Structural studies of metallic nanowires with synchrotron radiation

Mesoporous alumina membranes, with pore diameters between 5 and 100 nm, have been used as templates for the electrochemical deposition of nanowires of metals including gold, silver, iron and cobalt. Several absorption and scattering techniques using synchrotron X-ray radiation, including EXAFS, XANES, SAXS, WAXS and high-energy diffraction, have been used to study the structural features of the empty membranes and the confined metal nanowires. The results show that the membranes consist of arrays of highly-aligned parallel cylindrical pores with axes that are distributed on a disordered two-dimensional hexagonal lattice. Nanowires of gold, silver, and iron have the same lattice structures and almost identical interatomic distances as the corresponding bulk metals. Cobalt nanowires are composed of a mixture of h.c.p. and f.c.c. phases in a ratio depending on the pore diameter. The crystallites of the metallic nanowires show strong preferred orientation relative to the pore axis in the case of iron and cobalt but are more isotropic in the case of gold and silver. The results confirm the anisotropic behaviour of the materials but further theoretical work and additional modelling will be required to extract the full quantitative information from the available data. The powerful combination of complementary X-ray techniques shows the advantages of using third-generation synchrotron radiation for this type of structural investigation. (C) 2003 Elsevier B.V. All rights reserved.