Adatom islands

Figure 4.17 STM images from a video sequence showing oxygen adatom islands at Ru(0001) for a coverage of 0.09. (a) t = 0 (b) t = 0.17 s with the positions of the atoms illustrated m (c) and (d). Atoms marked brighter in (d) have moved with respect to those in (c) the arrows indicate examples of atom motions by which the size and shape of islands alter. (Reproduced from Ref. 30).

The first qualitative observation of vacancy-induced motion of embedded atoms was published in 1997 by Flores et al. [20], Using STM, an unusual, low mobility of embedded Mn atoms in Cu(0 0 1) was observed. Flores et al. argued that this could only be consistent with a vacancy-mediated diffusion mechanism. Upper and lower limits for the jump rate were established in the low-coverage limit and reasonable agreement was obtained between the experimentally observed diffusion coefficient and a theoretical estimate based on vacancy-mediated diffusion. That same year it was proposed that the diffusion of vacancies is the dominant mechanism in the decay of adatom islands on Cu(00 1) [36], which was also backed up by ab initio calculations [37]. After that, studies were performed on the vacancy-mediated diffusion of embedded In atoms [21-23] and Pd atoms [24] in the same surface. The deployment of a high-speed variable temperature STM in the case of embedded In and an atom-tracker STM in the case of Pd, allowed for a detailed quantitative investigation of the vacancy-mediated diffusion process by examining in detail both the jump frequency as well as the displacement statistics. Experimental details of both setups have been published elsewhere [34,35]. A review of the quantitative results from these studies is presented in the next subsections. 

Conrad et al. (S2) studied in detail the mutual interaction of coadsorbed O and CO on a Pd(l 11) surface. Some of their relevant results are summarized here. Oxygen adsorption is inhibited by preadsorbed CO. At coverages below Oco 1/3, LEED patterns show that O and CO form separate surface domains. However, the behavior is different when O is preadsorbed. CO can be adsorbed on the Pd(lll) surface covered with O which is less densely packed than a saturated CO layer. The O adatom islands are then suppressed to domains of a (v 3 x y/l)R30° structure (0 = 1/3), with a much larger local coverage than can be reached with O alone, which orders in a (2 x 2) structure (ff = 0.25). After further exposure, the LEED patterns s uggest the formation of mixed phases of Oads and CO ads (with local coverages of ffo = Oco = 0.5) which are embedded in CO domains. When these mixed phases are present, CO2 is produced even at temperature lower than room temperature. Coadsorption studies of other noble metal surfaces are consistent with this scenario preadsorbed CO inhibits the dissociative adsorption of oxygen, whereas CO is adsorbed on a surface covered with O. 

Figure 7 STM-image of PtaSnf 111) of the mixed p(2x2) and ( /3 x /3) R30° structure taken after annealing to lOOOK, size (44A), U =0.9V, 1 =1. OnA. The image has been differentiated to enhance contrast. The small adatom islands mark the p(2x2) domain whereas in the lower right corner ( /3 x /3) R30° areas with the honeycomb-network remain. The larger clusters may be due to residual contaminants, below the AES-detection-limit. From Ref. [35].

Fig. 58. Confinement nucleation of adatom islands on a dislocation network, (a) Ordered (25 X 25) dislocation network formed by the second Ag monolayer on Pt( 111) on deposition at 400 K and subsequent annealing to 800 K. The inset shows a model of this trigonal strain relief pattern, (b) A superlattice of islands is formed on Ag deposition onto this network at 110 K (coverage = 0.10 monolayers). The inset shows the Fourier transform of the STM image, (c) Island size distributions for random and ordered nucleation. The curve for ordered nucleation is a binominal fit. The curve labeled i = 1 shows the size distribution from scaling theory for random nucleation on an isotropic substrate. Size distributions were normalized according to scaling theory (5 is the island size in atoms, (S) its mean value, and Ns the density of islands with size s per substrate atom), (d) Zoom into image (b) [198]. Reprinted with permission from H. Brune et al.. Nature 344, 451 (1998), 1998, Macmillan Magazines Ltd.

Standing electron wave patterns of surface states also can be observed in defects. This was realized with adatom islands on Ag( 111) at low temperature [205]. Vacancy islands did not show electron confinement probably because of absorption losses via bulk transitions. 

Besides the (7x7) structure, the DAS model can easily be extended to (2n+1) X (2 +1) reconstructions with n= 1,2,3,4,... The corresponding unit cells possess n(n +1) adatoms and (n — 1) rest atoms. The domain walls separating the (2x2) adatom islands exhibit 3x dimers. Vanderbilt [56] suggested a relatively simple model for the energetics of the Si(lll) and Ge(lll) surfaces. For Si, he estimated the formation energy for the various stractural elements ... 

The consequence of these predictions is that single adatoms wUl tend to stay away from this type of surface step and growth will not occur there until the dimer bound to the lower step edge is broken. The repulsion of adatoms by the step would also enhance nucleation of new adatom islands in the surrounding region because they have two directions to move on the open terrace but only one near the step. This... 

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. 

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... 

The effect induced by different electronegative additives is more pronounced in the case where the additive adatoms occupy the most coordinated sites forming ordered structures (e.g Cl addition onNi(lOO)). In this case (Fig. 2.28) one modifier adatom affects 3-4 CO adsorption sites and complete disappearance of the CO p2-peak is observed above modifier coverages of -0.25 or less. The lack of ordering and the tendency of the modifier to form amorphous islands (e.g. P on Ni(100)) diminishes the effect. Thus in the case of P on Ni(100) the disappearance of the CO p2-peak is observed at P coverages exceeding 0.6. 

Interestingly, we have shown by postreaction LEED that those oxygen adatoms reside on the surface under reaction conditions in p(2xi)-0 islands of local coverage 0 = 0.5 on Ag(llO) or p(4x4)-0... 

Although it was not possible to distinguish between the specificity of oxygen states responsible for methoxy and formate formation, they were to be associated with isolated oxygen adatoms and oxygen states present at the periphery of the (2 x 1)0 islands. 

Nitric oxide is dissociatively chemisorbed at Ru(0001) at 295 K, with Zambelli et al.n establishing the role of a surface step in the dynamics of the dissociation process. Figure 8.3 shows an STM image taken 30min after exposure of the ruthenium surface to nitric oxide at 315 K. There is clearly a preponderance of dark features concentrated around the atomic step (black strip), which are disordered nitrogen adatoms, while the islands of black dots further away... 

Figure 8.3 STM image (380 x 330 A) of a Ru(0001) surface with a step after exposure to NO at 315 K the lower terrace is to the right of the step. The disordered dots are N adatoms the islands consist of oxygen adatoms. (Reproduced from Ref. 13).

A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. 

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. 

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