Untersuchungen zur selektiven Reaktivität von Ethen, Cyclooctin und Tetrahydrofuran mit Si(001)-Oberflächen

The site-selective reactivity of three prototypic organic molecules (ethylene, cyclooctyne and tetrahydrofuran) on a Si(001) surface was studied by means of scanning tunneling microscopy (STM). This technique permits the real-space study of clean and adsorbate covered surfaces with atomic resolution; detailed information on adsorbate geometry was obtained with respect to the underlying substrate structure. As a local probe, STM furthermore allows to distinguish between different adsorption geometries of one and the same adsorbate on the surface which may occur with varying frequency due to different site-selective reactivities. By means of careful coverage dependent measurements, the reactivity of different adsorption sites was deduced for all three molecules. Details of the adsorption mechanisms were accessible by varying the surface temperature during adsorption. Precoverage of atomic hydrogen leads to locally distorted configurations on the surface. The site-selective reactivity of these configurations for the respective molecules was studied in order to get deeper insight into the adsorption mechanism of the adsorbates. Ethylene is the smallest unsaturated organic molecule. The adsorption of ethylene on Si(001) surfaces has been studied extensively over the last years. It is well established that ethylene adsorbs non-dissociatively on one silicon dimer. In the present study, a second adsorption geometry was identified with an ethylene molecule adsorbed on two dimers. Although this second reaction path exhibits much lower reactivity on the clean Si(001) surface, nearly 20% of the molecules adsorb via this two-dimer pathway at high ethylene coverage. Preadsorption of atomic hydrogen is found to increase the site-selective reactivity of ethylene at locally distorted dangling bond configurations in comparison to the clean surface. Thus, also in the case of non-dissociative adsorption of an organic molecule, the site-selective reactivity can be controlled by changing the local electronic structure. This can be explained by a precursor mediated chemisorption process which was corroborated by means of Monte Carlo simulations. In the framework of this precursor model, the adsorbate distributions for different coverages were simulated and good agreement with the experimental results was obtained. In summary, a conclusive and advanced understanding of the adsorption process of ethylene on Si(001) has been achieved. Cycloalkynes are cyclic hydrocarbons with one triple bond leading to high ringstrain, especially in the case of smaller rings. The influence of these characteristics on the adsorption mechanism was studied for the adsorption of cyclooctyne on Si(001). Interestingly, the same adsorption behavior was found at low and at room temperatures, in contrast to most other organic adsorbates. This observation indicates a direct (barrierless) adsorption pathway. Two adsorption geometries were identified with the molecule situated symmetrically above one and two surface dimers, respectively. This leads to varying nearest neighbor distances of the adsorbed molecules of 1.5 and 2 dimer distances. At high coverage, cyclooctyne forms a well-ordered first layer on Si(001). Both the experimental results and Monte Carlo simulations yield a maximum coverage of 0.58 ML. Preadsorption of atomic hydrogen does not influence the site-selective reactivity of cyclooctyne at locally distorted dangling bond configurations. This result supports the interpretation of a direct adsorption mechanism. Based on these results, cyclooctyne is likely to show a higher chemical selectivity for the reaction with Si(001) than other molecules. As a consequence, cyclooctyne is a promising candidate for the synthesis of semiconductor-organic interfaces. The adsorption of tetrahydrofuran on Si(001) was studied in order to investigate possible reactions of organic solvents with semiconductor surfaces. Despite of the inert behavior of tetrahydrofuran in the liquid phase, an unexpectedly rich surface chemistry of tetrahydrofuran on Si(001) was observed. Entirely different adsorption geometries were identified at low temperature and at room temperature. Furthermore, at elevated surface temperatures, a complex reorganization occurs. The adsorption geometry at low temperature is likely to be a dative-bonded intermediate state. An irreversible conversion into the room temperature configuration is possible by thermal excitation of the low-temperature configuration. Further heating of the tetrahydrofuran covered surface to temperatures above room temperature leads to several new configurations which indicate decomposition of the molecule and in particular the insertion of oxygen into the substrate. In addition to the thermal excitation, a tip-induced conversion from the low temperature configuration into the room temperature adsorption geometry was identified at low temperature. Based on detailed investigations of its voltage and current dependence, this effect could be attributed to an electronic excitation.