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The adsorption of organic molecules on silicon has been the subject of intense research due to the potential applications of organic functionalization of silicon surfaces in semiconductor technology. The high reactivity of the silicon dangling bonds towards almost all organic functional groups, however, presents a major hindrance for the first basic reaction step of such a functionalization, i.e., chemoselective attachment of bifunctional organic molecules on the pristine silicon surface. Due to this high reactivity, the final adsorption products typically consist of a mixture of molecules adsorbed via different functional groups. For the preparation of well-ordered organic layers on silicon, it is thus important to learn how to control the reactions of the single functional groups.
Using various spectroscopic techniques, such as XPS, UPS, and nonlinear optics, in combination with scanning tunneling microscopy and molecular beam techniques, we investigated in detail the reaction mechanisms, kinetics, and dynamics of different functional groups on Si(001). Our main strategy for the controlled organic functionalization of Si(001) is then based on functionalized cyclooctynes: cyclooctyne’s strained triple bond is associated with a direct adsorption channel on the Si(001) surface, in contrast to almost all other organic molecules, which adsorb via weakly bound intermediates [1,2]. As a consequence, cyclooctyne derivatives with different functional side groups react on Si(001) selectively via the strained cyclooctyne triple bond while leaving the side groups intact. This second functional group is then used for the covalent attachment of further organic reagents on the road to well-defined molecular architectures on Si(001).
Electronic excitation [3] and hyperthermal energy distributions of the incoming molecules [4] are investigated as further means of control.

[1] M. Reutzel, et al., J. Phys. Chem. C 120, 26284 (2016).
[2] C. Länger, et al., J. Phys.: Condens. Matter 31, 034001 (2019).
[3] G. Mette, et al., Angew. Chemie Int. Ed. 58, 3417 (2019).
[4] T. Lipponer, et al., Surf. Sci. 651, 118 (2016).

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