adsorption heights are used to benchmark different density functional theory (DFT) schemes and determine the ones with predictive power for similar systems. This work investigates the geometric and chemical properties of different molecule/metal interfaces, relevant to molecular electronics and functional surfaces applications, primarily by means of the NIXSW technique. All NIXSW data are analyzed with the newly developed open source program Torricelli, which is thoroughly documented in the thesis. In order to elucidate the role played by the substrate within molecule/metal interfaces, the prototype organic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) is explored on the Ag(110) surface. The molecule results more distorted and at smaller bonding distances on the more reactive Ag(110) surface, in comparison with the Ag(100), the Ag(111) and Au(111) substrates. This conclusion follows from the detailed molecular adsorption geometry obtained from the differential analysis of nonequivalent carbon and oxygen species (including a careful error analysis). Subsequently, the chemisorptive PTCDA/Ag(110) interaction is tuned by the co-deposition of an external alkali metal, namely K. As a consequence, the functional groups of PTCDA unbind from the surface, which, in turn, undergoes major reconstruction. In fact, the resulting nanopatterned surface consists of alternated up and down reconstructed Ag terraces covered by PTCDA molecules partly unbound with respect to the pure molecular phase. This picture follows from a combination of NIXSW, XPS, UPS, LEED and STM experiments. Within the context of the functional surfaces, the interaction of the molecular switches azobenzene (AB) and 3,3',5,5'-tetra-tert-butyl azobenzene (TBA) adsorbed on the Ag(111) surface is investigated. The bonding distance of TBA, only slightly greater compared to AB, indicates that the desired geometric decoupling of the photochromic moiety to enable the trans to cis switching in the adsorbate state does not occur. In particular, the measured adsorption heights of nitrogen, in excellent agreement with the dispersion corrected DFT-PBE calculations, suggest that both molecules are in the trans isomerization. Moreover, an detailed adsorption geometry of AB and TBA, including the carbon atoms, is obtained by means of the Fourier vector analysis in the Argand diagram. This method allows the multiple molecular degrees of freedom of large and flexible molecules to be explored and provides structural parameters, e.g., the phenyl ring tilt angle and torsion angle, with unprecedented accuracy. Other functional surfaces that are appealing for molecular electronics applications are the 2D metal-organic networks. In this work, the self-assembled monolayer of the prototypical molecular ligand terephthalic acid (TPA) on the Cu(100) surface, prior to additional metal deposition, is examined. NIXSW data reveal a significantly distorted molecule with the carboxylate groups covalently bound to the Cu atoms underneath and the carbon backbone arc-like bent. This evidence suggests an intermolecular interaction mediated by the substrate, as also supported by previous HREELS measurements. Finally, the disagreement between the experimental adsorption geometry and the DFT-PBE prediction motivates further theoretical studies to improve the understanding of this prototypical molecule-metal interface