Reconstructed metals, surface structure

A breakdown of the structural results by type of surface shows results for nearly 50 clean, unreconstructed metal surfaces and about 10 alloys and reconstructed metal surfaces. The structures of about 65 atomic overlayers on metal surfaces have been determined, some 40 of these involving chalcogen atoms. Just over 20 molecular structures have been determined for metal surfaces, half of these being overlayers of undissociated carbon monoxide and the others various hydrocarbons. Turning to semiconductors, some 13 clean, usually reconstructed structures were determined, against nearly 10 atomic overlayer structures. In addition, about 15 insulator surface structures have been investigated. 

To conclude our discussion on semiconductor and metal surface structures, these surfaces tend not to retain their bulk-truncated structures and a huge variety of relaxations and reconstructions are possible. The interested reader may refer to Tables 2.3a and 2.3b of Somorjai s textbook on surface chemistry for a more extensive overview of the many and varied structures that clean solid surfaces can adopt [81]. 

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. 

Kolb and Franke have demonstrated how surface reconstruction phenomena can be studied in situ with the help of potential-induced surface states using electroreflectance (ER) spectroscopy.449,488,543,544 The optical properties of reconstructed and unreconstructed Au(100) have been found to be remarkably different. In recent model calculations it was shown that the accumulation of negative charges at a metal surface favors surface reconstruction because the increased sp-electron density at the surface gives rise to an increased compressive stress between surface atoms, forcing them into a densely packed structure.532... 

In genercil, cill structural changes vhich occur during a surface reaction (reconstruction, or removal of reconstruction) can have a marked effect on both the rate of adsorption and desorption. Possible candidates for these phenomena bo occur are all metal surfaces vhich can undergo reconstruction upon interaction with a chemicedly active ad-soj ate. Interesting systems here are (besides the cilready known Pt (100) orNi(IIO) faces) Ir(100)/0,00 or W(100)Al and Mo(100)/H. 

Subsequently, Mitchell s group in Vancouver, by means of a tensor-LEED study17 of the Cu (110)-(2 x 3)N surface structure, supported a reconstruction model in which the topmost layer is described as a pseudo-(100)-c(2 x 2)N overlayer with metal corrugation of about 0.52 A in the reconstructed layer. Each nitrogen adatom is almost coplanar with the local plane formed by the four neighbouring copper atoms. Of the four N atoms present in the unit mesh, three are also bonded to Cu atoms in the layer below and therefore are five coordinate. 

The structural behaviour of sulfur at metal surfaces has proven to be very rich. Large-scale reconstruction has been proposed in many cases and models for... 

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 EAM has been used to study the surface structure of other metals and metal alloys. For example. Daw has suggested that a missing row configuration is also the likely structure for the (2 x 1) reconstruction of the Pt(l 10) surface. Studies have also been made of the surface structures of various alloys, where for example surface segregation of one constituent over the other has been observed ° . In addition to studies of specific systems, the EAM formalism is also sufficiently general that it has been used to understand trends in surface reconstructions among various metals . ... 

The first successful first-principle theoretical studies of the electronic structure of solid surfaces were conducted by Appelbaum and Hamann on Na (1972) and A1 (1973). Within a few years, first-principles calculations for a number of important materials, from nearly free-electron metals to f-band metals and semiconductors, were published, as summarized in the first review article by Appelbaum and Hamann (1976). Extensive reviews of the first-principles calculations for metal surfaces (Inglesfeld, 1982) and semiconductors (Lieske, 1984) are published. A current interest is the reconstruction of surfaces. Because of the refinement of the calculation of total energy of surfaces, tiny differences of the energies of different reconstructions can be assessed accurately. As examples, there are the study of bonding and reconstruction of the W(OOl) surface by Singh and Krakauer (1988), and the study of the surface reconstruction of Ag(llO) by Fu and Ho (1989). 

Although the (111) face of fee metals is of the lowest surface free energy — a fact which may explain the reconstructions of the (100) and (110) faces —, the (111) face itself may also reconstruct Au(l 11) is normally reconstructed with a structure that may nevertheless still involve the hexagonally close-packed layer geometry (since extra sites of hexagonally arranged spots appear in LEED), but with a lattice constant different from that of the bulk . ... 

During the preceding decade the theoretical research has also shifted, possibly as a result of detailed experimental findings. The emphasis is now on microscopic-level reconstruction of surfaces. Advanced surface diffraction and imaging techniques allow detailed characterization of surface morphology at an atomic level. These studies show that metal surfaces contain high concentrations of atomic steps, usually one atom in height, separated by well-ordered terraces. Statistical mechanical theories were developed to explain how atomic-scale processes can lead to the formation of these structures. 

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