Reconstruction LEED study

Fig. XVI-8. (a) The quasi-hexagonal surface structure of clean Pt(lOO) surface, (b) Adsorption of CO lifts this reconstruction to give the structure corresponding to the termination of (100) planes (from LEED studies). [Reprinted with permission from G. Ertl, Langmuir, 3, 4 (1987) (Ref. 56). Copyright 1987, American Chemical Society.]...

Surface reconstruction, which had dominated much of surface science through LEED studies, was very much a central theme of STM in the early 1990s but with surprisingly little attention given to chemical reactivity and the origin of active sites in heterogeneous catalysis. This was in part due to the lack of in situ chemical information that could be directly related to the STM images and... 

Detailed LEED studies of ZnS (and other zincblende-structure compounds, including ZnTe and CdTe) have established that the (110) surface is reconstructed by movement of the Zn atoms inward (towards the bulk solid) and of the S atoms outward. Displacements of the cation and anion in the uppermost layer by <<0.5 A and in the second layer by 0.1 A compared to the bulk are involved (Duke, 1983). Harrison (1976, 1980) has explained this in terms of an electronic structure model by conversion of half-occupied dangling-bond hybrid orbitals at both Zn and S on the... 

In the present study on the stepped Pt(100) clean surface the ideal "hex" phase with its large unit cell has not been observed so far by field ion microscopy. LEED studies of the Pt[4(100)x(lll)]and Pt[9(100)x(111)] (ref.10) surfaces, however, give evidence for a reconstruction modulated by the steps. 

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. 

LEED is a very specific tool for examination of the geometric pattern of atoms on a surface. In electrochemistry, it has been used to study surface reconstruction. 

Pfniir H, Held G, Lindroos M, Menzel D. 1989. Oxygen induced reconstruction of a close-packed surface A LEED IV study on Ru(001)-p(2 x 1)0. Surf Sci 220 43. 

A series of LEED intensity studies, together with ion-scattering spectroscopy, established that a missing row structure was the correct model for the (1 x 2) phase,14 with some small subsurface relaxation and reconstruction.10... 

The group at Aarhus have reported carbon-induced structures at Ni(lll) and Ni(110) surfaces resulting from the dissociation of ethylene at high temperatures.27 Between 400 and 500 K, the Ni(l 10) surface is seen to form two carbidic structures with (4 x 3) and (4 x 5) domains present arising from surface reconstruction with substantial transport of nickel taking place. At higher temperatures (560 K), the surface becomes dominated by the (4 x 5) structure, which is well ordered and can be observed clearly by LEED. Ion scattering studies provide additional information which enables models to be constructed for both the (4 x 3) and (4 x 5) phases. 

In summary, LEED is most often used to verify the structure and quality of single crystal surfaces, to study the structure of ordered adsorbates and to study surface reconstructions. In more sophisticated uses of LEED one also determines exact positions of atoms, the nature of defects and the morphology of steps, as well as Debye temperatures of the surface. 

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