ReaxFF Molecular Dynamics Simulations of Large Gold Nanocrystals.

A systematic study of gold nanocrystals is carried out using molecular dynamics simulations with reactive force fields. The nanocrystal size is varied between 2 and 10 nm with methane and butane thiolate as ligands. The reactive force fields allow investigation of the formation of staples. The simulations explain several experimental observations such as the number of staples per thiolate of about 40% and the occupation of the top adsorption sites on the facets. They also show that the frequency of staples is increased on the edges, which leads to a desorption of gold atoms from the nanocrystal edges. In contrast to previous nonreactive simulations, no difference between the distances of the ligands on the nanocrystal edges and facets is observed. Except for the 2 nm particles, the nanocrystal size and the alkane chain length of the ligands have only a small influence on the nanocrystal properties. The occupation of adsorption sites and staple frequencies are very slowly converging properties, taking more than ns.

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.

Crack patterns in superlattices made of maghemite nanocrystals.

Here, 11 nm γ-Fe2O3 nanocrystals characterized by a low size distribution of 5% and dispersed in chloroform are deposited onto a substrate as well-defined films. The film thickness is controlled by the initial concentration of the colloidal solution. After drying, the film shows a network of well-defined cracks. It is demonstrated using scanning electron microscopy and small-angle X-ray diffraction that the nanocrystals within the film are self-assembled in ordered lattices. It is shown that the nanocrystals self-assemble in superlattices before the cracks appear. In contrast to what was previously observed with a disordered film of maghemite nanocrystals with large size distribution, the top surface of the films is covered with patterns formed before crack formation. The cracks preferentially follow the direction of the hexagonal array observed on the top surface of the film by using a field emission gun scanning electron microscope. The average crack distance as a function of the film height follows the same linear scaling law as in amorphous nanocrystal assemblies. The size of superlattices of nanocrystals evolves with film height, but is usually significantly smaller than the average crack distance except on the border of the films.

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 (J. Am. Chem. Soc.2012, 134, 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.

2D silver nanocrystal ordering modulated by various substrates and revealed using oxygen plasma treatment.

Here, 5 nm Ag nanocrystals are deposited, using the same procedure, on various substrates differing by their rms roughness, wetting properties and nanoparticle-substrate interactions leading, consequently, to different nanocrystal orderings. Theoretical calculations are carried out to understand how these parameters influence the size of the nanocrystal organizations on the substrate surface. When these nanocrystal arrays are subjected to an oxygen plasma treatment, the nanocrystals perfectly assembled in hexagonal networks remain intact, while the nanocrystals that are not well-packed coalesce to form larger particles independently on the used substrate. This phenomenon is observed on the entire substrate surface. This procedure gives an innovative way of using oxygen plasma generated by the reactive ion etching technique, as a new method to reveal defects in 2D Ag nanocrystal self-assemblies.

Do directional primary and secondary crack patterns in thin films of maghemite nanocrystals follow a universal scaling law?

Cracks due to a shrinking film restricted by adhesion to a surface are observed in nature at various length scales ranging from tiny crack segments in nanoparticle films to enormous domains observed in the earth's crust. Here, we study the formation of cracks in magnetic films made of maghemite (gamma-Fe2O3) nanocrystals. The cracks are oriented by an external magnetic field applied during the drying process which presents a new method to produce directional crack patterns. It is shown that directional and isotropic crack patterns follow the same universal scaling law with the film height varying from micrometer to centimeter scales. Former experimental studies of scaling laws were limited to small variations in height (1 order of magnitude). The large variation in height in our experiments becomes possible due to the combined use of nanocrystals and electron microscopy. A simple two-dimensional computer model for elastic fracture leads to structural and scaling behaviors, which match those observed in the experiments.

Cracks in magnetic nanocrystal films: do directional and isotropic crack patterns follow the same scaling law?

In this letter, we show that the use of nanocrystals enables new insights into the scaling law of crack patterns. Directional and isotropic crack patterns made of gamma-Fe2O3 nanocrystals follow the same scaling law, with the film height varying by 3 orders of magnitude. A simple two-dimensional computer model for elastic fracture also leads to the same scaling behavior for directional and isotropic cracks, in good agreement with the experiments.