Structure and Bonding of Metallic Nanowires Prepared in Nanoporous Alumina Membranes Studied by EXAFS, XANES and HED
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Structure and Bonding of Metallic Nanowires Prepared in Nanoporous Alumina Membranes Studied by EXAFS, XANES and HED M. Kröll1, R. E. Benfield2, J. C. Dore2, D. Grandjean2, G. Schmid3 1 Physics Department, Trinity College Dublin, Dublin 2, Ireland 2 School of Physical Sciences, University of Kent at Canterbury, CT2 7NR, United Kingdom 3 Institut für Anorganische Chemie, Universität GH Essen, Universitätsstr. 5-7, 45117 Essen, Germany ABSTRACT The structural properties of metallic nanowires prepared within the pores of nanoporous alumina membranes are investigated using EXAFS, XANES and High Energy X-Ray Diffraction (HED). Gold, silver, copper and iron nanowires show the same structure and atom-atom distances as the corresponding bulk metals. The structural properties of these metals do not change if the wire diameter is varied. Cobalt nanowires, however, show a size dependent mixture of fcc and hcp phases. The amount of fcc-Co, which in the bulk is only known as a high temperature modification, increases with decreasing wire diameter. This leads to interesting changes in the magnetic properties of Co nanowires.
INTRODUCTION Metallic nanowires have attracted considerable attention not only because of possible applications, for example as perpendicular magnetic storage devices [1,2], but also because of their interesting physical properties. The well-known physical properties of the bulk metal can change dramatically if the lateral dimensions of the sample are reduced down to the nanometer scale. Furthermore, an anisotropy may be introduced in the physical properties if only one or two dimensions of the material are reduced in size. A material can be considered as being onedimensional if the size of two dimension is reduced down to the range of, for example, the DeBroglie wavelengths of electrons, the excitation wavelengths of phonons or magnons or the magnetic domain wall width depending on the property under investigation. In order to prepare one-dimensional metallic nanowires using a bottom-up approach one needs to perform electrochemical or chemical reactions within the pores of a suitable host material. We use nanoporous alumina membranes as a template material and an AC plating procedure to prepare the nanowires within the pores. Anodically prepared alumina membranes offer adjustable pore diameters within a range of 5 to 250 nm (depending on the anodizing voltage applied), variable pore lengths (depending on the anodizing time) and a nearly hexagonal pore arrangement [3-5]. The structural properties of the nanowires may influence the physical properties as was shown for Co nanowires which show different forms of magnetic anisotropy depending on whether fcc- or hcp-Co was found inside the pores [6]. EXAFS and WAXS provide information about the local coordination geometry, atom-atom distances and the overall lattice structure. HED delivers data about structural anisotropies. XANES finally gives a qualitative measure on oxidation states.
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EXPERIMENTAL DETAILS Nanoporous alumina membranes are prepared by
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