Diffusion of gold nanoclusters on graphite

We present a detailed molecular-dynamics study of the diffusion and coalescence of large (249-atom) gold clusters on graphite surfaces. The diffusivity of monoclusters is found to be comparable to that for single adatoms. Likewise, and even more important, cluster dimers are also found to diffuse at a rate which is comparable to that for adatoms and monoclusters. As a consequence, large islands formed by cluster aggregation are also expected to be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling law for the dependence on size of the diffusivity of large clusters, we find that islands consisting of as many as 100 monoclusters should exhibit significant mobility. This result has profound implications for the morphology of cluster-assembled materials

Réseaux bidimensionnels d'agrégats magnétiques préformés en phase gazeuse

Ce travail de thèse porte sur l'organisation de nanoparticules sur des surfaces grâce à une approche originale basée sur le dépôt à faible énergie d'agrégats, préformés en phase gazeuse (approche Bottom-up), sur substrats de graphite HOPG, fonctionnalisés par des réseaux 2D de défauts créés par faisceau d'ions focalisés nano-FIB (approche Top-down). L'obtention de réseaux 2D d'amas d'agrégats est conditionnée par la diffusion des agrégats déposés et leur sensibilité aux défauts artificiels FIB. Le développement de cette approche passe tout d'abord par des simulations Monte-Carlo réalisées dans le cadre du modèle DDA (Dépôt-Diffusion-Agrégation). Ensuite, sa faisabilité expérimentale est testée avec succès en déposant des agrégats d'or. En particulier, cette étude montre la dépendance des propriétés morphologiques et structurales des défauts FIB en fonction de la dose d'ions. Par ailleurs, la sensibilité des agrégats aux défauts FIB, ainsi que la possibilité de contrôler la taille des amas, la période du réseau et sa géométrie sont analysées. Enfin, en se plaçant dans les conditions optimales permettant la meilleure organisation, notre approche est étendue aux agrégats magnétiques Co50Pt50, candidats potentiels au stockage d'information à ultra-haute densité. Une étude de l'influence du recuit sur les réseaux d'agrégats Co50Pt50 montre une amélioration de la qualité de l'organisation, et surtout une transition de phase des agrégats de la structure A1 vers la structure L10This thesis work focus on the organization of nanoparticles on surfaces by an original approach based on the deposition of preformed nanoclusters in a soft landing process (Bottom-up approach) onto functionalized HOPG-substrate with 2D arrays of defects created by Focused Ion Beam nano-FIB (Top-down approach). To obtain a 2D array of cluster-assembled nanoislands, the deposited nanoclusters must diffuse on the surface and be sensitive to FIB-defects. Then, in order to develop this approach, we first emphasize the predictions of Monte-Carlo simulations including DDA (Deposition-Diffusion-Agregation) model. Next, we successfully test its experimental feasibility by depositing gold preformed nanoclusters. Particularly, this study shows the dependence of morphological and structural properties of FIB-defects on the ion doses. Moreover, the sensitivity of deposited nanoclusters to FIB-defects and the possibility to control the periodicity and the geometry of the nanoparticle arrays as well as the size and the morphology of the cluster-assembled nanoislands were analyzed. Finally, choosing the set of experimental parameters responsible for the nicer organization, we extend our approach to Co50Pt50 magnetic nanoclusters, promising candidates for high-density data storage devices. Post-annealing treatments induce a drastic improvement of the quality of the organization as well as a phase transformation of the supported clusters from A1-phase to the L10-oneLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Dimensionality transition in submonolayer growth on carbon nanotubes

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Moiré induced organization of size-selected Pt clusters soft landed on epitaxial graphene

International audienceTwo-dimensional hexagonal arrays of Pt nanoparticles (1.5 nm diameter) have been obtained by deposition of preformed and size selected Pt nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moiré pattern) of graphene epitaxially grown on the Ir(111) surface. By means of complementary techniques (scanning tunneling microscopy, grazing incidence X ray scattering), the Pt clusters shapes and organization are characterized and the structural evolution during annealing is investigated. The soft-landed clusters remain quasi-spherical and a large proportion appears to be pinned on specific moiré sites. The quantitative determination of the proportion of organized clusters reveals that the obtained hexagonal array of the almost spherical nanoparticles is stable up to 650 K, which is an indication of a strong cluster-surface interaction

Intrinsic magnetic properties of bimetallic nanoparticles elaborated by cluster beam deposition

International audienceIn this paper, we present some specific chemical and magnetic order obtained very recently on characteristic bimetallic nanoalloys prepared by mass-selected Low Energy Cluster Beam Deposition (LECBD). We study how the competition between d-atom hybridization, complex structure, morphology and chemical affinity affects their intrinsic magnetic properties at the nanoscale. The structural and magnetic properties of these nanoalloys were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of the nanoalloys we observe different magnetic responses compared to their bulk counterparts. In particular, we show how specific relaxation in nanoalloys impacts their magnetic anisotropy; and how finite size effects (size reduction) inversely enhance their magnetic moment

Organization and magnetic properties of FePt nanoparticles deposited on graphene/Ir moiré pattern

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