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 C–S 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 S–H 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 C–S 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 S–H 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 Synthesis—A 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 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 air–liquid interface on the other hand were previously observed for the first time (<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 3714–3719). 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 air–liquid interface. It is also verified that the presence of thiol molecules at the air–solution 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