Adatom lattice structures

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... 

Fig.4 Examples for adsorbate lattice structures on (100) and (111) fcc-surfaces. The choices of unit vectors for adlattice and substrate lattice are indicated. Adatoms are shown in grey. Instead of the four-fold and three-fold coordination sites for the adsorbates, the same adlayer periodicities can result for sites with different coordination e.g., on top or bridge sites...

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. 4 Possible adatom (xmfigurations for the coadsorption of two atomic species (e.g. C,0) on the square lattices of preferred adsorption sites on (100) surfaces of b.c.c. transition metals. The two atomic species are denoted by small open or filled circles, respectively, (a) shows the top layer of the substrate and possible adsorption sites the solid lines connect centers of the substrate atoms in this layer, (b) shows the c(2 x 2) structure with random (xxupation of the sites by the two species (c) ordered structure I (the (2x1) structure) (d) ordered structure II [ordered c(2 x 2) structure] (e) and (f) show the disordered lattice gas and lattice liquid states, respectively. (From Lee and Landau .)...

The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species 62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractive forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity 65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption 66-68) which show that these binding sites represent the... 

Atomic processes that constitute the electrodeposition process, Eq. (6.93), can be seen by presenting the structure of the initial, (solution), and the final state, (lattice). Since metal ions in the aqueous solution are hydrated, the structure of the initial state in Eq. (6.93) is represented by [M(H20)J". The structure of the final state is the M adion (adatom) at the kink site (Fig. 6.13), since it is generally assumed that atoms (ions) are attached to the crystal via a kink site (3). Thus, the final step of the overall reaction, Eq. (6.93), is the incorporation of the adion into the kink site. 

At low adsorbate coverages the surface structure of the deposited metal is determined by the substrate periodicity. Thus, under these conditions the adsorbate-substrate interaction is predominant. At higher coverages the adsorbate may continue to follow the substrate periodicity or form coincidence structures with new periodicities that are unrelated to the substrate periodicity. The ordering geometry of large-radius metallic adatoms (especially K, Rb and Cs) shows relatively little dependence on the substrate lattice they tend to form hexagonal close-packed layers on any metal... 

Fig. 4.28 The atomic structure of the W (110) surface. Single adatoms sit at the lattice sites. Closely packed diatomic clusters have their centers of mass sitting in...

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