Buckling of the dimers

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 Si (100 surface reconstructs by forming dimers of (2x1) symmetry, which arrange themselves into parallel rows. It is now well established that these dimers buckle to form the higher-order p(2x2) and c(2x4) reconstructions. This buckling is due to a transfer of an electron from the lower to the upper atom of the dimer, which opens up a gap between the occupied and unoccupied states. At room temperatures or above, the buckled dimers oscillate in time, and therefore appear symmetric under... 

Figure 5.3. Models of the Si(100) and Ge(100) surface (Left) (2 x 1) dimer reconstruction involving symmetric dimers (Middle) c(4 x 2) dimer reconstruction with buckled dimers These two structures are observed for silicon at room temperature and lower temperature, respectively. For germanium, the structure at (Right), the p(2 x 2) dimer reconstruction with buckled dimers, is also observed at lower temperatures. In the top view model, the open circles represent the top layer atoms, with the larger and smaller circles designating the up and down atoms of the dimer, respectively. The filled circles represent the next layer of atoms.

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... 

The asymmetric reconstruction, the buckling of the ionic state Contrary to the covalent state, the ionic component should be stabilized by an asymmetric distortion if this one accommodates a positive charge on one side and a negative charge on the other one. The asymmetry allows to mix the first excited state (antisymmetric and ionic) with the upper component of the doubly excited state (symmetric and ionic) and thus to localize the electrons on a single-atom dimer. 

Experimentaly, the Si 2p core-level spectroscopies have been interpreted within the buckling model, the presence of two different charges on the two silicon atoms of the dimer leading to two core levels [21-24]. However, other explanations have been proposed within symmetric models. Artacho [9] and Redondo [12] have found an excellent agreement with experiment when the spin-orbit coupling is introduced. 

Fig. 4. Top view of the structures formed by RT Ag deposition onto the Si( 100)2x1 surface. The Si substrate dimerization is assumed to remain intact during RT Ag/Si(100) interface formation [92K2]. (a) structural model for isolated Ag adsorption. Ag atom occupies position between two Si dimers. Buckling of Si dimers is shown by variation in the size of the dimer atoms [93L6], Ag-Si bond length is 2.93 A and adsorption height 1.12 A [93Z3], (b) Possible structural model for Si(100)2xl-Ag reconstmction at 0Ag = 0.5 ML [94W1]. Ag atoms are shown in black and Si atoms in white.

The isoelectronic Ge(OOl) surface shows a similar reconstruction. However, the energy gain upon dimer buckling is substantially larger than that for Si. In contrast, the (001) surface of diamond is found to have no buckling of the surface dimers, which is explained by the differences in the surface electronic structure between C(OOl), Si(OOl), and Ge(OOl). ... 

Table 4.7 Structural parameters as defined in Figure 4.35 determined for the (2 X 1) reconstruction phases ofC(lll) and Si(lll). Buckling of the topmost dimer is seen for Si(l 11), whereas no buckling occurs on C(111). For the right columns, the...

The alternation of the dimer buckling is also observed for the Ge(OOl) surface. Zandvliet has estimated the energy gain for this structure in a simple electrostatic dipole model to be of the same order than the value obtained from a first-principle calculation [22]. He assumed a charge transfer of 0.1-0.15 eo from the lower to the upper atom of the dimer. 

Buckles and McGrew [J. Am. Chem. Soc. 88 (15), 1966] have studied the dimerization of phenyl isocyanate in liquid solution in the presence of a catalyst. 

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