Diffusion de surface

Résumé

En science des matériaux, la diffusion de surface ou diffusion superficielle est un phénomène impliquant le déplacement d'adatomes, de molécules, et d'agrégats atomiques sur la surface de matériaux solides. Les mécanismes par lesquels cette diffusion s'effectue ont une certaine importance, car ils conditionnent le développement de matériaux ayant des propriétés intéressantes. Le processus peut être considéré comme des particules sautant d'un site d'adsorption d'une surface à une autre, comme schématisé sur la figure 1. Comme pour la plupart des mécanismes de diffusion de la matière, ce mouvement est caractéristique d'un processus thermique dont la fréquence augmente avec la température. Il a été mis en évidence que de nombreux systèmes présentent un comportement de diffusion, qui dérive d'un modèle conventionnel de sauts aux voisins proches. La diffusion à effet tunnel est un exemple particulièrement intéressant de mécanisme non conventionnel, dans lequel il a été montré que de l'hyd

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This thesis reports on the design and construction of a novel experimental setup for the study of surface magnetic systems. This setup consists of a scanning tunneling microscope (STM) designed for ultra high vacuum (UHV) operating at T ≲ 1 K and strong magnetic fields (0 - 8.5 T). Such extreme experimental conditions have influenced the choice of the construction materials and components as well as the general design principles. This work comprises the design, realization and first tests of the STM, cryostat, damping system, sample preparation UHV chambers and transfer system. In parallel to this new setup realization, we have investigated the magnetic properties of dilute transition metal impurities deposited on non-magnetic metal surfaces. Measurements of those systems were performed with x-ray magnetic circular dichroism (XCMD). We have started by adressing the host electronic density effect on magnetic properties of the impurities. In order to make contact with the Friedel [1] and Anderson [2] models, 3d impurities (Fe and Ni) were deposited on alkali metal substrates (K, Na et Li) whose electronic structure is close to the Fermi electron gas. The gradual increase of the host electronic density with decreasing atomic weight going from K to Li allow us to study the effects of s - d electron hybridization on the magnitude magnetic moment of the impurity . In particular, we show that the magnetic moment of Fe is strongly reduced in going from K to Li, while the Ni moment disappears on Na and Li. The XMCD data also show that the orbital magnetic moment is atomic-like on K, but compared to the spin moment, significantly reduces as the substrate electron density increases. Finally, we report on a variable temperature STM study of the nucleation and growth processes of Pd/Pt(111). The experimental results are analyzed on the basis of mean field theory applied to surface diffusion processes and solved by numerical integration. This study allows us to highlight the different atomistic processes that contribute to the nucleation and growth of a prototype epitaxial system.

EPFL2005

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Recent progress in surface NMR-electrochemistry

Jean-Philippe Ansermet, Pierre-André Vuissoz

NMR spectroscopy is one of the newer spectroscopic techniques for investigating the static, dynamic and electronic structures of molecules adsorbed onto metal catalyst surfaces. We review recent progress in the application of solid-state NMR methods to the investigation of molecules adsorbed onto metal surfaces in an electrochemical environment: in the presence of electrolyte, and at an electrified interface under external potentiostatic control. While at a very early stage of development, the NMR-electrochemistry approach has considerable potential for investigating otherwise inaccessible aspects of electrode and adsorbate structure, and should enable a comparison of results obtained from different spectroscopies, in particular from IR spectroscopy. We present a brief review of the development of the subject, followed by details of the instrumentation necessary for NMR-electrochemistry studies. We show how spin-spin relaxation can give information on surface structure and surface diffusion, how spin-lattice relaxation can give information on the presence of conduction electron spillover onto the adsorbate, and how the NMR of surface species responds to an externally applied electric field. The C-13-NMR of CO on Pt in an electrochemical environment is compared with the C-13-NMR of CO on Pt catalysts in vacuum, which are well characterized. In the case of CN on Pt, we show large spectral shifts of the resonance as the electrode potential is varied, providing an independent measurement of the effects of the electrified interface on the chemisorption bond. Spectral sensitivity is also now adequate to observe nuclei which produce even weaker signals than C-13, such as N-15. The NMR-electrochemistry method thus opens up a broad new array of possibilities for probing static structures (from T-2), surface diffusion (from the temperature dependence of T-2) as well as electronic properties of the chemisorption bond (from T-1, and from electrode potential effects) at electrochemical interfaces, and for studying reactive intermediates and poisons on high-surface-area catalysts, such as those utilized in hydrogen and organic fuel cells.

1997

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Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

Raphaël Butté, Jean-François Carlin, Gatien Cosendey, Nicolas Grandjean

Thanks to its high refractive index contrast, band gap, and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790424]

Amer Inst Physics2013

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