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Abstract

The control over the work function of surfaces and interfaces is one of the most important issues of modern surface science and nanotechnology, e.g. in context of organic electronics and photovoltaics. The goal of this work was to look for new ways to control the work function of metal substrates by using molecular self-assembly. Two different strategies were used. The first strategy was to use aliphatic and aromatic molecules which contain an embedded dipolar group (midchain functionalization). Such self-assembled monolayers (SAMs) allow for tuning the substrate work function in a controlled manner, independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface. In the case of aliphatic films, we used alkanethiols functionalized with an embedded ester dipole, with the length of both top and bottom segments as well as the direction of the embedded dipole being varied. In the case of aromatic systems, we used terphenyl based thiols functionalized with an embedded pyrimidine dipolar group, with the direction of the dipole being varied. The electronic and structural properties of these embedded-dipole SAMs were thoroughly analyzed using a number of complementary characterization techniques combined with quantummechanical modeling. It is shown that such mid-chain-substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core-level energies in the regions above and below the dipoles relative to one another. Particularly imptortant, in context of the present work, is the fact that the mid-chain functionalized films are indeed well suited to adjust the work function of metal substrates. This could be e.g. done by varying the orientation of the dipolar group but also by mixing the molecules with differently oriented dipoles as was demonstrated in the present work. Within the second strategy, we used photoresponsive systems, viz. azobenzene substituted alkanethiols, having a specially designed architecture to control the packing density and carrying an additional dipolar tail group. These novel SAMs were studied in detail by using spectroscopic and microscopic techniques. Performing photoisomerization experiments we obtained a reproducible, stimuli-responsive change in the work function which was, however, limited to some extent due to the strong steric hindrance effects. In order to reduce these effects, we diluted the azobenzene molecules with short spacer molecules, which resulted in an improvement in the photoswitching behavior.