R. Ferrando - Academia.edu (original) (raw)

Papers by R. Ferrando

The jump-rate problem in a periodic potential is exactly solved in the whole damping range by a F... more The jump-rate problem in a periodic potential is exactly solved in the whole damping range by a Fourier analysis of the energy width of the first-Brillouin-zone dynamic structure factor, numerically calculated by the matrix-continued-fraction method. An analytical solution of the escape problem from a bimetastable potential is obtained and its relationship with the solution of the periodic problem is discussed.

The energetics of nanowires of several materials is studied by quenched molecular dynamics with t... more The energetics of nanowires of several materials is studied by quenched molecular dynamics with the aim of comparing wires pertaining to two different types of structures, fcc and pentagonal, which are expected to be among the most favorable ones for wires of intermediate thickness. A detailed study is made in the case of nickel, copper, and silver wires. These metals

Journal of Physics: Condensed Matter, 2014

The nanoparticle phase diagram of an immiscible system is studied at the atomic level. Cu-Ag clus... more The nanoparticle phase diagram of an immiscible system is studied at the atomic level. Cu-Ag clusters with sizes 1000 and 2000 atoms, resulting from a global minimum search and belonging to icosahedral and crystalline structural motifs, are considered. We present the statistical analysis of the effect of temperature on the solubility of the two elements based on Metropolis Monte Carlo importance sampling. Our results suggest that the relevance of bulk phase diagrams to nanoparticles is limited to cases where the internal stress distribution does not deviate very much from uniform (e.g. sufficiently large crystalline clusters). In the general case, the principal interdependence between partial phase compositions and the overall cluster composition in nanoparticle phase diagrams need to be taken into account.

Nanoscale, 2015

All nanostructures are metastable – but some are more metastable than others. Size-selected gold ... more All nanostructures are metastable – but some are more metastable than others. Size-selected gold nanoclusters present icosahedral, decahedral or fcc structures. Fcc are the most stable; the others are locked in by template growth.

Journal of Physics: Condensed Matter, 2013

Equilibrium structures, chemical ordering and thermal properties of Pt-Ni nanoalloys are investig... more Equilibrium structures, chemical ordering and thermal properties of Pt-Ni nanoalloys are investigated by using basin hopping-based global optimization, Monte Carlo (MC) and molecular dynamics (MD) methods, based on the second-moment approximation of the tight-binding potentials (TB-SMA). The TB-SMA potential parameters for Pt-Ni nanoalloys are fitted to reproduce the results of density functional theory calculations for small clusters. The chemical ordering in cuboctahedral (CO) Pt-Ni nanoalloys with 561 and 923 atoms is obtained from the so called semi-grand-canonical ensemble MC simulation at 100 K. Two ordered phases of L12 (PtNi3) and L10 (PtNi) are found for the CO561 and CO923 Pt-Ni nanoalloys, which is in good agreement with the experimental phase diagram of the Pt-Ni bulk alloy. In addition, the order-disorder transition and thermal properties of these nanoalloys are studied by using MC and MD methods, respectively. It is shown that the typical perfect L10 PtNi structure is relatively stable, showing high order-disorder transition temperature and melting point among these CO561 and CO923 Pt-Ni nanoalloys.

Supramolecular nanostructures with tunable dimensionalities are fabricated by deposition of benze... more Supramolecular nanostructures with tunable dimensionalities are fabricated by deposition of benzene−carboxylic acids on the Cu(110) surface. By tailoring the number and position of the functional moieties, the structure of the final molecular assemblies can be rationally modified ranging from isolated one-dimensional chains to compact two-dimensional islands. Molecular units are chosen that can assemble through metal−organic and electrostatic interactions. The hierarchy between these intermolecular forces guarantees that a primary organization level, constituted by metal−organic polymeric chains, is developed by all molecular units while the secondary interchain interactions can be arbitrarily adjusted. Scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations are used to characterize and rationalize the experimental findings.

