Composite Cathode with Boroxine Ring Developed for All-Solid-Polymer Lithium Cell
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of nanowires. As reported in the November 29, 2004, issue of Applied Physics Letters (p. 5337), the research team discovered that fragmentation of copper nanowires occurs as a function of temperature and is driven by the Rayleigh instability. The Rayleigh instability concept, introduced over a century ago to explain the instability of a liquid jet, is now being applied to describe the fragmentation of copper nanowires; it is driven by surface atomic diffusion in conjunction with any periodic variations in wire diameter. The researchers observed, by high-resolution scanning electron microscopy (HRSEM), different stages of fragmentation after annealing the nanowires at elevated temperatures and reported a clear dependence of the decay process on temperature. According to the researchers, the wires started to fragment at ~400°C, form shorter segments with an increase to ~500°C, and decayed into chains of nanospheres at 600°C. All of these processes occurred at temperatures far below the melting point of copper. The nanowires employed by Toimil Molares and colleagues for the annealing experiments were fabricated by electrochemical deposition of copper into the pores of etched ion track membranes. The templates were created by irradiating thin polycarbonate foils with 2 GeV Au heavy ions. The damage zones produced by the ions along their paths were then dissolved in NaOH solution. The scientists were also able to show that these pores have a slightly biconical shape, with diameters varying between 30 nm at the center and 50 nm at the ends. These pores were subsequently filled with copper by electrochemical deposition, and the surrounding matrix was dissolved in dichloromethane for SEM investigations. Toimil Molares said that the observed mechanism of the copper nanowire fragmentation agrees qualitatively with previous theoretical work, such as Monte Carlo calculations of fragmentation of Ge nanowires due to the Rayleigh instability. The resulting size and spacing of the nanospheres has also been linked to additional theoretical predictions. “Our research clearly indicates that for nanoscale device applications, technological problems arising from the thermal instability of materials must be taken into account if reliable devices must be produced,” said Toimil Molares. The observed phenomenon could also be used to develop new materials processing techniques, for example, to fabricate nanospheres of copper, silver, or gold. “This new kind of nanoscopic string of spheres could also find applications in 6
nanophotonics, since it may be used to guide light below the diffraction limit via coherent coupling of surface plasmon polaritons,” Toimil Molares said. This increased understanding of how copper nanowires behave as a function of temperature should have an impact on both fundamental sciences and technological applications, and it could lead to substantial future studies—for instance, ab initio atomistic simulations of the mechanism to reveal more of the underlying atomistic details of the processes. MARKUS J. BUEHLER
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