Adsorbate-induced reconstructions

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

FIGURE 4.3 Left The unreconstructed surface of 50% C/fccCo (100). Right The clock reconstructed surface of 50% C/fcc Co (100). The darker spheres represent cobalt atoms and the lighter ones (in the fourfold hollow sites) represent carbon atoms. (Reprinted from Ciobica. I. M.. van Santen, R. A., van Berge, P. J., and van de Loosdrecht, J., Adsorbate Induced Reconstruction of Cobalt Surfaces, Surface Science, 602, 17-28. Copyright 2008, with permission from Elsevier.)... 

The changes in local coordination of the majority specie (the substrate) at the surface (e.g. adsorbate induced reconstruction). It all depends on the detection mode and on the absolute surface sensitivity of the most surface sensitive detection mode. The way to proceed is to measure differential SEXAFS spectra on one adsorption edge of the substrate material, where the difference is made between the clean substrate and the exposed substrate, or between a bulk sensitive detection of the EXAFS, (TY, FY, high energy AEY) and a surface sensitive measure (low energy AEY). Attempts have been made on this groxmd, but final reports have not yet appeared in the literature. 

In addition to the strong reconstructions of the A1 substrate in these single and multi-layer surface alloys, further weak reconstructions are found, in which A1 atoms in the first few layers are subject to both vertical and lateral displacements from their bulk positions. Accurate treatment of these weak reconstructions is important in both LEED and DFT analyses in order to achieve agreement with experiment. Thus the alkali on aluminum system exhibits a wide spectrum of adsorbate-induced reconstructions of the substrate. 

Jenkins SJ (2006) Dissociative adsorption and adsorbate-induced reconstruction on Ee 211. Surf Sci 600 1431... 

Another complication is that the substrate often does not possess a rigid structure but responds to the forces exerted by the adsorbate by some local deformation. Such an adsorbate-induced relaxation of the topmost substrate layer (with a corresponding change of lattice parameters but no change of lattice symmetry) or adsorbate-induced reconstruction (with a corresponding change of lattice symmetry of the topmost substrate layer) will not be discussed further here. 

Proper identification of the order parameter of a particular system often needs detailed physical insight, and sometimes is complicated because different degrees of freedom are coupled. For example, there are many reports in the literature that an order-disorder transition of adsorbates on loose-packed substrates causes an adsorbate-induced reconstruction of the substrate surface. In such a situation, the order parameter of the adsorbate order-disorder transition is the primary order parameter whereas the lattice distortion of the substrate surface is a secondary order parameter . However, for pure surface reconstruction transitions (i.e. structural phase transitions of the surface of crystals where no adsorbates are involved) all considered degrees of freedom are atomic displacements relative to positions of higher symmetry. The proper distinction between primary and secondary order parameters is then much more subtle. 

Molecular Beam Studies the Mechanism of Adsorbate-Induced Reconstruction... 

A plot of rg vs. 0 Q is strongly non-linearl6,l7 the rate expression governing the adsorbate-induced reconstruction is ... 

Rotational, translational, and mirrored domains are prevalent in the presence of adsorbate superstructure or adsorbate-induced reconstruction. A surface structure with symmetry lower than the symmetry of the substrate can produce such domains by applying all symmetry operations of the substrate. Since different domains with the same internal structure have the same structural energy, these domains should have an equal existence probability on a surface. If the areas of any domains presented on the surface are larger than the lateral coherence length of the incident electron beam, which is typically in the order of few tenths of a nanometer, the LEED pattern becomes the sum of the individual diffraction patterns of each domain. However, in cases where the symmetry of the surface is distorted by steps or by a strain, it is possible to eliminate certain domains selectively and one domain becomes dominant. Figure 7A shows an example of a well-oriented surface that contains three rotational domains, and Figure 7B displays a one-domain sample obtained using a stepped surface. 

Currently, theoretical studies on this process are mainly focused on the adsorbate-induced reconstruction. Ciobica et al. have studied the reconstruction process systematically. One interesting phenomenon... 

J.K. Norskov, Theory of adsorption and adsorbate-induced reconstruction. Surf. Sci. 299(1-3), 690-705 (1994)... 

Vitus, C. M. Chang, S. C. Schardt, B. C. Weaver, M. J. 1991. In situ scanning tunneling microscopy as a probe of adsorbate-induced reconstruction at ordered monocrystaUine electrodes Carbon monoxide on Pt(lOO). J. Phys. Chem. 95 7559-7563. 

KRU Kruse, N., Voss, C. Surface Reactions and Adsorbate-Induced Reconstruction CO and NO onRh Crystals Z. Phys. Chem. 202 (1997) 213. 

Table 9 lists the adsorbate-induced changes on fcc(lOO). Adsorbates induce reconstructions of fcc(lOO) perhaps more frequently than for the hexagonal surfaces, but the statistics are poor, and the choice of metals studied probably not random. However, the adsorbates that cause reconstructions tend to be the same as on the hexagonal surfaces primarily alkali adatoms, some other metal adatoms that form a one-layer alloy with the substrate, and oxygen. While sulfur induces reconstructions on fcc(lll) and fcc(llO), none is evident on fcc(lOO). On the other hand, adsorbates can also remove a clean-surface reconstruction (few such cases are listed in Table 9, but examples are known qualitatively by the change in their LEED patterns). 

Figure 4.5 Superstructures formed by adsorbates with (a-c) no substrate changes induced (except for some modification of layer relaxation) and (d-e) with different kinds of substrate reconstructions induced. Panel (f) displays schematically the lifting of a reconstruction by the adsorbate and panel (g) an adsorbate-induced reconstruction switch.

However, there is almost always a substantial interaction between adsorbate and substrate, so that the latter s structure is modified and the same holds for the adsorbate (when it is a molecule). The substrate s modification may be only by a change of the multilayer relaxation or, more drastically, by an adsorbate-induced reconstruction. The latter can come, as indicated in panels (d) and (e) of Figure 4.5, simply by induced displacements of substrate atoms or by chemical reactions (including replacements of atoms). Also, the adsorbate can an existing reconstruction of a clean surface or make it switch to another type of reconstraction, as indicated in panels (f) and (g), respectively. In rare cases, it has also been found that the adsorbate is incorporated in deeper surface layers (subsurface). 

Figure 4.45 Adsorbate-induced reconstructions of Ni(lOO) by atomic displacements induced by oxygen in the phase Ni(100)-c(2x2)-0 (top panels) and by carbon in the phase Ni(100)-p4g(2x2)-C. Panels (a) and (c) provide a perspective view, while panels (b) and (d) give side and top views, respectively.

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