Thesis defense of Malte Schulte

1,3,7-Trimethylxanthine (C8H10N4O2), better known as ca eine, has an asymmetric and achiral structure. These properties result in two enantiomers on the surface upon adsorption and affect the structural arrangement of the molecules on a crystalline substrate. Therefore, a structural analysis of caffeine molecules in mono- and submonomolecular coverage on weakly interacting substrates allows to study this influence. In this work, the formation of caffeine molecules on a Au(111) surface is investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The thin caffeine films are prepared by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) and studied at room temperature. In monomolecular coverage, the caffeine molecules assemble in a quasi-hexagonal phase on Au(111). Thereby, two mirrored domains are measurable with respect to the substrate surface in STM and LEED experiments. From these measurements, a periodic layer with only one molecule in the unit cell is initially deduced. In XPS experiments, no strong interaction with the substrate is observed. This motivates a theoretical analysis of the caffeine monolayer structure, which is performed with ab-initio DFT simulations. From the simulation results, it can be concluded that a caffeine monolayer formation with at least three molecules in different chiralities per unit cell is preferred. This hypothesis is in accordance with all recorded measurement data. In addition to the room temperature study, a structural analysis is performed at low temperatures focusing on submonolayer coverage of caffeine molecules on the Au(111) surface. In monolayer coverage, the quasi-hexagonal order of the caffeine molecules on the gold substrate can be reproduced at low temperatures. Also, it can be shown that more than one molecule is present in the adsorbate unit cell and the three-molecule unit cell hypothesis is strengthened. In contrast to the room temperature STM measurements, it is possible to perform investigations of the submonolayer coverage at low temperatures. There, distinct assemblies of caffeine molecules are found in defined orientation to the substrate. These can be identified as building blocks of chiral motifs on the surface. For higher coverage the transition to the dense quasi-hexagonal monolayer is observable. Thus, the assembled formations of the building blocks disappear and the aforementioned monolayer formation recurs. These results contribute to the understanding of the formation process of caffeine monolayers on crystalline substrates.

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