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/
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