Quantitative Analysis of Adsorbate Induced Segregation at Bimetallic Surfaces:  Improving the Accuracy of Medium Energy Ion Scattering Results

Abstract

Bimetallic systems respond dynamically to the nature of the gas phase in contact with the surface. The development of a surface analytical tool which probes the layer by layer composition of a bimetallic surface covered by an adsorbate would be of great benefit in understanding complex catalytic and corrosion processes. This paper examines critically the possibility of using medium energy ion scattering to obtain such information. The unique shadowing and blocking capabilities of this technique make it possible, in principle, to selectively illuminate integer numbers of surface layers. As the physics of the incident ion-surface atom collision is well established, it should then be possible to effectively count the number of each atom type present in the first layer, the top two layers, the top three layers, etc., of the bimetallic surface, thereby giving quantitative compositional information. Using a combination of experiment and simulation, we investigate how the number of layers illuminated depends on the extent of the surface layer relaxation, the adsorbate and the surface Debye temperature. We calculate the layer compositions of the top three layers in the Cu 50 Pd 50 (110)/C 2 H 4-x Cl x system. We show that hydrocarbon adsorption produces a measurable Pd enrichment in the surface layer, while Cu enrichment is produced by the presence of Cl(ads) and C(ads).

Alloy formation in the Au{111}/Ni system – An investigation with scanning tunnelling microscopy and medium energy ion scattering

“…In previous studies of CuPd{110} [16], Ni{1 1 1}/Au [25,26] and Pd{1 1 1}/Au [27], we have used ion intensity versus energy plots at well-defined scattering angles to quantify the layer by layer composition of the bimetallic surfaces by measuring peak areas and carrying out appropriate normalisation procedures [15] to establish the mole fraction of each element visible in each double alignment geometry. In the present case, it is clear that the ion intensity versus energy spectra are more complex.…”

“…1, a three layer incident geometry is achieved and, by utilising the phenomenon of blocking, 1-, 2-and 3-layer specific outgoing directions can be achieved. In reality, it is not possible to achieve perfect layer specificity even with a single crystal surface of a pure metal since surface layer relaxation and thermal vibrations of surface and sub-surface atoms act to enhance the visibility of sub-surface layers [16].…”

Adsorption of (S)-glutamic acid induces segregation of Ni at bimetallic Au/Ni surfaces

“…For a single crystal bimetallic surface, by following appropriate normalisation procedures [14], one may quantify the composition in each layer of the near surface by comparison of the intensity of each elemental peak in particular blocking directions. We have used such an approach to investigate the layer by layer composition of CuPd{1 1 0} [17].…”

Ultra-thin films and surface alloying of Pd on Cu(111) investigated by medium energy ion scattering

“…Therefore, it was impossible to safely determine the layer-by-layer composition using this or similar methods [19]. However, the variation of composition with depth may be determined by full simulation of the blocking curves, including the effect of twinning.…”