Jungiusstr. 11, Hörsaal AP
SFB 668 - Kolloquium
Prof. B. Poelsema (University of Twente):
Quantum size effects and their influence on stability, topography and crystalline structure of ultrathin films
Quantum Size Effects (QSEs), defined as standing wave patterns of Fermi electrons inside and perpendicular to a thin film, have a distinct influence on its stability. Prime examples are given by preferred heights observed for thin Pb-films on several substrates. Such features were believed to exist only (well) below room temperature. Our recent LEEM study of Pb/Ni(111) demonstrates, however, that QSEs are decisively present up to about 500 K, leading to well defined film thicknesses. Heating these films to 525 K results in their ultrafast collapse into compact hemispheres.
Surprisingly, QSEs can even lead to allotropism in thin films as we found for Bi/Ni(111) using LEEM. Not only do we observe crystallites with preferred thicknesses, but also with different, well defined structures. We have identified at least three different ones. Here, QSEs are believed to impose the structures, which are also stable at temperatures up to about 500 K.
Finally as a third example, these QSEs are not only dictating the thickness and structure of specific thin films, but can also be responsible for preferred lengths of nanowires. I will illustrate this for the system Ir/Ge(001) studied by STM/STS. We can directly image the standing waves, enabled by their specific, favourable geometry.