Interacting Interactions: A Study on the Interplay of Molecule-Molecule and Molecule-Substrate Interactions at Metal-Organic Interfaces

In this work a surface science study on metal-organic interfaces is presented toresolve their geometric and electronic properties and study the interplay of moleculemoleculeand molecule-substrate interactions. The organic molecules benzene, azobenzene,3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA), and terephthalicacid (TPA) are deposited on low index Ag and Cu surfaces to form monolayer andsub-monolayer structures which are investigated by normal incidence X-ray standingwaves and angle resolved photoemission spectroscopy, which leads to severalsurprising findings.Investigating the adsorption of benzene, we find it physisorbed in a flat geometry forbenzene on Ag(111). Enhancing the molecule-substrate interaction by exchangingAg(111) with the stronger interacting Cu(111) is expected to simply lower the adsorptionheight. However, we find flat molecules at an elevated adsorption height forbenzene/Cu(111), which seem to be stabilized via intermolecular interactions due tothe coexistence with upright standing benzene molecules.The interplay of molecule-molecule and molecule-substrate interactions is furtherexplored on a metal-organic network formed by codeposition of TPA and Fe atomson Cu(100). The coordination of TPA molecules by the Fe atoms reduces the TPAsubstrateinteraction. An additional sitespecific adsorption of oxygen again altersthis balance.In case of PTCDA a comprehensive study for its adsorption on low index Ag surfacesis presented. From linking the geometric and electronic stucture properties,it is understood that the electron density spill-out of the surface and its uptake bythe adsorbing molecule is a decisive molecule-substrate interaction channel. Thisexplains the finding that the resulting binding energies of the lowest unoccupiedmolecular orbital (LUMO) as well as the adsorption height of PTCDA on Ag aredetermined by the work function.IIIMoving to the archetypal molecular switch azobenzene, which is studied on Cu(111),three different azobenzene monolayer phases which are formed along with a coveragedependent dissociation of the molecule are revealed. The higher the densityof molecules get, the stronger molecule-molecule interactions become and force themolecule to bend. However, its strong molecule-substrate bond prevents a conformationalchange and the resulting stress ultimately leads to a dissociation.The surprising results of this work show that the understanding of interactions atmetal-organic interfaces is still only rudimentary and stress the importance of furtherfundamental research