Surface layer atomic density changes

For many surfaces, the displacements of the atoms from their bulk truncated positions are more pronounced than a simple relaxation in which only the interlayer spacings change. These may involve lateral displacements of atoms within the surface layers and/or a change in the surface layer atomic density. First, we discuss some examples of reconstructions at metal surfaces and then some examples at semiconductor surfaces. 

Charges in surface layer atomic density Large-scale surface reconstructions producing changes in the surface layer density have been identified on several metal surfaces. Notably, as described in Chapter 1, the top layer of the (100)... 

At S/V and LI V surfaces, the atomic density decreases sharply within a few layers from its value in the bulk phase to zero. This leads to a high excess energy of solid atoms at the surface and hence to a high surface energy. At S/L interfaces of pure metals, the change of atomic density is comparatively small, a few percent, so the value of ctSl for a given metal is generally much lower than those of [Pg.164]

Simple calculation gives a comparable distribution of the electrode potential in the two layers, (64< >h/64( sc) = 1 at the surface state density of about 10cm" that is about one percent of the smface atoms of semiconductors. Figure 5—40 shows the distribution of the electrode potential in the two layers as a function of the surface state density. At a surface state density greater than one percent of the surface atom density, almost all the change of electrode potential occurs in the compact layer, (6A /5d )>l, in the same way as occurs with metal electrodes. Such a state of the semiconductor electrode is called the quasi-metallic state or quasi-metallization of the interface of semiconductor electrodes, which is described in Sec. 5.9 as Fermi level pinning at the surface state of semiconductor electrodes. 

Surface reconstruction is driven by stabilization of the adsorbate after adsorption of carbon atoms on more reactive surface atoms. Ciobica et al. (74) demonstrated that an overlayer of Cads leads to the Co(lll) to Co(lOO) reconstruction on fee cobalt (the stable phase of small cobalt particles). Because of the change in metal atom density in the surface layer, the reconstruction may be associated with faceting and hence creation of step-edge sites, which are highly active in the Fischer-Tropsch reaction (5). Hence, surface reconstruction and formation of a stable carbide overlayer may actually be the processes occurring during the initial activation of the catalyst. This phenomenon has been described by Schulz (101) as self-organization. 

Figure 2.18 reveals that the threefold adsorption sites are not yet completely occupied, suggesting the possibility to increase the coverage even beyond 0 = 1. This is indeed the case and saturation is reached only at0 = 1.5,forwhichtheassociatedl x 2structureis reproduced in Fig. 2.19. The H atoms are no longer adsorbed in identical sites, and rows of Ni atoms in the topmost layer have been shifted together by 0.6 A to allow occupation of opposite threefold adsorption sites. As a consequence, the structures of even deeper layers are affected. This is a case of displacive reconstruction in which the symmetry of the surface unit cell is affected without a change in the atomic density.

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