The stability and electronic structure of fully H or F terminated and mixed H and F terminated diamond (111) surfaces were studied using first principles calculations. It was found that F atoms on the surface, like H, formed sp3 type bonding with C atoms, which resulted in a more stable 1×1 configuration rather than the π-bonded 2×1 construction of clean diamond. A phase diagram showing the stable surface composition regions was constructed as a function of chemical potentials of H and F. The diagram shows that the surface with 75% F (25% H) termination was unstable. The F terminated surface was more stable than H termination due to the formation of strong ionic C-F bonding and the close packing of the large F atoms. Due to the attractive forces developed between F atoms, a close packed surface was formed. Additionally, the exposure of C to the environment became restricted because of the large size of F atoms. Hence, F terminated diamond surface was more chemically inert compared to H terminated surface. To bring two F terminated surfaces together, a large repulsive force was required due to the negative charge on F atoms, and this led to low adhesion between two F terminated diamond surfaces compared to two H terminated surfaces.

Nucleic acid nanoparticles can self-assembly through the formation of complementary loop-loop interactions or stem-stem interactions. Presence and concentration of ions can significantly affect the self-assembly process and the stability of the nanostructure. In this paper we use explicit molecular dynamics simulations to examine the variations in cationic distributions around DNA and RNA helices and loop-loop interactions with identical sequence except for Thymine to Uracil substitution. Our simulations show that the ionic distributions are different around RNA and DNA motifs which could be related to the discrepancy in stability of loop-loop complexes.

First-principles calculations have been applied to investigate the interactions between Ptn (n=1˜13) clusters and a graphene sheet to model the Pt/C fuel-cell catalytic electrode. For the small clustesr (n<7), planar configurations vertically adsorbed on graphene (V-2D) are more stable than the three-dimensional (3D) configurations. For the large clusters (n>7), the 3D clusters are more stable than the V-2D clusters. In order to investigate the effects of defects and dopants in a graphene sheet, the interactions between the Pt13 cluster and the graphene sheet with an atomic vacancy and an dopant atom, such as boron and nitrogen atoms have been examined. For the atomic vacancy, the Pt atom is directly adsorbed on the C atom vacant site. For the Pt13 cluster adsorbed on the graphene sheet doped with the B or N atoms, the Pt atom is not directly adsorbed on the impurity atoms. The adsorption of the Pt13 cluster above the vacancy of the graphene sheet is more stable that that on the doped graphene sheet in the present calculations.

We present a method which allows to calculate gas sorption in complex polymers where, as slow processes, gas induced plasticization and volume dilation are important factors. Since the relaxational swelling of the polymer matrix that is observed at elevated gas concentrations takes hours or days, the swelling process is orders of magnitudes too slow to simulate the respective molecular dynamics in reasonable time and effort. To address this apparent incompatibility of experiment and simulation, we use single representative reference states from experiment and construct atomistic packing models according to these specifications. Gas sorption of CO2 and CH4 was successfully calculated on polysulfone, a 6FDA-polyimide, and a polymer of intrinsic microporosity, PIM-1, at 308 K and pressures up to 50 bar.

Morphological evolution of two metallic clusters of different elements at coalescence is investigated using molecular-dynamics (MD) simulation. All the pair combinations of the elements Ni, Cu, Au, Ag, Pt, and Pd are considered. The final structures of united bimetallic clusters are classified into three categories: epitaxial, core-shell, and alloyed. Which type of structure appears via coalescence depends on the size and temperature of clusters, which can be summarized in an observed structure map.

The work presents some results of an ongoing research program aimed at building a material database and material models for specific types of polymers. Results for three thermoplastics are the focus of the present article: polycarbonate, polypropylene, and acrylonitrile-butadiene-styrene. Uniaxial compression / tension tests at room temperature and different strain rates have been performed to characterize their mechanical response. A rate-dependent material model has been developed and implemented in a finite element code to predict such mechanical behavior. The model predictions have shown good agreement with the tests results.

Recently, heteromolecular crystals of fullerene C60 and cubane (C8H8) have been synthesized. For some temperatures the C60 molecules are free to rotate whereas cubanes behave like a static bearing in a so-called rotor-stator phases. In this work we report classical and tight-binding molecular dynamics simulations in order to investigate the rotor-stator dynamics and polymerization processes. Our results show that, for 200 K and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations within the timescale of our simulations, while C60 fullerenes show rotational motions. Fullerenes perform “free” rotational motions at short times (< 1 ps), small amplitude hindered rotational motions (librations) at intermediate times, and rotational diffusive dynamics at long times (> 10 ps). Random copolymerization among cubanes and fullerenes were observed when temperature is increased, leading to the formation of a disordered structure.

The low thermal conductivities of silica aerogels have made them of interest to the aerospace community for applications such as cryotank insulation. Recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight materials for impact absorption as well. In this work, we perform multiscale computer simulations to investigate the tensile strength and failure behavior of silica and polymer-coated silica aerogels. The gels' nanostructure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) procedure. The procedure produces fractal aggregates that exhibit fractal dimensions similar to those observed in real aerogels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which is composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect. We investigate tensile failure via the application of a uniaxial tensile strain to the DLCA clusters. In computing the energetics of tensile strain, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by Morse pair potentials, representing the strain energetics of the silica gel and the polymer coating, parameterized such that the potential ranges are much smaller than the secondary particle size. The Morse parameters are obtained by separate atomistic simulation of models of the interparticle bridges and polymer coatings, with the tensile behavior of these bridges modeled via molecular statics. We consider the energetics of tensile strain and tensile failure, and compare qualitative features of low-and high-density gel failure.

It is well known that the mechanical behavior of nanoscale multilayered composites is strongly governed by single dislocation mechanisms and dislocation-interface interactions. Such interactions are complex and multiscale in nature. In this work, two such significant effects are modeled within the dislocation dynamics-continuum plasticity framework: elastic properties mismatch (Koehler image forces) and interface shearing in the case of weak interfaces. The superposition principle is used to introduce the stress fields due to both effects solved for by finite elements. The validation of both methodologies is presented. Furthermore, it was found that the layer-confined threading stress of a dislocation in hair-pin configuration increases if the layer is surrounded by layers made of a stiffer material and that this strengthening effect grows more significant as the layer thickness decreases. The observation made through molecular dynamics, that weak interfaces act as dislocation sinks, was also captured with our approach. A dislocation is attracted to the interface independent of its sign or character. Also the force increases sharply as the dislocation approaches the interface. These findings agree with published molecular dynamics simulations and dislocation-based equilibrium models of this type of interaction.

The statistics of internal elastic fields and dislocation density tensor associated with arbitrary 3D dislocation distributions have been modeled using probability density function and pair correlations. Numerical results for these quantities have been obtained for dislocation structures generated by the method of dislocation dynamics simulation.