Overlayers, structured

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Figure 2 Examples of overlayer structures with appropriate notation, (a) fee (100) p(2X2) (b) fee (100) c(2X2) (c) fee (111) p(73 X 73>R30< (d) bee (110 ) p(2 X 1) and bee (110) p(1 X 2). The two orientations for the unit mesh in (d) are both possible because of the symmetry of the substrate they have the same free energy and are called degenerate.

Figure4.25. LEED pattern obtained from N atoms on Fe(l 11) at Ep = 42 eV. The surface is estimated to have a coverage of 0.96 Ml nitrogen and is reconstructed into a 5x5 overlayer structure. The dark shadow in the...

In the majority of cases where adsorbates form ordered structures, the unit cells of these structures are longer than that of the substrate they are referred to as superlattices. Two notations are used to describe the superlattice, the Wood notation and a matrix notation.18 Some examples of overlayer structures at an fcc(llO) surface are as follows ... 

There has been sustained interest in the behaviour of sulfur at surfaces over several decades and a huge literature base exists. The early studies by LEED quickly established a picture of the range of structures that sulfur created at different surfaces and most of these have since been verified with more advanced techniques. However, over the last 20 years our understanding of the nature of these structures has changed significantly. Very few have turned out to be simple overlayer structures, the majority involving surface... 

Figure 6.10 illustrates LEED patterns of the clean Rh(lll) surface, and the surface after adsorption of 0.25 monolayers (ML) of NH3 [22]. The latter forms the primitive (2x2) overlayer structure (see Appendix I for the Wood notation). In the (2x2) overlayer, a new unit cell exists on the surface with twice the dimensions of the substrate unit cell. Hence the reciprocal unit cell of the adsorbate has half the size of that of the substrate and the LEED pattern shows four times as many spots. 

Overall reaction, 32 281, 285-287 see also Simple overall reactions Overlayer structure, adsorbates on metal surfaces, 29 7-8... 

Selected Overlayer Structures for Adsorbates on Metal Surfaces... 

Figures 5 and 6 also show each overlay structure of the tigloyl derivatives lc-2c and ld-2d, in which all structures are determined by X-ray structural...

Porter and coworkers [98] who have used pc-Au electrodes and have examined monolayers of -alkanethiols CnH2n+lSH with n = 3-10, 12, 16, and 18, have found the surface coverage to be independent of n and equal to 9.3 ( 0.6) xl0 mol cm . Assuming that the real surface area of pc-Au electrode is 1.2 times larger than the geometric surface area, this is consistent with structures proposed on the basis of different experiments in which monolayers of various long-chain n-alkanethiols were found to form a (. 3 X overlayer structure on Au... 

We will now compare the N2 system to the much more studied isoelectronic CO molecule adsorbed on Ni(100). Like N2, CO adsorbs in a c(2 x 2) overlayer structure on Ni(100), occupying on-top sites with the carbon end down with a C—Ni distance of 1.73 A, see Chapter 1 for details. However, the adsorption energy of 1.2 eV [63] is much higher in comparison to that of N2. It is therefore very interesting to see how the difference in electronegativity of the carbon and oxygen atoms influences the surface-chemical bond in comparison to the isoelectronic N2. 

Figure 4.13. Calculated shifts in the d band centers for a number of overlayer structures. The shifts are calculated relative to the d band center for the pure overlayer metal surface. The shifts therefore reflect the change in reactivity of the overlayer relative to the pure metal. Adapted from ref. [44].

Only very few investigations have been performed with surfaces not belonging to the most densely packed planes. On Pd(210) the formation of 1 x 1- and 1 x 2-overlayer structures were observed for which the proposed structure models are reproduced in Fig. 9 (52, 60). For the 1 x 1 structure a C—O stretch frequency of 1878 cm"1 was observed which is, interestingly,... 

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