585 research outputs found
Metallic nanoparticles meet Metadynamics
We show how standard Metadynamics coupled with classical Molecular Dynamics
can be successfully ap- plied to sample the configurational and free energy
space of metallic and bimetallic nanopclusters via the implementation of
collective variables related to the pair distance distribution function of the
nanoparticle itself. As paradigmatic examples we show an application of our
methodology to Ag147, Pt147 and their alloy AgshellPtcore at 1:1 and 2:1
chemical compositions. The proposed scheme is not only able to reproduce known
structural transformation pathways, as the five and the six square-diamond
mechanisms both in pure and core-shell nanoparticles but also to predict a new
route connecting icosahedron to anti-cuboctahedron.Comment: 7 pages, 8 figure
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Structure, thermodynamics, and rearrangement mechanisms in gold clusters-insights from the energy landscapes framework.
We consider finite-size and temperature effects on the structure of model AuN clusters (30 ≤ N ≤ 147) bound by the Gupta potential. Equilibrium behaviour is examined in the harmonic superposition approximation, and the size-dependent melting temperature is also bracketed using molecular dynamics simulations. We identify structural transitions between distinctly different morphologies, characterised by various defect features. Reentrant behaviour and trends with respect to cluster size and temperature are discussed in detail. For N = 55, 85, and 147 we visualise the topography of the underlying potential energy landscape using disconnectivity graphs, colour-coded by the cluster morphology; and we use discrete path sampling to characterise the rearrangement mechanisms between competing structures separated by high energy barriers (up to 1 eV). The fastest transition pathways generally involve metastable states with multiple fivefold disclinations and/or a high degree of amorphisation, indicative of melting. For N = 55 we find that reoptimising low-lying minima using density functional theory (DFT) alters their energetic ordering and produces a new putative global minimum at the DFT level; however, the equilibrium structure predicted by the Gupta potential at room temperature is consistent with previous experiments
A diffusion Monte Carlo study of small para-Hydrogen clusters
Ground state energies and chemical potentials of parahydrogen clusters are
calculated from 3 to 40 molecules using the diffusion Monte Carlo technique
with two different pH2-pH2 interactions. This calculation improves a previous
one by the inclusion of three-body correlations in the importance sampling, by
the time step adjustement and by a better estimation of the statistical errors.
Apart from the cluster with 13 molecules, no other magic clusters are
predicted, in contrast with path integral Monte Carlo results
Ethanol chemisorption on core-shell Pt-nanoparticles: an ab-initio study
By means of ab-initio calculations, we have investigated the chemisorption
paroperties of ethanol onto segregating binary nanoalloys. We select
nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer
over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent
composition, there is an increse of the ethanol binding energy. This is not
merely due to observed shortening of the Pt-O distance but depends on the
nanoparticle distortion after ethanol adsorption. This geometrical distortion
within the nanoparticle can be interpreted as a radial breathing, which is
sensitive to the adsortion site, identified by the O-anchor point and the
relative positions of the ethyl group. More interestingly, being core-dependent
-larger in Pd@Pt and smaller in Ni@Pt-, it relates to an effective electron
transfer from ethanol and the M-core towards the Pt-shell. On the view of this
new analysis, Pd@Pt nanoalloys show the most promissing features for ethanol
oxidation
Lead clusters: different potentials, different structures
The lowest-energy structures of lead clusters interacting via a Gupta
potential are obtained for N<151. Structures based on Marks decahedra dominate
at the larger sizes. These results are very different from those obtained
previously using a lead glue potential, and the origins of the differences are
related back to differences in the potential.Comment: 6 pages, 4 figures, TAMC4 proceeding
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Thermodynamics of CuPt nanoalloys.
The control of structural and chemical transitions in bimetallic nanoalloys at finite temperatures is one of the challenges for their use in advanced applications. Comparing Nested Sampling and Molecular Dynamics simulations, we investigate the phase changes of CuPt nanoalloys with the aim to elucidate the role of kinetic effects during their solidification and melting processes. We find that the quasi-thermodynamic limit for the nucleation of (CuPt)309 is 965 ± 10 K, but its prediction is increasingly underestimated when the system is cooled faster than 109 K/s. The solidified nanoparticles, classified following a novel tool based on Steinhardt parameters and the relative orientation of characteristic atomic environments, are then heated back to their liquid phase. We demonstrate the kinetic origin of the hysteresis in the caloric curve as (i) it closes for rates slower than 108 K/s, with a phase change temperature of 970 K ± 25 K, in very good agreement with its quasi-thermodynamic limit; (ii) the process happens simultaneously in the inner and outer layers; (iii) an onion-shell chemical order - Cu-rich surface, Pt-rich sub-surface, and mixed core - is always preserved
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