Abstract
 
STM on atomic Resolution
 
Peter Varga
 
Institut für Allgemeine Physik, TU Wien, Austria, Wiedner Hauptstr.8-10/134, A-1040 Vienna

After a short introduction into surface science the principle of scanning tunneling microscopy (STM) will be discussed. Scanning tunnelling microscopy is a method which has proofed its power for determining crystallographic structures in the last decade although the imaging process is still not very well understood in detail. It is well established that it is the electronic structure of a surface which is imaged and not the atomic position but especially for metals it is agreed that in almost all cases the tunnelling current reaches its maximum if the tip is on top of an atom. Only two examples are known until now where the highest electron density does not coincide with the atoms position i.e. Fe and Cr(100) surfaces where it was shown that the position for the highest tunnelling current is the fourfold hollow side.
 
Generally speaking STM is an imaging method which reflects the real space and has the possibility to analyse locally crystallographic surface structures and not only structures with long range order as methods based on diffraction. It has been shown that with high resolution STM at least chemical discrimination of different species on alloy surfaces can be achieved without loosing atomic resolution . This possibility makes the STM to an unique instrument to analyse locally chemical composition of multicomponent surfaces as well as the crystallographic structure. We have studied in the last years several alloys mainly Pt- alloys like PtNi, PtRh, PtCo, PtFe, PtCr and PtCu but also other alloys like AgPd, AuPd, FeSi and AlNi. For all this surfaces we could achieve chemical discrimination although this chemical discrimination was not always very stable and reproducible.
 
To learn to understand the imaging process better, simulation calculations from the respective surfaces have been performed. If applying the most simple STM theory (Tersoff Haman) the electron density around the Fermi edge at the position of the tip apex atom is what is imaged in constant current STM. This electron density of states can be calculated today easily by ab initio methods based on the density functional theorem and compared to constant current STM images.
In this talk not only examples will be given where chemical discrimination on low index single crystal surfaces of alloys have been achieved, but also how this resolution can be used to learn more about the fundamental process of growth modes of ultra thin films of metals and insulators on metal surfaces, site dependent adsorption on alloys (Ligand effect) and the formation of oxide layers.
 
 
More information can be seen in the STM Gallery of our home page:
http://www.iap.tuwien.ac.at/www/surface/