From metastability to equilibrium during the sequential growth of Co-Ag supported clusters: a real-time investigation.

Atomic motions and morphological evolution of growing Co-Ag nanoparticles are followed in situ and in real time, by wide and small angle X-ray scattering obtained simultaneously in grazing incidence geometry (GISAXS and GIWAXS), in single or multi-wavelength anomalous modes. The structural analysis of the experimental data is performed with the aid of equilibrium Monte Carlo simulations and of molecular-dynamics simulations of nanoparticle growth. Growth is performed by depositing Co atoms above preformed Ag nanoparticles. This growth procedure is strongly out of equilibrium, because Ag tends to surface segregation, and generates complex growth sequences. The real time analysis of the growth allows to follow the nanoparticle evolution pathways almost atom-by-atom, determining the key mechanisms during Co deposition: starting with the incorporation of Co atoms in sub-surface positions, to the off-center Co domain formation, then by which the nanoparticles finally approach their equilibrium quasi-Janus then core-shell structures.

Reversed size-dependent stabilization of ordered nanophases.

The size increase of a nanoscale material is commonly associated with the increased stability of its ordered phases. Here we give a counterexample to this trend by considering the formation of the defect-free L11 ordered phase in AgPt nanoparticles, and showing that it is better stabilized in small nanoparticles (up to 2.5 nm) than in larger ones, in which the ordered phase breaks in multiple domains or is interrupted by faults. The driving force for the L11 phase formation in small nanoparticles is the segregation of a monolayer silver shell (an Ag-skin) which prevents the element with higher surface energy (Pt) from occupying surface sites. With increasing particle size, the Ag-skin causes internal stress in the L11 domains which cannot thus exceed the critical size of ~2.5 nm. A multiscale modelling approach using full-DFT global optimization calculations and atomistic modelling is used to interpret the findings.

Experimental determination of the energy difference between competing isomers of deposited, size-selected gold nanoclusters.

The equilibrium structures and dynamics of a nanoscale system are regulated by a complex potential energy surface (PES). This is a key target of theoretical calculations but experimentally elusive. We report the measurement of a key PES parameter for a model nanosystem: size-selected Au nanoclusters, soft-landed on amorphous silicon nitride supports. We obtain the energy difference between the most abundant structural isomers of magic number Au561 clusters, the decahedron and face-centred-cubic (fcc) structures, from the equilibrium proportions of the isomers. These are measured by atomic-resolution scanning transmission electron microscopy, with an ultra-stable heating stage, as a function of temperature (125-500 °C). At lower temperatures (20-125 °C) the behaviour is kinetic, exhibiting down conversion of metastable decahedra into fcc structures; the higher state is repopulated at higher temperatures in equilibrium. We find the decahedron is 0.040 ± 0.020 eV higher in energy than the fcc isomer, providing a benchmark for the theoretical treatment of nanoparticles.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen.

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

Computation of shear viscosity of colloidal suspensions by SRD-MD.

The behaviour of sheared colloidal suspensions with full hydrodynamic interactions (HIs) is numerically studied. To this end, we use the hybrid stochastic rotation dynamics-molecular dynamics (SRD-MD) method. The shear viscosity of colloidal suspensions is computed for different volume fractions, both for dilute and concentrated cases. We verify that HIs help in the collisions and the streaming of colloidal particles, thereby increasing the overall shear viscosity of the suspension. Our results show a good agreement with known experimental, theoretical, and numerical studies. This work demonstrates the ability of SRD-MD to successfully simulate transport coefficients that require correct modelling of HIs.

Interpretation of surface diffusion data with Langevin simulations: a quantitative assessment.

Diffusion studies of adsorbates moving on a surface are often analyzed using 2D Langevin simulations. These simulations are computationally cheap and offer valuable insight into the dynamics, however, they simplify the complex interactions between the substrate and adsorbate atoms, neglecting correlations in the motion of the two species. The effect of this simplification on the accuracy of observables extracted using Langevin simulations was previously unquantified. Here we report a numerical study aimed at assessing the validity of this approach. We compared experimentally accessible observables which were calculated using a Langevin simulation with those obtained from explicit molecular dynamics simulations. Our results show that within the range of parameters we explored Langevin simulations provide a good alternative for calculating the diffusion procress, i.e. the effect of correlations is too small to be observed within the numerical accuracy of this study and most likely would not have a significant effect on the interpretation of experimental data. Our comparison of the two numerical approaches also demonstrates the effect temperature dependent friction has on the calculated observables, illustrating the importance of accounting for such a temperature dependence when interpreting experimental data.

