Surfaces islands

Bustos, V., Ufiac, R., Zaera, F. et al. (2003) Lattice-gas study of the kinetics of the catalytic NO-CO reaction on rhodium surfaces. II. The effect of nitrogen surface islands , J. Chem. Phys., 118, 9372. 

When the attachment of the substrate to the precipitate to be formed is strong, the clusters tend to spread themselves out on the substrate and form thin surface islands. A special limiting case is the formation of a surface nucleus on a seed crystal of the same mineral (as in surface nucleation crystal growth). As the cohesive bonding within the cluster becomes stronger relative to the bonding between the cluster and the substrate, the cluster will tend to grow three-dimensionally (Steefel and Van Cappellen, 1990). 

In future work we plan to extend this approach to consider the effect of external fields due to applied potentials or adsorbed species, as well as the modifications when surface islands can change their mean size (ripening or decaying). Correspondingly, there is noteworthy current work on the effect of sublimation or deposition on the step fluctuations of a vicinal surface (E.g. Pierre-Louis and Misbah, 1996). It would also be interesting to consider the effects of weak pinning potentials. 

Many different methods have been presented for finding MEPs and saddle points, Since a first order saddle point is a maximum in one direction and a minimum in all other directions, methods for finding saddle points invariably involve some kind of maximization of one degree of freedom and minimization in other degrees of freedom. The critical issue is to find a good and inexpensive estimate of which degree of freedom should be maximized. Below, we give an overview of several commonly used methods in studies of transitions in condensed matter. We then compare their performance on the surface island test problem. 

Figure 9 Thefrequency at which the various saddle points for the surface island transitions (illustrated in figure 8) are found with the Dimer method. The lowest saddle points are found with the highestjrequency. Also shown are the number of iterations required to go from the intial state to the saddle point to within a force tolerance of0.001 eV/AFor the more practical 0.01 eV/A tolerance, the average number offeree evaluations was a little under 300. The error bars show the standard deviation.

An adlayer with a variety of adspecies may become covered with patches of surface islands. 

As a second example we will discuss the microscopic basis of noble metal catalyzed reactions. Essential here is recognition of the importance of surface-composition and diffusion to the overall reaction. Since surface adatoms and adsorbed molecules and their fragments show a strong non-ideality in their mixing behavior, classical kinetics approaches do often not apply. This holds especially when strong lateral interaction lead to surface island formation. As an... 

The formation of an overlayer of adatoms or molecules can lead to reconstruction of the surface metal layers. This will reduce strain in the surface layer due to the altered metal-metal atom interactions. Often ordered surface phases are formed, in which the adatoms have reduced reactivity, because of the increased interaction with the reconstructed surface atom overlayer. The reactivity of the adsorbate overlayer is then limited to the boundary atoms of the overlayer surface islands. Once ordered overlayers are formed and the surface concentration of adatoms or molecules is further increased, bonding in... 

At low coverages, thiol molecules are mobile on gold and silver. With increasing surface coverage, islands with the alkane carbon chain tails parallel to the surface plane form and nucleate to cover the surface. Islands with the alkane tails normal to the surface plane consequently form and nucleate to form a complete mono-layer structure. If thiol islands exist on the substrates used in this work prior to lipid deposition, lateral hydrophobic interactions between alkane carbon tails and the lipids, deoxycholate, and cytochrome c oxidase may disperse the islands, leading to a thiol submonolayer with the thiol molecules more evenly spaced over the electrode surface. 

Allen et al. [221] have used both OM and TEM in a study of lubrication of LDPE containing 5000 ppm oleamide and stearamide. The primary fatty amides migrate to the polymer surface islands of amide gradually appear and then coalesce into a uniform coating. 

The QUASES software [124] is a powerful tool for analyzing AES and XPS spectral peak shapes to obtain information on the near-surface sample morphology [91]. By analyzing the shapes of inelastic features adjacent to the main peak of electrons that were not inelastically scattered, it is possible to distinguish surface islands, thin films, and buried layers and to obtain quantitative information on the amount of material in each phase. Software from XPS International (Spectral Data Processor) [123], Casa Software (CasaXPS) [125], the University of Leipzig (Unifit)... 

Atomic diffusion on GaAs surfaces has been examined in the context of coarsening of surface islands on a flat surface. [21] They and others have found that coarsening does not proceed as one would expect for normal attachment/detachment and diffusion processes on surfaces. The data is consistent with a higher than usual concentration of adatoms on the surface, suggesting that the attachment rate of atoms to surface islands is relatively slow compared to the rate at which atoms leave the clusters. Likewise, it suggests that nucleation of new islands is even slower. 

---