Electron diffraction surface structure

For an example of electron diffraction in structure determination of a membrane protein, see Y. Kimura, D. G. Vassylyev, A. Miyazawa, A. Kidera, M. Matsushima, K. Mitsuoka, K. Murata, T. Hirai, and Y. Fujiyoshi, Surface of bacteriorhodopsin revealed by high-resolution electron crystallography, Nature 389,206-211, 1997. 

Many of the Cu 100 based surface alloys discussed in this chapter are relatively well characterised in terms of their layerwise compositional profile, geometric structure and thermal stability. However, it is clear that the majority of structural studies performed to date have made the (often necessary) assumption that a single homogeneous structural phase with a somewhat idealised compositional profile is present. In many cases, particularly for adsorbates which exhibit considerable bulk solubility in copper this may be a oversimplification. Future work to investigate the sensitivity of quantitative probes of surface structure and composition such as LEED, ion scattering spectroscopies and photo-electron diffraction to structural heterogeneity will be invaluable. 

Figure 8b. The surface structure of ethylidyne adsorbed on Pt(l 11) as obtained by low-energy electron diffraction surface crystallography.

EEED Eow Energy Electron Diffraction Surface Mono-energetic electron beam 10-1000 eV Diffracted electrons 0.4-2 nm <6 pm Crystallographic structure of surface resolution 0.01 nm 36... 

Over the past twenty years over 250 surface structures have been solved, most of them by low energy electron diffraction-surface crystallography(9). From these studies an entirely different model of the structure of surfaces emerged on the atomic scale. This model indicates a dynamic structure, a structure where the atomic positions of the surface atoms are different from that predicted by the rigid lattice model when the surface is clean and change again when chemisorption occurs. 

Figure 11b. Low energy electron diffraction surface crystallography determination of the structure of a graphite adsorbed layer on the platinum (111) crystal face. The presence of two layers of carbon is clearly visible. (Reproduced with permission from Lawrence Berkeley Laboratory.)...

Low-energy electron diffraction LEED Elastic backscattering low-energy electrons Atomic surface structure of surfaces and of adsorbed gases... 

HEED High-energy electron diffraction [104] Diffraction of elastically back-scattered electrons (-20 keV, grazing incidence) Surface structure... 

LEED Low-energy electron diffraction [62, 75, 105] Elastic backscattering of electrons (10-200 eV) Surface structure... 

RHEED Reflection high-energy electron diffraction [78, 106] Similar to HEED Surface structure, composition... 

The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

Such ideal low mean free paths are the basis of FEED, the teclmique that has been used most for detennining surface structures on the atomic scale. This is also the case of photoelectron diffraction (PD) here, the mean free path of the emitted electrons restricts sensitivity to a similar depdi (actually double the depth of FEED, since the incident x-rays in PD are only weakly adenuated on this scale). 

One fiirther method for obtaining surface sensitivity in diffraction relies on the presence of two-dimensional superlattices on the surface. As we shall see fiirtlrer below, these correspond to periodicities that are different from those present in the bulk material. As a result, additional diffracted beams occur (often called fractional-order beams), which are uniquely created by and therefore sensitive to this kind of surface structure. XRD, in particular, makes frequent use of this property [4]. Transmission electron diffraction (TED) also has used this property, in conjunction with ultrathin samples to minimize bulk contributions [9]. 

The diffraction pattern observed in LEED is one of the most connnonly used fingerprints of a surface structure. Witii XRD or other non-electron diffraction methods, there is no convenient detector tliat images in real time the corresponding diffraction pattern. Point-source methods, like PD, do not produce a convenient spot pattern, but a diffrise diffraction pattern that does not simply reflect the long-range ordermg. 

Ichimiya A, Ohno Y and Horio Y 1997 Structural analysis of crystal surfaces by reflection high energy electron diffraction Surf. Rev. Left 4 501-11... 

Takayanagi K 1990 Surface structure analysis by transmission electron diffraction—effects of the phases of structure factors Acta. Crystalloger A 46 83-6... 

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