Reconstructed surfaces of metals

The (110) surfaces of the transition metals Au, Ir. Pt display both a stable (1x2) and a metastable (1x3) reconstruction. The structural details of these reconstructions are listed in table 2 and the (1x2) reconstructed surface is illustrated in fig. 2. Both the (1x2) and (1x3) reconstructions are of the missing-row type which involve the removal of every second (1x2) or third (1 x3) row of atoms from the top atomic plane of the bulk termination. The removal of this row is accompanied by significant atomic relaxations of at least the first three atomic planes perpendicular to the surface, see table 2. Both Pt(110)(lx2), Pt(110)(lx3) and Au(110)(lx2) have relaxations of a similar magnitude but the relaxations of Ir(110)(lx2) are significantly smaller. In addition to the planar relaxations, all of these surfaces exhibit lateral motions of the atoms within the second atomic plane out towards the valleys left by the missing rows. In addition, the removal of the atomic row causes a buckling of the third atomic layer which conforms with the hill and valley structure of the missing-row surface. 

The missing row reconstruction may be induced in ordinarily unreconstructed fee metals by driving electrons into the surface region, either electrochemically or by alkali-metal adsorption. For example, a (1x2) missing row reconstruction of Cu(110) and Pd(110) may be created by K and Cs adsorption (Barnes et al., 1985 Hu et al., 1990). The structural parameters of these surfaces are included in table 2 and show the smaller normal relaxations that are qualitatively similar to the stable missing-row forms of Ir(110). 

The Ir(100)(lx5) reconstruction is caused by a lateral distortion of the top layer of Ir atoms along the (10) direction (Lang et al., 1983). This distortion allows the top layer of Fr atoms to form a quasi-hexagonal two-dimensional lattice which is commensurate with the underlying (100) plane formed by the 

Below room temperature, the W(100)c(2x2) reconstructed surface is created by lateral movements of the W atoms in first W layer which propagate into at least the second layer of the surface (fig. 3). Alternate atoms move along the (Oil) direction to form zig-zag chains. A LEED structural study (Pendry et al., 1988) determines the amplitude of the lateral movements to be 0.24 0.04 A in the top W layer and 0.028 0.007 A in the second W layer. The top layer relaxes into the surface by -7.0 2.0% of the bulk interlayer spacing, the second layer spacing expands by +1.2 2.0%. These structural parameters are in reasonable agreement with a recent X-Ray diffraction (XRD) determination (Altmann et al., 1988) which finds that the amplitude of the lateral movements is 0.24 0.05 A in the top W layer and 0.10 0.05 A in the second W layer. By XRD, the top layer is found to relax into the surface by -4.0 1.0% of the bulk interlayer spacing. 

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Reconstructed surfaces of metals are also perturbed by the Ceo molecules, as has been shown for bare Au(l 10)-p(l x 2) where a Au(110)-p(6 x 5) superstructure is induced (Pedio et al., 2000). Again, the adsorption of Ceo is accompanied by important displacements of underlying gold atoms. 

LEED has been used to determine the structure of a wide variety of surfaces, including clean and reconstructed surfaces of metals and semiconductors, and atomic and molecular physisorption and chemisorption on many different substrates (see part 5). As the theoretical and experimental tools of LEED have improved, the structure of systems with larger and more complex unit cells have been determined. Successful LEED structure determinations have been carried out for systems with several molecules adsorbed in unit cells up to 16 times larger than the substrate unit cell,/1 / and for reconstructed surfaces where the structural rearrangement involves several surface layers./2/... 

In Figs. 5.2-4 and 5.2-6, some accepted models of reconstructed surfaces of metals are shown. Table 5.2-4 gives the parameters for the 2 x 1 missing-row reconstruction of Au(llO), Ir(llO), and Pt(llO). The missing-row reconstruction stabilizes the surface by relieving the elastic stress. 

The LEED pattern of the clean (1x2) reconstructed Au(llO) is presented in Fig. 16 [72] together with a schematic representation of the missing row reconstructed surface. The peculiar geometry of the missing row reconstructed surfaces of fee metals offers a priori a unique way to generate linear structures of adatoms and to measure their specific properties. However, as will be seen below, in the present case, the situation is more complex. 

