Adatom

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.)...

Fig. XVni-8. (a) Work function change for Pt(lU) as a function of oxygen adatom coverage. From Ref. 82. b) Same, for potassium. The corresponding sequence of LEED structures is indicated. [Reprinted with permission from R. G. Windham, M. E. Bartram, and B. E. Koel, J. Phys. Chem., 92, 2862 (1988) (Ref. 83). Copyright 1988, American Chemical Society.]...

LID) see Ref. 139. In this last method, a small area, about 0.03 cm radius, is depleted by a laser beam, and the number of adatoms, N(t), that have diffused back is found as a function of time. From Pick s second law of diffusion ... 

Many surfaces have additional defects other than steps, however, some of which are illustrated in figure A1.7.1(b). For example, steps are usually not flat, i.e. they do not lie along a single low-mdex direction, but instead have kinks. Terraces are also not always perfectly flat, and often contain defects such as adatoms or vacancies. An adatom, is an isolated atom adsorbed on top of a terrace, while a vacancy is an atom or group of atoms missing from an otiierwise perfect terrace. In addition, a group of atoms called an island may fonn on a terrace, as illustrated. 

Figure Al.7.6. Schematic diagrams of the DAS model of the Si(l 11)-(7 x 7) surface structure. There are 12 adatoms per unit cell in the outennost layer, which each have one dangling bond perpendicular to the surface. The second layer, called the rest layer, also has six rest atoms per unit cell, each with a perpendicular dangling bond. The comer holes at the edges of the nnit cells also contain one atom with a dangling bond.

Figure Al.7.7. Atomic-resolution, empty-state STM image (100 A x 100 A) of the reconstmcted Si(l 11)-7 7 surface. The bright spots correspond to a top layer of adatoms, with 12 adatoms per unit cell (courtesy of Alison Baski).

The field ion microscope (FIM) has been used to monitor surface self-diflfiision in real time. In the FIM, a sharp, crystalline tip is placed in a large electric field in a chamber filled with Fie gas [14]. At the tip. Fie ions are fonned, and then accelerated away from the tip. The angular distribution of the Fie ions provides a picture of the atoms at the tip with atomic resolution. In these images, it has been possible to monitor the diflfiision of a single adatom on a surface in real time [15]. The limitations of FIM, however, include its applicability only to metals, and the fact that the surfaces are limited to those that exist on a sharp tip, i.e. difhision along a large... 

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]. 

Bonig L, Liu S and Metiu FI 1996 An effective medium theory study of Au islands on the Au(IOO) surface reconstruction, adatom diffusion, and island formation Surf. Sot 365 87... 

Wahnstrdm G, Lee A B and Strdmquist J 1996 Motion of hot oxygen adatoms on eorrugated metal surfaees J. Chem. Phys. 105 326... 

Figure Bl.19.25. AFM image of Si(l 11)-(7 x 7) taken in the AC mode. Contrast ean be observed between inequivalent adatoms. Image eourtesy of R Erlandsson. (Taken from [217], figure 4.)...

Nakagiri N, Suzuki M, Okiguchi Kand Sugimura FI 1997 Site discrimination of adatoms in Si(111)-7 7 by noncontact atomic force microscopy Surf. Sc/. 375 L329... 

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.

Figure Bl.26.24. The change of work fimction of the (100) plane of tungsten covered by Na, K and Cs, and work fiinction of alkali metals (dashed-dotted line) versus adatom concentration n (Kiejna A and Wojciechowski 1981 Prog. Surf. Sci. 11 293-338).

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Tyliszczak T, Hitchcock A, Wu S, Chen A, Szymanski G and Lipkowski J 1998 X-ray absorption studies of mixed overlayers formed by copper adatom co-adsorbed with anions at the Au(111) electrode surface Synchrotron Radiat. News 11 31-8... 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

B Surface vacant site C Single vacancy kink D Adatom E Kinked ledge F Terrace... 

Figure 4.2 Terraces, ledges and kinks on a solid surface, together with an emerging screw dislocation, a vacant site, and an adatom...

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