Two-layer cluster

To model the copper (100) surface a two-layer cluster of C4V symmetry, with 5 copper atoms in one layer and 4 copper atoms in the other layer, has been used. In this cluster, all the 9 metal atoms were described by the LANL2DZ basis set. The LANL2DZ basis set treats the 3s 3p 3d 4s Cu valence shell with a double zeta basis set and treats all the remainder inner shell electrons with the effective core potential of Hay and Wadt [33]. The non-metallic atoms (C and H) were described by the 6-3IG basis set of double zeta quality with p polarization functions in... 

In our relativistic density-functional study of mixed Pt-M nanoparticle surfaces is represented by a two-layer cluster with seven surface and three second-layer atoms, Ptio-nMn(7,3) [6]. The subnano cluster model does not simulate bulk surface properties because of its limited size and undercoordinated metal atoms. However, the model is suitable for simulating the properties of nanoscale particle catalysts, e.g., Pt-Ru alloy nanoparticles wife an fee surface. Catalytically much more active than bulk metal surfaces, these nanocrystals exhibit a transition from metallic to insulator properties [48]. The cluster model is also suitable for rough Pt-M electrode surfaces that exhibit a high surface density of reactive Pt-M sites [49]. 

Significant differences are also found between the one-layer and the two-layer results for the second group clusters. Adding three Pt-atoms to (Ptn) as the second layer leads to a decrease of -0.2 eV in Eads, an increase of -0.05 A in Rp,.c, and a reduction of -1 Ncm" in kpt.c- Then, the Eads exhibits a very slight decrease (0.07 eV) fi-om the two-layer cluster to the three-layer one. The variation of Eads... 

Lipid bilayer (Section 26 4) Arrangement of two layers of phospholipids that constitutes cell membranes The polar termini are located at the inner and outer membrane-water interfaces and the lipophilic hydrocarbon tails cluster on the inside... 

It is observed that the CA dynamics, with these rules, results in a pattern in which clusters of X are scattered among clusters of Y. This is not what is seen in the laboratory. In reality, two immiscible liquids will separate into two layers, each responding to a different influence from gravity. What the CA model shows is the separation experienced in the weightlessness of space. A gravity rule must be added. 

Although Ni(CO)4 was discovered many years ago, no neutral Ni2(CO)x compound has ever been synthesized in macroscopic amounts. However, several communications report ionic species such as [Ni2(CO)8l+, [Ni2(CO)7], and [Ni2(CO)6]+, where structures with one or two bridging carbonyls are proposed.2418 Plausible structures for neutral Ni2(CO)x (x = 5, 6, 7) have been investigated by theoretical methods, and decomposition temperatures well below room temperature have been predicted.2419,2420 Tetra-, penta-, and hexanuclear nickel carbonyl clusters have been investigated by means of molecular orbital theory. It is found that the neutral forms are more stable than the corresponding anionic forms but the anionic forms gain in stability as the nuclearity rises.2421 Nickel carbonyl cluster anions are manifold, and structural systematics have been reviewed.2422,2423 An example includes the anion [Ni9(CO)i6]2- with a close-packed two-layer metal core.2424... 

It is evident that the four manganese atoms assume a two-by-two disposition in the two layers formed by the atoms of manganese and oxygen. The most important structural aspect of this Mn2inMn2IV cluster is, as already discussed, the relative proximity of the two coordinated water molecules (Ow, Ow0-... 

Figure 5-11 shows a simple model of the compact double layer on metal electrodes. The electrode interface adsorbs water molecules to form the first mono-molecular adsorption layer about 0.2 nm thick next, the second adsorption layer is formed consisting of water molecules and hydrated ions these two layers constitute a compact electric double layer about 0.3 to 0.5 nm thick. Since adsorbed water molecules in the compact layer are partially bound with the electrode interface, the permittivity of the compact layer becomes smaller than that of free water molecules in aqueous solution, being in the range from 5 to 6 compared with 80 of bulk water in the relative scale of dielectric constant. In general, water molecules are adsorbed as monomers on the surface of metals on which the affinity for adsorption of water is great (e.g. d-metals) whereas, water molecules are adsorbed as clusters in addition to monomers on the surface of metals on which the affinity for adsorption of water is relatively small (e.g. sp-metals). 

Nucleation of two-dimensional clusters on the surface, which expand and merge to form new layers... 

Nucleation of two-dimensional clusters mechanism. In cases where the crystal face is smooth, growth can occur by either a two-dimensional nucleation mechanism or by a spiral-growth mechanism. For two-dimensional nucleation, growth occurs by attachment of molecules to the edge of a nucleus on the surface. Under ideal conditions, the growing step on a crystal surface will advance across the crystal face until that particular layer is complete. Before another layer starts, a center of crystallization has to form via surface nucleation. The growth rate forthis mechanism is exponentially dependent on the driving force ... 

In order to assign the red and near infrared PL spectra observed experimentally for heat-pretreated MSs and silica nanoparticles (shown in Figure 4.4), we considered model clusters with various combinations of two types of the point defects, NBO and OV, with OV located 2-5 layers of Si-0 bonds away from NBO [57-59]. In our calculations, we used much larger clusters as compared to those in the previous studies, the defect areas were surrounded by at least two layers of Si-O bonds (Figure 4.5), and only... 

Figure 1. Top panel, TEM cross section of a multilayer, showing the quasiordered arrangements of the Co cluster particles (dark circles). Lower panel, 3-dimensional schematic view of two layers.

Fig. 26. Perspective view of the (010)-surface of the vanadium pentoxide with different mutual arrangements of neighboring pyramidal [VOs] units (Panel a). Panels b and c correspond to neutral, stoichiometric surface cluster including two layers of the (010) surface pyramids (126 atoms) in Panel b, which illustrates the SINDO AIM net-charge distribution, the bipyramidal subsystems I and II are shown (see Table 1) Panel c represents the AIM FF distribution diagram...

Table 1. Rigid and relaxed condensed hardnesses (a.u.) of the complementary subsystems X and Y of the stoichiometric, two- 010)-layer cluster of vanadium pentoxide including 126 atoms (Fig. 26) here X denotes the bipyramidal model of the surface active site (I or II) and Y stands for the corresponding remainder of the cluster [V205] = (X Y).

Similar surface structures should exist in large clusters, beginning with a two-layer 55 atom cuboctahedron. The question arises, whether or not the best coverage of a metal surface can be realized by many small ligands or by fewer bulky ligands, such as phosphines. In contrast to CO, phosphines and related ligands can easily be dosed, in order to prevent formation of smaller clusters. 

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