C. Bichara

Tight-binding potential for atomistic simulations of carbon interacting with transition metals: Application to the Ni-C system

We present a tight-binding potential for transition metals, carbon, and transition-metal carbides, which has been optimized through a systematic fitting procedure. A minimal basis, including the s and p electrons of carbon and the d electrons of the transition metal, is used to obtain a transferable tight-binding model of the carbon-carbon, metal-metal, and metal-carbon interactions applicable to binary systems. The Ni-C system is more specifically discussed. The successful validation of the potential for different atomic configurations indicates a good transferability of the model and makes it a good choice for atomistic simulations sampling a large configuration space. This approach appears to be very efficient to describe interactions in systems containing carbon and transition-metal elements. By way of example, we present results concerning the epitaxial growth of graphene sheets on (111) Ni surfaces, as well as the catalytic nucleation of carbon nanotubes.

Understanding amorphous phase-change materials from the viewpoint of Maxwell rigidity

Phase-change materials (PCMs) are the subject of considerable interest because they have been recognized as potential active layers for nonvolatile memory devices, known as phase-change random access memories. By analyzing first-principles molecular-dynamics simulations we develop a method for the enumeration of mechanical constraints in the amorphous phase and show that the phase diagram of the most popular system (Ge-Sb-Te) can be split into two compositional regions having a well-defined mechanical character: a tellurium rich flexible phase and a stressed rigid phase that encompasses the known PCMs. This sound atomic scale insight should open new avenues for the understanding of PCMs and other complex amorphous materials from the viewpoint of rigidity.

Peierls instabilities in covalent structures. I. Electronic structure, cohesion and the Z=8-N rule

The vast majority of the molecular, crystalline or liquid structures of groups V, VI and VII of the periodic table and their compounds obey the octet rule (Z = 8 - N). The structure and stability of those structures are discussed in a simple tight-binding approximation. In this framework we show that the Peierls electronic instability of a simple cubic structure leads to the octet rule. This instability does not rely upon the periodicity and consequently may occur in crystalline, amorphous or liquid matter. In a general discussion on the stability of covalent structures, we show that the existence of a Peierls distortion is governed by the balance between the attractive (band) term and the empirical repulsive term. As the hardness of the latter increases when going down the periodic table, this explains why the Peierls distortion is stronger for the light elements.

Hydrogen storage enhanced in Li-doped carbon replica of zeolites: A possible route to achieve fuel cell demand

We first report the atomistic grand canonical Monte Carlo simulations of the synthesis of two realistic ordered microporous carbon replica in two siliceous forms of faujasite zeolite (cubic Y-FAU and hexagonal EMT). Atomistic simulations of hydrogen adsorption isotherms in these two carbon structures and their Li-doped composites were carried out to determine their storage capacities at 77 and 298 K. We found that these new forms of carbon solids and their Li-doped versions show very attractive hydrogen storage capacities at 77 and 298 K, respectively. However, for a filling pressure of 300 bars and at room temperature, bare carbons do not show advantageous performances compared to a classical gas cylinder despite of their crystalline micropore network. In comparison, Li-doped nanostructures provide reversible gravimetric and volumetric hydrogen storage capacities twice larger (3.75 wt % and 33.7 kg/m(3)). The extreme lattice stiffness of their skeleton will prevent them from collapsing under large external applied pressure, an interesting skill compared to bundle of carbon nanotubes, and metal organic frameworks (MOFs). These new ordered composites are thus very promising materials for hydrogen storage issues by contrast with MOFs.

