Silicon dimer

Figure 5.2. The clean Si(100)—2 x 1 surface, with rows of silicon dimers lining the surface. The buckling of the dimers is shown in this figure. These dimers play an important role in the chemistry of organic molecules at this surface.

The first examples of what can be categorized as [2 + 2] type cycloaddition product formed by reaction between an alkene and a silicon surface were reported in the late 1980s. Alkenes such as ethylene, as well as the related alkyne molecule acetylene, were reacted with the clean Si(100)-2 x 1 surface in vacuum [196-213]. The adsorption of these unsaturated C2 molecules (ethylene and acetylene) on Si(100)-2 x 1 is also discussed in Chapter 1. The alkenes were found to chemisorb at room temperature, forming stable species that bridge-bonded across the silicon dimers on the surface. The reaction proceeded by formation of two new a bonds between Si and C atoms, hence the bonding was referred to as di-sigma bonding. In addition, it was shown that while the bonds of the alkene and of the Si—Si dimer are... 

Figure 5.13. Cycloaddition products at the silicon dimer of the Si(100)-2 x 1 surface, (a) shows the [2 + 2] cycloaddition product formed in the reaction with ethylene, and (b) shows the [4 + 2], or Diels-Alder, cycloaddition product formed in the reaction with 1,3-butadiene.

The simplest hydrocarbon molecule is acetylene HC=CH, which in vacuum possesses a triple carbon carbon bond. If this molecule attaches to a clean silicon surface, it has essentially two options it can either adsorb on the tip of a silicon dimer, where the C-C bond in this case is reduced to a double bond or it can attach to two adjacent dimers, if the C-C bond is reduced to a single bond. There was some controversy, a few years ago, about the preferred adsorption site. Different methods seemed to reach a different conclusion concerning the actual adsorption geometry under different thermal conditions (for an outline of the discussion, see [57]). There were essentially two diverging opinions (i) There are only two adsorption... 

Keywords Chemisorption surface structure cycloaddition silicon dimer semiconductor functionalization 7r bonding free-radical reactions. 

The description of the chemisorption in terms of cycloaddition reactions is useful if it leads to reliable predictions of the reaction products for most reactions, a variety of products are possible, yet only one will result from a particular cycloaddition mechanism. Central to the applicability of such schemes is the notion that the silicon dimers contain a weak 7r bond responsible for the enforced concerted motion of the two electrons involved. However, in reality there is little evidence to support the presence of even a weak 7r bond within the dimers. While DFT calculations that enforce spin pairing depict the bond as a singlet biradical [32], spin-polarized calculations predict a triplet ground state for the unbuckled dimer [33] with no 7T character whatsoever. The decoupling of the two silicon electrons means that their motion is not likely to be concerted so that a [2+2] cycloaddition reaction becomes better represented as an independent [1 + 2+1] process, a notation that recognizes the independence of the silicon free radicals. This mechanism is also illustrated in Fig. 3. In practice such a reaction is unlikely to proceed in a concerted fashion, and a key signature for it would be the... 

The reactions of ethylene and acetylene with Si( 100)-(2 x 1) were initially described as being [2 + 2] cycloadditions, with the di-configuration predicted by this mechanism believed to provide the dominant reaction products. A variety of alternate reaction products could actually be formed as a result of the chemisorption, with, e.g., the organic molecule spanning silicon atoms in different dimer rows, adhering above a row oriented perpendicular to the silicon dimers, or adhering above a row and parallel to the dimers. As reviewed in Sec. 3, a variety of alternate structures have now indeed been found for chemisorbed acetylene. Hence, while the [2 + 2] cycloaddition mechanism appears apt for ethylene chemisorption, its applicability to similar processes in acetylene is questionable. 

Fig. 9. 1,3,5,7-cyclooctatetrene and its adsorption geometry on Si(100) for majority species comprising two C=C bonds reacting each in the di-<7 configuration with adjacent silicon dimers. 

Here we describe the results of the reaction of the diketoamine ligand 1 with several halosilanes, which furnish the novel silicon dimers 6-11. 

Si(lOO) reconstmcts as well, yielding a (1 x 2) surface phase that is formed when adjacent silicon atoms bond through their respective dangling bonds to form a more stable silicon dimer. This reconstructed bonding results in a buckling of the surface atoms. Furthermore, because Si-Si dimer bonds are weaker than bulk silicon bonds, the reconstruction actually facilitates etching. For a comprehensive discussion on STM studies... 

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