Atomic-layer by atomic-layer compositional depth profiling: Surface segregation and impurity cosegregation of Pt-Rh and Pt-Ru alloys

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Abstract
Absolute composition depth profiles of true single-atomic-layer depth resolution have been obtained for the near surface layers of the (001) and (111) planes of Pt-Rh and Pt-Ru alloys in surface segregation and cosegregation with sulfur of these alloys. For both alloys, the segregation species is found to be Pt, and its concentration approaches the bulk value nonmonotonically; the composition of the second layer is reversed. We did not detect a significant dependence of surface segregation on crystal planes and also find the plane-edge effect to be very small for Pt-Rh alloy. Also no reversed segregation at the two dilute ends of the alloy composition of Pt-Rh is found. For Pt-Rh, if the sample contains a very small amount of sulfur, then sulfur and Rh cosegregate to the surface, with sulfur forming an adsorption overlayer. This reversal of segregation species by impurity sulfur is not observed for Pt-Ru alloys. We believe that atom-probe composition depth profiles can also be used as references for calibrating other surface analytical techniques.