Adatom lattice

Fig. XVII-18. Contours of constant adsorption energy for a krypton atom over the basal plane of graphite. The carbon atoms are at the centers of the dotted triangular regions. The rhombuses show the unit cells for the graphite lattice and for the commensurate adatom lattice. (From Ref. 8. Reprinted with permission from American Chemical Society, copyright 1993.)...

As an example of adatom-lattice defect interactions on a surface, we will describe here a study of the interaction between an adatom and an impurity atom in a surface layer. An impurity atom, like any other lattice defect, in the surface layer can perturb the periodicity of the surface both electronically and elastically. Such a perturbation will change the potential energy of a diffusing adatom on the surface. A study of adatom-... 

Figure Bl.21.3. Direct lattices (at left) and corresponding reciprocal lattices (at right) of a series of connnonly occurring two-dimensional superlattices. Black circles correspond to the ideal (1 x 1) surface structure, while grey circles represent adatoms in the direct lattice (arbitrarily placed in hollow positions) and open diamonds represent fractional-order beams m the reciprocal space. Unit cells in direct space and in reciprocal space are outlined.

Fig. 2. A graphic of a nanotube showing a pulled-out atomic wire and several stabilizing spot-welds. Only two layers have been shown for clarity, although typical multiwalled nanotubes have I0-I5 layers. The spot-weld adatoms shown between layers stabilize the open tip conformation against closure. The atomic wire shown was previously part of the hexagonal lattice of the inner layer. It is prevented from pulling out further by the spot-weld at its base.

At the same time, many lattice dynamics models have been constructed from force-constant models or ab-initio methods. Recently, the technique of molecular dynamics (MD) simulation has been widely used" " to study vibrations, surface melting, roughening and disordering. In particular, it has been demonstrated " " " that the presence of adatoms modifies drastically the vibrational properties of surfaces. Lately, the dynamical properties of Cu adatoms on Cu(lOO) " and Cu(lll) faces have been calculated using MD simulations and a many-body potential based on the tight-binding (TB) second-moment aproximation (SMA). " ... 

In order to study the vibrational properties of a single Au adatom on Cu faces, one adatom was placed on each face of the slab. Simulations were performed in the range of 300-1000"K to deduce the temperature dependence of the various quantities. The value of the lattice constant was adjusted, at each temperature, so as to result in zero pressure for the bulk system, while the atomic MSB s were determined on a layer by layer basis from equilibrium averages of the atomic density profiles. Furthermore, the phonon DOS of Au adatom was obtained from the Fourier transform of the velocity autocorrelation function. ... 

The overall reaction pathway is probably similar to what has been presented for the reaction of SnBu4 with Pt particles, that is first hydrogenolysis of the As-C bond to form Nis[AsPh2] , species, which rapidly evolves into Nis[AsPh]jy, Nis[As]jy to give finally an alloy by migration of the As adatoms into the Ni lattice as evidenced by the formation of Nickeline (NiAs) according to XRD studies (Scheme 36). 

Dynamics of Crystal Growth hi the preceding section we illustrated the use of a lattice Monte Carlo method related to the study of equilibrium properties. The KMC and DMC method discussed above was applied to the study of dynamic electrochemical nucleation and growth phenomena, where two types of processes were considered adsorption of an adatom on the surface and its diffusion in different environments. 

What became evident was that interactions between adsorbed particles can also exert an influence on their surface mobility and therefore the residence time at a particular site. The mean residence time of an isolated oxygen adatom at the Ru(0001) surface varies from 60 to 220 ms when a second oxygen adatom is located two lattice constants a0 apart from the first but only 13 ms when the... 

The existence of active sites on surfaces has long been postulated, but confidence in the geometric models of kink and step sites has only been attained in recent years by work on high index surfaces. However, even a lattice structure that is unreconstructed will show a number of random defects, such as vacancies and isolated adatoms, purely as a result of statistical considerations. What has been revealed by the modern techniques described in chapter 2 is the extraordinary mobility of surfaces, particularly at the liquid-solid interface. If the metal atoms can be stabilised by coordination, very remarkable atom mobilities across the terraces are found, with reconstruction on Au(100), for example, taking only minutes to complete at room temperature in chloride-containing electrolytes. It is now clear that the... 

The considerations in Section 18.3 apply to a single adsorbed atom. At higher coverages the interaction of the adatoms with each other is important. We consider the case of adsorbed alkali ions. If the adsorbates form a regular lattice, this interaction takes the form ... 

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