Surface dimer

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],... 

The second dimer-opening mode observed resulted when a surface atom diffused to the site of a surface dimer and bumped a nearby atom into the center of the dimer. This mechanism resulted in atoms which occupied lattice sites, and produced a surface dimer which remained open for the course of the simulation. Because the final atomic positions corresponded to lattice sites, and because a high rate of surface diffusion was required to produce the bump , this mechanism was associated with the high-temperature epitaxial growth mode identified by Gossmann and Feldman. 

Fi. 8. Insertion mechanism of dimer opening as a function of time in femtoseconds (fs). The adatom started with 7.5 eV of kinetic energy and oriented perpendicular to the surface. The hatched circles represent the adatoms, the shaded circles the original surface dimer atoms and the open circles the substrate atoms. Only four layers of the ten us in the simulation are shown. (From Ref. 172.)... 

Figure 5.4. The highest occupied molecular orbital of a Si911,2 dimer cluster. The top two silicon atoms comprise the surface dimer, and the remaining seven Si atoms contain three subsurface layers which are hydrogen terminated to preserve the sp3 hybridization of the bulk diamond cubic lattice. The up atom is nucleophilic and the down atom is electrophilic.

Due to the historical importance of the initial stages of silicon oxidation to microelectronics fabrication, there has been a great deal of interest in the reaction of the water oxidant on the Si(100)-2 x 1 surface. A number of studies have shown that water adsorbs in a dissociated state consisting of OH(a) and H(a) species adsorbed on the Si surface dimer at room temperature [60-69]. More recent studies have closely investigated the mechanism of water oxidation. A series of density functional theory calculations (DFT) calculations by Konecny and Doren indicated that water first molecularly adsorbs through one of its lone pairs in a weakly bound precursor state, then transfers a proton to form OH(a) and H(a) species on the surface dimer [43]. The pathway to proton transfer is found to be unactivated with respect to the entrance channel, which suggests that OH(a) and H(a) are the dominant surface species at room temperature, in agreement with the previous experimental work [60-69]. 

Eggins and McNeill compared the solvents of water, dimethylsulfoxide (DMSO), acetonitrile, propylene carbonate, and DMF electrolytes for C02 reduction at glassy carbon, Hg, Pt, Au, and Pb electrodes [78], The main products were CO and oxalate in the organic solvents, while metal electrodes (such as Pt) which absorb C02 showed a higher production for CO. In DMF, containing 0.1 M tetrabutyl ammonium perchlorate and 0.02 M C02 at a Hg electrode, Isse et al. produced oxalate and CO with faradaic efficiencies of 84% and 1.7%, respectively [79], Similarly, Ito et al. examined a survey of metals for C02 reduction in nonaqueous solution, and found that Hg, Tl, and Pb yielded primarily oxalate, while Cu, Zn, In, Sn, and Au gave CO [80, 81]. Kaiser and Heitz examined Hg and steel (Cr/Ni/Mo, 18 10 2%) electrodes to produce oxalate with 61% faradaic efficiency at 6 mA cm-2 [82]. For this, they examined the reduction of C02 at electrodes where C02 and reduction products do not readily adsorb. The production of oxalate was therefore explained by a high concentration of C02 radical anions, COi, close to the surface. Dimerization resulted in oxalate production rather than CO formation. 

The previously discussed hydrolysis and oligomerization are evident from the spectrum of the air cured sample (figure 9.19 b). The silane region shows clear contributions from the hydrolyzed monodentate (-46 ppm) and oligomerized (tridentate, -66 ppm) forms. The APDMS is found to be almost entirely (90%) in the bidentate form. Because of the bifunctionality of the APDMS this is the most plausible form. Any hydrolysis and condensation between silane molecules can at most lead to surface dimers. A minor contribution of the monodentate form (-12.4 ppm) is visible. 

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.

EXAFS data characterizing Si02-supported molybdenum species made from [Mo2(allyl)4] have led to precise structural models, including those of surface dimers (with Mo-Mo bonds) and pair sites (without Mo-Mo bonds) (Iwasawa, 1987), but the structures seem to be sensitive to undefined chemistry of the Si02 support surfaces and could be difficult to reproduce. Catalytic data for ethylene hydrogenation and butadiene hydrogenation with these samples and with samples expected to have isolated Mo sites point to a catalytic role of the neighboring sites (Iwasawa, 1987). 

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. 

FIGURE 10. (a) Schematic illustration of surface hydrogen bonding between dissociatively adsorbed OH of two surface dimers on an Si(100)-2 x 1 surface, (b) Stmcture of surface hydrogen bonding between dissociatively adsorbed OH of four surface dimers on an Si(100)-2 x 1 surface... 

Figure 2 Schematic diagram of a single-layer-stepped Si or Ge(00 1) surface. The dumbbells represent surface dimers. The two differenttypesofsingle-layerstepsarelabelledS AandS B (ao = 5.43 A and a = 3.84AforSi(00 1) and do = 5.64 A and a = 4.00 A for Ge(00 1)).

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