Motivated by recent experimental findings, we have set up a model of a stepped Au(111) surface un... more Motivated by recent experimental findings, we have set up a model of a stepped Au(111) surface under the effect of an intense scanning tunneling microscope (STM) electric field. Atoms diffusing across the surface are represented by means of a lattice gas, and their kinetics are simulated by a kinetic Monte Carlo algorithm. Despite the approximations inherent in our simple model,

We have studied by means of Monte-Carlo simulation and exact finite-size analysis, the spin-1 Blu... more We have studied by means of Monte-Carlo simulation and exact finite-size analysis, the spin-1 Blume Capel model with Glauber and Kawasaki dynamics. The Kawasaki spin exchange process flows energy into the system from an external source. Some phase diagrams of the model are presented. For some values of the parameters, the system displays a kind of self organization phenomenon within the disordered phase.

The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-... more The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-principles simulations using 1D models. Nanostructures with L1(0) chemical ordering and cube-on-cube epitaxy are predicted to possess large magnetic moments and easy magnetization axis perpendicular to the surface. However, their magnetic anisotropy energy is roughly halved with respect to the bulk alloy due to a peculiar mixing of particle and support electronic states. The general factors at play in determining this behavior and the implications of these findings are discussed in view of designing room-temperature magnetic bits.

The growth modes of the Ag(1 1 0) surface are studied near room temperature by means of Kinetic M... more The growth modes of the Ag(1 1 0) surface are studied near room temperature by means of Kinetic Monte Carlo simulations. For the elementary diffusion processes, we use energy barriers determined by quenched molecular dynamics simulations where silver has been modeled by many-body potentials. Numerical results show evidence of layer growth under Molecular Beam Epitaxy conditions. The island number density and the adatom density at small coverages display a power-law dependence on the diffusion length.

The structure of Au clusters adsorbed on MgO(001) is studied by density-functional calculations. ... more The structure of Au clusters adsorbed on MgO(001) is studied by density-functional calculations. The results of two different exchange-correlation functionals, the local density approximation (LDA) and the Perdew-Burke-Ernzerhof (PBE) functionals, are compared. The size range 11<=N<=24 atoms is considered. It is shown that a complex competition among a variety of structural motifs takes place. These motifs comprise two-dimensional leaflets, open

An efficient numerical method for the calculation of the collective diffusion coefficient is deve... more An efficient numerical method for the calculation of the collective diffusion coefficient is developed. The method is based on the Bortz-Kalos-Lebowitz algorithm, with local updating of the particle lists for each process, coupled to the memory expansion for the calculation of the center-of-mass diffusion coefficient. The method is applied to the diffusion in a two-dimensional lattice gas model of square symmetry with repulsive lateral interactions. The numerical results are compared to two popular approximations, the Darken equation and the dynamical mean-field theory, whose respective merits are discussed. Finally, the decay of the dynamic structure factor with time is investigated.

We report a systematic study of island adsorption and single-adatom diusion on free silver nanocl... more We report a systematic study of island adsorption and single-adatom diusion on free silver nanoclusters, and discuss the consequences on the growth. In our calculations, silver is modelled by semiempirical many-body potentials. We consider magic non-crystallographic structures at dierent sizes: icosahedra (Ih) at 55, 147 and 309 atoms; Markstruncated decahedra (m-Dh) at 75, 146 and 192 atoms. We calculate the map of adsorption sites and the energy barriers for the dierent diusion processes. We ®nd that, due to purely geometrical reasons, medium-size (from 6±8 to 30±40 atoms depending on the cluster) islands on the cluster (1 1 1) facets prefer the hcp stacking on both Ih and Dh structures, while both smaller and larger islands are better placed on fcc stacking. Interfacet diusion is easy on both Dh and Ih, indicating that large islands are easily grown; in particular, there are multi-atom diusion processes which allow fast diusion among the two caps of Dh clusters. For Dh clusters, islands on hcp stacking may lead to the appearance of new ®vefold symmetry points, and to the transformation of the cluster into an icosahedron.

The aggregation process of a two-component dilute system (3 vol %), made of alumina submicrometer... more The aggregation process of a two-component dilute system (3 vol %), made of alumina submicrometer particles and silica nanoparticles, is studied by Brownian dynamics simulations. Alumina and silica particles have very different sizes (diameters of 400 and 25 nm, respectively). The particle-particle interaction potential is of the DLVO form. The parameters of the potential are extracted from the experiments. The simulations show that the experimentally observed aggregation phenomena between alumina particles are due to the silica-alumina attraction that induces an effective driving force for alumina-alumina aggregation. The experimental data for silica adsorption on alumina are very well reproduced.

