Chemical discrimination on atomic level by STM

Chemical information with spatial atomic resolution on multicomponent surfaces (especially alloys) can be achieved by STM (scanning tunnelling microscopy) with constant current imaging. Therefore STM can not only be used for determination of the crystallographic structure of single crystal surfaces but also for finding the chemical composition of bimetallic surfaces. This possibility makes the STM a unique instrument to find out the local chemical structure of multicomponent surfaces on an atomic scale. This ability can be used for studying in great detail segregation processes on metal surfaces. Examples of chemical discrimination between different metals on low index single crystal surfaces of bulk alloys we have seen so far are PtNi, PtRh, PtCo, PtAu and AgPd. For surfaces where the identification of the alloy constituents in the STM images is ambiguous (e.g., because of unknown or equal concentrations), it will be shown how ab initio calculations of the electron density of states using the FLAPW (full potential linearized augmented plane waves) method can help to interpret constant current STM topographs just by following the simple theory of Tersoff-Hamann. On the other hand, tip changes (e.g., adsorbates) can strongly influence the chemical contrast on constant current STM images. (C) 1999 Elsevier Science B.V. All rights reserved.