Overlayer, surface structures

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

Jentz D, Rizzi S, Barbieri A, Kelly D, Van Hove M A and Somorjai G A 1995 Surface structures of sulfur and carbon overlayers on Mo(IOO) a detailed analysis by automated tensor LEED Surf. Sc 329 14-31... 

Other topics recently studied by XPS include the effects of thermal treatment on the morphology and adhesion of the interface between Au and the polymer trimethylcy-clohexane-polycarbonate [2.72] the composition of the surfaces and interfaces of plasma-modified Cu-PTFE and Au-PTFE, and the surface structure and the improvement of adhesion [2.73] the influence of excimer laser irradiation of the polymer on the adhesion of metallic overlayers [2.74] and the behavior of the Co-rich binder phase of WC-Co hard metal and diamond deposition on it [2.75]. 

For overlayer thicknesses of a few atomic or molecular layers, the supporting metal can produce surface-enhanced fields at the surface of the overlayer. Then, composition and structure of the overlayer surface can be analyzed by SERS spectroscopy [4.291]. 

The Wood notation, as this way of describing surface structures is called, is adequate for simple geometries. However, for more complicated structures it fails, and one uses a 2x2 matrix which expresses how the vectors al and a2 of the substrate unit cell transform into those of the overlayer. 

At Rh(lll) under the same conditions as for CO adsorption at Pt(lll), the surface structures observed vary with the pressure. At the lowest pressure (10 8 Torr) a (2 x 1) overlayer forms but with increasing pressure the (v/7 x /7) 7 19° appears in the 10 7-l Torr region, above which (up to 700 Torr) only a (2 x 2) structure is present6. 

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 real-space characterization of the CDW-induced modulation of a 2D surface lattice can be ideally performed with variable temperature STMs. The temperature-dependent modulation can be classified according to the HFW model introduced in Section 4.2 taking the ideal 1x1 surface structure as the reference lattice (as and bs) and the projected CDW-modified structure as the overlayer system (uo and bo). In the case of TTF-TCNQ = a and h = b and for the images taken at 63 K... 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

When adsorption takes place on an ordered metal-crystal surface, the adsorbed material forms ordered surface structures. The root cause of this is in mutual atomic interactions, which may be categorized into adsorbate-adsorbate and adsorbate-substrate interactions. In case of chemisorption, the former is considerably the weaker of the two. The possible long-range ordering of the overlayer formed is dominated by adsorbate-adsorbate interaction, however. Ordering of the adsorbed material is also dependent on the degree of surface coverage. Thus, for instance, at... 

Motoo [305] has also been able to show that an order-disorder transition in the overlayer can affect the activity of the surface (this may be related to the homogeneous adatom surface distribution vs cluster, or island formation), and that the elec-trocatalytic activity depends solely on the surface structure and not on bulk properties. The latter point has been demonstrated [306] by noting the similarity in the behavior of Au on Pt and of Pt on Au. Limited synergetic effects have been observed in both cases. 

One can thus capture a spectrum, convert it for ChemText, read it in and overlay a structural diagram, arrows, boxes, text, etc, to make interpretation of the spectrum easier for the reader. (See Fig. 1.9) Similarly, one can generate a space-filling model, or surface diagram in CHEMLAB and incorporate that as an image, (see Fig. 10)... 

The structure of the hydrogen-poor carbon deposit has not been clarified by surface science and is referred to as the carbonaceous overlayer. The structural nature of the hydrogen-rich carbon deposit, however, was amenable to a suite of surface science techniques and the unique structure of alkylidines [61] was derived. These adsorbate species, which have molecular coun-... 

Fig. 3. Relationship between unit cell (mesh) vectors of a LEED pattern and unit mesh vectors of the corresponding surface structure, x = Integral-index LEED spots (due to substrate) x = fractional-index LEED spots (due to overlap) O = (2v/3 x 2N/3)R30° overlayer lattice (mesh) Grid = substrate mesh A, B = unit cell (mesh vectors of LEED pattern a, b = unit mesh vectors of overlayer structure I = unit mesh vectors of substrate LEED pattern and i = unit mesh vectors of substrate surface.

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. 

The technique of low energy electron diffraction (LEED) has been the most widely used tool in the study of surface structure. LEED experiments involve the scattering of monoenergetic and collimated electrons from a crystal surface and detection of elastically diffracted electrons in a backscattering geometry (Figure 2). The characteristic diffraction pattern in LEED arises from constructive interference of electrons when scattered from ordered atomic positions. The diffraction pattern represents a reciprocal map of surface periodicities and allows access to surface unit cell size and orientation. Changes in the diffraction pattern from that of a clean surface can be indicative of surface reconstruction or adsorbed overlayers. 

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