Rest atom

Figure Al.7.6. Schematic diagrams of the DAS model of the Si(l 11)-(7 x 7) surface structure. There are 12 adatoms per unit cell in the outennost layer, which each have one dangling bond perpendicular to the surface. The second layer, called the rest layer, also has six rest atoms per unit cell, each with a perpendicular dangling bond. The comer holes at the edges of the nnit cells also contain one atom with a dangling bond.

The DAS model has the least number of dangling bonds (19) among all the models ever proposed. There are 12 dangling bonds at the adatoms, 6 at the rest atoms, and 1 at the center atom deep in the corner hole. The 19 dangling bonds are at different energy levels. 

The different surface atoms on the Si(lll)-7 x 7 surface have chemically distinct properties. Because of the electron redistribution, each rest atom and corner hole is negatively charged, while each of the adatoms has a partial positive charge [19,47]. Consequently, the rest atoms and corner hole can be considered nucleophilic, and the adatoms electrophilic. An adjacent rest atom-adatom pair hence forms a dipolar or diradical entity although the closest rest atom-adatom spacing on Si(lll)-7 x 7 (4.5 A) is much larger than the dimer distance on Si(100)-2 x 1 (2.4 A) [48], this pair may be expected to show some similarities in reactivity to the dimer on the... 

Butadiene has also been reported to form a [4 + 2] cycloaddition-like product at the Si(l 11)—7 x 7 surface. Theoretically, the reaction has been predicted to occur between a rest atom-adatom pair via a step-wise, diradical path pathway that is unactivated [19,249]. Recent STM studies of butadiene adsorbed on Si(l 11)—7 x 7 have observed the formation of either a [4+ 2]-like or a [2+ 2]-like product with the Si adatom-rest atom pair, but were not able to definitely assign the product as one or the other [250]. Interestingly, the STM images also reveal that about a third of the cycloaddition product forms between two Si adatoms. Because both adatoms are positively charged, the stepwise reaction does not seem likely, and the authors suggest that this reaction occurs in a concerted fashion [250]. 

Some of the most dramatic STM images have been recorded for the Si(l 11) 7X7 reconstruction, as depicted in Figure 3.14.138 These images have been recorded at several different biases (see Spectroscopy and Chemical Selectivity, below) and provide one of the best examples of how STM can be used to better understand the chemistry of such surfaces ( specifically, the electrophilicity and nucleophilicity of the individual surface atoms). Clearly depicted in these results are the orbitals associated with surface atoms, rest atoms, and backbonds. Such studies have continued and been greatly extended into the exploration of a variety of chemical reactions that occur on silicon surfaces. These studies are described in detail in a recent review.135... 

Fig. 9a, b. Structural model of (a) the c(2x8) Gieflll) reconstruction and (b) the (7x7) Si(lll) reconstruction. The small, dark shaded balls represent second layer atoms and the larger shaded balls, first or rest layer atoms the fighter shaded large balls represent rest atoms that have a dangling bond. The partially transparent circles represent the Si or Ge adatoms. The heavy solid fines fiighfigfit the surface unit cells. 

Figure 12.14a shows a side-view of the planes perpendicular to the [111] direction in the body-centered cubic crystal. The displacements of the atomic planes is similar to Figure 12.6c. Atoms in planes 1 and 4 are at rest, atoms in planes 2 and 3 displace in the opposite direction. From the figure it can be seen that the...

Fig. 5.2-16 Top view of Ge( 111 )c(2 x 8) simple adatom model. Large and small empty circles represent adatoms and rest atoms respectively. Both ideal shaded) and c(2 X 8) unit cells are shown [2.28]...

The structure of the top layers of the reconstructed Si( 111) crystal surface differs considerably from that of the bulk. There are adatoms in the uppermost layer protruding from the surface and centered in a tetrahedron where they have only three neighbors at the corners and an additional atom just below them forming a fourth, strongly stressed, bond. Other types of tri-coordinated centers, called rest atoms, can be observed in the first reconstructed bilayer that possess enhanced affinity toward small molecules. 

Table 2 Calculated MEF Values and Ammonia Binding Energies at 300 pm Distance above the Si(l 11) Surface. Experimental Binding Preferences are Rest Atom > Center Adatom > Comer Adatom, Faulted > Unfaulted. Incorrectly Predicted Preference is Denoted by Italics...

QMs) are used for the description of the core region and the rest atoms of the system are treated by force-field methods. Nevertheless, the latter can be any lower-level method, which is faster than that used for the core. 

H3 site, (c) One adatom occurs for every four surface atoms in a (2x2) unit cell, leaving one atom with a dangling bond, known as rest atom. Electron transfer from the dangling bond of the adatom to the rest atom causes the formation of a lone pair in the dangling bond orbital of the rest atom. 

State, while the dangling bond of the rest atom becomes s-like, which lowers its energy. This causes an electron transfer from the adatom to the rest atom, leaving a filled lone pair orbital on the rest atom. The total energy is lowered, a gap in the surface states opens, and the surface becomes semiconducting and there by is fulfilling Duke s principle III of structure formation on semiconductors. 

Adatom T4-site Rest atom 1 St layer atom 2nd layer atom... 

Figure 9.23 Saturation of dangling bonds by adatoms and rest atoms in a c(2x8) reconstruction on Ge(lll). Adatoms are in T4 sites. Four adatoms occur for every 16 surface atoms in the c(2x8) unit cell, leaving four atoms with a dangling bond, that is, four rest atoms. Electron transfer from...

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