65 research outputs found
Melting of metallic nanoclusters: Alloying and support effects
No full textAlloying and support effects on melting properties of metallic clusters were studied via molecular dynamics simulations. We used a tight binding semi-empirical potential to model the metal-metal interactions and a surface energy potential fitted to ab initio calculations for the metaloxide ones in the case of Pd/MgO(100). As expected, the support interaction increases the melting temperature as compared to free clusters. However, when referred to the size of the particle (taking into account the morphological change between free and supported clusters), we show that the linear dependence of the melting temperature with the reciprocal cluster radius remains unchanged between free and supported cluster. Inversely, the alloying effect is expected to decrease the melting temperature because of the eutectic behaviour in the phase diagram. However we show that stress relief in certain bimetallic clusters drives to a higher melting point than for the pure equivalent cluster
Modeling free and supported metallic nanoclusters: Structure and dynamics
No full textWe compare atomic structure and dynamics of free and supported metallic clusters via molecular dynamics simulations using tight-binding semiempirical potentials for metal-metal interactions and a potential fitted to ab initio calculations for the metal-supported ones, the support being essentially the MgO(100) surface in the case of a nonreactive metal-oxide interface. The structural transition for free Ni, Pd, Pt, Cu, Ag, Au clusters with noncrystalline structures (mainly icosahedral and decahedral) at small sizes to FCC truncated octahedrons for larger sizes is reported as well as the variation of the critical size of transition from 3d to 5d metals. In the case of Pd clusters on the MgO(100) surface, we analyze the substrate-induced modifications in morphology and atomic structure and follow their evolution as a function of cluster size. The mechanism of strain release by misfit interfacial dislocations in 3D clusters is described at the atomic level. Dynamics of growth and melting of free silver clusters are discussed and some effects of the oxide substrate in melting transition are pointed out, notably the delay in melting induced by the epitaxial relation with the support
Atomistic simulation of Ag thin films on MgO(100) substrates: A template substrate for heterogeneous adsorption
No full textThe nanostructuration of Ag thin films deposited on the MgO(100) substrate is simulated by classical molecular dynamics using a tight-binding many-body potential for the metal-metal bonds and a potential fitted to ab initio calculations for the metal–oxide ones. Due to the lattice mismatch between the Ag deposit and the MgO(100) substrate, the silver film is strained. The stress is partially released by the introduction of misfit dislocations at the interface. These dislocations form a network with a periodicity of about 10 nm, which varies for ultrathin films according to the film thickness. The strain induced by the interfacial dislocation cores propagates across the silver film up to the surface driving to the nanostructuration of the surface. The atomistic results are compared to the predictions of the elasticity theory. The theoretical results are in a nice agreement with recent experiments obtained by grazing incidence small angle x-ray scattering revealing a self-organization of Co clusters adsorbed on a thin film of Ag/MgO(100) [F. Leroy, G. Renaud, A. Letoublon, R. Lazzari, C. Mottet and J. Goniakowski (unpublished)]. We show that the preferential Co adsorption site is obtained on top of tensile surface sites and that the periodicity of the clusters' self-organization can be tuned by the Ag film thickness
Polarity effects in unsupported polar nanoribbons
No full textWe analyze the characteristics of polarity in unsupported nanoribbons with zigzag edges, by a combination of analytic models, semiempirical Hartree-Fock simulations, and first-principles approach. We consider two materials with widely different ionic-covalent character, MgO and MoS2, and two polarity healing mechanisms: the so-called electronic compensation in ribbons with pristine edges, and ionic compensation in ribbons with an adequately chosen density of missing edge ions. The general expression of compensating charges, the edge metallization and spin polarization in the electronic mechanism, and the efficiency of the ionic mechanism are similar to those known in thin films and at polar surfaces. Differences, however, exist and are related to the low dimensionality, the atomic structure, and the strong undercoordination of edge atoms in nanoribbons. Polarity signatures are specified and a discussion of the possible origins of metallic edge states in these low dimensional objects is provided.Fil: Güller, Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; ArgentinaFil: Llois, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Goniakowski, J.. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Centre National de la Recherche Scientifique; Francia. Universite de Paris VI; FranciaFil: Noguera, C.. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Centre National de la Recherche Scientifique; Francia. Universite de Paris VI; Franci
Investigation of the interaction of water with the calcite {1014} surface using ab-initio simulation
No full textDensity functional theory calculations were employed to explore the interaction between
water and the {1014} surface of calcite. In addition a defective {1014} surface
and stepped surfaces in contact with water were investigated. A series of percentage
water coverages and water con�figurations were explored, including dissociated water
states. Static relaxations found associated water to be favourable on the {1014}
surface, although a metastable dissociated state 1.77eV higher in energy was found.