Our discussion in the previous part of this section showed some of the variety associated with surface reconstructions on the surfaces of metals with both the fee and bcc structures. Just as such surfaces have served as a proving ground for our understanding of metals, semiconductor surface reconstructions have played a similar role in the study of covalent systems. The starting point for the analysis in this section is a synthesis of a wide variety of experimental data in the form of a phase diagram for the Si surfaces. The objective in the remainder of the section will be to try to rationalize, even predict, such phases on the basis of microscopic analysis. Beyond their academic interest to surface science, the reconstructions in Si also impact the nature of the growth processes that occur when atoms are deposited on exposed surfaces. 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

Two surfaces are conditioned the aforementioned InP(lll) A-face with In atoms protruding a quarter of a monolayer and the In-rich InP(lOO) (2x4) reconstructed surface of a thin homoepitaxial layer, prepared by metal organic vapor-phase epitaxy (MOVPE) on a crystalline InP wafer substrate [241], Whereas the former surface has been successfully conditioned for the development of an efficient and stable photovoltaic photoelectrochemical solar cell (see Section 2.5.2), the latter surface has been employed in the development of an efficient photoelec-trocatalytic structure for light-induced hydrogen generation (see Section 2.6.2). 

In Tables 6-8 we have listed the submonolayer and monolayer stractures of metals on the principal low index surfaces of metal substrates from 2- to 4-fold rotational symmetry. This compilation comprises heteroepitaxial systems only since the structure of homoepitaxial systerrrs is in most cases trivial. For umeconstructed surfaces the bulk stacking is pseudomorphically continued and for reconstructed ones the reconstruction is lifted below the adlayer and at the same time taken on by the adsorbate layer. Only few homoepitaxial cases are worth mentioning since their reconstructions can metastably be lifted, as seen for Au/Au(110)-(lx2) [97Giin], or a reconstruction can be induced at a lower tenqreratrrre by homoepitaxial adsorption, as seen for Pt/Pt( 111) [93Bot]. 

Adsorbate structures and reconstructions, particularly of metal surfaces. The high sensitivity of MEIS to movements of substrate atoms parallel to the surface makes the technique very useful in the study of such systems. MEIS allows one to quantify the number of displaced atoms and the number of reconstructed layers. 

FIGURE 20.8 Structure of the reconstructed surface of Au(lll). (Reprinted from Prog. Surf. Sci., 51, Kolb, D.M., Reconstruction phenomena at metal-electrolyte interfaces, 109-173, Copyright 1996, with permission from Elsevier.)... 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

Since then, STM has been established as an insttument fot foteftont research in surface physics. Atomic resolution work in ultrahigh vacuum includes studies of metals, semimetals and semiconductors. In particular, ultrahigh-vacuum STM has been used to elucidate the reconstructions that Si, as well as other semiconducting and metallic surfaces undergo when a submonolayer to a few monolayers of metals are adsorbed on the otherwise pristine surface. ... 

In 1985 Car and Parrinello invented a method [111-113] in which molecular dynamics (MD) methods are combined with first-principles computations such that the interatomic forces due to the electronic degrees of freedom are computed by density functional theory [114-116] and the statistical properties by the MD method. This method and related ab initio simulations have been successfully applied to carbon [117], silicon [118-120], copper [121], surface reconstruction [122-128], atomic clusters [129-133], molecular crystals [134], the epitaxial growth of metals [135-140], and many other systems for a review see Ref. 113. 

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

The first STM evidence for the facile transport of metal atoms during chemisorption was for oxygen chemisorption at a Cu(110) surface at room temperature 10 the conventional Langmuir model is that the surface substrate atoms are immobile. The reconstruction involved the removal of copper atoms from steps [eqn (1)], resulting in an added row structure and the development of a (2 x 1)0 overlayer [eqn (2)]. The steps present at the Cu(llO) surface are... 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

The BET surface area of the catalysts is summarised in Table 3. The enhancement could be explained in case of MO-s with the reconstruction of the lamella structure. The reason of enhancement in the presence of 212 is still not known. All the other cases significant decrease can be observed. The surface area of metallic part of the used RNi-s shows increase from A to C, with the increasing temperature of the catalyst production, indicating growing Ni distribution. 

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