Theory

Domain I: Molecular Bonding and Spectroscopic Signatures

First-principles methods, specifically based on DFT, shall be used to determine adsorption geometries and energies for molecules on pristine, defective, and pre-structured oxide surfaces (funCOS Molecular and Oxide Toolboxes). Calculations of energy landscapes will provide first insight into geometric structures of adsorbate systems and related interaction mechanisms. Systematic studies of binding properties will support the experiments in funCOS 1 and funCOS 3 to identify linker groups with selective bonding to specific sites. Vibrational frequencies will be calculated and used to interpret IRAS data obtained in funCOS 3. Also, XPS chemical shifts will be predicted and compared to those measured in funCOS 2 and 3. Moreover, STM and AFM images from funCOS 1 will be simulated based on structures, orbitals, and eigenvalues obtained in DFT calculations. The interplay of Domain I with funCOS 1 and funCOS 3 shall result in a comprehensive characterization and understanding of basic adsorbate–substrate interactions of organic molecules on oxides.

Domain II: Excited States and Photoexcitations

The electronic structure and photophysics of molecules (funCOS Molecular Toolbox) adsorbed on oxides (funCOS Oxide Toolbox) will be investigated by first-principles methods within the framework of DFT, time-dependent DFT, and many body perturbation theory within GW and BSE approaches. Electron transfer processes between the adsorbate and the metal oxide shall be analyzed based on calculated quasiparticle spectra and excited states including their lifetime. The resulting insights into electronic structure will provide a microscopic understanding of the effect of linker groups on excited state properties and on electron transfer processes (investigated in funCOS 2). Electron transfer with metal oxides may also include low-coordinated surface sites (e.g. oxygen vacancies and steps) as sinks or sources of electrons. Adsorbates interacting with such sites may show distinct electron transfer and optical properties; aspects that shall be included in photophysical investigations. The latter will be closely linked with funCOS 2 and will contribute to optical characterization in funCOS 5. It will build on insights from funCOS 1 and the other domains of funCOS 6.

Domain III: Intermolecular Interactions and Structure Formation

Domain III will focus on understanding growth processes and structure formation. DFT calculations on aggregates of small test molecules (funCOS Molecular Toolbox) with linker groups shall yield first information on basic adsorbate–adsorbate interactions (direct or substrate mediated), in parallel with experimental studies in funCOS 1, funCOS 2 and funCOS 3. These calculations will be extended to assemblies of larger organic molecules (e.g. porphyrins, funCOS Showcases) using semiempirical, parameterized tight-binding methods, supplemented by empirical force field terms to take into account van-der-Waals interactions. The coverage dependence of binding energies and geometries will yield information on intermolecular interactions in relation to the direct molecule–substrate bonding. We aim at providing guidelines for funCOS 4 and funCOS 5 on how to control structure formation and self-organization in adsorbate layers by linker or spacer groups. From an analysis of the thermodynamic stability of layer structures it can be deduced how film formation may be influenced by the preparation conditions. This also includes, for instance, surface reduction or hydroxylation as a result of the environmental conditions during growth. Finally, transition states will be calculated in order to obtain insight into the kinetics of early stages of structure formation, growth, and self-organization, experimentally investigated in funCOS 4 and 5.

Objectives

The proposed theory project aims at a comprehensive theoretical understanding at the microscopic, i.e., quantum mechanical, level of the adsorbate-substrate and adsorbate-adsorbate interactions of functionalized organic molecules on structured oxide surfaces, which eventually shall enable controlled formation of organic films with specific structural, electronic and optical properties and rationally designed functionalities in areas such as catalysis, sensoring, or photo-chemistry. To that end methods based on DFT supplemented by means to treat Van-der-Waals interactions and, where necessary, strong correlation shall be applied. For large systems DFT-derived semi-empirical tight-binding methods shall be employed. Specifically, we aim at understanding

• adsorbate–substrate interactions of organic test molecules, e.g, benzene derivatives with different linker groups, with pristine oxide surfaces and with low-coordinated sites on structured oxide substrates;
• adsorbate–substrate interactions of larger functionalized (metal)organic molecules, e.g., metalloporphyrines, with pristine and defective oxide surfaces;
• charge transfer processes and modifications of electronic and optical properties of functionalized organic molecules upon adsorption on structured oxide surfaces;
• adsorbate-adsorbate interactions of functionalized organic molecules on structured oxide surfaces;
• spectroscopic and imaging data (IRAS, XPS, STM, STS, AFM, EPR, or 2PPE) by analyzing and simulating experimental data on the basis of electronic structure calculations;
• how the formation of functional surface structures depends on the interplay of adsorbate-substrate and adsorbate-adsorbate interactions of functionalized organic molecules on structured oxide surfaces;
• the reaction and activation of small molecules with metalorganic molecules like metalloporphyrins arranged on a structured oxide surface.