Commensurate reconstruction

A special notation is used to describe surface reconstructions and surface overlayers and is described in books on surface crystallography (Clarke, 1985). The lattice vectors a and b of an overlayer are described in terms of the substrate lattice vectors a and b. If the lengths la I = mlal and Ib l = nibl, the overlayer is described as mXn. Thus, a commensurate layer in register with the underlying atoms is described as 1 X 1. The notation gives the dimension of the two-dimensional unit cell in terms of the dimensions of an ideally truncated surface unit cell. 

Figure 1.5. Plan view of Cu(lll)/CH3S- pseudo-(lOO) reconstruction assuming a commensurate [ 4 ] registry of the overlayer and substrate. The methylthiolate species are represented by the S head-group atoms alone, shown as the darkest spheres. The Cu atoms of the reconstructed pseudo-(lOO) layer are shown more darkly shaded than those of the underlying substrate. For clarity the reconstructed overlayer has been omitted from the lower right-hand side of the diagram, exposing the outermost unreconstructed Cu(l 11) layer.

FIGURE 3.2. (a) Chemical structure of octanethiol. (b) A constant current STM image of octanethiol SAM on Au(l 11). Au reconstruction is lifted and alkanethiols adopt commensurate crystalline lattice characteriized by a c(4 x 2) superlattice of a (a/3 x V3)R30°. (c) Model of commensuration condition between alkanethiol monolayer (large circles) and bulk-terminated Au surface (small circles). Diagonal slash in large circles represents azimuthal orientation of plane defined by all-trans hydrocarbon chain. (Reprint with permission from Ref.25 G. E. Poirier, Chem. Rev., 97, 1117-1127 (1997). Copyright 1997 American Chemical Society.)... 

Unless there is reconstruction, this clean surface is labeled (lxl) to denote that the surface lattice is the same as it would be if the bulk lattice were cut along a crystallographic plane (the terminated bulk surface ). In the case of reconstruction or with overlayers, the surface symmetry and periodicity is often different from the (lxl) lattice. Often this new periodicity is commensurate ... 

W21 beamline and diffractometer [33]. A large number, 366 of which 267 were non-equivalent, of in-plane reflections arising from the reconstruction were measured. All peaks were exactly centred at the expected positions to within 0.001° of azimuthal rotation, which showed that the surface reconstruction is perfectly commensurate with the underlying bulk lattice. Their width and Lorentzian shape indicated an exponential decay in correlations with the decay length of -500 A. Several reconstruction diffraction rods were also measured. The absence of symmetry of the rod intensity with respect to =0 showed that the reconstruction has the minimal hexagonal symmetry p3. 

The Ir(100)(lx5) reconstruction is caused by a lateral distortion of the top layer of Ir atoms along the (10) direction (Lang et al., 1983). This distortion allows the top layer of Fr atoms to form a quasi-hexagonal two-dimensional lattice which is commensurate with the underlying (100) plane formed by the... 

The following tables contain experimentally-determined commensurate structure parameters for alkali metal adsorption systems (only simple structures are listed). The temperatures quoted are the measurement temperature. The bond length quoted is the chemisorption bond length. Effective r (Eff r) is the chemisorption bond length minus the metalHc radius of the substrate atom. Excess r (Exc. r) is the Effective r minus the ionic radius of the alkali metal atom N is the coordination number of the alkali adatom. A coordination number denoted as indicates that due to surface reconstruction, an unambiguous assigmnent carmot be made. 

The situation was however quite different when metallic substrates were used [130, 131]. For 4T films on Ag(lll) the covalent bonding of the rr-system led to a commensurate structure in the first molecular layer [131, 138]. Within this bonding the TT-system and hence the 4T molecule arranged parallel to the surface. This preferentially lying orientation can be maintained also for thicker films of some tens of A [98, 99]. A parallel orientation was also found in the case of 6T deposited onto a 2 X 1 reconstructed Au(l 10) surface [139]. Other studies on 2T adsorbed on a more reactive surface, namely Ni(l 11), showed that a strong bonding occurs which may even lead to a strong distortion of the 2T molecule [140]. 

---