Geometric structure surface reconstructions

For organic thin films on metals, one might be able to monitor the order of the underlying substrate if the metallic response dominates over the overlayer nonlinearity. One generally assumes that the overlayer does not perturb the underlying surface of the substrate but there are currently no other direct experimental probes to verify this. As with the reconstruction data, it is always important to consider how the electronic properties of the surface may be altered by the overlayer in addition to the geometrical structure as measured by the symmetry of the response. 

Silicon forms a diamond lattice. As Si single crystals are cut, some of the sp3 bonds that stabilize the diamond structure are cleaved, resulting in the creation of unsaturated, or dangling bonds at the surface. To reduce its surface energy, the system tends to minimize the number of dangling bonds. In this process, new bonds between adjacent Si atoms are formed, associated with geometric surface reconstruction. The reactivity of specific Si surface sites strongly depends on their... 

Fig. 13. STM images of the reconstructed Pt4gNigQ(100) surface showing (a) the geometric structure and (b) a superposition of geometry and chemical contrast [40]. Image (b) shows that the rows of atoms in hollow sites (some of them marked "hollow") consist of 100% Pt, while a large number of Ni atoms (brightest atoms of the rows) is found in the rows of on-top atoms. Line scans along two rows marked "A" and "B" in the lower image are shown on the right-hand side.

Besides single atomic steps double steps are also observed on Si(OOl) surfaces. Surfaces with solely biatomic steps exhibit the same dimer orientation on all terraces, that is, may form a single-domain (2x1) reconstruction. The surface is built only from one of the two inequivalent fee sublattices of the diamond structure. For the geometric structure of double steps, again there are two possible configurations as sketched in Figure 9.37. 

The existence of active sites on surfaces has long been postulated, but confidence in the geometric models of kink and step sites has only been attained in recent years by work on high index surfaces. However, even a lattice structure that is unreconstructed will show a number of random defects, such as vacancies and isolated adatoms, purely as a result of statistical considerations. What has been revealed by the modern techniques described in chapter 2 is the extraordinary mobility of surfaces, particularly at the liquid-solid interface. If the metal atoms can be stabilised by coordination, very remarkable atom mobilities across the terraces are found, with reconstruction on Au(100), for example, taking only minutes to complete at room temperature in chloride-containing electrolytes. It is now clear that the... 

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