A nonmonotonic concentration depth profile of Pt–Rh alloys: A surface segregation study using the atom-probe field ion microscope

Abstract

Equilibrium concentration depth profiles with true single atomic layer depth resolution have been obtained for the surface segregation of Pt–Rh alloys using a time-of-flight atom-probe field ion microscope. The depth profiles obtained for the (001) plane of Pt–Rh alloys of five different bulk composition all show an enrichment of Rh to the top surface layer. It is also observed that there is a significant Rh depletion in the second layer. An additional feature of cosegregated sulfur impurity and overlayer formation is observed. The question of the possible nonequilibrium depth profiles has been resolved by a careful experimental testing. The surface segregation of Rh is consistent with several theoretical models, whereas the nonmonotonic concentration profile may indicate an oscillatory structure in the potential of single ions in the near surface layers.