Chemical modification of Si surfaces has attracted much attention from
the view point of engineering such as coating, sensors, molecule-based
microelectronics and so on. In particular, cycloaddition of
unsaturated organic molecules to a Si(100) clean surface is one of the
most important family of chemical reactions, because two stable
covalent bonds are formed between each molecule and the surface.
The Si(100) clean surface is regularly covered with silicon asymmetric
dimers, and partial charge transfer occurs from a down dimer atom (Sd)
to an up dimer atom (Su) for each dimer. Namely, these Sd and Su sites
have ability as Lewis acid and base, respectively, and it is expected that electrophilic and nucleophilic attack will occur at these sites.
We have investigated how these Sd/Su sites on Si(100) surface interact with unsaturated hydrocarbon molecules during the cycloaddition process based on experiments and first-principles calculations, and found a mechanism similar to a well-known chemical reaction in organic chemistry.
This paper consists of two parts. First, we report that regioselective cycloaddition obeying "Markovnikov's rule" is observed for asymmetric alkene molecules such as propene and 2-methylpropene, and discuss the mechanism based on atomic configuration and electronic structure of the precursor state. This provides us with a clear and simple concept of the cycloaddition reactions of relatively small molecules at low temperature (~90K). However, the actual reaction paths can be affected by temperature and steric hindrence. So next, we also discuss how these effects appear in the chemisorption processes of 1,4-cyclohexadiene on a Si(100) surface.
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