Surface crystallography 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. 

Early LEED studies [97] indicated that the basic structure of a clean 100 surface was 2x1. (For a discussion of the nomenclature of surface crystallography, see ref. 113.) Three different models have been proposed to account for this form of reconstruction the vacancy model [78, 79, 114] assumes that half the surface atoms are missing, while the other two suggestions invoke surface dimerization [80, 114] and the formation of complex conjugated chain structures [115, 116], respectively. We will consider each of these in a little more detail. 

Fractographic analyses of CF must be improved, including quantitative measurement of crack surface crystallography [119], and computerized image analysis methods to characterize and reconstruct the CF process [120]. 

Let us first concentrate on the surfaces of flat, low-Miller-index planes. Figure 8 shows the (100) crystal face of platinum. When clean, this surface is reconstructed and its diffraction pattern indicates the presence of a 5 x20 surface structure. When the surface is impure or has a fraction of a monolayer of adsorbates, the square unit cell shown in figure 8 is obtained, which is what one would expect from projection of the bulk unit cell up to the surface. While this 5 x20 surface structure was first detected in our laboratory in 1965 [9], it was actually solved by surface crystallography in 1981 [10]. 

Fig. 9. Structure of the reconstructed Pt(lOO) crystal face as solved by surface crystallography...

Figure 11 shows the beautiful diffraction patterns exhibited by one of the more stable structures on the reconstructed silicon (111) surface. This is the 7x7 structure which has a complex unit cell which is still not resolved by surface crystallography. It is hoped that a resolution in this surface structure will be forthcoming within a year. It should be noted that the silicon (100) and (111) surfaces are frequently used as substrates for electronic circuitries. As a result, the atomic surface structure of these surfaces is of utmost importance in the integrated circuitry technology, since the electronic transport properties are clearly dependent on the location of atoms at the surface. 

The ordered phase formed on Fe(lOO) (classified as c2 x 2 structure according to the standard nomenclature of surface crystallography has structural parameters similar to those found with the (002) plane of Fe4N without distortion of the substrate lattice this is the reason why in the case of Fe(lOO) no reconstruction occurs. The structure of this phase as derived from quantitative LEED analysis is reproduced in Fig. It has a coverage = 0.5, and the N atoms occupy... 

To understand the meaning of the STM images of silicon surfaces, we review some basic facts of the crystallography of silicon. We will discuss the simpler Si(lll) surfaces first, then the complicated 7X7 reconstruction. In fact, the STM imaging of the simple Si(lll) surface is the most elementary case of imaging semiconductors, and perhaps the most instructive one. 

Takayanagi, K. et al. Structure Analysis of the Silicon(lll) 7x7 Reconstructed Surface by Transmission Electron Diffraction/5 Surface Science, 164, 367 (1985). Tromp, R.M. and E.J. van Loenen Ion-Beam Crystallography on Silicon Surfaces III. Si(lll),5 Surface Science, 155, 441 (1985)... 

Much is known about the growth, crystallography and reconstruction of semiconductor (SC) surfaces from STM ultrahigh vacuum experiments. In this environment the first atomic resolution image was obtained for a semiconductor subslralc" before it was done for metal substrate. In the case of air and in particular in liquid enviromnent the situation is different. This is because of the presence of the native oxides formed in air on SC surface, and corrosion processes taking place easily in solution. 

Figure 6 A comparison of the rendered surfaces of TEM reconstructions of the active RecA-DNA filament (left) and the inactive RecA-DNA filament (right) with the RecA crystal surface (center). The RecA crystal surface, obtained by X-ray crystallography, has been rendered at low resolution, to be comparable with the TEM reconstructions. Both the active and inactive filament reconstructions are from averages of frozen-hydrated specimens imaged with cryo-EM. (Reproduced with permission from Egelman EH and StasiakA (1993) Electron microscopy of RECA-DNA complexes Two different states, their functional significance and relation to the solved crystal structure. Micron 24 309-324.)...

Combining the TDS profiles and LEED crystallography of the O-Rh(llO) surface, Schwarz et al. [7] correlated the TDS peaks to the LEED patterns of various reconstructed phases. Comelli et al. [6] reinterpreted the TDS data of the O-Rh(llO) surface based on the known adsorbate s locations. The most stable structure corresponds to the p2mg phase and so the adsorbate binds to the surface more strongly. Due to the repulsive interactions, the adsorbate in the unreconstructed troughs binds to the surface weakly. 

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