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LEED crystallographic studies for chemisorption-induced relaxations and reconstructions of copper and rhodium surfaces

University of British Columbia

Low-energy electron diffraction (LEED) has been used to investigate some structures formed by electronegative atoms (O and P) on copper and rhodium single crystal surfaces. The motivation is to provide new atomic-scale details, to develop fundamental understanding of surface structural chemical principles, and to help establish approaches for determining relatively complex surface structures using tensor LEED. A detailed analysis for the Cu(110)-c(6x2)-O surface indicates that this structure is composed of double-stranded chains built from 12-membered rings formed by alternating O and Cu atoms. This analysis provides direct information on the structural details, which support the view that the driving force for this structure relates to sufficient O being incorporated in order to stabilize the surface as a modified (211) plane of bulk Cu20. Together with the structural details from LEED analyses for two other oxygen on copper surfaces, designated as Cu(100)-(2xl)-O and Cu(100)-(2√2x√2 )R45°-O, certain common features are identified. It is found that all these structures receive some stability from being able to fit O-Cu-0 three-atom building blocks, of the sort needed to construct bulk Cu20, on to the clean surfaces, while ensuring that each O atom maintains its favorable four-coordinate status with reasonable O-Cu bond lengths (average close to 1.90 Å). A study of the Cu(110)- [formula which cannot be rendered here; see abstract] (2 2\ ,-1 1. -P surface establishes a new type of reconstruction in which all P atoms occupy identical six-coordinate sites; the reconstruction involves 0.25 ML of "added Cu" atoms at hollow sites, each bonded to two neighboring P atoms, which line up along the [112] direction. Appreciable relaxations, both vertical and lateral, are carefully rationalized; the average P-Cu bond length is determined to be 2.33 A. An analysis for the Rh(111)-( √7x√7 )R19.1°-P surface has identified a novel reconstruction for the topmost metallic layer, which involves a packed arrangement of Rh pentagons and triangles. Each P atom has an identical environment and bonds to eight neighboring Rh atoms with P-Rh bond lengths ranging from 2.17 to 2.97'A. The nearly flat, densely-packed Rh-P layer (with five Rh and three P per seven substrate atoms in the unit mesh) appears to give this surface a high general stability. This represents the first successful determination for a complex √7x√7 surface structure. The model type identified may have wider applicability, for example the analogous system formed by S on Pd(111); suggestions are also made in the latter case on how the structural form may influence chemical reactivity, such as for the cyclization of acetylene to thiophene.

Thesis/Dissertation

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2009-02-19

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http://hdl.handle.net/2429/4786

Chemistry

University of British Columbia

UBCV

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