Electron tunneling between vibrating atoms in a copper nano-filament

Nanowires, atomic point contacts, and chains of atoms are one-dimensional nanostructures, which display size-dependent quantum effects in electrical and thermal conductivity. In this work a Cu nanofilament of a defined resistance and formed between a Cu and Pt electrode is heated remotely in a controlled way. Depending on the robustness of the conductive filament and the amount of heat transferred several resistance-changing effects are observed. In case of sufficiently fragile nanofilament exhibiting electrical quantum conductance effects and moderate heating applied to it, a dramatic increase of resistance is observed just after the completion of the heating cycle. However, when the filament is allowed to cool off, a spontaneous restoration of the originally set resistance of the filament is observed within less than couple tens of seconds. When the filament is sufficiently fragile or the heating too excessive, the filament is permanently ruptured, resulting in a high resistance of the cell. In contrast, for robust, low resistance filaments, the remote heating does not affect the resistance. The spontaneous restoration of the initial resistance value is explained by electron tunneling between neighboring vibrating Cu atoms. As the vibrations of the Cu atoms subside during the cooling off period, the electron tunneling between the Cu atoms becomes more likely. At elevated temperatures, the average tunneling distance increases, leading to a sharp decrease of the tunneling probability and, consequently, to a sharp increase in transient resistance.

A flexible platform for controlled optical and electrical effects in tailored plasmonic break junctions

Mechanically controllable break junctions are one suitable approach to generate atomic point contacts and ultrasmall and controllable gaps between two metal contacts. For constant bias voltages, the tunneling current can be used as a ruler to evaluate the distance between the contacts in the sub-1-nm regime and with sub-Å precision. This ruler can be used to measure the distance between two plasmonic nanostructures located at the designated breaking point of the break junction. In this work, an experimental setup together with suitable nanofabricated break junctions is developed that enables us to perform simultaneous gap-dependent optical and electrical characterization of coupled plasmonic particles, more specifically bowtie antennas in the highly interesting gap range from few nanometers down to zero gap width. The plasmonic break junction experiment is performed in the focus of a confocal microscope. Confocal scanning images and current measurements are simultaneously recorded and exhibit an increased current when the laser is focused in the proximity of the junction. This setup offers a flexible platform for further correlated optoelectronic investigations of coupled antennas or junctions bridged by nanomaterials.

Electromigration-induced directional steps towards formation of single atomic Ag contacts.

Background:The process of electromigration is still not quantitatively understood. We showed recently that it can be used reliably for formation of single atomic point contacts in pre-structured Ag nanostructures. Results:The process of formation of nanocontacts by electromigration (EM) down to a single atomic point contact was investigated for ultrathin (5 nm) Ag structures at 100\,K. In this paper, we compare the structures with constrictions below the average grain size of Ag layers (15 nm), where the contribution of a single grain dominates, with structures of much larger constrictions of around 150 nm with multiple grains at the centre constriction during the initial steps of EM. The latter initially form filamentous structures. Despite these clear morphological differences, the conductance traces of both types of structures suggest that finally, i.e., in the quantized conduction regime, only one atomic point contact was formed. To analyse the thinning process within the semi-classical regime in detail, we used experimental conductance histograms in the range between 2 G0 and 15G0 and their corresponding Fourier transforms (FT). The FT analysis of the conductance histograms exhibits a clear preference for thinning along the [100] direction. Using well-established models, both atom-by-atom steps and ranges of stability, presumably caused by electronic shell effects, can be discriminated. A large range (5 to 14G0) of unstable conductance values was found in these electromigrated contacts that has not been reported by other techniques. It was observed irrespective of the initial geometry. Conclusion: Although the directional motion of atoms during EM leads to specific properties like the instabilities mentioned, similarities to mechanically opened contacts with respect to cross sectional stability were found.