Segregation and surface chemical ordering - an experimental view on the atomic scale

M. Schmid and P. Varga

Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Austria

Chapter 4 in: The Chemical Physics of Solid Surfaces, Volume 10: Alloy surfaces and surface alloys; ed. by. D. P. Woodruff, Elsevier 2002, ISBN 0-444-51152-0, pp. 118-151

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 atom imaged.

The composition and chemical order on surfaces is determined by an interplay of ordering and segregation. If the chemical ordering is weak, segregation is mainly determined by the differences in surface energies. On the surfaces of alloys with a strong tendency towards ordering, segregation and ordering can either compete, e.g., in cases where preserving the bulk chemical order requires the surface to assume the bulk composition, or ordering can enhance segregation, e.g. in cases where bulk ordering allows a pure-metal termination. In cases where only short-range chemical ordering occurs at the surface, the trends in surface chemical ordering were found to correspond well to the ordering tendency observed in the bulk.

We also show 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. We could also 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. We could thereby demonstrate the ligand effect, i.e., the dependence of adsorption strength on the atoms neighbouring an adsorption site.

Corresponding author: P. Varga (varga< encoded email address >).


Contents of this chapter:

1. Introduction

2. Chemical discrimination on bimetallic surfaces with atomic resolution by STM
2.1. True topographic effect
2.2. Difference in local electronic density of states
2.3. Tip-surface interaction

3. Segregation on alloys - surface and subsurface composition

3.1. Segregation
3.2 Preferential sputtering and segregation in the altered layer

4. Chemical Ordering of alloy surfaces

4.1. Bulk chemical order
4.2. Fundamentals of surface chemical order
4.3. Chemical order of close-packed alloy surfaces
4.4. fcc(100) surfaces
4.5. Site-specific segregation

5. Implications for adsorption on alloys

5.1. Chemical Affinity
5.2. The ensemble effect
5.3. The ligand effect

6. Conclusions