Junior Research Group of Dr. Regina Hoffmann
We investigate the structural and electronic properties of metallic nanocontacts. The electronic properties of nanocontacts are often explained by making assumptions about their atomistic structure. Our goal is to study the structure using scanning probe microscopy. The nanocontacts are made by controlled electromigration. This will allow us to better understand and tailor the properties of the leads and contacts to small electronic building blocks such as CMOS transistors and molecular electronics functional units.
Metallic nanocontacts and molecular electronics
If one thins a metallic wire further and further, one finds deviations from Ohms law. The conductance of the contact no longer varies continuously with the dimensions of the wire. For these metallic nanocontacts the atomic configuration and the chemical nature of the metal atoms on one hand and the wave nature of the electrons on the other hand determine the conductance.Metallic nanocontacts can be prepared by thermally assisted electromigration. For this method, a thin metallic wire is heated by applying an electric current such that the thermally activated atoms diffuse under the influence of electromigration forces such that the contact becomes thinner. During this process, the temperature of the contact that forms is controlled.
This method is also used to contact molecules. In this case a molecule must be placed on an insulating surface. Single molecules on insulating surfaces can be imaged with high-resolution scanning force microscopy. The sensitivity and resolution is similarly good as for the scanning tunneling microscope. In this way self-organisation of molecules and the epitaxial relationship of molecular islands with the substrate can be investigated. Conclusions can be drawn about the interaction of the molecules with other molecules and with the substrate.
Scanning force microscopy in the dynamic mode allows to image insulating, semiconducting and metallic surfaces with atomic resolution. The force between a tip that is attached to a cantilever and a clean and ordered surface is determined by measuring the displacement of the cantilever. To measure the force with higher precision, we use a dynamic mode in which the tip is oscillated. The dynamic mode is also the best way of avoiding a hard contact between the tip apex and the surface and the resulting tip damage. If such a damage is avoided, chemical bonding forces between the tip and the surface can be measured directly.
In addition to the measurement with atomic resolution in the lateral direction parallel to the surface, one can measure the chemical bonding forces as a function of tip-sample distance. From this information one can generate three-dimensional vector fields of the forces above a few atoms of the sample. We compare the measured forces with atomistic calculations. Suitable models for the surface are well-known. Since little is known about the tip, we use several models for the tip. A comparison between simulations and measurements allows to better understand the origin of the measured contrast and to refine the models of the tip, for example to determine the polarity of the frontmost tip atom.