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The effect of crystallinity on the thermal instability of gold nanowires S. Karim, W. Ensinger Metallic nanowires are an important component of future nanoscale devices. Among other important factors, the thermal stability of nanowires is crucial for a reliable performance of nanowire based devices. Recent experimental and theoretical results demonstrate that the so-called Rayleigh instability causes the decay of a cylindrical nanowire into nanospheres in particular at elevated temperatures where atomic movement by diffusion becomes significant. This pose a serious obstacle to the sustained reliability of nanowire based devices. For this reason, the Rayleigh instability of gold nanowires was investigated. The emphasis was on their crystallographic properties. Cylindrical gold nanowires with diameters D = 87 nm were prepared by electrochemical deposition in Polycarbonate templates. The polymer foils of thickness 30 µm were irradiated normal to the surface at the UNILAC linear accelerator of GSI, Darmstadt, with uranium ions of energy 2 GeV at a fluence 10 8 ions/cm 2 . Nanoporous templates were then produced by chemically etching the latent ion tracks. After deposition of a conductive Cu back-electrode, wires were grown electrochemically in the templates. By systematically varying the deposition conditions, polycrystalline (PC) and single-crystalline (SC) gold nanowires with wellcontrolled crystallographic characteristics were obtained. Characterization of wires by X-ray diffraction (XRD), and transmission electron microscopy (TEM) confirmed that the SC gold wires contained no grain boundaries and possess a strong 〈110〉 preferred orientation. The PC wires exhibit a bamboo-like structure with the grain sizes varying between several hundred nanometers and few micrometers . Figure 1 shows representative XRD diffractograms of both types of samples. Intensity (a.u.) 111 Cu 200 Cu 220 311 222 40 60 80 100 2 Theta (deg) Fig. 1: XRD patterns of (a) single-crystalline and (b) poly-crystalline wires of diameter 87 nm. Cu reflections are ascribed to the Cu back-electrode. For the annealing experiments, the Polycarbonate matrix was dissolved. The samples were introduced in a vacuum furnace (~5×10 -4 Pa), heated up to a temperature - 92 - Cu (a) (b)
Ta = 600 o C, held at Ta for a certain annealing time ta, and then cooled down to room temperature. The samples were examined using a high-resolution scanning electron microscope (HRSEM). The micrographs in Figure 2 illustrate two sequences of the morphological transformation of (left) poly- and (right) single-crystalline cylindrical gold wires caused by the Rayleigh instability. For both type of wires thickness undulations are first developed along the wire axis leading to fragmentation. Eventually, the fragments change to spheres. Figures 2a and 2e display the morphology of the as-prepared poly- and single-crystalline wires prior to annealing. The polycrystalline wires developed axial perturbations after one hour annealing (Fig. 2b), and the wires fragmented, and decayed partially into nanospheres after ta = 2 h (Fig. 2c). Finally after ta = 2.5 h, the wires had been transformed completely into chains of nanospheres (Fig. 2d). The decay of the single-crystalline wires is depicted in Figures 2e-2h. The single-crystalline wires proved to be significantly more resistant to Rayleigh instability compared to the polycrystalline ones. First radial perturbations appeared after ta = 2.5 h, fragmentation was observed after ta = 3 h, and complete decay into spheres occurred only after an annealing time of ta = 5 h. 200nm RT (a) (b) (c) (d) 2µm 5µm 2µm 500nm t a =1h t a =2h t a =2.5h 200nm (e) 200nm (f) RT 2µm t a =3.5h (g) (h) 5µm t a =2.5h t a =5h Fig. 2: The HRSEM images show the stages of the decay of poly- (left), and single-crystalline (right) gold nanowires of diameter 87 nm into nanospheres upon annealing for different times ta at 600 o C. In summary, the results that the nanowire crystallinity influences the Rayleigh instability considerably. The single crystalline wires are found to be more resistant against the Rayleigh instability than polycrystalline ones of the same thickness. Acknowledgment: The authors would like to thank Drs. T.W. Cornelius, R. Neumann (GSI), and A.G. Balogh (Thin Films) for the fruitful collaboration. - 93 -
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