The structural variation in tungsten nanocontacts (NCs) during a pulsed-voltage application was observed in situ by high-resolution transmission electron microscopy. The direction of electromigration in the NCs changed from the well-known direction to the opposite direction at a critical voltage of 0.9 V. Upon applying a higher pulsed voltage of 2.5 V, the NC structure changed to amorphous, with an average conductance density decreased to 82% of that of the crystalline NCs. We demonstrated that the external shape and texture of tungsten NCs can be controlled with an atomic precision through electromigration and amorphization by a pulsed-voltage application.

The amorphization and crystallization of the molybdenum (Mo) nanocontacts (NCs) via pulsed voltage application were observed in situ at the atomic resolution by transmission electron microscopy. Simultaneously, the variation in conductance of NCs during structural transformation was measured. The structural transformation could be controlled by the height of applied pulse waves. The amorphization was accelerated when NCs have longer constriction regions, which has not been found in NCs comprised of other metallic elements. The amorphized states of NCs were maintained during the observation at room temperature. The conductance ratio of amorphous Mo NCs to crystal Mo NCs was 0.62, which is lower than the ratio of liquid to solid Mo and is the lowest among those of other metallic NCs, leading to applications to advanced switching and memory functions.

The atomistic structural variation in gold nanocontacts (NCs) due to applied voltage pulses was examined inside a transmission electron microscope. This examination was accompanied by simultaneous conductance measurements. The NCs melted during the passage of 4 ns long voltage pulses, with voltages in the 0.75-1.30 V range. Electromigration occurred in the molten state, and was directed from the positively-biased electrode to the negatively-biased electrode, which opposes the observed direction convention of most solid state metals. Voltage pulses higher than 1.30 V resulted in the formation of nanoscaled gaps. This study demonstrates the ability to control the external shape of NCs and to induce nanoscaled gap formation by utilizing the molten state electromigration phenomenon using voltage pulsing.

We demonstrate the creation of single-atom-sharpened tungsten tips via nanosecond pulse application under tensile stresses. The formation process and resultant tip structures were observed in situ at the atomic resolution by high-resolution transmission electron microscopy. It was found that the single-atom-sharpened tips have high stability sufficient for the dramatic turnaround in various fine tip techniques.

Surface reconstructions are caused by structural stabilization resulting from the modulation of surface atomic positions. Studies on surface reconstruction have been conducted for substantially large surfaces, rather than at the size of reconstructed surface unit cells. Hence, well-known surface reconstruction manners may not be applicable for the surfaces of nanometer-sized isolated crystals, such as nanoclusters, nanowires and nanotubes. This is because they have high surface area-to-interior volume ratios exceeding several tens of percent, and their surface structures significantly affect the stabilization of their entire structures. In this study, we demonstrate the inherent surface reconstruction of gold nanowires via nanosecond-pulsed electromigration with the application of tensile stresses. The results lead to evolutions in basic studies relating to surface reconstruction and nanostructures and in applications of nanowires, for which stabilization is essential when they are used in extremely miniaturized integrated circuits for next-generation electronics.