Adsorbate superstructures

Fig. 10. Adsorbate superstructures on 100) surfaces of cubic crystals. Atoms in the top-layer of substrate are shown as white circles, while adsorbate atoms are shown as full black circles. Upper part shows the two possible domains of the c(2x2) structure, ohtained by dividing the square lattice of preferred adsorption sites into two sublattices following a checkerboard pattern either the white sublattice or the black sublattice is occupied with adatoms. The (2x1) structure also is a 2-sublattice structure, where full and empty rows alternate. These rows can be interchanged and they also can run either in -t-direction (middle part) or y-direction (lower part), so four passible domains result and one has a two-component order parameter.

Rotational, translational, and mirrored domains are prevalent in the presence of adsorbate superstructure or adsorbate-induced reconstruction. A surface structure with symmetry lower than the symmetry of the substrate can produce such domains by applying all symmetry operations of the substrate. Since different domains with the same internal structure have the same structural energy, these domains should have an equal existence probability on a surface. If the areas of any domains presented on the surface are larger than the lateral coherence length of the incident electron beam, which is typically in the order of few tenths of a nanometer, the LEED pattern becomes the sum of the individual diffraction patterns of each domain. However, in cases where the symmetry of the surface is distorted by steps or by a strain, it is possible to eliminate certain domains selectively and one domain becomes dominant. Figure 7A shows an example of a well-oriented surface that contains three rotational domains, and Figure 7B displays a one-domain sample obtained using a stepped surface. 

This relationship is true for each direction parallel to the surface independently. It is particularly useful for determining the size of adsorbate islands which lead to extra superstructure spots. A good introduction (in German) to spot profile analysis is given by Henzler and Gopel [2.249]. 

In the last decade two-dimensional (2D) layers at surfaces have become an interesting field of research [13-27]. Many experimental studies of molecular adsorption have been done on metals [28-40], graphite [41-46], and other substrates [47-58]. The adsorbate particles experience intermolecular forces as well as forces due to the surface. The structure of the adsorbate is determined by the interplay of these forces as well as by the coverage (density of the adsorbate) and the temperature and pressure of the system. In consequence a variety of superstructures on the surfaces have been found experimentally [47-58], a typical example being the a/3 x a/3- structure of adsorbates on a graphite structure (see Fig. 1). 

LEED, namely one with a, c(2x2) and one with a, p(2x2) superstructure. They are compatible with CusPt and CusPta layers. The first atomic layer was in both cases found by means of photoemission of adsorbed xenon to be pure copper. Details of the experimental work can be found in ref. 9 and 10. A schematic view of both structures can be seen in figure 1. Both consist of alternating layers of pure copper and of mixed composition. In the CuaPt case, the second and all other evenly numbered layers have equal numbers of copper and platinum atoms, whereas in the CusPta case the evenly numbered layers consist of thrice as many platinum as copper atoms. 

The most recent advances in structure determination by LEED make use of holographic effects. In short, adsorbed atoms in an ordered superstructure on the surface act as beam splitters, reflecting a reference wave and transmitting a wave that reflects from the surface as the object wave. Both waves together constitute the holographic image, from which the adsorption geometry can in principle be reconstructed [25]. 

The structure factor S(q as defined in Eq. (54) in terms of the Ising pseudospins Si, in the framework of the first Bom approximation describes elastic scattering of X-rays, neutrons, or electrons, from the adsorbed layer. SCq) is particularly interesting, since in the thermodynamic limit it allows to estimate both the order parameter amplitude tj/, the order parameter susceptibility X4, and correlati length since for q near the superstructure Bragg reflection q we have (k = q— q%)... 

In many theories, nevertheless, the (1 — 0) law has been used as the foundation on which to build elaborate superstructures, which must necessarily fall if the (1 — 0) law is invalid. Even from a theoretical analysis of what should happen when an incoming molecule strikes a surface which is partially covered, one should become suspicious of the (1 — 0) law. Should one not expect that an incoming molecule, which collides with an adsorbed atom (or molecule), is deflected by this atom and strikes a near-by bare spot where it can be adsorbed The view that the incoming molecule bounces off the surface if it collides with an adsorbed atom is in our opinion far too naive. 

Fig. 6. Experimental results of oxygen adsorbed on Ru(0001) with half a monolayer coverage. While the clean surface has hexagonal symmetry (a), the oxygen covered surface shows a 2 X 2 superstructure (b). The shape of the ensuing features depends on the tunneling conditions (c) it changes from circular to triangular as surface and tip move closer together.

A Cu 100 -c(2x2)-Au superstructure was first reported by Palmberg and Rhodin upon deposition of 0.5 ML of Au on Cu 100 [14], Palmberg and Rhodin argued that the c(2x2) periodicity originated fi-om an ordered two-dimensional CuAu alloy due to the tendancy for transition metals to form close packed monolayers when adsorbed as overlayers to enhance co-ordination when surface alloying is absent [1,2]. 

A clean silver surface will adsorb oxygen in a dissociated form. LEED studies show that on the Ag(l 11) face oxygen adsorbs to form a stable (4 x 4) superstructure. This has been interpreted as a coincidence lattice between the Ag(lll) plane and the (111) plane of silver(i) oxide. There is evidence that oxygen adsorption on faces other than Ag(lll) results in the formation of Ag(l 11) facets. " Incorporation of oxygen into the subsurface appears to be rather slow in the presence of molecular oxygen, but alternate cycles of oxidation and reduction with CO results in the build up of a thin subsurface layer of oxidized silver. ... 

There appear some small peaks at 2.7 and 3.3THz for K/Pt(lll), and they are assigned to surface phonon modes of Pt substrate. These phonon modes are at the zone boundary on the clean surface and they are optically inactive without adsorbates. Since the K adsorbate forms a ( /3 x /3) superstructure at the coverage, the Brillouin zone of a clean surface is reduced such that the zone boundary at the K point is folded back to the T point and so the zone boundary phonon modes become optically active. 

Adsorption. Oxygen is adsorbed on clean tungsten surfaces in a variety of atomic and molecular states. At low temperature (<0 °C), oxygen is adsorbed molecularly, but at room temperature this adsorption is a precursor state to the atomic adsorption. A covered surface shows an ordered oxygen superstructure. If the temperature is increased, a more extensive coverage occurs and oxide-like structures are formed. The surface layer can be described as adsorbed oxide. 

It is of interest to consider the ground state (temperature T = 0) behaviour of this model [121] prior to the discussion of its properties at finite temperatures. The adsorbed layer unit cell, corresponding to the given superstructure, labelled by m, is characterized by the unit vectors e and and... 

Different superlattices with -v/S X /3 periodicity have been imaged. This periodicity has been related to rotation of graphite lattice [17]. These superlattices can be produced by either a multiple tip effect [17b] or electronic perturbations caused by adsorbed molecules [17c]. A hexagonal superlattice with a 4.4 nm periodicity, rotated 30° with respect to the HOPG lattice, and 0.38 nm corrugation has also been reported [17a]. This superlattice was also attributed to rotation of the surface layer of graphite. As this type of superstructures is most frequendy observed for thin layers of material, they have been associated with charge density waves [14, 18]. 

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