Study of the B1-B2 transition in colloidal clusters.

The possible mechanisms for the B1 (NaCl-type) to B2 (CsCl-type) transition in crystalline colloidal clusters of equally sized particles are studied by means of two computational techniques: metadynamics and nudged elastic band calculations. The system is modelled by a screened Coulomb potential. Different interaction ranges are considered. The transition from a perfect NaCl cubic cluster to a full CsCl cluster is forced by metadynamics, revealing a transition path with intermediate metastable configurations in which planes are shifted one by one. The presence of metastable configurations in the transition path, corresponding to a certain number of NaCl planes turned into CsCl, has clear analogies with the known Hyde and O'Keeffe mechanism for ionic crystals, with some important differences due to finite-size effects. These comprise the fact that the transition starts by shifting a surface plane by means of a row-by-row mechanism that has no analog in bulk crystals. The energy barriers between the local minima in the transition path are calculated, showing that the barriers strongly depend on the screening length, in such a way that the B1 metastable phase can have very long lifetimes when the interaction is sufficiently long-ranged.

Atomic Details of Interfacial Interaction in Gold Nanoparticles Supported on MgO(001).

Atomic-scale imaging using aberration-corrected scanning transmission electron microscopy reveals direct evidence for semicoherent interfacial epitaxy and coordinate-dependent surface contraction for the fcc (001) oriented Au nanoparticles (2-3 nm in diameter), suggesting that their interaction with the substrate is weaker than previously assumed. A significant change in interfacial separation distance from 2.47 ± 0.12 Å for the fcc (001) oriented Au nanoparticles to 3.07 ± 0.11 Å for the fcc (111) oriented Au nanoparticles has also been observed. These results are used to verify the atomistic models generated by the global optimization calculations, which shed further light on the intricate relation between the interfacial energy and the atomic structure of the nanoparticle and their combined effect on the inhomogeneous surface structural relaxation of supported nanoparticles.

Aggregation in colloidal suspensions: evaluation of the role of hydrodynamic interactions by means of numerical simulations.

Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne-Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

Kinetically driven ordered phase formation in binary colloidal crystals.

The aggregation of binary colloids of the same size and balanced charges is studied by Brownian dynamics simulations for dilute suspensions. It is shown that, under appropriate conditions, the formation of colloidal crystals is dominated by kinetic effects leading to the growth of well-ordered crystallites of the sodium-chloride (NaCl) bulk phase. These crystallites form with very high probability even when the cesium-chloride (CsCl) phase is more stable thermodynamically. Global optimization searches show that this result is not related to the most favorable structures of small clusters, which are either amorphous or of the CsCl structure. The formation of the NaCl phase is related to the specific kinetics of the crystallization process, which takes place by a two-step mechanism. In this mechanism, dense fluid aggregates form at first and then crystallization follows. It is shown that the type of short-range order in these dense fluid aggregates determines which phase is finally formed in the crystallites. The role of hydrodynamic effects in the aggregation process is analyzed by stochastic rotation dynamics - molecular dynamics simulations, and we find that these effects do not play a major role in the formation of the crystallites.

Aggregation kinetics and gel formation in modestly concentrated suspensions of oppositely charged model ceramic colloids: a numerical study.

Aggregation kinetics and gel formation in aqueous suspensions that undergo heteroaggregation are studied by means of Brownian dynamics simulations. The simulated system, described in a previous paper [M. A. Piechowiak, A. Videcoq, F. Rossignol, C. Pagnoux, C. Carrion, M. Cerbelaud, R. Ferrando, Langmuir, 2010, 26(15), 12540-12547.], is constituted of two kinds of synthesized, almost equally sized colloids: silica particles that are negatively charged and alumina-coated silica particles that are positively charged. The interactions between colloids are modeled by the DLVO potential. Several compositions are analyzed, from silica-rich to alumina-rich cases. The particle volume fraction φ is varied in the range 6-12%. The study of the aggregation kinetics allows us to clarify the effect of those variations on the clustering process. Gelation is analyzed by detection of spanning clusters in each x-, y-, z-direction of the cubic simulation box. Percolating networks start to be observed from φ = 7%, a low value of the volume fraction close to the solid volume fraction experimentally measured in sediments of those suspensions.

