Burn-out current density of bismuth nanowires

In the recent past, extensive studies were performed on nanoscale materials. In particular, bismuth nanowires attracted large interest because of their extraordinary properties and possible application in thermoelectric devices. Regarding future implementations, the maximal current density jmax nanowires are able to carry is an important aspect. High current densities can cause voids in the wires or even lead to wire breakage. Bulk metals fail because of Joule heating at current densities of 10 to 10 A/cm [1]. Bulk Bi is expected to fail at lower jmax, since the specific electrical resistivity of this material is more than fifty times higher than in conventional metals and the melting temperature amounts only to 271 °C. In microstructures another effect electromigration – was found to cause failure [2]. This is a thermally activated process where atoms diffuse mostly at grain boundaries so that voids are formed. Failure occurs when voids coalesce so that they finally cause a breakage along the whole wire width. In order to investigate the burn-out current density, single Bi nanowires were prepared electrochemically in ion track-etched polycarbonate (PC) membranes. The templates were fabricated by irradiating PC foils with single swift heavy ions at the UNILAC. Subsequently, the foils were etched in 2M NaOH at 50 °C. After coating one side of the membrane with a conductive layer, the wire was grown electrochemically in the single nanopore [3]. The deposition was continued until a cap was formed on top of the wire. After rinsing and drying the sample, an additional gold layer was sputtered onto the cap for electrically contacting the nanowire [4]. In order to determine jmax the current was ramped up in a two-contact configuration until failure occurred. Fig. 1 displays the I-V curve of a single bismuth nanowire with diameter 230 nm. The breakage being indicated by the arrow is observed at I = 900 μA, i.e., jmax = 2.2*10 A/cm.