Templated growth and properties of iron nanowires: anisotropic electron states of the quasi 1D copper substrate, and morphological and magnetic properties of the iron adsorbate

2007/2008 Low dimensional and nano-scale material systems display quantum behavior due to the confinement in one or more dimensions of the electron states. The goal of nanoscience is to understand and exploit such behavior. Reaching such goal implies growing or fabricating nano-scale systems with atomic precision, not achievable by statistical top down methods. Self-organized and self-assembled systems on single crystal surfaces are prototypical atomically precise systems since they can be reduced to 1 (quantum wire) or 0 (quantum dot) dimension, maintaining the atomic precision of crystal. The surface reconstruction of stepped surfaces, induced by locally selective chemical reactions, is exploited in the present work to produce: a) a quasi 1D stripes array of single crystal terraces that can be studied both by local probes to establish the morphology and the local density of states as well as by means of extended electronic structure techniques to probe the anisotropic confinement of the electronic states; b) to provide chemically selective substrates for the growth of atomically precise quasi 1D metallic wires. This work presents a study on the morphology and on the electronic states of the Cu(332) surface as selectively oxidized to form a quasi 1D terraced surface. This surface provides then a template for the growth of 1D iron wires. The oxygen exposition of vicinal Cu(n,n,n-1) surfaces induces a reconstruction of stripes, parallel to the step edges, consisting of alternating uncontaminated Cu(111) terraces and Cu(110)-O(2x1)facets. We show that the control of the surface synthesis conditions, i.e. oxygen dose and substrate temperature, allows to tailor the periodicity of the reconstruction from 3 to 10 nm on the same primitive vicinal surface. The structural characterization of the surfaces was accomplished by means of Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM) with atomic resolution, in ultra-high vacuum. The Angular Resolved Photoemission (ARPES) data, obtained with polarized synchrotron radiation at the APE-INFM beamline at Elettra, show important changes in the Cu(111) L-gap surface states as a function of stripe width (reconstruction periodicity) due to quantum confinement: the surface state in the oxygen treated Cu(332) is confined in the (111) terraces and displays a large asymmetry with respect to the terrace axes. The Cu surface state of the clean Cu(332) surface is little perturbed by the monoatomic steps, and behaves as an average-surface-like electron state. The surface state of the oxygen treated surface is, on the other hand, characteristic of a quasi 1D terraces system, and display anisotropic dispersion with clear evidence of quantum confinement in the direction of the terrace width. The growth of iron on the O/Cu(332) nanostructured surface is ruled by the affinity for the oxygen-induced (110) facets that run parallel to the Cu(111) terraces. Well ordered Fe nanowires can be produced on such facets even at room temperature (RT). Such ultrathin 1D iron nanowires have been characterized by X-ray absorption spectroscopy (XAS), by magneto-optic Kerr effect (in situ) and by X-ray magnetic circular dichroism, by using circularly polarized radiation from the APE-High Energy beamline at Elettra. XXI Ciclo 1979