ABSTRACT The recently observed formation of cross-channel ripples in the growth of Ag/Ag(110) at ... more ABSTRACT The recently observed formation of cross-channel ripples in the growth of Ag/Ag(110) at low temperatures and their rotation at higher temperatures are studied by kinetic Monte Carlo simulations. The key microscopic mechanisms of both ripple formation and rotation are singled out.

The multilayer growth of Ag/Ag(1 1 0) is studied by kinetic Monte Carlo simulations based on mode... more The multilayer growth of Ag/Ag(1 1 0) is studied by kinetic Monte Carlo simulations based on model in the full fcc geometry with realistic activation barriers. The model is able to describe both the island rotation in the submonolayer regime and the ripple rotation in multilayer growth, in very good quantitative agreement with the experiments. The ripple wavelength and slope are calculated, and the results are compared also to simple analytical estimates, finding a good agreement in the case of the low-temperature cross-channel ripples. The role of the different elementary microscopic mechanisms is studied by varying one by one the values of the activation barriers; this has allowed to set limits on the values of the barrier themselves.

By means of quenched molecular dynamics, we have studied the elementary diffusion processes (intr... more By means of quenched molecular dynamics, we have studied the elementary diffusion processes (intralayer and interlayer diffusion) relevant for the epitaxial growth of Ag on Ag(ll0). Silver has been modeled by many-body potentials derived in the framework of the second-moment approximation to the tight-binding model. Energy barriers for diffusion on the fiat surface are found strongly anisotropic. The proximity to steps alters these barriers considerably. The adatom descent from islands takes place by different mechanisms depending on the orientation and some of those mechanisms depend strongly on the island size and shape. In particular, the adatom descent at kinks is much easier on small islands than on large ones. This fact may have a strong influence on the growth character at low temperatures. Finally, the energetics of small adatom clusters is presented.

It is shown by molecular-dynamics simulations that correlated processes involving both jumps and ... more It is shown by molecular-dynamics simulations that correlated processes involving both jumps and exchanges become important in the diffusion of Ag/Ag\(110\) at temperatures around 600 K. Silver is modeled by many-body potentials derived in the framework of the tight-binding model.

The jump-rate problem in a periodic potential is exactly solved in the whole damping range by a F... more The jump-rate problem in a periodic potential is exactly solved in the whole damping range by a Fourier analysis of the energy width of the first-Brillouin-zone dynamic structure factor, numerically calculated by the matrix-continued-fraction method. An analytical solution of the escape problem from a bimetastable potential is obtained and its relationship with the solution of the periodic problem is discussed.

The energetics of nanowires of several materials is studied by quenched molecular dynamics with t... more The energetics of nanowires of several materials is studied by quenched molecular dynamics with the aim of comparing wires pertaining to two different types of structures, fcc and pentagonal, which are expected to be among the most favorable ones for wires of intermediate thickness. A detailed study is made in the case of nickel, copper, and silver wires. These metals

Journal of Physics: Condensed Matter, 2014

The nanoparticle phase diagram of an immiscible system is studied at the atomic level. Cu-Ag clus... more The nanoparticle phase diagram of an immiscible system is studied at the atomic level. Cu-Ag clusters with sizes 1000 and 2000 atoms, resulting from a global minimum search and belonging to icosahedral and crystalline structural motifs, are considered. We present the statistical analysis of the effect of temperature on the solubility of the two elements based on Metropolis Monte Carlo importance sampling. Our results suggest that the relevance of bulk phase diagrams to nanoparticles is limited to cases where the internal stress distribution does not deviate very much from uniform (e.g. sufficiently large crystalline clusters). In the general case, the principal interdependence between partial phase compositions and the overall cluster composition in nanoparticle phase diagrams need to be taken into account.

Nanoscale, 2015

All nanostructures are metastable – but some are more metastable than others. Size-selected gold ... more All nanostructures are metastable – but some are more metastable than others. Size-selected gold nanoclusters present icosahedral, decahedral or fcc structures. Fcc are the most stable; the others are locked in by template growth.

