Mono-atomic cluster

Fig. 3.24. Sublimation rate (in prn/s) as a function of surface temperature for C and W. For C, two cases are shown (1) mono-atomic evaporation predominates, (2) C3 cluster evaporation dominates [3]...

An alternative type of tip-induced nanostructuring has recently been proposed. In this method, a single-crystal surface covered by an underpotential-deposited mono-layer is scanned at a close tip-substrate distance in a certain surface area. This appears to lead to the incorporation of UPD atoms into the substrate lattice, yielding a localized alloy. This procedure works for Cu clusters on Pt(l 11), Pt(lOO), Au(l 11), and for some other systems, but a model for this type of nanostructuring has not been available until now. (Xiao et al., 2003). 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

For two and three dimensions, it provides a crude but useful picture for electronic states on surfaces or in crystals, respectively. Free motion within a spherical volume gives rise to eigenfunctions that are used in nuclear physics to describe the motions of neutrons and protons in nuclei. In the so-called shell model of nuclei, the neutrons and protons fill separate s, p, d, etc orbitals with each type of nucleon forced to obey the Pauli principle. These orbitals are not the same in their radial shapes as the s, p, d, etc orbitals of atoms because, in atoms, there is an additional radial potential V(r) = -Ze2/r present. However, their angular shapes are the same as in atomic structure because, in both cases, the potential is independent of 0 and (f>. This same spherical box model has been used to describe the orbitals of valence electrons in clusters of mono-valent metal atoms such as Csn, Cu , Na and their positive and negative ions. Because of the metallic nature of these species, their valence electrons are sufficiently delocalized to render this simple model rather effective (see T. P. Martin, T. Bergmann, H. Gohlich, and T. Lange, J. Phys. Chem. 95, 6421 (1991)). 

The chiral discrimination in cyclic dimers and trimers of mono-substituted sulfoxides and thioperoxides (Scheme 3.21) has been studied by means of DFT (B3LYP/6-31+G ) and ab initio (MP2/6-311+G ) calculations [5]. In addition, the inter- and intramolecular proton transfer processes that interconvert these two classes of compounds have been considered for the isolated molecules and the clusters. These two classes of compounds present different kinds of chirality, while the sulfoxides show a stereogenic sulfur atom, the thioperoxides present axial chirality. 

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

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