Surfaces dimer reconstruction

Approximate first-order kinetics has been observed on the Ge(100)-2 x 1 surface, which also has a surface dimer reconstruction [44, 55]. D Evelyn et al. [44] showed that the prepairing model reproduces deviations from first-order behavior on Ge, again with an of about 5 kcal/mol. This preliminary evidence hints at wider generality for the prepairing model. Several groups have found indications of first-order kinetics on the diamond (100)-2 X 1 surface [56-58] as well, though the kinetic parameters are not well determined. 

Diels-Alder reactions are allowed by orbital symmetry in the delocalization band and so expected to occur on the surface. In fact, [4-1-2] cycloaddition reaction occurs on the clean diamond (100)-2 x 1 surface, where the surface dimer acts as a dienophile. The surface product was found to be stable up to approximately 1,000 K [59, 60], 1,3-Butadiene attains high coverage as well as forms a thermally stable adlayer on reconstructed diamond (100)-2 x 1 surface due to its ability to undergo [4h-2] cycloaddition [61],... 

As discussed above, the Si(001) surface is reconstructed into dimers, a side view of which is shown in Fig. 7. In addition to the reconstruction of the surface atoms there is significant distortion of the subsurface region. This distortion blocks channels which enter into the bulk, and causes an excess of backscattered ions over what would be expected from atomic rows in the ideal crystal. This local distortion is the property monitored by backscattered ion intensities. The dimer pairs also tend to form in rows, which is the source of the (2 X 1) LEED pattern observed for this surface. 

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.

Although Si(100) and Ge(100) undergo similar dimer reconstructions, the Ge(l 11) surface reconstructions differ from those of Si(lll). As described above, Si(lll) reconstructs into a (7 x 7) structure that contains 49 surface atoms in the new unit cell. Ge(lll) is found in various reconstructed forms depending on surface preparation, but the most common reconstruction under vacuum is Ge(lll)-c(2 x 8) [51-53]. This structure involves charge transfer from adatoms to restatoms [5]. On the other hand, most of the passivation and functionalization studies reviewed here lead to the Ge(lll)-1 x 1 surface structure. This structure, in which the surface Ge atoms retain their bulk positions, can be achieved by hydrogen, chlorine, or alkyl termination of the surface (discussed below). The structure is analogous to that for H-terminated Si(lll). 

Fig. 1. Models of the silicon(lOO) surface, (a) The clean reconstructed Si(100)-(2 X 1) surface lined with rows of symmetric dimers, (b) The tilted-dimer model of the surface. Note that the actual periodicity is c(4 X 2). (c) The monohydride-passivated Si(001)-(2 X 1)-H surface, Dimers are symmetrized upon hydrogen adsorption.

FIGURE 2. (a) Si(100) surface before reconstruction. Topmost layers are represented by the largest circle, (b) Si(100) surface after the 2 x 1 reconstruction, (c) Symmetric structure of surface dimer. 

Both the Si(100) and Ge(100) surfaces have stable (2x1) reconstructions which involve the saturation of dangling bonds by the formation of" dimers between the atoms in the top layer of the bulk termination of the solid. In the case of Si(100)(2xl), although the existence of surface dimers is no longer controversial, there have been contradictory reports of dimers oriented parallel to the surface (the symmetric dimer model) or tilted in the plane perpendicular to the surface (the asymmetric dimer model), see fig. 10. 

Figure 6.8 Atomic force microscopic image of a reconstructed (lOO)-diamond face. The ordered structure of the surface dimers is evident ( APS 1993).

However, this is only true for hydrogenated surfaces. With the diamond featuring a different surface structure, other effects come to the fore. Reconstructed surfaces, for instance, bear tt-bonds that may be arranged in various ways on the main crystallographic faces (Section 6.2.2). The surface dimers present on the (lOO)-plane, for example, cause the respective orbitals to split into a n- and a n -... 

Depending on the method of preparation, the surface of a diamond film is either functionalized aheady, for example, covered by hydrogen atoms, or it exhibits an array of so-called surface dimers (it-bonds arising from reconstruction). The latter case is mostly found for samples that have been subject to a secondary thermal treatment to remove their initial, usually inhomogeneous surface functionalization. 

Early LEED studies [97] indicated that the basic structure of a clean 100 surface was 2x1. (For a discussion of the nomenclature of surface crystallography, see ref. 113.) Three different models have been proposed to account for this form of reconstruction the vacancy model [78, 79, 114] assumes that half the surface atoms are missing, while the other two suggestions invoke surface dimerization [80, 114] and the formation of complex conjugated chain structures [115, 116], respectively. We will consider each of these in a little more detail. 

Fig. 8.28. Plots of surface energy density versus biaxial mismatch strain for (105) vicinal surfaces with steps aligned with the [010] direction on (001) Si. The stepped edges are serrated in both cases by removing every other step-edge atom, which leaves two possibilities for dimer reconstruction on the terraces. In the configuration proposed by Khor and Das Sarma (1997), the dimer nearest the step edge rebonds directly into the step face, whereas the configuration proposed by Mo et al. (1990) presumes the alternate dimer reconstruction. The plot is based on compntations by Shenoy et al. (2002) using the Tersoff potential.

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