Monatomic substrate

Consider the situation where two isolated atoms a and b, with electronic energies ea and /, are located above the — n and m sites in a monatomic substrate, with site (bond) energy a (/ ) (see Fig. 8.1(a)). Upon chemisorption... 

The system we wish to investigate consists of a single atom a interacting with a semi-infinite monatomic chain. The site (bond) energy of the chain is a( ), while that of the a-atom is aa (fja). Initially, the substrate is represented by a cyclic chain of N atoms, whose GF is given by (2.49). A semi-infinite chain is then formed by breaking the bond between the n = 0 and N — 1 atoms (Fig. 3.1). 

Figure 21. STM images of Au( 111) in O.OSAf H2SO4 + 1 xiM CUSO4 at two diffeient potential regions, (a) Bare Au( 111), (b) and (c) Au( 111) at potentials in the middle of two underpotential deposition peaks, (c) Monatomic steps of the substrate. (From Ref. 67 with the permission of the Royal Chemical Society.)...

Fig. 7.139. In situ STM image showing the frazzled appearance of a monatomic step on Aa(111) substrate. System Ag(111)/10 MCuS04 + 5x1(T2 M H2S04 at E= 60 mV vs. SCE and T = 298 K. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, p. 22, copyright 1996John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.

The kinetics by which UPD layers form are qualitatively the processes already discussed. There are the electron transfer kinetics from the metal substrate to the depositing ion and the surface diffusion of the adions formed to edge sites on terraces. Complications occur, however, for there is the adsorption of ions to take care of and that brings up questions of which isotherm to use (Section 6.8). Three kinds of UPD formations are shown in Fig. 7.146. Thus Fig. 7.146 (c) shows ID phase formation along a monatomic step in the terraces on the single ciystal Fig. 7.146 (b) shows 2D nucleation at a step, and Fig. 7.146 (a) shows 2D nucleation on an atomically flat plane. 

Finally, one has to distinguish between underpotential deposition of M on S and alloy formation. Alloys can be formed electrochemically (Brenner, 1942). Underpotential deposition is usually a monatomic step affair. With the alloy, the foreign atoms go on building up until they form part of the new substrate, the alloy. 

In this method, the electrode snrface is snbjected to a beam of low-energy (50 to 500 eV) electrons, and the elastically back-scattered electrons are collected onto a phosphor screen. The appearance of distinct diffraction spots (LEED patterns) on the screen indicates an ordered near-surface region. For known monatomic and small-molecule adsorbates, the adlayer stractural symmetry may be deduced readily from the LEED pattern, especially when information on the surface coverage is available from other experiments. For complex molecules, extraction of the substrate-adsorbate interfacial stracture from digitized LEED data is a nontrivial computational task. °... 

Complex adsorbates and substrates. The majority of LEED studies have concentrated on monatomic or diatomic adsorbates on low-index surfaces but attention is now being turned to less simple systems. This effort is illustrated by the work of Somorjai and his colleagues who have examined acetylene and ethylene on Pt(l 11), normal paraffins and cyclohexane on Pt(l 11) and Ag(l 11), benzene on Pt(l 11)," ... 

Figure 2.6 In situ STM images of a freshly prepared Au(lll) substrate showing the initial thermally induced reconstruction rows (visible as stripes) for different surface areas [2.10]. System Au(lll)/ 10 M H2SO4 at = - 150 mV vs. SCE and T = 298 K. (a) top view of an atomically smooth surface, and (b) 3D representation of a face with a monatomic step. Reprinted by permission of Kluwer Academic Publishers.

Figure 2.10 In situ STM image showing the frazzled appearance of a monatomic step on Ag(lll) substrate 2.22). SystemAg(lll)/10 M CuS04 + 5 x lO M H2SO4at =60 mVvs. SCEandT=298 K. Reprinted from Surface Science Letters, Vol. 327, M. Dietterle, T. Will, D.M. Kolb, Step dynamics at the Ag(lll)-electrolyte interface, p. L495,1995, with kind permission of Elsevier Science.

A comparison between cyclic voltammograms using electrochemically grown and real silver single crystal substrates showed a significant influence of the density of monatomic steps on F at the adsorption peak Ai in Fig. 3.4 [3.93-3.95, 3.109]. Therefore, the assumption of an expanded superlattice structure Ag(lll)-(2 x 2) Pb at low For high AE (Table 3.1) is unrealistic. New experimental results have shown that a better approach is to assume a step decoration at low Fin the potential range of the adsorption peak Ai. 

Consequently, the formation of the Pb adlayer in this underpotential range can be considered as an 1/2 localized adsorption on a square lattice. In this case each adatom in the compact monolayer covers effectively two adsorption sites. Thus, domains with an Ag(100)-c(2 x 2) Pb structure located on different substrate sublattices Oike white and black fields of a chessboard) separated by mismatch boundaries are obtained as shown, for example, by Monte Carlo simulation (cf. Section 8.4) of 1/2 adsorption on a square lattice [3.214], The fit of experimental coverage data of the first Pb adsorption step on Ag(lOO) (cf. Fig. 3.9) by Monte Carlo simulation is illustrated in Fig. 3.30. From this fit, a lateral attraction energy between the Pb adatoms of V Pbads-Pbads 2.5 X 10 J (corresponding to 1.5 x 10 J mole ) can be estimated [3.184, 3.190, 3.191, 3.214]. Preferential Me adsorption on surface heterogeneities like monatomic steps was disregarded in the fit procedure. 

TTie interatomic distance of nearest Pb neighbors in this STM image is d = 0.35 + 0.01 nm. It can be assumed that in this UPD range the overlayer most probably consists of coexisting domains of Ag(l(K))-hcp Pb and Ag(l(X))-c(2 x 2)Pb overlayer structures. The structural transformation from a com-mensurate quadratic Ag(lOO)-c(2 X 2) Pb overlayer to an Ag(100)-hcp Pb monolayer, which most probably starts at mismatch boundaries between adlayer domains and/or monatomic steps of the substrate, is completed at relatively low AE. In the UPD range 0 mV < A < 30 mV, only the hep Pb overlayer structure could be observed by in situ STM. 

The surfaces of real substrates are inhomogeneous and exhibit surface defects such as steps, kinks, pits etc. These defects do significantly influence not only the energetics of 2D nucleation [3.249], but also the overlapping of growing 2D islands. Thus, the assumptions of the Avrami equation (3.65) are not fulfilled in this case. In the following, the influence of surface inhomogeneities such as monatomic steps on the kinetics of 2D Meads phase formation is briefly discussed. 

Figure 3.42 Formation of 2D and ID Meads phases on stepped foreign substrates, (a) 2D nucleation on atomically flat terraces (b) 2D nucleation at monatomic steps (c) ID Meads phase formation along...

Figure 3.44 Schematic representation of i(f) transients of condensed 2D Meads phase formation at monatomic steps, (a) substrate surface with regular step spacing (b) substrate surface with irregular step spacing (arbitrary step distribution).

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