We show scanning probe microscopy measurements of metallic nanocontacts between controlled electromigration cycles. The nanowires used for the thinning process are fabricated by shadow evaporation. The highest resolution obtained using scanning force microscopy is about 3 nm. During the first few electromigration cycles the overall slit structure of the nanocontact is formed. The slit first passes along grain boundaries and then at a later stage vertically splits grains in the course of consuming them. We find that first the whole wire is heated and later during the thinning process as the slit forms the current runs over several smaller contacts which needs less power.Comment: 4 pages, 4 figure

FIG. 5. STM set-up used in the literature for studying nanoscale metallic junctions with TEM. The set-up very much resembles a standard STM setup, except that this instrument is used under the electron beam of a TEM. Reprinted with permission from Ohnishi et al., Nature 395, 780 (1998). FIG. 6. (a) If microscopic and macroscopic effects are to be summarized in one picture, diffusion under the influence of electromigration forces can be understood as hopping in a tilted washboard-potential. (b) Energy landscape for a hopping atom with a definition of its energy parameters.

In the field of molecular electronics thin films of molecules adsorbed on insulating surfaces are used as the functional building blocks of electronic devices. A control of the structural and electronic properties of the thin films is required for a reliable operating mode of such devices. Here, noncontact atomic force and Kelvin probe force microscopies have been used to investigate the growth and electronic properties of pentacene on KBr(001) and KCl(001) surfaces. Mainly molecular islands of upright standing pentacene are formed, whereas a new phase of tilted molecules appear near step edges on some KBr samples. Local contact potential differences (LCPD) have been studied with both Kelvin experiments and density-functional theory calculations. Large LCPD are found between the substrate and the differently oriented molecules, which may be explained by a partial charge transfer from the pentacene to the surface. The monitoring of the changes of the pentacene islands during dewetting shows that multilayers build up at the expense of monolayers. Moreover, in the Kelvin images, previously unknown line de- * To whom correspondence should be addressed fects appear, which unveil the epitaxial growth of pentacene crystals.

Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, [Formula: see text] and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. [Formula: see text] nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of [Formula: see text] islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules 'climb' to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.

We determine magnetoresistance effects in stable and clean Permalloy nanocontacts of variable cross section, fabricated by UHV deposition and in situ electromigration. To ascertain the magnetoresistance (MR) effects originating from a magnetic domain wall, we measure the resistance values with and without such a wall at zero applied field. In the ballistic transport regime, the MR ratio reaches up to 50% and exhibits a previously unobserved sign change. Our results can be reproduced by recent atomistic calculations for different atomic configurations of the nanocontact, highlighting the importance of the detailed atomic arrangement for the MR effect.