Atomic structure in nanoscale systems

Recently, there is considerable interest in ultrathin layers of transition metal oxide, mainly due to their particular magnetic and electronic properties. However, little is known about their atomic structure, although it is of fundamental importance for understanding their properties. In most cases the atomic structure of nanosystems is assumed to be the same as the bulk structure of the material, although one must expect major deviations, because a large number of atoms is located at the surface and the interface similar to simple surfaces. Two techniques will be used in order to determine the atomic structure of ultrathin films of nanometer thickness, namely scanning probe microscopy (SPM) and low energy electron diffraction (LEED). Since the analysis of LEED I-V curves still is a demanding task, we will initially limit the effort to well characterized systems: Ultrathin layers of CoO and NiO on noble metal single crystals. SPM has been proven to be well suited for determining the nanoscale growth mode and morphology of the system, i.e., the roughness and thickness of films as well as size and shape of clusters and islands. In the case of atomic resolution, the lateral arrangement of the top layer atoms can be determined in addition. There will be a close collaboration with project B5, in which the same basic technique is used to investigate films of transition metals. The second aim is to determine the positions of individual atoms in the nanoscale system. Two-dimensional ultra-thin layers grown epitaxially on single crystal substrates exhibit long-range order parallel to the surface. Whereas the lateral distances are expected to be strongly related to the substrate, the resulting strain should lead to more pronounced relaxations perpendicular to the surface. The most suited method for tackling this question is the analysis of I-V curves of low-energy electron diffraction (LEED). The investigations will benefit considerable from additional information about the systems of other projects. In particular the investigations of projects B1 and B2 will nicely complement the results obtained here with respect to a comprehensive understanding of the physical properties of such systems.