205 route de Narbonne - 31077 Toulouse
Thin magnesium oxide films supported on metal surfaces have become an important substrate for metal clusters, which electronic but also geometric properties can be tuned by choosing the properties of the support (e.g. film thickness). Sodium chloride thin films on metal surfaces are used to decouple the intrinsic electronic properties of atoms and molecules from metal surfaces.
In the last 5 years a dramatic increase of work can be observed, where especially noncontact AFM (nc-AFM) and Kelvin probe force microscopy (KPFM) have been used to characterize the surface morphology but also electronic properties of such thin film systems. For studying thin films with the AFM it is of utmost importance to understand the contrast formation in first place.
In my talk I will discuss the basic mechanisms of the contrast formation in NC-AFM and KPFM on thin film systems. It is shown that the topography contrast strongly depends on the potential of the tip's last imaging atoms. Furthermore it is shown that the mean KPFM voltage can be used to directly identify the polarity of the tip’s potential, which helps to distinguish charged tips from neutral ones but also positive from negative tips. The electrostatic potential of the tip is mainly determined by single ions or dipoles at the tip apex, which has been verified by theory. Results of MgO but also NaCl thin films grown on Ag(001) are presented.
In the second part of my talk I will present recent progress in the use of nanostructured bulk insulator surfaces, which can be used to confine the growth of clusters and molecules. It is shown that the nanostructured Suzuki surface of Cd doped NaCl crystals can be used to confine the growth of palladium clusters and functionalized brominated pentahelicene molecules into only one of the two existing surface regions (pure NaCl and Suzuki regions). The growth of palladium results into nanometre sized clusters, which form two-dimensional cluster arrays inside the regions containing the Cd impurities (Suzuki region). The clusters exhibit a high cluster density, are uniform in size, and due to the specific Suzuki structure the clusters are polarized or charged. Brominated pentahelicene molecules completely decorate the Suzuki regions and form two-dimensional islands whereas no molecules can be found inside the pure NaCl regions. The molecules are in a flat configuration, which finds strong support from first principle calculations. It is shown that changing the functional group of the helicene molecules leads to different adsorption characteristics, selfassembly phenomena and different surface dipoles.