Oppositely charged model ceramic colloids: numerical predictions and experimental observations by confocal laser scanning microscopy.

Fluorescent silica and alumina-like spherical particles with almost equal sizes are synthesized. Dilute aqueous suspensions are prepared with various ratios of those colloidal particles that exhibit opposite surface charges. These suspensions undergo heteroaggregation for a wide range of compositions. The structure of the formed aggregates is analyzed by means of confocal microscopy. The experimental results are compared to those of Brownian dynamics simulations in which the interactions between colloids are modeled by the DLVO potential. Good agreement between experiments and simulations is obtained.

Simulations of heteroaggregation in a suspension of alumina and silica particles: effect of dilution.

The influence of dilution on the aggregation process of suspensions composed of two kinds of oxide particles (alumina positively charged particles d(1)=400 nm and silica negatively charged particles d(2)=250 nm) has been studied by computer simulations. Two kinds of simulations have been performed: Brownian dynamics simulations to study the aggregation process and its kinetics and global minimization searches to find the most stable configurations of aggregates. We show that the rate of dilution has a strong influence on the structure and on the shape of aggregates in Brownian dynamics simulations. By confronting these aggregates with the stable aggregates found by global minimization, we demonstrate that they are metastable and their shape is explained by the competition between the kinetics of aggregate coalescence and the kinetics of aggregate reorganization into more stable configurations.

Movimientos anormales psicógenos a propósito de casos clínicos / Psychohenic movement disorders in relation to medical cases

El presente es un trabajo de revisión y puesta al día de la literatura respecto al tema de los movimientos anormales de etiología psicógeno. Se comunica un caso clínico de distonia psicógena clínicamente establecida, analizando su historia desde el punto de vista neurológico y psiquiátrico, fundamentos el diagnóstico positivo y analizando los diagnóstico diferentes. Concluimos en la necesidad del abordaje neuropsiquiátrico cuando se abordan pacientes que padecen movimientos anormales.

This document is a work revision and an update of the psychogenic etiology of movement disorders’ literature. A medical case of dystonia psychogenia clinically established is reported, the medical history is analyzed from a neurological and psychiatric perspective, the positive diagnosis is explained, and the different diagnosis analyzed. Finally, we conclude the necessity of neuropsychiatric treatment when those patients suffer movement disorders.

A low-energy core-collapse supernova without a hydrogen envelope.

The final fate of massive stars depends on many factors. Theory suggests that some with initial masses greater than 25 to 30 solar masses end up as Wolf-Rayet stars, which are deficient in hydrogen in their outer layers because of mass loss through strong stellar winds. The most massive of these stars have cores which may form a black hole and theory predicts that the resulting explosion of some of them produces ejecta of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel. An alternative origin for low-energy supernovae is the collapse of the oxygen-neon core of a star of 7-9 solar masses. No weak, hydrogen-deficient, core-collapse supernovae have hitherto been seen. Here we report that SN 2008ha is a faint hydrogen-poor supernova. We propose that other similar events have been observed but have been misclassified as peculiar thermonuclear supernovae (sometimes labelled SN 2002cx-like events). This discovery could link these faint supernovae to some long-duration gamma-ray bursts, because extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce such long gamma-ray bursts, the afterglows of which do not show evidence of associated supernovae.

Surface-supported gold cages.

It is shown, by density-functional theory calculations, that gold clusters on the MgO(001) surface prefer cage structures in the size range between 23 and 42 atoms. These structures belong to a new structural family, the open pyramidal hollow cages, which has no counterpart in gas-phase clusters. These cages are possible because of the peculiar features of the Au-Au and Au-MgO interactions, which include strong many-body and directional effects. These effects reinforce the tendency of Au to produce cage structures with respect to the gas phase.

[Regional cerebral blood flow changes in Parkinson's disease: correlation with disease duration]. / Cambios de la perfusión cerebral en la enfermedad de Parkinson: relación con la duración de los síntomas.

