The authors present the real-time atomic force microscopy (AFM) imaging of structural changes in gold (Au) nanowires during the feedback-controlled electromigration (FCE) process. The resistance increases during the FCE process and is associated with drastic changes in the nanowire morphology, suggesting successful control of electromigration (EM) through the FCE scheme. Moreover, the AFM images taken after performing FCE indicate a redeposition of matter along the nanowire in the direction of the anode side. The grains show faceting structures at the anode side. Furthermore, to obtain quantitative information on the height of structures, cross-sections of the nanowire obtained from the AFM images during FCE were investigated. The height evolution of the narrowest part of the wire perpendicular to the electron flow was obtained, showing that void nucleation and void growth along the grain boundaries, which are located on the border of the nanowire, start in the vicinity of the nanowire constriction at the cathode side. Furthermore, a maximum relative mass transport value of 19% and a mass transport rate of 106 atoms/s were found. These results imply that the in situ AFM technique provides insight into the behavior of EM-induced voids in metal nanowires during FCE.
In this study, the electrical properties of thin graphite wires were investigated for strain sensors. The thin graphite wires were simply and easily fabricated from pyrolytic graphite sheet, which can be formed by firing a polymer film (such as a polyimide film) at high temperatures. The resistance of the thin graphite wires increased under increasing tensile bending strains and decreased under increasing compressive bending strains. Notably, the sensitivity of the sensors increased when the thickness of the thin graphite wires was reduced. This property was investigated via modeling of the strain-induced changes in the overlap area and conduction pathways of the graphite flakes. Multiple-cycle tests were carried out to evaluate the long-term stability of the thin graphite wires; specifically, the electrical response was monitored under repeated cycling, for approximately 1000 cycles. The thin graphite wires were assembled on ultrathin gloves to fabricate data gloves that could detect finger motions. The results of this study indicate that the thin graphite wires that were simply and easily fabricated from pyrolytic graphite sheet have great potential for a wide range of applications, including human motion detectors.
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