Surface structure Clean

The atomic relaxations visualized here are small repositionings of surface atoms. This is shown by the ease with which such relaxation transformations can take place. For example, Palmberg 346) has made the observation that the Ge(lll)-(2 x 8) and Ge(lll)-(1 x 2) clean surface structures relax to (1 X 1) when sodium is deposited with the germanium crystal held at the very low temperature of — 195°C. 

A striking example is diffusion of nickel atoms on a Si(lll) surface. It is known (340a, 383) that Ni causes the complicated Si(lll)-7 clean surface structure to change to a V19 structure. When a Si(lll) crys-... 

After a C(2 x 2)-NH2 layer is formed on W(IOO) by flashing a room temperature NHg deposit to 525°C, more NH3 can be adsorbed on the cooled surface. The C(2 x 2) half-monolayer becomes a (1 X 1) full mono-layer of NHa after a second heating to 525°C. Thermal degradation of this W(100)-(l X l)-NHa causes appearance first of a (6x6)-NH2, then the C(2 X 2)-NH2 and finally a clean surface. Structural models of the transformation via (6 X 6) were not given by Estrup and Anderson. 

The (7 X 7) reconstruction on Si(l 11) is by far the most complex and most famous clean surface structure in the history of surface science. Employing the (7x7) reconstruction on Si(lll), we demonstrate how creative nature is in finding sophisticated strategies for the saturation of dangling bonds and the reduction of the surface free energy. It took more than 25 years from its discovery in 1957 by Farnsworth et al. [47] until its solution in 1985 by the dimer-adatom-staddng fault (DAS) model of Takayanagi et al. [52, 53]. 

It might be imagined that the structure of a clean surface of, say, a metal... 

I.P.P.D and its relatives have become standard procedures for the characterization of the structure of both clean surfaces and those having an adsorbed layer. Somoijai and co-workers have tabulated thousands of LEED structures [75], for example. If an adsorbate is present, the substrate surface structure may be altered, or reconstructed, as illustrated in Fig. VIII-9 for the case of H atoms on a Ni(llO) surface. Beginning with the (experimentally) hypothetical case of (100) Ar surfaces. Burton and Jura [76] estimated theoretically the free energy for a surface transition from a (1 x 1) to a C(2x 1) structure as given by... 

Some fascinating effects occur in the case of CO on Pt(lOO). As illustrated in Fig. XVI-8, the clean surface is reconstructed naturally into a quasi-hexag-onal pattern, but on adsorption of CO, this reconstruction is lifted to give the bulk termination structure of (110) planes [56]. As discussed in Section XVIII-9E very complicated changes in surface structure occur on the oxidation of CO... 

Fig. XVI-8. (a) The quasi-hexagonal surface structure of clean Pt(lOO) surface, (b) Adsorption of CO lifts this reconstruction to give the structure corresponding to the termination of (100) planes (from LEED studies). [Reprinted with permission from G. Ertl, Langmuir, 3, 4 (1987) (Ref. 56). Copyright 1987, American Chemical Society.]...

Surfaces are found to exliibit properties that are different from those of the bulk material. In the bulk, each atom is bonded to other atoms m all tliree dimensions. In fact, it is this infinite periodicity in tliree dimensions that gives rise to the power of condensed matter physics. At a surface, however, the tliree-dimensional periodicity is broken. This causes the surface atoms to respond to this change in their local enviromnent by adjusting tiieir geometric and electronic structures. The physics and chemistry of clean surfaces is discussed in section Al.7.2. 

The study of clean surfaces encompassed a lot of interest in the early days of surface science. From this, we now have a reasonable idea of the geometric and electronic structure of many clean surfaces, and the tools are readily available for obtaining this infonnation from other systems, as needed. 

Because LEED theory was initially developed for close packed clean metal surfaces, these are the most reliably determined surface structures, often leading to 7 p factors below 0.1, which is of the order of the agreement between two experimental sets of 7-V curves. In these circumstances the error bars for the atomic coordinates are as small as 0.01 A, when the total energy range of 7-V curves is large enough (>1500 eV). A good overview of state-of-the-art LEED determinations of the structures of clean metal surfaces, and further references, can be found in two recent articles by Heinz et al. [2.272, 2.273]. 

In figures 1, 2, and 3 we present the phonon DOS of Au single adatoms on Cu(lOO), Cu(llO), and Cu(lll) respectively for the three directions x, y, and z at 300"K. It is interesting to note that the structure and position of these modes are completely different from those of the corresponding clean surfaces of Cu. ... 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

A catalyst used for cleaning exhaust gases from automobiles consist, among other things, of Rh particles on an AI2O3 support material. The Rh particles expose primarily Rh(lll) and secondarily Rh(lOO) surface structures. Rh is a FCC metal with a lattice distance of a = 0.381 nm. 

Figure 3.3 Phase diagram showing the free energy for different surface structures for water at pH = 0 in contact with Au(lll), Pt(lll), and Ni(lll). The figure is based on the free energy values in Table 3.1. All free energies are shown relative to those of the clean surface with the water bilayer.

Figure 10.17 STM images of the changes in surface structure observed when meth-anethiol is adsorbed at a Cu(110) surface at room temperature, (a) Clean surface with terraces approximately lOnm wide separated by multiple steps, (b) After exposure to 2 L of methanethiol there has been considerable step-edge movement. On the terraces a local c(2 x 2) structure is evident, (c) After a further 7 L exposure, a view of a different area of the crystal shows rounded short terraces these still retain the c(2 x 2) local structure, (d) After 60 L gross changes to the surface are evident and the STM is unable to image at high resolution.

If the tip is contaminated, its apex is most likely attached to a hydrogen molecule or H atoms. As a consequence, the conductance of this tip should be much lower than that of a clean tungsten tip. Since this conductance change has not been reported, it can be concluded that the reduced 0—0 distance is not the effect of a contaminated tip. Surface-tip interactions are evaluated by calculating the interaction between the reacted surface and a tungsten cluster at low distance. Here, the calculations indicate that there is no substantial relaxation due to interactions between the two leads. Consequently, the only possibility left is that the electronic surface structure somehow changes the appearance of the oxygen positions. 

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