Auger electron spectroscopy on cylindrical single crystals

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Abstract

The aim of this work is to investigate the effect of surface crystallography, and in particular the effect of a certain type of surface defect, the monatomic step, on the chemical reactivity of surfaces. There are basically two approaches to this problem. The most common approach is to investigate a particular reaction on several different samples all with different surface orientations. An alternative approach (the one used in this work) is to investigate a given chemical reaction on a sample which exhibits a range of surface crystallographies. This latter approach has many advantages and some disadvantages as will become apparent in the work which follows.

For this investigation, copper and nickel cylindrical single crystals were used which had been cut with a <110> axis. Thus, all three low index planes (001), (111) and (110) were made available as well as the (113) face and a continuum of vicinal surfaces. A unique property of <110> axis cylindrical single crystals of face centered cubic metals is that all step ledges run parallel to the cylinder axis, and this means that for a perfect crystal, the kink density is zero. Furthermore, the step density varies in a linear manner between the planes (113) and (001), (113) and (111) and between (110) and (111). This allows the investigation of the effect of monatomic steps, and in particular, variations in step density, on the chemical reactivity of surfaces. Such a sample also allows rather accurate comparisons to be made of the properties of the various low index planes.

The analytic technique used in this work was Auger electron spectroscopy which allowed accurate determination of surface coverages in the sub-monolayer regime with good spatial resolution. The reactions studied are carbon and sulphur segregation in nickel (chapter 4), and the redox properties of the copper cylinder (chapters 5 and 6). In chapter S oxidation experiments on the copper cylinder are described in which two different oxidizing agents, oxygen and N2O, are used, and the reactions are compared and contrasted. The oxygen adsorption experiments also provide a basis for the re-evaluation of two models proposed by Armitage describing the kinetics of adsorption on all the surfaces provided by the <110> axis copper cylinder. A simple model describing the crystallographic variation of the initial reaction probability of N2O decomposition on copper surfaces is also described in this chapter, and it is found that the initial reaction probability shows a dependence on step density which is non-linear.

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