Structure and faceting of low-symmetry metal surfaces

Walko, Donald Alan

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Description

Title

Structure and faceting of low-symmetry metal surfaces

Author(s)

Walko, Donald Alan

Issue Date

2000

Doctoral Committee Chair(s)

Robinson, Ian K.

Department of Study

Physics

Discipline

Physics

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• X-ray diffraction
• crystalline surfaces
• Gallium (010)
• Copper(115)

Language

en

Abstract

X-ray diffraction has, within the past two decades, developed into a powerful analytical tool for determining the atomic structure of crystalline surfaces. The dual advantages of high intensity and high resolution have made this technique thrive at synchrotron radiation sources. The experiments in this thesis extend surface x-ray diffraction to metal surfaces with particularly low symmetry: the multilayer surface structure of a-Ga(OlO), complicated by the low symmetry of the bulk, and Cu(115), a regularly stepped surface which spontaneously facets when exposed to oxygen. The equations describing surface x-ray diffraction are derived, with attention paid to assumptions made during the derivation and limitations of the technique. In particular, the theory of scattering from a rough surface is generalized to permit various models of roughness. The surface diffraction chamber in which these experiments were performed is briefly described, along with the load lock which allows samples to be inserted without opening the entire vacuum chamber to atmosphere. We have determined the surface structure of a-Ga(OlO) near its melting point using xray diffraction. Due to the low symmetry of the a-Ga bulk structure, two distinct bulk truncations of the (010) surface are possible. Of these two ways, we find the true surface is formed by cutting through dimer bonds (i.e., between metallic bilayers). The contraction of the metallic bonds and expansion of the covalent bonds at the surface imply that the surface is more metallic than the bulk. Our results suggest that a-Ga is fundamentally composed not of Ga2 dimers, but of corrugated metallic bilayers which can be modeled as deltahedral clusters. Cu(OOl) vicinal surfaces facet when exposed to 0. We have studied this process on Cu(115), which transforms from a clean surface to 104 and 113 facets, using surface ray diffraction. Unlike a-Ga(010), the low symmetry of these surfaces is due to their high Miller indices, i. e., the stepped nature of the surfaces. The Cu(115) surface exhibits a complex interlayer relaxation accounted for by basic elasticity theory; the vertical displacements of the three surface atoms correlate to those of the subsurface atoms directly below. The O/Cu(104) facets do not, as previously proposed, involve any missing Cu rows, but the top three rows are expanded away from the bulk; the Cu-0 chains which stabilize this surface are similar to those present on other 0 on Cu reconstructions. A complete structure determination was not possible for the 0 /Cu(113) facets, due to significant disorder, but an unambiguous (3x 1) reconstruction was observed. Besides being instrumental in determining the static structure of surfaces and facets, surface x-ray diffraction allows us to noninvasively observe, in situ, the evolution of the faceting surface. We find that the faceting is driven by the formation of O/Cu(104) facets: 0 exposure induces spinodal decomposition of the (115) surface into (104) and (014) facets, which form spontaneously, and also disordered, stepped facets, whose orientation gradually changes from (115) to (113) as the (104) facets grow. We identify three temperature regimes which have qualitatively different faceting processes, shedding light on the temperature dependence of the equilibrium crystal shape for part of the 0-covered Cu system. During the faceting process, the time evolution follows a slow dynamic scaling behavior, consistent with either a logarithmic or power-law dependence. Throughout this thesis, comparisons are made with results obtained by other surfacesensitive techniques. The complimentarity of these techniques is worth emphasizing; despite the power of surface x-ray diffraction in solving crystal structures, its ability to interpret and explain the properties of these surfaces is greatly enhanced by microscopy, spectroscopy, and other diffraction techniques, as well as theoretical and numerical studies.

Type of Resource

text

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©2000 Walko

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