Chemical discrimination on atomic level by STM

Chemical information with spatial atomic resolution on multicomponent surfa ces (especially alloys) can be achieved by STM (scanning tunnelling microsc opy) 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 chemica l structure of multicomponent surfaces on an atomic scale. This ability can be used for studying in great detail segregation processes on metal surfac es. Examples of chemical discrimination between different metals on low ind ex single crystal surfaces of bulk alloys we have seen so far are PtNi, PtR h, 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 eq ual concentrations), it will be shown how ab initio calculations of the ele ctron density of states using the FLAPW (full potential linearized augmente d 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.