Journal of Physics: Condensed Matter, 2013

Equilibrium structures, chemical ordering and thermal properties of Pt-Ni nanoalloys are investig... more Equilibrium structures, chemical ordering and thermal properties of Pt-Ni nanoalloys are investigated by using basin hopping-based global optimization, Monte Carlo (MC) and molecular dynamics (MD) methods, based on the second-moment approximation of the tight-binding potentials (TB-SMA). The TB-SMA potential parameters for Pt-Ni nanoalloys are fitted to reproduce the results of density functional theory calculations for small clusters. The chemical ordering in cuboctahedral (CO) Pt-Ni nanoalloys with 561 and 923 atoms is obtained from the so called semi-grand-canonical ensemble MC simulation at 100 K. Two ordered phases of L12 (PtNi3) and L10 (PtNi) are found for the CO561 and CO923 Pt-Ni nanoalloys, which is in good agreement with the experimental phase diagram of the Pt-Ni bulk alloy. In addition, the order-disorder transition and thermal properties of these nanoalloys are studied by using MC and MD methods, respectively. It is shown that the typical perfect L10 PtNi structure is relatively stable, showing high order-disorder transition temperature and melting point among these CO561 and CO923 Pt-Ni nanoalloys.

Supramolecular nanostructures with tunable dimensionalities are fabricated by deposition of benze... more Supramolecular nanostructures with tunable dimensionalities are fabricated by deposition of benzene−carboxylic acids on the Cu(110) surface. By tailoring the number and position of the functional moieties, the structure of the final molecular assemblies can be rationally modified ranging from isolated one-dimensional chains to compact two-dimensional islands. Molecular units are chosen that can assemble through metal−organic and electrostatic interactions. The hierarchy between these intermolecular forces guarantees that a primary organization level, constituted by metal−organic polymeric chains, is developed by all molecular units while the secondary interchain interactions can be arbitrarily adjusted. Scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations are used to characterize and rationalize the experimental findings.

Motivated by recent experimental findings, we have set up a model of a stepped Au(111) surface un... more Motivated by recent experimental findings, we have set up a model of a stepped Au(111) surface under the effect of an intense scanning tunneling microscope (STM) electric field. Atoms diffusing across the surface are represented by means of a lattice gas, and their kinetics are simulated by a kinetic Monte Carlo algorithm. Despite the approximations inherent in our simple model,

We have studied by means of Monte-Carlo simulation and exact finite-size analysis, the spin-1 Blu... more We have studied by means of Monte-Carlo simulation and exact finite-size analysis, the spin-1 Blume Capel model with Glauber and Kawasaki dynamics. The Kawasaki spin exchange process flows energy into the system from an external source. Some phase diagrams of the model are presented. For some values of the parameters, the system displays a kind of self organization phenomenon within the disordered phase.

The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-... more The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-principles simulations using 1D models. Nanostructures with L1(0) chemical ordering and cube-on-cube epitaxy are predicted to possess large magnetic moments and easy magnetization axis perpendicular to the surface. However, their magnetic anisotropy energy is roughly halved with respect to the bulk alloy due to a peculiar mixing of particle and support electronic states. The general factors at play in determining this behavior and the implications of these findings are discussed in view of designing room-temperature magnetic bits.

The growth modes of the Ag(1 1 0) surface are studied near room temperature by means of Kinetic M... more The growth modes of the Ag(1 1 0) surface are studied near room temperature by means of Kinetic Monte Carlo simulations. For the elementary diffusion processes, we use energy barriers determined by quenched molecular dynamics simulations where silver has been modeled by many-body potentials. Numerical results show evidence of layer growth under Molecular Beam Epitaxy conditions. The island number density and the adatom density at small coverages display a power-law dependence on the diffusion length.

The structure of Au clusters adsorbed on MgO(001) is studied by density-functional calculations. ... more The structure of Au clusters adsorbed on MgO(001) is studied by density-functional calculations. The results of two different exchange-correlation functionals, the local density approximation (LDA) and the Perdew-Burke-Ernzerhof (PBE) functionals, are compared. The size range 11<=N<=24 atoms is considered. It is shown that a complex competition among a variety of structural motifs takes place. These motifs comprise two-dimensional leaflets, open

An efficient numerical method for the calculation of the collective diffusion coefficient is deve... more An efficient numerical method for the calculation of the collective diffusion coefficient is developed. The method is based on the Bortz-Kalos-Lebowitz algorithm, with local updating of the particle lists for each process, coupled to the memory expansion for the calculation of the center-of-mass diffusion coefficient. The method is applied to the diffusion in a two-dimensional lattice gas model of square symmetry with repulsive lateral interactions. The numerical results are compared to two popular approximations, the Darken equation and the dynamical mean-field theory, whose respective merits are discussed. Finally, the decay of the dynamic structure factor with time is investigated.

