Molekulare Funktionalisierung von Oberflächen für photoelektrochemische Sensoren und ihre mikroskopische Charakterisierung

Photoelectrochemical sensors represent one means for the detection of biochemical analytes in solution, while they perceive its selective interaction with specifically sensitized surfaces. The signal generation follows the photoexcitation of semiconducting nanoparticles (quantum dots), which are coupled to the base electrode via a molecular interlayer. The design enables integration of multiple sensor schemes into one device and thus successive detection of several species. The feasibility of this concept is demonstrated in present work, however, primarily subsystems of this layered sensor design and their interfaces are investigated. Using complementary techniques self-assembling monolayers (SAMs) of stilbenedithiol on gold surfaces are characterized. The immobilization of quantums dots on such wet chemically prepared films requires a sufficient degree of ordering within the layer and the availability of free thiol groups on top, which soon is impeded by their potential oxidation. The film quality shows a strong dependence on preparation parameters, such as used solvent, temperature and ambient conditions during and subsequent to the preparation. Further, a remarkably high thermal stability is found which is ascribed to bridging of individual molecules. One widely used technique used for the characterization of SAMs is given by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. On base of the dichroism observed on the carbon K-edge it allows determination of the average molecular orientation with respect to the sample surface. In contrast to pi-conjugated systems, which exhibit distinct pi*-resonances with collinear transition dipole moments and, thus, allow a rather straightforward analysis, aliphatic systems such as alkanethiols do not show such lines. However, the relative polarization and oscillator strength of various transitions can be determined employing density functional theory calculations. In present study they are applied in order to identify the resonances underlying to the NEXAFS spectra of the model system octadecanethiol (ODT). On that base a refined strategy for the deduction of the molecular orientation from an experimentally observed dichroism is developed and applied to ODT SAMs on Au(111) as well as polycrystalline gold surfaces. Furthermore, organic thin films on GaAs(001) surfaces are analyzed. In addition to ODT, which forms well-ordered SAMs, aromatic benzenethiol and its derivatives are addressed, intending a specific variation of the substrate work function. However, NEXAFS data imply low coverage, isotropic orientation as well as strong interaction of the aromatic system and the substrate material. Such strong interaction is also deduced for thin films of pentacene, whose prime layer shows a substantially stronger binding to clean GaAs(001) surface as compared to subsequent layers.