Paper SS2-TuM9
Surface Segregation on Composition Spread Alloy Films

Tuesday, November 10, 2009, 10:40 am, Room N

Surface segregation has been studied in Pd_xCu_1-x alloys used for hydrogen purification membranes. Surface segregation influences the performance of these alloys for hydrogen purification and furthermore, segregation at the surfaces of these materials is a sensitive function of the concentration of contaminants such as sulfur. Segregation is not just restricted to the topmost atomic layer of the alloy. The concentration of one component in the top few layers of the surface may differ from that of the bulk. Because surface segregation is a continuous function of bulk composition, x, a complete understanding of segregation in a binary alloy requires the development of high throughput methods that allow concurrent measurements of surface segregation at all possible values of bulk composition.

Surface segregation has been studied in a Pd₇₀Cu₃₀ alloy using both x-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The results show that the topmost atomic layer is rich in Cu. On the other hand, the near-surface region consisting of the immediate subsurface layers is rich in Pd. Furthermore, the adsorption of sulfur on the surface causes the complete elimination of Cu from the topmost layer.

A high throughput method for study of surface segregation has been developed that is based on the deposition of thin Composition Spread Alloy Films (CASFs) that contain all possible bulk compositions of the Pd_xCu_1-x alloy. Spatially resolved surface analysis of the surface of the Pd_xCu_1-x CASF has been achieved using XPS. The results show that the near-surface region of the alloy is Pd rich over a wide range of bulk Pd concentrations.