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Croissance de nanofils métalliques en présence d'une pointe et étude des propriétés physiques associées

Abstract : This PhD concerns the growth and the study of physical properties of nanometric structures that can be obtained on stepped surfaces. Previous experimental and theoretical studies showed that with self-organized methods it is possible to obtain a large number of nanostructures, but in general the quality of the nanowires depends strongly on the preparation and on the deposition statistic. The original idea of this work consist in assisting the growth using a STM tip. In order to characterize the specific properties of the structures, it is necessary to describe the substrate and its interactions with the adsorbates very precisely. The DFT code FLEUR is used to characterize energetically and structurally different platinum surfaces. In the continuity of this work, the determination of magnetic properties associated with Co chain (free or supported on vicinal Pt surfaces) is investigated. In the first part, a study related to growth phenomenon conducted numerically with a Kinetic Monte Carlo (KMC) model is presented. In the case of the Xe/Cu(110) system, we show that the presence of a fixed STM tip, depending on its height relative to the substrate can locally modify the atomic diffusion and allow the measure of diffusion coefficients related to the motions of the atoms. In a more general frame, it was shown that it is possible to improve the profile of nanowire deposited on vicinal surfaces using a moving STM tip in a repulsive or in an attractive mode. The perfection of the wires depends on the number, the type of the sweepings and also on the considerered temperature range. Then, the electronic properties of platinum surfaces were investigated. After the study of the three flat platinum surfaces namely (111), (100) and (110) surfaces, we focused on the (233) vicinal surface. This one presents the same geometry as the steps of the experimental Pt(997) surfaces. Surface energies, step energies, local density of states or bandstructures have been calculated. The calculations of STM pictures were done in order to interpret the STM pictures of the Pt(997) surface obtained at the EPFL (Lausanne, team of K. Kern and H. Brune). Neither the relaxation (internal) nor the local density of states near the step edge can explain the protusion observed at this location on some of the experimental images. But, this does not mean that our calculations are wrong. The protusion can be due to the experimental set up which does not follow fast enough in time the evolution of the LDOS. A second application of the calculation of STM pictures is relative to the adsorption of Ir atoms on a Ir(111) substrate. FLAPW calculations and the experiments of Thomas Michely's team show that the tip can distinguish unequivocally two different stackings (hcp or fcc) of comparable energie but with different density of states around the Fermi level. Finally, the study of the magnetic properties of Co nanowires unsupported or adsorbed at the foot of Pt step edges is approached. For the unsupported system, the magnetic moment of the cobalt atom decreases from 3 uB in the case of an isolated atom to 2.33 uB for the chain, 2.09 uB for the monolayer and 1.65 uB in the bulk. Calculations were also conducted on periodic arrays of Co wires supported on Pt(233) and eventually relaxed systems. It is shown that the influence of the substrate on the magnetic moment of the Co atom is not very big. On the other hand, a strong quenching of the orbital moments is observed. The magnetic and orbitals moments are equal respectively to 2.105 uB and [0.058-0.091] uB depending on the orientation of the spin quantization axis. The determination of the magnetic anisotropy shows that the easy axis is along the chain direction when the system is not relaxed. Only the relaxation can explain the fact that the easy axis goes out of the chain axis.
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https://tel.archives-ouvertes.fr/tel-00010835
Contributor : Stéphanie Baud <>
Submitted on : Sunday, October 30, 2005 - 8:29:50 PM
Last modification on : Thursday, April 8, 2021 - 3:39:07 AM
Long-term archiving on: : Friday, April 2, 2010 - 10:46:13 PM

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Stéphanie Baud. Croissance de nanofils métalliques en présence d'une pointe et étude des propriétés physiques associées. Physique [physics]. Université de Franche-Comté, 2005. Français. ⟨tel-00010835⟩

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