Surface cluster distribution

Summary. Surface-enhanced Raman spectrsocopy (SERS) can be used as an in-situ method for monitoring the development of surface morphology, nucleus formation, and crystal growth. The correlation between the true surface area and the Raman intensity was investigated. The splitting of the CN stretch vibration is interpreted as a representation of a surface cluster distribution. 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

Figure 2. Evidence for an atom addition mechanism of cluster growth is provided by analysis of the Ni Cry cluster distribution produced by vaporization of a nichrome surface. The simulated distribution below assumes that the probability of Ni or Cr occurring in a cluster is related only to its composition in the source material. Reproduced from Ref. 8, Copyright 1985, American Chemical Society.

Since the first electron-microscopical observation of a heavy atom on a surface different studies have looked at effects related to individual atomic adsorbates. These include diffusion along the surface (atoms can be tracked in real time), giving results in agreement with equivalent FIM observations, and pair spacing distributions, showing for example a peak in the distribution near 4-5 A for uranium atoms on a carbon surface Clustering can be studied in some cases as well. 

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...

The surface properties of colloids depend on the organization and structuring of their aqueous environment, which is best modeled, at the present time, using explicit water clusters. The orientation of bound water molecules depends on a number of factors including the surface charge distribution, polarization and... 

Shrinkage porosity Gas porosity Appears as a localized honeycomb or mottled pattern due to improper pouring temperature or alloy composition (e.g., A1 alloys) Round or elongated smooth, dark spots located individually or in clusters distributed randomly in the casting due to release of gas during solidification or evaporation of moisture from volatiles from the mold surface... 

A word of cantion is in order. A better connectivity in SSC cannot be instantly eqnated to higher condnctivity. In the simulations of Nafion and SSC, the same amount of water is distributed in the same volume. A change in connectivity is almost certainly accompanied by a change in the characteristic dimension and geometry of the cluster channel. In other words, if one stretches out clusters in SSC in order to better connect them, under the constraint that the volume of the aqueous domain is the same as that in Nafion, then one must accept that the dimension of a channel in a clusters in SSC is smaller. (That the characteristic channel width in SSC is smaller than in Nafion has also been observed experimentally at least for the medium and high water contents.) This change in dimension can affect the environment of the water and hydronium ions. If the channel is smaller and more spread out in SSC PFSA membrane than in Nafion, then it has more surface area with the hydrophobic phase per unit volume. This additional interaction with the hydrophobic phase can be characterized as additional confinement. The effects of confinement on both the diffusion of water and the vehicular and structural components of diffusion of the proton are not fully understood. Thus it is important to corroborate the suggestions of this water cluster distribution analysis with other measures of structure and transport. 

Figure 2 The (a) mass- and (b) surface-averaged distribution of atoms on the (111) and (100) crystal faces and on the edges and corner sites of a cubo-octahedral cluster model. (From Ref. 4.) Mass-averaged and surface-averaged distributions are based on calculations using cubo-octahedron cluster model and represent number of different crystallographic planes divided by the (a) mass or (b) the surface area of the cluster (at the corresponding particle size). Hence (e- -c) in (a) represents edge and kink positions and (100) (111) the normal cubic crystal planes.

Silver photography is based on the formation, during the exposure, of the latent image made of silver clusters distributed at the surface of each silver halide crystal embedded in the gelatine and containing from 0 to 10 atoms. 

Electron microscopic studies in the 1940s proved that supported catalysts possess a crystalline structure, dispelling earlier conjecture of amorphicity. However, practical catalysts are never uniform, exhibiting particle size distribution, lattice defects (Frenkel or Schottky), and dislocations. The following questions then arise Are all lattice surfaces equally active Do surface clusters of particles and surface atoms have comparable activity Does catalytic activity depend on particle size Is there an optimal particle size or distribution These questions remain, in general, still unanswered. However, in recent electrocatalytic studies concern about these effects is shown, following similar concern in conventional heterogeneous catalysis. 

The estimation of the depth of interaction revealed that not only the implantation of the Cu ions into the matrix lattice with forming isolated ions is possible, but the formation of small surface clusters (CuO)x with highly covalent Cu—O bonds. The distribution of catalytically active component between the free oxide, clusters, and ions implanted into the matrix lattice depends both on the conditions of formation and on the composition of the catalytic system as well as on the type of cement-containing agent. 

Fig. 2.2. Space partitioning in EPE embedded cluster calculations. I - internal region treated at a QM level II - shell model enviromnent of the QM cluster subdivided into regions of explicit optimization (Ila), of the effective (Mott-Littleton) polarization (lib) and of the external area (lie). The sphere indicates an auxiliary surface charge distribution which represents the Madelung field acting on the QM cluster (dashed line).

The first thing to note is the difference in cluster distribution between the defect and the control simulations Conformations in the top three clusters of the defect simulation make up about 81 % of the total probability of surface-bound states, whereas conformations in the first cluster alone in the control simulation have a similar probability of existing on the surface of just over 78 %. As Fig. 6 shows, this is because areas of shortened alkyl chain lengths caused by depressions in the gold substrate below the SAM surface dramatically disrupt the helical structure that LKal4 normally adopts at interfaces, leading to a wide array of unfolded structures. Nearly, all secondary structure, indicated by the color of the peptide s backbone (i.e., magenta, cyan, and purple indicate turns, coils, and alpha helical residues, respectively), is lost with the addition of the surface defects. Unlike the central... 

---