INTRODUCTION: Changes in regional cerebral blood flow (rCBF) have been reported in idiopathic Parkinson's disease (PD). Nonetheless, their typical pattern still remains controversial regarding some features, such as basal ganglia involvement and the main cortical regions affected. Functional neuroimaging makes it possible to identify the brain dysfunctions of the neural circuits underlying the disease. Voxel-based analysis methods make it possible to increase the reliability of the results. OBJECTIVE: To assess the rCBF changes in patients with PD and their relation with disease duration. MATERIALS AND METHODS: Thirty PD adult patients without dementia underwent evaluation with (99m)Tc-ECD SPECT. SPM5 was used for statistical comparison with 25 normal controls of similar ages. The disease course duration in years was added as a covariate. Additionally, patients with a 6-year evolution or less and those with more than 6 years were compared separately with normal controls. RESULTS: Significant hypoperfusion was detected in bilateral premotor and posterior parietal cortex and increase of perfusion was present in the cerebellum. These changes correlated with the years of evolution of the illness. Patients with longer evolution also presented thalamic, subthalamic and basal ganglia hypoperfusion. CONCLUSIONS: We describe rCBF changes in PD in neural circuits related with control of movements. These changes are more manifest in patients with a longer duration of the disease.

Cambios de la perfusión cerebral en la enfermedad de Parkinson: relación con la duración de los síntomas / Regional cerebral blood flow changes in Parkinson's disease: correlation with disease duration

Introducción: En la enfermedad de Parkinson idiopática (EP) se han observado alteraciones del flujo sanguíneo cerebral regional (FSCr) cuyo patrón característico aún presenta aspectos controvertidos, como la existencia de alteraciones gangliobasales y las áreas corticales más afectadas. La neuroimagen funcional permite observar las disfunciones de circuitos neuronales existentes en estos pacientes. Los métodos de análisis estadístico basado en vóxeles permiten incrementar la validez de los resultados. Objetivo: Investigar los cambios de la perfusión cerebral existentes en pacientes con EP y su relación con la duración de los síntomas. Materiales y métodos: Treinta pacientes adultos con EP sin demencia fueron estudiados mediante SPECT cerebral con 99mTc-ECD. Se utilizó SPM5 para su comparación estadística con un grupo control de 25 sujetos sanos de edades similares. Se introdujo, como covariable en dicha comparación, el tiempo de evolución en años y se analizaron por separado los pacientes con 6 años o menos de evolución y aquellos con más de 6 años. Resultados: Se detectó hipoperfusión significativa en la corteza premotora y parietal posterior bilateral y aumento del flujo en el cerebelo. Estas alteraciones se correlacionaron con los años de evolución de la enfermedad. Los pacientes con evolución más prolongada presentaron además hipoperfusión talámica, subtalámica y gangliobasal. Conclusiones: Describimos alteraciones del FSCr en la EP que se relacionan con los circuitos implicados en el control del movimiento. Las mismas son más evidentes en los pacientes con evolución más prolongada de la enfermedad(AU)

Introduction: Changes in regional cerebral blood flow (rCBF) have been reported in idiopathic Parkinson's disease (PD). Nonetheless, their typical pattern still remains controversial regarding some features, such as basal ganglia involvement and the main cortical regions affected. Functional neuroimaging makes it possible to identify the brain dysfunctions of the neural circuits underlying the disease. Voxel-based analysis methods make it possible to increase the reliability of the results. Objective: To assess the rCBF changes in patients with PD and their relation with disease duration. Materials and methods: Thirty PD adult patients without dementia underwent evaluation with 99mTc-ECD SPECT. SPM5 was used for statistical comparison with 25 normal controls of similar ages. The disease course duration in years was added as a covariate. Additionally, patients with a 6-year evolution or less and those with more than 6 years were compared separately with normal controls. Results: Significant hypoperfusion was detected in bilateral premotor and posterior parietal cortex and increase of perfusion was present in the cerebellum. These changes correlated with the years of evolution of the illness. Patients with longer evolution also presented thalamic, subthalamic and basal ganglia hypoperfusion. Conclusions: We describe rCBF changes in PD in neural circuits related with control of movements. These changes are more manifest in patients with a longer duration of the disease(AU)

Simulation of the heteroagglomeration between highly size-asymmetric ceramic particles.

Aggregation phenomena of dilute suspensions composed of two kinds of oxide particles (alumina d(1)=400 nm and silica d(2)=25 nm) have been studied by computer simulations. These particles are oppositely charged and so are prone to heteroagglomerate. The interaction between particles has been modeled by the DLVO potential. Two kinds of simulations have been performed: Brownian dynamics simulations to study the aggregation kinetics and global minimization searches that permit the examination of the most stable configurations of agglomerates. We demonstrate that aggregation should occur also for quite large fractions of added silica (even when 200 silica particles are adsorbed on each alumina particle) and that aggregates are likely to present chainlike shapes. Both findings are in agreement with experiments.