Atomistic Structural Variation via Electromigration in Molten-State Gold Nanocontacts

Abstract

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.

Structure control of tungsten nanocontacts through pulsed-voltage application

“…Nanosecond-pulsewave application is one of the candidates of such processing methods. [9][10][11] However, the studies on the pulse application method hover at an early stage, with a simple observation of the structural variation between crystal and amorphous states in NCs of only several elements. In this study, we demonstrate an atomistic control of the external shape and texture of tungsten (W) NCs through electromigration (EM) and amorphization using the pulse-wave-application method.…”

“…Such directional change of the EM during the pulse application has been also observed in gold and tantalum (Ta) NCs. 10,11) The EM direction is determined by the magnitude relationship between direct forces along the positive-to-negative direction (F el ) acting on the ion cores and electron wind forces acting on the atoms (F wd ) along the opposite direction; negative-to-positive direction for |F wd |=F el > 1 and opposite direction for |F wd |=F el < 1. 19,20) The ratio |F wd |=F el is equal to |Z wd |=Z el , where Z wd and Z el are the effective charge number owing to the electron wind and effective charge number, respectively.…”

“…|Z wd |=Z el decreases below 1 above the specific temperature of 5,400 K, which is 1,700 K higher than the melting point of W. This shows that when a pulsed voltage higher than 1.0 V is applied, the temperature of the NCs exceeds the specific temperature, leading to a directional change of the EM. For a Au NC, |Z wd |=Z el decreases below 1 when the temperature exceeds its melting point, 11) indicating that the direction switching above the specific temperature is a characteristic feature of W. Amorphization occurs when the quenching rate exceeds the critical rate for glass (amorphous) transformation. 9,[25][26][27] Zhong et al reported that NCs comprised of high-meltingpoint metals amorphized during the application of pulse waves with a height of 0.5-3.0 V and width of 3.6 ns.…”

Surface reconstruction in gold nanowires

“…Thus, electromigration occurred from the positively biased side to the negatively biased side, resulting in growth of a NW via pulse application. The electromigration direction implies that the constriction region melts once during the pulse application 17 . The NW length increased to 3.1 nm after 18 pulses, as shown in Fig.…”

“…The surface reconstruction observed in this study was caused by electromigration via pulsed voltage application under tensile stress conditions. Electromigration occurs predominantly at surfaces 17 , 27 . As observed in Fig.…”