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Accueil du site > Séminaires > A venir ...

A venir ...

 
Jeudi 19 Juillet 2018
Abdelkader Kara
Department of Physics, University of Central Florida
Interface Characteristics of Organic Materials on Metal Surfaces
Extended materials consisting of organic molecules fall between ‘hard’ and ‘soft’ materials categories with promising novel and useful functionalities. Their growth through self-assembly is, however, not well understood due to the lacking of a detailed understanding of an accurate account of different operative forces, which are responsible for the assembly of materials with varying physical/chemical properties. I will present results of a detailed computational study of the adsorption of selected groups of organic molecules on metal surfaces with varying geometries and elemental composition. The targeted systems have the potential to serve as efficient devices for solar energy harvesting and for solid-state lighting. The computational studies use both standard density functional theory (DFT) as well as exploiting the self-consistent inclusion of dispersive forces (Van der Waals interactions), with the aim of obtaining the effects of dispersive forces and their dependence on the surface chemical properties. I will present the results on how the interface characteristics between the organic materials and metal surfaces change with the characteristics of molecules, the degree of reactivity, as well as the geometry of the surfaces [1-5]. The results obtained for the adsorption of acenes, thiols and other molecules with CN groups on several transition metal surfaces (Au, Ag, Cu, Ni, Pd, Pt and Rh) will be presented. * Work supported by a DOE grant No DE-FG02-11ER16243. [1] K. Muller et al, J. Phys. Chem C 116, 23465 (2012) [2] H. Yildirim and A. Kara J. Phys. Chem C 117, 2893 (2013) [3] H. Yildirim, T. Greber and A. Kara J. Phys. Chem C 117, 20572 (2013) [4] W. Malone, J. Matos and A. Kara, Surf. Sci. 669, 121 (2018) [5] W. Malone, H. Yildirim, J. Matos and A. Kara, J. Phys. Chem. C, 121, 6090 (2017)

Jeudi 06 Septembre 2018
Christian Brandl
Institute for Applied Materials, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany
Advanced engineering materials with tailored functionality at the nanoscale: Insights from computer simulations & nanoscale experiments
In advanced engineering materials the state of microstructure determines the materials behaviour during synthesis and during operation in devices. Especially strength and deformation in metals depend on the state and evolution of the microstructure, which constitutes the spatial arangment of materials defects from the atomic-scale defect core structure to the mesoscale defect morphology. The focus of the talk will be on the emergence of novel thermomechanical behaviour by microstructure processes focusing on defect nucleation and defect migration at the nanoscale. The synergies of molecular dynamics (MD) simulations and experimental work are highlighted for the notch-insensitive deformation behaviour of gold nanowires, stress-driven grain boundary migration and self-healing gold nanoparticles. The aspect of defect nucleation is discussed in context of flaw-insensitivity of Au nanowires. The predicted local deformation behaviour at the flaws and the underlying deformation mechanisms from MD simulations are validated by simulation-inspired nanomechanical testing complemented by (high-resolution) transmission electron microscopy. The combined approach allowed to deduce the origin of unique defect structures as a result of dislocation nucleation, grain boundary (GB) formation and accompanied stress-driven GB migration. The role of GB migration during plastic deformation is further evaluated in context of traditional GB migration at high temperature. The transition from a “diffusive” mobility regime at high temperatures to a pinning-depinning (“yield”) GB mobility regime is illustrated for different GBs. The underlying mechanisms are the nucleation and/or migration of disconnections in the interface. The simulations reveal, moreover, the same operating atomistic mechanisms, although the emerging kinetic signatures (mobility) change. The implications are analysed in context of failure suppression by GB motion in extreme environments, the associated structural ingredients for mobile GBs and a mesoscale GB mobility model for a multiscale modelling approach. The role of surface steps (in analogy to the above interface disconnections) will be considered for the novel self-healing mechanisms of deformed gold nanoparticles. The detailed mechanisms, kinetic regimes and the structural prerequisites are deduced from the size- dependent mechanical behaviour and the kinetic mismatch of possible diffusion pathways on the nanoparticle surface. I will conclude with the discussion on how computer simulations allow to deduce and predict guiding principles to tailor materials with defect-dominated properties.

Jeudi 04 Octobre 2018
Boris Le Guennic
nstitut des Sciences Chimiques de Rennes, UMR CNRS 6226 Université de Rennes 1, 263 Av. du Général Leclerc, 35042 Cedex Rennes, France
Ab initio calculations of lanthanide complexes: from single molecule magnets to circularly polarized luminescence
Complexes containing trivalent lanthanide ions are of great interest in numerous fields due to their specific spectroscopic and magnetic characteristics. In particular, they can give rise to single molecule magnets (SMM) with slow magnetic relaxation, uniaxial magnetic anisotropy and high-energy barrier to the reversal of the magnetic moment. These features originate from the subtle interplay between the spin-orbit coupling and the crystal field interaction created by the ligands surrounding the lanthanide ion. Few general rules for building SMM have been formulated to date but no fundamental magneto-structural relationship explaining the behavior of lanthanide-based SMM has been proposed yet. To this end, and in addition to experimental evidences (SQUID magnetometry, EPR spectroscopy, polarized neutron diffraction...) ab initio calculations (SA-CASSCF/PT2/SI- SO) are one of the most appropriate theoretical tools to get reliable insights into the electronic structure of these compounds. Herein, the recent elucidation of the magnetic behavior of several lanthanide-based complexes is reported.[1] The limits of this computational approach is discussed. Finally, recent advances in the calculations of circularly polarized luminescence (CPL) and paramagnetic NMR (pNMR) in lanthanide complexes are also presented. [1] a) T. T. da Cunha, J. Jung, M.-E. Boulon, G. Campo, F. Pointillart, C. L. M. Pereira, B. Le Guennic, O. Cador, K. Bernot, F. Pineider, S. Golhen, L. Ouahab J. Am. Chem. Soc. 2013, 135, 16332. b) F. Pointillart, B. Le Guennic, O. Cador, O. Maury, L. Ouahab, Acc. Chem. Res., 2015, 48, 2834. c) J.-K. Ou-Yang, N. Saleh, G. Fernandez Garcia, L. Norel, F. Pointillart, T. Guizouarn, O. Cador, F. Totti, L. Ouahab, J. Crassous, B. Le Guennic Chem. Commun. 2016, 52, 14474. d) F. Pointillart, O. Cador, B. Le Guennic, L. Ouahab Coord. Chem. Rev. 2017, 346, 150. e) M. Xémard, A. Jaoul, M. Cordier, F. Molton, O. Cador, B. Le Guennic, C. Duboc, O. Maury, C. Clavaguéra, G. Nocton Angew. Chem. Int. Ed. 2017, 56, 4266. f) L. Norel, L. E. Darago, B. Le Guennic, K. Chakarawet, M. I. Gonzalez, J. H. Olshansky, S. Rigaut, J. R. Long Angew. Chem. Int. Ed. 2018, 57, 1933.