Surface reconstructions

Much surface work is concerned with the local atomic structure associated with a single domain. Some surfaces are essentially bulk-temiinated, i.e. the atomic positions are basically unchanged from those of the bulk as if the atomic bonds in the crystal were simply cut. More coimnon, however, are deviations from the bulk atomic structure. These structural adjustments can be classified as either relaxations or reconstructions. To illustrate the various classifications of surface structures, figure A1.7.3(a ) shows a side-view of a bulk-temiinated surface, figure A1.7.3(b) shows an oscillatory relaxation and figure A1.7.3(c) shows a reconstructed surface. 

Figure Al.7.3. Schematic illustration showing side views of (a) a biilk-tenninated surface, (b) a relaxed surface with oscillatory behaviour, and (c) a reconstructed surface.

The surface unit cell of a reconstructed surface is usually, but not necessarily, larger than the corresponding bulk-tenuiuated two-dimensional unit cell would be. The LEED pattern is therefore usually the first indication that a recoustnictiou exists. However, certain surfaces, such as GaAs(l 10), have a recoustnictiou with a surface unit cell that is still (1 x i). At the GaAs(l 10) surface, Ga atoms are moved inward perpendicular to the surface, while As atoms are moved outward. 

When atoms, molecules, or molecular fragments adsorb onto a single-crystal surface, they often arrange themselves into an ordered pattern. Generally, the size of the adsorbate-induced two-dimensional surface unit cell is larger than that of the clean surface. The same nomenclature is used to describe the surface unit cell of an adsorbate system as is used to describe a reconstructed surface, i.e. the synmietry is given with respect to the bulk tenninated (unreconstructed) two-dimensional surface unit cell. 

STM found one of its earliest applications as a tool for probing the atomic-level structure of semiconductors. In 1983, the 7x7 reconstructed surface of Si(l 11) was observed for the first time [17] in real space all previous observations had been carried out using diffraction methods, the 7x7 structure having, in fact, only been hypothesized. By capitalizing on the spectroscopic capabilities of the technique it was also proven [18] that STM could be used to probe the electronic structure of this surface (figure B1.19.3). 

Taking advantage of the intrinsic physical and chemical differences of surfaces introduced by the discontinuity of the bulk enviromuent. Specifically, most solids display specific structural relaxations and reconstructions, surface... 

The atomic structure of a surface is usually not a simple tennination of the bulk structure. A classification exists based on the relation of surface to bulk stnicture. A bulk truncated surface has a structure identical to that of the bulk. A relaxed surface has the synnnetry of the bulk stnicture but different interatomic spacings. With respect to the first and second layers, lateral relaxation refers to shifts in layer registry and vertical relaxation refers to shifts in layer spacings. A reconstructed surface has a synnnetry different from that of the bulk synnnetry. The methods of stnictural analysis will be delineated below. 

It is well known that the catalytic oxidation of CO on certain Pt surfaces exhibits oscillatory behavior, within a restricted range of pressures and temperatures, which are coupled with adsorbate-induced surface phase transitions [16,17]. In fact, in their clean states the reconstructed surfaces of some crystallographic planes, e.g. Pt(lOO) and Pt(llO), are... 

Here is the energy gain or loss when a site reconstructs. The lateral interaction energies and V2s between nearest (a) and next nearest (b) (and further) neighbors are most likely attractive to favor the growth of domains that are either reconstructed or unreconstructed. If V2s were repulsive then a c(2 x 2) pattern of alternately reconstructed and unreconstructed cells would be favored. A gas phase particle can adsorb either on the unreconstructed ui = 0 or 1) or the reconstructed surface (r, = 0 or 1) subject to the constraints... 

The first terms in (99) and (100) say that adsorption can take place either on the remaining sites of the reconstructed surface or on those surface sites that are neither reconstructed nor occupied. The first term in (101) allows for reconstruction from the unreconstructed area, 1 — 9, but also says that this reconstruction may be hindered or helped if there is an adsorbate on the unreconstructed surface. A similar interpretation holds for the last term in (101) describing the lifting of the reconstruction. For the adsorption coefficients Wy, Ws, etc., one writes expressions analogous to (47). 

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... 

For Au( 111) in nearly nonadsorbing solutions (0.1 MHC104)at E = -0.3 V (SCE), a reconstruction similar to that existing in UHV has been detected by STM, while at a > Othe (1 x 1) structure has been observed.188,467,538 At a > 0, strings and clusters of atoms disappearing with time have been found on the deconstructed surface. Therefore the more positive. 0 value is probably related to the reconstructed surface (Table 9). 

