Paper SS2-FrM8
Site-selective Reactivity of Ethylene on Si(001) induced by Local Electronic Distortions

Friday, November 13, 2009, 10:40 am, Room N

The adsorption of organic molecules on semiconductor surfaces is of special interest with respect to surface functionalisation and its use in molecular electronics. Due to the localized electronic states of a semiconductor surface, its reactivity strongly correlates with the local electronic properties of the dangling-bond states. Dissociative adsorption of H₂ on Si(001), e.g., shows pronounced site-selective reactivity at steps or preadsorbed atomic hydrogen [1].

In this study, we test the concept of site-selective reactivity at locally distorted configurations for more complex, organic molecules. On that account, the adsorption of ethylene on clean and hydrogen precovered Si(001) surfaces has been investigated by means of scanning tunneling microscopy. On the clean surface, two ethylene adsorption geometries were identified with ethylene adsorbed on one and two dimers, respectively. The latter adsorption geometry shows significantly lower reactivity and has not been observed so far. Preadsorption of atomic hydrogen and the concomitant distortion of the electronic states is found to increase the reactivity of this two-dimer adsorption pathway by almost two orders of magnitude. Its site selective reactivity thus surpasses that of the one dimer configuration on the clean surface.

The results are rationalized in the framework of a precursor mediated adsorption process. Our experiments indicate that the conversion barrier between precursor and final chemisorbed state can be efficiently controlled by changing the local electronic structure of the surface. Thus, locally distorted dangling-bond configurations allow for the control of site-selective reactivity also in the case of barrierless, non-dissociative adsorption of an organic molecule.

[1] M. Dürr and U. Höfer, Surf. Sci. Rep. 61, 465 (2006)