Investigation and Application of Self-Assembly Processes

The interest in self-assembled monolayers (SAMs) has steadily increased over the last years. Due to their unique properties, they open up new perspectives for preparation of chemically modified surfaces with application in various fields like sensors, electrodes, biomaterials, nano-structuring, and tribology. Although a broad spectrum of SAM systems, ranging from organosulfur compounds on metals to alkylphosphonic acids on oxidic surfaces, is documented in the literature, a better knowledge on the quantitative influence of various deposition parameters and the exact role of the surface chemistry of the substrate on the adsorption mechanism is still needed. Such information is particularly important in order to make accessible new substrates which at the time being appear not suitable for coating with SAMs. Thus, the goal of this project is the investigation of the adsorption of alkyltrichlorosilanes on various substrates with controlled number and distribution of reactive hydroxyl binding sites for the film molecules. The respective substrates will be prepared by modification of silicon and mica surfaces both through conventional activation and deactivation steps (e.g. UV/ozone, HF) and through adsorption of different molecules via self-assembly with and without subsequent chemical transformation of the adsorbed molecules at the surface. Further investigations with different deposition solvents and experiments performed under controlled electrical potential of the substrate will be targeted at the role of electrostatic phenomena on the adsorption kinetics. It is expected that the proposed research will significantly contribute to the mechanistic understanding of self-assembly adsorption processes with numerous applications in science and technology, but also to analytical-methodological developments.

In this project the formation of thin organic films through self-assembly adsorption from solutions has been investigated. Self-assembly means spontaneous formation of highly ordered structures without human aid and can be frequently found in nature. For instance, membranes of living cells can be formed by self-assembly processes. In the laboratory this principle can be used to produce self-assembled monolayers which are a way to modify the chemical composition of the outmost layer of a solid. This can be used to control a wide variety of properties (e.g. resistivity against corrosion or tribological properties) for different applications. The goal of this project was to achieve a better understanding of the mechanisms behind the formation of self- assembled monolayers starting from alkyltrichlorosilane precursor molecules on silicon surfaces. Atomic force microscopy (AFM) has been used to obtain information on the structure of sub-monolayer films. Ellipsometry has been used to determine film thicknesses and infrared spectroscopy has been applied to elucidate the ordering of molecules in the film. It has been found that various parameters have a very strong influence on the growth behavior of the films. For instance, increasing temperature leads to smaller islands and lower surface coverages, whereas increasing water concentration in the adsorption solution leads to larger islands and higher surface coverages. Furthermore, the activity of the substrate surface (i.e. the availability of chemically reactive anchoring groups) strongly affects the shape of the deposited islands. For example, a low number of active anchoring groups means a higher mobility of the adsorbed species giving them the opportunity to arrange into round islands with smooth edges, whereas at a high number of anchoring groups the species arriving at the surface are trapped quickly and typical dendritically shaped islands are formed. It should also be mentioned that a complex interplay between the effects of different growth parameters has been observed. Moreover, characterization of the adsorption solution (e.g. existence of ordered species in the solution) has contributed to the understanding of the processes observed macroscopically. Summarizing, the project has significantly contributed to the understanding of the growth mechanism of self- assembled monolayers which is an important prerequisite to produce such layers in a reproducible manner.