The Growth and Mechanical Properties of Gold Nanowires
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The Growth and Mechanical Properties of Gold Nanowires Rui Dou, and Brian Derby Materials Science Centre, The University of Manchester, Grosvenor Street, Manchester, M1 7HS, United Kingdom ABSTRACT We have measured the yield strength of gold nanowire forests with mean diameter 30, 60 and 70 nm fabricated by electro-deposition into porous alumina templates. All nanowire sizes showed yield strengths much greater than expected from polycrystalline gold specimens with the 30 nm specimens having a yield stress in excess of 1.4 GPa. We found no significant work hardening at plastic strains up to 30%. The strength of the nanowires as a function of wire diameter follows the same trend as has been reported for the compression strength of larger gold columns reported in the literature. TEM observations of deformed wires are consistent with mechanisms of dislocation induced deformation. INTRODUCTION Uniaxial compression studies have shown that the yield strength of sub-micron gold columns increases dramatically with decreasing column diameter [1-3]. Possible explanations for this size effect in the deformation of small crystalline structures have been reviewed by Nix et al and can be ascribed to two mechanisms [4], either the presence of strain gradients, in which a geometrically stored population of dislocations leads to strengthening or dislocation starvation, where the presence of a free surface and the associated image forces remove dislocations from the structure and the generation of new dislocations are required to maintain deformation. However, uniaxial compression does not introduce significant strain gradients and therefore, the size effects reported in the deformation of sub-micron gold columns are most likely attributed to the dislocation starvation mechanism. In previous reported work, gold columns have been fabricated by the focused ion beam (FIB) machining of bulk gold specimens, with the smallest diameter columns having a diameter of 180 nm [3]. Here we use the porous alumina templates to fabricate gold nanowires with smaller diameters in the range 30 – 80 nm by electro-deposition. The mechanical properties of gold nanowires were investigated by uniaxial compression testing. EXPERIMENT Fabrication of gold nanowire forests Our gold nanowires were fabricated by electrodepositing gold into the pores of a highly ordered anodic aluminium oxide (AAO) template with controlled pore diameter and spacing. We used the two-step anodization process of Masuda to make hexagonally ordered porous alumina membranes [5,6]. High purity (99.999%) aluminium foils were degreased in acetone and electropolished in a mixture of perchloric acid and ethanol (1:4 by volume). The aluminium foil was anodised at a constant voltage of 40 V in 0.3 M oxalic acid solution at 10°C for 12 hours. Then a mixture of chromic acid (0.1 M) and phosphoric acid (0.1 M) was used at 60°C for more than 1 hour to eliminate the AAO film formed by the first anodization process. The AAO film
with regular pores was obtained by a second anodizing ste
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