We report a systematic study of island adsorption and single-adatom diusion on free silver nanocl... more We report a systematic study of island adsorption and single-adatom diusion on free silver nanoclusters, and discuss the consequences on the growth. In our calculations, silver is modelled by semiempirical many-body potentials. We consider magic non-crystallographic structures at dierent sizes: icosahedra (Ih) at 55, 147 and 309 atoms; Markstruncated decahedra (m-Dh) at 75, 146 and 192 atoms. We calculate the map of adsorption sites and the energy barriers for the dierent diusion processes. We ®nd that, due to purely geometrical reasons, medium-size (from 6±8 to 30±40 atoms depending on the cluster) islands on the cluster (1 1 1) facets prefer the hcp stacking on both Ih and Dh structures, while both smaller and larger islands are better placed on fcc stacking. Interfacet diusion is easy on both Dh and Ih, indicating that large islands are easily grown; in particular, there are multi-atom diusion processes which allow fast diusion among the two caps of Dh clusters. For Dh clusters, islands on hcp stacking may lead to the appearance of new ®vefold symmetry points, and to the transformation of the cluster into an icosahedron.

The aggregation process of a two-component dilute system (3 vol %), made of alumina submicrometer... more The aggregation process of a two-component dilute system (3 vol %), made of alumina submicrometer particles and silica nanoparticles, is studied by Brownian dynamics simulations. Alumina and silica particles have very different sizes (diameters of 400 and 25 nm, respectively). The particle-particle interaction potential is of the DLVO form. The parameters of the potential are extracted from the experiments. The simulations show that the experimentally observed aggregation phenomena between alumina particles are due to the silica-alumina attraction that induces an effective driving force for alumina-alumina aggregation. The experimental data for silica adsorption on alumina are very well reproduced.

ABSTRACT The recently observed formation of cross-channel ripples in the growth of Ag/Ag(110) at ... more ABSTRACT The recently observed formation of cross-channel ripples in the growth of Ag/Ag(110) at low temperatures and their rotation at higher temperatures are studied by kinetic Monte Carlo simulations. The key microscopic mechanisms of both ripple formation and rotation are singled out.

The multilayer growth of Ag/Ag(1 1 0) is studied by kinetic Monte Carlo simulations based on mode... more The multilayer growth of Ag/Ag(1 1 0) is studied by kinetic Monte Carlo simulations based on model in the full fcc geometry with realistic activation barriers. The model is able to describe both the island rotation in the submonolayer regime and the ripple rotation in multilayer growth, in very good quantitative agreement with the experiments. The ripple wavelength and slope are calculated, and the results are compared also to simple analytical estimates, finding a good agreement in the case of the low-temperature cross-channel ripples. The role of the different elementary microscopic mechanisms is studied by varying one by one the values of the activation barriers; this has allowed to set limits on the values of the barrier themselves.

By means of quenched molecular dynamics, we have studied the elementary diffusion processes (intr... more By means of quenched molecular dynamics, we have studied the elementary diffusion processes (intralayer and interlayer diffusion) relevant for the epitaxial growth of Ag on Ag(ll0). Silver has been modeled by many-body potentials derived in the framework of the second-moment approximation to the tight-binding model. Energy barriers for diffusion on the fiat surface are found strongly anisotropic. The proximity to steps alters these barriers considerably. The adatom descent from islands takes place by different mechanisms depending on the orientation and some of those mechanisms depend strongly on the island size and shape. In particular, the adatom descent at kinks is much easier on small islands than on large ones. This fact may have a strong influence on the growth character at low temperatures. Finally, the energetics of small adatom clusters is presented.

It is shown by molecular-dynamics simulations that correlated processes involving both jumps and ... more It is shown by molecular-dynamics simulations that correlated processes involving both jumps and exchanges become important in the diffusion of Ag/Ag\(110\) at temperatures around 600 K. Silver is modeled by many-body potentials derived in the framework of the tight-binding model.