Multi-L1₀ domain CoPt and FePt nanoparticles revealed by electron microscopy

EMC2012: The 15th European Microscopy Congress, 16th - 21st September 2012, Manchester Central, United Kingdo

Multi-L1₀ domain CoPt and FePt nanoparticles revealed by electron microscopy

The atomic structure of CoPt and FePt nanoparticles (with a diameter between 2 and 5 nm) has been studied by transmission electron microscopy. The particles have been produced by a laser vaporization cluster source and annealed under vacuum in order to promote chemical ordering. For both alloys, we observe a coexistence of crystalline and multiply twinned particles with decahedral or icosahedral shapes. In addition to particles corresponding to a single L1₀ ordered domain, we put into evidence that even small particles can display several L1₀ domains. In particular, the chemical order can be preserved across twin boundaries which can give rise to spectacular chemically ordered decahedral particles made of five L1₀ domains. The stability of such structures, which had been recently predicted from theoretical simulations, is thus unambiguously experimentally confirmed. © 2013 American Physical Society.F. Tournus, K. Sato, T. Epicier, T. J. Konno, and V. Dupuis, Phys. Rev. Lett. 110, 055501, 2013

Bottom-up strategies for the assembling of magnetic systems using nanoclusters

International audienceIn the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context twenty, years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the Low Energy Cluster Beam Deposition (LECBD) technique. In this paper, we focus our attention on the typical case of Co clusters embedded in various matrices to study interface magnetic anisotropy and magnetic interactions as a function of volume concentrations, and on still current and perspectives through two examples of binary metallic 3d-5d TM (namely CoPt and FeAu) clusters assemblies to illustrate size-related and nanoalloy phenomena on magnetic properties in well-defined mass-selected clusters. The structural and magnetic properties of these cluster assemblies 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 both NPs and matrix, we observe different magnetic responses compared to their bulk counterparts. In particular, we show how finite size effects (size reduction) enhance their magnetic moment and how specific relaxation in nanoalloys can impact their magnetic anisotropy

Low Temperature Ferromagnetism in Chemically Ordered FeRh Nanocrystals

In sharp contrast to previous studies on FeRh bulk, thin films, and nanoparticles, we report the persistence of ferromagnetic order down to 3 K for size-selected 3.3 nm diameter nanocrystals embedded into an amorphous carbon matrix. The annealed nanoparticles have a B2 structure with alternating atomic Fe and Rh layers. X-ray magnetic dichroism and superconducting quantum interference device measurements demonstrate ferromagnetic alignment of the Fe and Rh magnetic moments of 3 and 1 mu(B), respectively. The ferromagnetic order is ascribed to the finite-size induced structural relaxation observed in extended x-ray absorption spectroscopy. DOI: 10.1103/PhysRevLett.110.08720

Ab initio study of benzene adsorption on carbon nanotubes

The adsorption of a benzene molecule on carbon nanotubes (CNTs) with various diameters and chiral angles is investigated within the ab initio framework. The physisorption of such an organic molecule is an example of noncovalent functionalization involving pi-stacking interactions and corresponding to a weak binding energy. Our calculations show that for small diameter tubes, the most favorable adsorption site is one type of C-C bond. The disparities between the inequivalent bonds of a CNT are discussed in terms of the pi orbital axis vector misalignment. Moreover, the curvature and the chirality effect on benzene adsorption are analyzed, showing that large diameter nanotubes are the most reactive ones

Mutual orientation of two C-60 molecules: An ab initio study

The orientational dependence of the interaction between two C-60 molecules is investigated using ab initio calculations. The binding energy, computed within density functional theory in the local density approximation, is substantially smaller than the one derived from the experimental heat of sublimation of fullerite, which calls into question the nature of inter-C-60 bonding. According to our calculations, the experimentally observed orientation with a C-60 presenting a hexagon-hexagon bond to a pentagonal face of the other C-60 is not really favored. Some other configurations are very close in energy and in fact a pentagon facing a pentagon and a hexagon facing a hexagon-hexagon bond are found to be slightly more favorable situations. Our results are compared to previous ones obtained either with previous empirical intermolecular potentials or to existing ab initio studies of crystalline C-60. In addition, the stacking of C-60 in a crystal and in a decahedral (C-60)(7) cluster is discussed. (C) 2005 American Institute of Physics

Chemical composition dispersion in bi-metallic nanoparticles: Semi-automated analysis using HAADF-STEM

cited By 7International audienceWe present a method using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) to determine the chemical composition of bi-metallic nanoparticles. This method, which can be applied in a semi-automated way, allows large scale analysis with a statistical number of particles (several hundreds) in a short time. Once a calibration curve has been obtained, e.g., using energy-dispersive X-ray spectroscopy (EDX) measurements on a few particles, the HAADF integrated intensity of each particle can indeed be directly related to its chemical composition. After a theoretical description, this approach is applied to the case of iron-palladium nanoparticles (expected to be nearly stoichiometric) with a mean size of 8.3 nm. It will be shown that an accurate chemical composition histogram is obtained, i.e., the Fe content has been determined to be 49.0 at.% with a dispersion of 10.4 %. HAADF-STEM analysis represents a powerful alternative to fastidious single particle EDX measurements, for the compositional dispersion in alloy nanoparticles. © Springer Science+Business Media B.V. 2012

π-stacking interaction between carbon nanotubes and organic molecules

The π -stacking interaction between various planar organic molecules is investigated within the framework of ab initio calculations. The adsorption of these molecules on the sidewall of the cylindrical carbon structure induces a small binding energy compared to conventional covalent functionalization. Such a weak interaction is found to be only physisorption and leads to minor and predictable modifications of the electronic structure. These changes in the electronic behavior of the host carbon nanotube are ruled by the relative positions of the molecular levels of the isolated molecule and both the valence and conduction bands of the perfect tube. © 2005 The American Physical Society