Figure 2.14. Reaction mechanisms for the oxidation of CO on the (110) surface of platinum (left) and on the reconstructed surface (right). See text for explanation.

Further STM and SXS smdies [Wu et al., 1998] concerning this phenomenon indicated that the presence of specifically and nonspecifically adsorbing anions as well as organic molecules (e.g., pyridine, bipyridine, and uracil) may also lift the reconstructed surface by exhibiting a structural transition, and it has been extensively studied and reviewed in [Kolb, 1996]. 

While in previous ab initio smdies the reconstructed surface was mostly simulated as Au(lll), Feng et al. [2005] have recently performed periodic density functional theory (DFT) calculations on a realistic system in which they used a (5 x 1) unit cell and added an additional atom to the first surface layer. In their calculations, the electrode potential was included by charging the slab and placing a reference electrode (with the counter charge) in the middle of the vacuum region. From the surface free energy curves, which were evaluated on the basis of experimentally measured capacities, they concluded that there is no necessity for specific ion adsorption [Bohnen and Kolb, 1998] and that the positive surface charge alone would be sufficient to lift the reconstmction. 

Figure 5.7 shows the modified surface free energies y calculated for unreconstructed and reconstmcted Au(lOO) using DFT. The details can be found in [Jacob, 2007a]. The plot shows that the hexagonal reconstructed surface is more stable over... 

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... 

FIGURE 4.3 Left The unreconstructed surface of 50% C/fccCo (100). Right The clock reconstructed surface of 50% C/fcc Co (100). The darker spheres represent cobalt atoms and the lighter ones (in the fourfold hollow sites) represent carbon atoms. (Reprinted from Ciobica. I. M.. van Santen, R. A., van Berge, P. J., and van de Loosdrecht, J., Adsorbate Induced Reconstruction of Cobalt Surfaces, Surface Science, 602, 17-28. Copyright 2008, with permission from Elsevier.)... 

As an example we consider the Au(100) surface of a single crystal Au electrode [3]. This is one of the few surfaces that reconstruct in the vacuum. The perfect surface with its quadratic structure is not thermodynamically stable it rearranges to form a denser lattice with a hexagonal structure (see Fig. 15.3), which has a lower surface energy. In an aqueous solution the surface structure depends on the electrode potential. In sulfuric acid the reconstructed surface is observed at potentials below about 0.36 V vs. SCE, while at higher potentials the reconstruction disappears, and the perfect quadratic structure is ob-... 

Figure 2.3 A simple model depicting reconstruction on the surface of a material (a) ideal surface and (b) reconstructed surface.

One important aspect not discussed above is the change in atomic structure at a surface. Contrary to the schematic picture of the Si(lll) surface shown in Fig. 14.6, a solid surface is usually not just the end of a perfect crystal. Surfaces reconstruct in response to the changes in the electronic distribution caused by the surface itself. Again, all these changes occur selfconsistently, and in principle, if the total energy for various configurations of atomic structures at a surface could be evaluated, the shifts in the positions of the atoms and the electronic structures of the surface could be determined theoretically. This approach will be discussed in the next section, but the first calculations for reconstructed surfaces were done using experimental determinations of the atomic positions. 

Surface structure can also be determined in a similar fashion to what is described here for the bulk structure analysis. One begins with an ideal, non-reconstructed surface and calculates the net forces on the atoms or the changes in energy when a surface atom is moved. For example, if there is an energy decrease when an atom is... 

The reconstructed surface showed a polycrystalline—like hydrogen adsorption-desorption voltanunogram. Therefore, the reconstructed Pt(lll) surface cannot be regarded as PtClll) any more. 

The more densely packed reconstmcted surface has a higher work function and a more positive pzc than the unreconstructed one. During cyclic polarization, the shape of voltammograms changes markedly if the scan enters higher positive potentials. The current charge associated with the removal of the reconstruction must be accounted for in the electrochemical studies on reconstructing surfaces. 

The strength of the lattice instability near the Fermi vector depends on the magnitude of the electron-phonon coupling and on the phase space available for electron-hole pair excitation around 2kf. Thus, a reconstructive surface phase transition has to fulfill the following requirements in order to be ascribed to an electronically driven lattice instability ... 

---