Molecular dynamics (MD) simulations of low water coverage reveal fluctuations in
the H-O water bond when the H atom is directed towards a surface CO3 ion. Desorption
of an H2O molecule was observed in simulations above 900K. Water was found
to be strongly bound to the perfect {1014} surface, with an adsorption energy of
-0.91eV. MD simulations of a defective {1014} surface found water to favour dissociation
at CO3 vacancies. However, water at Ca vacancies di�used across the surface
to form a bond with the nearest surface Ca ion. Water was also found to favour
an associated state at both acute and obtuse steps. On all these imperfect surfaces
water was found to adsorb strongly to the surface, with adsorption energies ranging
from -0.99eV to -1.60eV
Modeling Free and Supported Metallic Nanoclusters: Structure and Dynamics
No full textWe compare atomic structure and dynamics of free and supported metallic clusters via molecular dynamics simulations using tight-binding semiempirical potentials for metal-metal interactions and a potential fitted to ab initio calculations for the metal-supported ones, the support being essentially the MgO(100) surface in the case of a nonreactive metal-oxide interface. The structural transition for free Ni, Pd, Pt, Cu, Ag, Au clusters with noncrystalline structures (mainly icosahedral and decahedral) at small sizes to FCC truncated octahedrons for larger sizes is reported as well as the variation of the critical size of transition from 3d to 5d metals. In the case of Pd clusters on the MgO(100) surface, we analyze the substrate-induced modifications in morphology and atomic structure and follow their evolution as a function of cluster size. The mechanism of strain release by misfit interfacial dislocations in 3D clusters is described at the atomic level. Dynamics of growth and melting of free silver clusters are discussed and some effects of the oxide substrate in melting transition are pointed out, notably the delay in melting induced by the epitaxial relation with the support
Melting of metallic nanoclusters: alloying and support effects
No full textAlloying and support effects on melting properties of metallic clusters were studied via molecular dynamics simulations. We used a tight binding semi-empirical potential to model the metal-metal interactions and a surface energy potential fitted to ab initio calculations for the metal-oxide ones in the case of Pd/MgO(100). As expected, the support interaction increases the melting temperature as compared to free clusters. However, when referred to the size of the particle (taking into account the morphological change between free and supported clusters), we show that the linear dependence of the melting temperature with the reciprocal cluster radius remains unchanged between free and supported cluster. Inversely, the alloying effect is expected to decrease the melting temperature because of the eutectic behaviour in the phase diagram. However we show that stress relief in certain bimetallic clusters drives to a higher melting point than for the pure equivalent cluste
Prediction of structural and metal-to-semiconductor phase transitions in nanoscale MoS2, WS2, and other transition metal dichalcogenide zigzag ribbons
No full textWhile MoS2 and WS2 nanostructures gain an increasing importance in a number of recent technological applications, the control of their structure as a function of their size and their environment appears of prominent importance. In the present study which relies on first-principles simulations, we predict the dimerized 1T′ structural phase to be the actual ground state of MoS2, WS2, and MoSe2 zigzag nanoribbons of small width and monolayer thickness. We assign this result to the competition between edge energy - which favors the nonpolar 1T′ edges over the polar 1H edges - and the energy of atoms in the center of the ribbons - which favors the 1H ground state of the infinite monolayers. A metal-to-semiconductor transition accompanies the structural transition. At variance, ZrS2 zigzag ribbons are predicted to display the 1T structure whatever their width. In compounds of major technological importance, such structural and electronic flexibility associated with polarity effects opens the possibility for controlling the ribbon type during synthesis.Fil: Güller, Francisco. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; ArgentinaFil: Llois, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Goniakowski, J.. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Centre National de la Recherche Scientifique; Francia. Universite de Paris VI; FranciaFil: Noguera, C.. Laboratorio Internacional Franco-Argentino en Nanociencias; Argentina. Centre National de la Recherche Scientifique; Francia. Universite de Paris VI; Franci
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