Random site surfaces

The above theoretical data obtained for quasi-continuous surfaces can be used as reference states for the interpretation of particle deposition at random site surfaces, as discussed below. 

The random site surface (RSS) regime is pertinent to nanoparticle (colloids, proteins, polyelectrolytes) deposition at surfaces bearing isolated centers such as ions, smaller particles or macromolecules able to bind the solute particles irreversibly. 

Therefore, the results shown in Figure 11.11 support the hypothesis that one can regulate the composition and structure of surface clusters formed in deposition on random site surfaces by the ionic strength. 

A simple mapping enables tire RSA fonnalism to be applied to binding of a ligand L to receptors R embedded in a surface (tire RSA-random site (RSA-RS) model) [149,... 

The active sites from which a reaction in a solid spreads are known as nuclei. It is known that nuclei may grow in one, two, or three dimensions, and each case leads to a different form of the rate law. If the nuclei form in random sites in the solid (or perhaps on the surface), the rate laws are known random nudeation rate laws that have the form... 

Waibel et al. [459] have studied deposition of Pt on unreconstructed Au(lll) and Au(lOO) applying CV and in situ STM. STM studies revealed that PtCh " complex is adsorbed on both surfaces and Pt is further deposited at rather high overpotential. Nucleation of Pt started mainly at the defects, like step edges, at low deposition rates. Due to the high overpotential, some nuclei also appeared on terraces at the random sites. 

Focusing our interest towards special sites available for adsorption, W g(Ar) for a nearest-neighbour vector Ar directly yields the fraction of (two-fold) bridge-sites between unlike atoms. Unfortunately, A(Ar) or W gCAr) do not directly yield the number of three-fold or four-fold hollow sites neighbouring, e.g., only atoms of type A. Nevertheless, we will see that the value of also gives an indication whether such sites are more or less common than in a random alloy surface. 

Finally, we should note that molecules cannot cover a surface completely. As a first approximation, the random site model [37] assumes that they can be represented by impenetrable spheres, adsorbing sequentially on a uniform surface, in locations selected randomly (see Figure 3.5). If the molecule adsorbing on a new trial position overlaps with a previously adsorbed molecule, it does not stick to the surface otherwise, it is adsorbed. An initially fresh surface covers quickly. 

B (l-0)2-p0= when the adsorbing surface have a random site distribution, and... 

For a random heterogeneous surface Eqs. (77) and (79) also apply. However, area fractions or local site densities are not relevant and in this case fj is simply the fraction of sites Sj. 

Jin X, Wang NHL, Tarjus G and Talbot J 1993 Irreversible adsorption on nonuniform surfaces the random site model J. Phys. Chem. 97 4256-8... 

Above expression is valid when the clusters form at random sites within the bulk of original phase (so-called homogeneous nucleation). In the case of heterogeneous nucleation, the clusters are formed on foreign surfaces placed within the system (impurities, walls of the vessel, wafers or certain base materials - e g. nanofibers) and the necessary energy of clusters formation on flat surface reads (see, e g. [4]) ... 

The site of ouabain inhibition of the Na pump has been studied in some detail using site-specific and random-site mutagenesis. Ouabain binds to the outside surface of the pump in a partially K -competitive manner. It was well known that rat Na,K ATPase was relatively resistant to ouabain compared to other spedes. When various Na pumps were sequenced, it was found that the... 

Simulations of random site deposition can be carried out either for point-like sites distributed perfectly random over a homogeneous surface (Jin et td. [51]) or for sites having finite dimensions and the shape of hard disks of diameter la incorporated into the substrate (Figure 11.2a) [49] or hard spheres attached to the surface, (Figure 11.2b) [50]. 

Adsorbed atoms and molecules can also diflfiise across terraces from one adsorption site to another [33]. On a perfect terrace, adatom diflfiision could be considered as a random walk between adsorption sites, with a diflfiisivity that depends on the barrier height between neighbouring sites and the surface temperature [29]. The diflfiision of adsorbates has been studied with FIM [14], STM [34, 35] and laser-mduced themial desorption [36]. 

Diffraction is not limited to periodic structures [1]. Non-periodic imperfections such as defects or vibrations, as well as sample-size or domain effects, are inevitable in practice but do not cause much difSculty or can be taken into account when studying the ordered part of a structure. Some other forms of disorder can also be handled quite well in their own right, such as lattice-gas disorder in which a given site in the unit cell is randomly occupied with less than 100% probability. At surfaces, lattice-gas disorder is very connnon when atoms or molecules are adsorbed on a substrate. The local adsorption structure in the given site can be studied in detail. 

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