Abstract
This chapter describes segregation and surface chemical ordering—an experimental view on the atomic scale. Scanning tunneling microscopy with atomic resolution and chemical contrast offers unique possibilities in studying segregation and chemical ordering of alloy surfaces. Chemical contrast in STM can have three different reasons (a) true topographic effects, (b) different density of states of the alloy constituents, and (c) tip–sample interaction depending on the chemical identity of the imaged atom. The composition and chemical order on surfaces is determined by interplay of ordering and segregation. The chapter also shows that the study of surface composition and chemical order is essential for understanding adsorption on alloy surface. Even weak ordering can lead to significant changes in the availability of some adsorption sites. It is also possible to obtain STM images of an alloy surface with chemical contrast and images of adsorbates in the same surface area, revealing the chemical structure of adsorption sites. The chapter thereby demonstrates the ligand effect, that is, the dependence of adsorption strength on the atoms neighboring an adsorption site.
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