Atom-Probe and Field Ion Microscope Studies of the Atomic Structure and Composition of Overlayers on Metal Surfaces

The field ion microscope can give a two-dimensional view of the atomic structure of a solid surface. The composition of the top and deeper surface layers can be analyzed atomic layer by atomic layer with a time-of-flight atom probe If an adsorption layer structure is commensurate with the substrate, then its structure can be derived reliably from the field ion image. We have observed an adsorption layer superstructure, silicon on W(110), which can be directly correlated to the experimentally measured pair energies of silicon adatom-ad-atom interactions. The true atomic layer by atomic layer compositional analysis of surfaces can be best illustrated by a study of surface segregation of platinum base binary alloys, where the number of surface layers enriched with the segregants is found to vary from one system to another, ranging from 1 to 4 atomic layers. For some alloys, the second atomic layer is depleted with se gregants. This result is consistent with an oscillatory structure in the single-ion potential in the near surface layers.