Ether-Moleküle und bifunktionale Cyclooctine auf Si(001) - vom mikroskopischen Verständnis der Adsorption zur chemoselektiven Anbindung

In this thesis, the functionalization of semiconductor surfaces with organic molecules was investigated. By analyzing the adsorption configurations, kinetics, and dynamics of ether molecules on Si(001), a understanding of the adsorption process on the microscopic level could be obtained. Based on these results, further studies of our group, and methods adopted from biological chemistry, a concept for the chemoselective adsorption of bifunctional organic molecules on Si(001) was developed. For tetrahydrofuran (THF) and diethyl ether (Et2O), it was shown that these ether molecules adsorb in a nonactivated process via an intermediate state on Si(001). In the datively bonded intermediate state, electron density of the oxygen lone pairs is donated into an empty Ddown-state of a zwitterionic silicon dimer. At 80 K, the conversion rate into the final state is negligible, the intermediate state could thus be characterized in STM-, XPS- and UPS-experiments. Thermal activation leads to the cleavage of the O-C bond of the ether group and thus the formation of covalent Si-O and Si-C bonds on two neighboring dimer rows. As both, the ringlike and the linear ether molecule adsorb over two neighboring dimer rows, arguments based on a geometrically more favorable final state can be excluded for this unusual final state. Instead, this final state might be preferred by the nature of the surface mediated reaction mechanism, e.g., due to an interaction with the Dup-state of the neighboring dimer row. The fragmentation of diethyl ether into two parts further leads to the observation of a tip induced hopping-process. The key parameters of the potential energy curve of Et2O, the barrier Ea for the conversion into the final state and the binding energy Ed of the intermediate state, were measured by a combination of optical second-harmonic generation and molecular beam techniques: Using optical second-harmonic generation, the conversion rates from the intermediate state into the final state were measured as a function of surface temperature; the barrier Ea=0.38+-0.05 eV was determined. By measuring the initial sticking coefficient as a function of surface temperature, the energy difference between desorption and conversion barrier, Ed-Ea=0.24+-0.03 eV, and thus the binding energy of the datively bonded intermediate state Ed=0.62+-0.08 eV was determined. The insights obtained for these model systems were applied to the functionalization of semiconductor surfaces with bifunctional organic molecules. The major hinderance for the chemoselective adsorption of such bifunctional molecules is the high reactivity of the Si(001) surface. In STM- and XPS-experiments, a chemoselective adsorption process of cyclooctyne ether and cyclooctyne ester with the strained triple bond was observed; the second functionality (ether, ester) remained unreacted. This chemoselectivity can be explained with the qualitatively different adsorption dynamics of both functional groups: The strained triple bond of cyclooctyne adsorbs directly into the final state of the underlying potential energy curve. If the second functionality reaches the surface first, the molecule is trapped in an intermediate state with a finite lifetime. In the intermediate state, the molecule can sample the surface with the strained triple bond, thus enabling a conversion into the direct pathway of the strained triple bond with the concomitant cleavage of the weak bonding of the second functionality. The qualitatively different potential energy curves thus enable the chemoselective adsorption of bifunctional molecules Si(001). In analogy to bio-orthogonal chemistry, we term this type of selectivity surface-orthogonal chemistry. Such chemoselective adsorption of bifunctional molecules is the first step for building molecular architectures on semiconductor surfaces.