Atomization step

All real surfaces will contain defects of some kind. A crystalline surface must at the very least contain vacancies. In addition, atomic steps, facets, strain, and crystalline subgrain boundaries all can be present, and each will limit the long-range order on the surface. In practice, it is quite difficult to prepare an atomically flat surface. 

What gives rise to streaks in a RHEED pattern from a real surface For integral-order beams, die explanation is atomic steps. Atomic steps will be present on nearly all crystalline surfaces. At the very least a step density sufficient to account for any misorientation of the sample from perfeedy flat must be included. Diffraction is sensitive to atomic steps. They will show up in the RHEED pattern as streaking or as splitdng of the diffracted beam at certain diffraction conditions that depend on the path difference of a wave scattered from atomic planes displaced by an atomic step height. If the path difference is an odd muldple of A./2, the waves scattered... 

The first four steps in our procedure lead to a provisional Lewis structure that contains the correct bonding framework and the correct number of valence electrons. Although the provisional stmcture is the correct structure in some cases, many other molecules require additional reasoning to reach the optimum Lewis structure. This is because the distribution of electrons in the provisional structure may not be the one that makes the molecule most stable. Step 3 of the procedure places electrons preferentially on outer atoms, ensuring that each outer atom has its full complement of electrons. However, this step does not always give the optimal configuration for the inner atoms. Step 5 of the procedure addresses this need. 

Figure 8.3. Potential energy diagram based on DFTcalculations. Notice how the reaction pathway is strongly modified by the presence of atomic steps on the Ni (112) surface. First of all, steps lowerthe barrier for the initial methane dissociation. Although this barrier...

In the following we consider nitrogen atoms adsorbed on a ruthenium surface that is not completely flat but has an atomic step for each one hundred terrace atoms in a specific direction. The nitrogen atoms bond stronger to the steps than to the terrace sites by 20 kj mok. The vibrational contributions of the adsorbed atoms can be assumed to be equal for the two types of sites. (Is that a good assumption ) Determine how the coverage of the step sites varies with terrace coverage. 

Nitric oxide is dissociatively chemisorbed at Ru(0001) at 295 K, with Zambelli et al.n establishing the role of a surface step in the dynamics of the dissociation process. Figure 8.3 shows an STM image taken 30min after exposure of the ruthenium surface to nitric oxide at 315 K. There is clearly a preponderance of dark features concentrated around the atomic step (black strip), which are disordered nitrogen adatoms, while the islands of black dots further away... 

Pco = 5 x 10 8 mbar). The times refer to the start of the CO exposure. The structure at the upper left corner is an atomic step of the Pt surface. Image sizes, 180A x 170A Vt = 0.5 V /t = 0.8nA. (Reprinted with permission from Ref. [58]. Copyright 1997, The American Association for the Advancement of Science.)... 

Figure 6.4 (a) Large-area STM image (620A x 620 A, + 0.48V, 1.4 nA) of a singledomain lepidocrocite nanosheet on (1 x 2)-Pt(l 1 0). The central brighter area is separated from the lower terrace by a substrate mono-atomic step. Inset (14 x 4) LEED pattern. 

Another example of the flexibility of the Pt catalyst is the reconstruction of a stepped Pt(l 11) crystal with adsorbed sulfur upon exposure to CO [25]. Single-crystal Pt(l 1 1) cut at an angle of approximately 5° from the (1 1 1) direction consists of numerous terraces with a width of 20-60 A separated by steps with single-atom height. The adsorption of sulfur atoms restructures the clean stepped Pt(l 1 1) surface with single-atom steps into a sulfur-adsorbed surface with double-atom... 

For metals and crystals, cleavage can attempt similar feats, but the results are not as good. Metal surfaces formed by cleavage are usually not atomically flat. When an Au wire is flame-annealed in a hydrogen-air flame, the Au(lll) face is formed preferentially, since it has a lower surface energy than the Au(100) or Au(110) faces, but these Au(lll) faces resemble New Mexico mesas the atomically flat region may be only 50 x 50 nm, and is surrounded by one- or two-atom steps leading down to the plain, and then on to the next mesa. 

When the hydrogen atoms released from methoxy species in step 8.14 are trapped with extra oxygen atoms (step 8.10), step 8.11, and step 8.12 cannot be discriminated from each other. Alternatively, the hydrogen atoms react with OeH(a) to produce H20(g) ... 

Divalent dissolution is initiated by a hole from the bulk approaching the silicon-electrolyte interface which allows for nucleophilic attack of the Si atom (step 1 in Fig. 4.3). This is the rate-limiting step of the reaction and thereby the origin of pore formation, as discussed in Chapter 6. The active species in the electrolyte is HF, its dimer (HF)2, or bifluoride (HF2), which dissociates into HF monomers and l ions near the surface [Okl]. The F ions in the solution seem to be inactive in the dissolution kinetics [Se2], Because holes are only available at a certain anodic bias, the Si dissolution rate becomes virtually zero at OCP and the surface remains Si-H covered in this case, which produces a hydrophobic silicon surface. 

Step 2 Draw any single, double, or triple bonds between the carbon atoms. Step 3 Add the branches to the appropriate carbon atoms of the main chain. Step 4 Add hydrogen atoms so that each carbon atom forms a total of... 

Step 1 The main chain is hexane. Therefore, there are six carbon atoms. Step 2 This compound is an alkane, so all carbon-carbon bonds are single. Step 3 The ethyl group is attached to carbon number 3. Tbe metbyl group is attached to carbon number 2. 

Recently the previously developed Rh(lll) EAM potential has been employed to model the ejection process from Rh(331), a stepped surface that consists of (111) terraces three atoms wide with a one-atom step height. In this surface there are atoms that are both more and less coordinated than on the (111) surface. The agreement between the experimental and calculated angular distributions is excellent . This same EAM potential was used for the Rh interactions in the 0/Rh(l 11) study discussed in section 2. 

Ideal Surfaces, A model of an ideal atomically smooth (100) surface of a face-centered cubic (fee) lattice is shown in Figure 3.13. If the surface differs only slightly in orientation from one that is atomically smooth, it will consist of flat portions called terraces and atomic steps or ledges. Such a surface is called vicinal. The steps on a vicinal surface can be completely straight (Fig. 3.13a) or they may have kinks (Fig. 3.13b). 

The interpretation of this data on metals in terms of microscopic mechanisms of surface atom transport is not totally understood. The original papers[ 11] proposed that during surface transport the controlling process was adatom terrace diffusion between steps with the adatom concentration being that in local equilibrium with the atomic steps. This may indeed be the case, but in light of other experiments on adatom diffusion[13] and exchange processes at steps[14] the possibility of step attachment/detachment limited kinetics caimot be raled out. 

Figure 13. Schematic showing the probable configuration of atomic steps surrounding the (001) terrace at a saddle point of a 2-D grating on Si(OOl). Note that if the surrounding steps are mono-atomic they are all of the same type and will prefer to be all of the low energy type(SA) if the saddle point moves up or down by one step unit the surrounding steps will all be of the high energy type. This is believed to be the basic reason for the preference in type of saddle point terrace.

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