Abstract: Determination of the surface vs bulk composition of Ag/Au alloys by low‐energy ion‐scattering spectroscopy

Abstract

Recently there has been considerable interest in measuring the surface composition of binary alloys. This interest has been generated by a variety of other phenomena (such as catalysis) which are related to the surface composition. Thermodynamic arguments predict that the surface of a multicomponent system will be enriched in the constituent which has the lowest surfacefree energy.1,2 However, due to a number of experimental difficulties, many of the measurements of the surface enrichment which have been made by Auger electron spectroscopy(AES) have led to confusing results. By using a technique such as low‐energy ion‐scattering spectroscopy (ISS) which has monolayersurface sensitivity, many of the problems encountered in the AESmeasurements are eliminated. This paper is a report on the measurement of the surface composition of two Ag/Au alloys by ISS. The measurements were made with 1500‐V 4He+ and 20Ne+ as the ion probes. The ion beam was rastered over a 16‐mm2 area to minimize the sputtering which occurred during the time it took to make a measurement (typically 30 s). Pure gold and silver samples which had been polished to a 5‐μm finish were used as sensitivity standards. The only surface treatment which the alloys received was several cycles of sputter cleaning followed by annealing at 400 °C. In Table I are listed the bulk composition measured by an electron microprobe, the surface compositionmeasured by ISS, and the surface composition predicted by the regular solution model.1 The present data do not agree with previously reported results for the Ag/Au system where no enrichment was observed.3 However, they are in reasonable agreement with the enrichment predicted by the regular solution model.