16 research outputs found
ReaxFF Simulations of Self-Assembled Monolayers On Silver Surfaces and Nanocrystals
The self-assembled monolayers of alkane thiolates on Ag (111) surfaces and
nanoparticles are studied using molecular dynamics. Reactive force fields allow
simulations of very large systems such as nanoparticles of 10 nm. Stable
(sqrt(7) X sqrt(7))R19.1{\deg} assemblies are obtained as experimentally
observed for these systems. Only nanoparticles smaller than 4 nm show a
spontaneous restructuration of the metallic core. The preferred adsorption site
is found to be in an on-top position, in good agreement with recent X-ray
absorption near edge structure experiments. Moreover, similar distances between
the sulfur headgroups are found on the facets and edges
Optimization of a New Reactive Force Field for Silver - Based Materials
A new reactive force field based on the ReaxFF formalism is effectively parametrized against an extended training set of quantum chemistry data (containing more than 120 different structures) to describe accurately silver- and silver-thiolate systems. The results obtained with this novel representation demonstrate that the novel ReaxFF paradigm is a powerful methodology to reproduce more appropriately average geometric and energetic properties of metal clusters and slabs when compared to the earlier ReaxFF parametrizations dealing with silver and gold. ReaxFF cannot describe adequately specific geometrical features such as the observed shorter distances between the under-coordinated atoms at the cluster edges. Geometric and energetic properties of thiolates adsorbed on a silver Ag20 pyramid are correctly represented by the new ReaxFF and compared with results for gold. The simulation of self-assembled monolayers of thiolates on a silver (111) surface does not indicate the formation of staples in contrast to the results for gold-thiolate systems.</div
Influence of Force-Field Parameters on the Atomistic Simulations of Metallic Surfaces and Nanoparticles
The
influence of the interaction model on the adsorption of butanethiolate
on gold surfaces and nanocrystals has been studied with molecular
dynamics simulations. The results obtained for three different head
group sizes are compared to experiments. The use of the largest head
group induces new organizations of the ligands in the case of nanocrystals
and Au(100) surfaces, while no such difference is observed for Au(111).
As a consequence, this model does not reproduce the higher surface
coverage experimentally observed for nanocrystals. Our results show
that the evaluation of the quality of force fields cannot be restricted
to the study of specific surfaces. Some properties such as the occupation
frequencies of adsorption sites markedly depend on the nanocrystal
size
Butanethiol adsorption and dissociation on Ag (111): A periodic DFT study
International audienceThe molecular and dissociative adsorption of butanethiol (C4H9SH) on regular Ag (111) surfaces has been studied by means of periodic ab initio density functional techniques. In molecular form, butanethiol is bound to the surface only by weak polarization-induced forces with the CâS axis tilted by 38° relative to the normal surface. The S atom occupies a position between a hollow fcc and a bridge site. In the dissociative adsorption process, the SâH bond breaks leading to butanethiolate. The S atom of the thiolate also occupies a threefold position, slightly displaced to a hollow fcc site compared to the thiol adsorption case. The CâS axis of the thiolate is tilted by about 37°. The calculated adsorption energies show that the butanethiol and butanethiolate have similar adsorption ability. The computed reaction pathway for the SâH dissociation gives an activation energy of 0.98 eV indicating that the thiolate formation from thiol, although not spontaneous at room temperature, might be feasible on silver surfaces. The dissociation process induces both adsorbate and surface polarization with a significant charge transfer from the substrate to the adsorbate
Solvent Effects on Cobalt Nanocrystal SynthesisâA Facile Strategy To Control the Size of Co Nanocrystals
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Atomistic Simulations of Self-Assembled Monolayers on Octahedral and Cubic Gold Nanocrystals
International audienceThis paper reports on a molecular dynamics investigation of the molecular organization of alkanethiolates (from ethane to dodecanethiolate) on octahedral and cubic gold nanocrystals with diameters up to 10 nm. We show that the average surface per adsorbed thiolate only slightly depends on the nanocrystal shape and the alkane chain length. Two different organizations of thiolates are observed on the facet centers and edges of octahedral nanocrystals, while on cubic nanocrystals only one appears. This explains the closer distance between thiolates on the edges of octahedral nanocrystals, which is not observed for nanocubes. The enhanced surface coverage of thiolates on nanocrystals is explained by the new organization for octahedral nanocrystals and can be attributed to the occupation of adsorption sites on the edges for cubic nanocrystals. Small differences observed in the molecular organizations on icosahedral and octahedral nanocrystals can be mainly explained by the larger facets of the latter ones
Atomistic Simulations of Self-Assembled Monolayers on Octahedral and Cubic Gold Nanocrystals
Atomistic Simulations of the Surface Coverage of Large Gold Nanocrystals
Here, the adsorption of alkanethiols
(from ethane to dodecanethiol)
on icosahedral gold nanocrystals with diameters up to 10 nm is studied
by molecular dynamics simulations in a vacuum. The surface coverage
of the nanocrystals obtained in the simulations is in good agreement
with experimental data. We show that the average surface per adsorbed
thiol does not markedly depend on the nanocrystal size and ligand
and is only about 10% lower than the value observed on a flat Au(111)
surface. We observe two different molecular organizations of the thiolates
on the edges and in the centers of the nanocrystal facets. The incompatibility
between both organizations explains the fact that the formation of
self-assembled monolayers usually observed on flat Au(111) surfaces
is hindered for nanocrystals smaller than 6 nm. We also show that
the organization of thiolates on the edges is at the origin of the
lower average surface per adsorbed thiol found for the nanocrystal
Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations
Under solvent saturation, a precipitation of full-grown
supracrystals
on the one hand and the formation of well-defined supracrystalline
films at the airâliquid interface on the other hand were previously
observed for the first time (<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 3714â3719). Here, these
two simultaneous growth processes are studied by additional experiments
and by Brownian dynamics simulations. The thickness of the supracrystalline
films and the concentration of free nanocrystals within the solution
are measured as a function of the nanocrystal size. The simulations
show that the first process of supracrystal growth is due to a homogeneous
nucleation favored by solvent-mediated ligand interactions, while
the second one is explained in terms of a diffusion process caused
by a decrease in the surface energy when the particles penetrate the
airâliquid interface. It is also verified that the presence
of thiol molecules at the airâsolution interface does not hinder
the formation of supracrystalline films
Formation of colloidal crystals of maghemite nanoparticles: Experimental and theoretical investigations
International audienceNovel colloidal crystals made of maghemite nanocrystals are fabricated by a co-evaporation method with a mixture of ethanol/hexane. Through a series of comprehensive characterization performed by grazing incidence small-angle X-ray scattering (GISAXS) and field emission gun scanning electron microscope (FEG-SEM), we show the first example of well-defined face-centered cubic (fcc) colloidal crystals. In order to obtain a clear picture of the crystal formation, the amount of ethanol in the solution is monitored using gas chromatography. In parallel, the interactions between the nanocrystals are calculated by statistical mechanics theory using solubility parameters. Theory predicts the formation of colloidal crystals at quite high amounts of ethanol around 15%, in perfect agreement with experimental results. We show that the theory can further be applied to predict the optimal experimental conditions for the formation of colloidal crystals using other solvent mixtures