Interaction of carbon clusters with Ni(100): Application to the nucleation of carbon nanotubes

In order to understand the first stages of the nucleation of carbon nanotubes in catalytic processes, we present a tight-binding Monte Carlo study of the stability and cohesive mechanisms of different carbon structures deposited on nickel (1 0 0) surfaces. Depending on the geometry, we obtain contrasted results. On the one hand, the analysis of the local energy distributions of flat carbon sheets, demonstrate that dangling bonds remain unsaturated in spite of the presence of the metallic catalyst. Their adhesion results from the energy gain of the surface Ni atoms located below the carbon nanostructure. On the other hand, carbon caps are stabilized by the presence of carbon atoms occupying the hollow sites of the fcc nickel structure suggesting the saturation of the dangling bonds

Structure of liquid II–IV compounds: CdTe

Abstract The structure factor of CdTe is studied by neutron diffraction just above its melting point (1100°C). Short wavelength neutrons (λ = 0.35 A ) have been used in order to avoid the strong absorption of Cd around λ = 0.7 A . The structure factor and the pair correlation function differ substantially from those of group IV semiconductors (Si, Ge) or III–V compounds (GaAs, InSb) in the liquid state. The first peak of the pair correlation function coincides with the nearest neighbour distance in crystalline CdTe and the coordination number lies between 3 and 4. We suggest that the structure of liquid CdTe is similar to the structure of the continuous random network (Polk or Connell-Temkin model). This is consistant with the increase of the electrical resistivity with temperature.

Testing the feasibility of using the density functional theory route for pore size distribution calculations of ordered microporous carbons

The pore size distribution (PSD) characterization of microporous carbon materials is traditionally obtained from the analysis of N2 adsorption isotherms at 77 K. In this work, we aim at testing the feasibility of using the density functional theory (DFT) route for PSD calculations of interconnected carbon pore structures. The first step of this study was to generate using an atomistic simulation approach, an ordered porous carbon material with well-defined porosity using NaY zeolite as a templating matrix. For this purpose, we used the grand canonical Monte-Carlo (GCMC) technique in which the carbon–carbon interactions were described within the frame of a newly developed tight binding approach and the carbon–zeolite interactions assumed to be characteristic of physisorption. We calculated the PSD of such a carbon porous material. At a second stage, we calculated nitrogen adsorption isotherms at different temperatures. These data were subsequently used as inputs for DFT calculation to obtain the PSD. Comparisons between DFT–PSD and MC–PSD are made. In particular, we show that with an appropriate wall thickness of two graphene layers, the PSD obtained from DFT calculation agrees well with that from direct analysis of the simulated structure.

Stability of porous carbon structures obtained from reverse monte carlo using tight binding and bond order hamiltonians

The constrained Reverse Monte-Carlo (RMC) technique [1,2] was used to generate atomic configurations of disordered microporous carbons in a previous work. However, a carbon structure obtained from RMC is a result of the fitting to some structural data such as obtained from X-ray diffraction; it does not guarantee the stability of the resulting models when a realistic interatomic potential is used. In the present work, we studied the stability of these RMC structures using canonical Monte-Carlo simulations. Two different descriptions of the carbon-carbon and carbon-hydrogen interactions are used, both encompassing the bonding processes characteristic of carbon chemistry. The first approach is based on a bond-order potential while the second considers a tight binding model. We found that the structures obtained from RMC simulations undergo local structural changes upon relaxation, however the porous structure of the models remains intact.

Local order and phase separation in sulphur–tellurium melts: a neutron scattering study

Abstract A neutron scattering investigation of the local order in liquid sulphur–tellurium was performed for alloys at xs=0.2, 0.3, 0.4, 0.6 and 0.8, at three temperatures. A detailed analysis of the structure in terms of partial coordination number is performed by means of a modelling of the structure factors at large momentum-transfer range. Local order changes are essentially caused by tellurium whose coordination number decrease from 2.6 to 1.8 in a 20% composition range. The change in the local atomic environment is shown to be responsible for the observed miscibility gap that occurs around xs=0.4.

A thermodynamic investigation of selenium confined in silicalite zeolite

In this paper, we study the practical feasibility of selenium adsorption in silicalite-1 zeolite by performing Grand Canonical Monte Carlo (GCMC) simulations on a simulation box including the porous matrix and its outer surface. This work aims at gaining insight on the stability of semi-conductor wires in microporous materials. The simulations at two different temperatures show two distinct behaviors: adsorption occurs inside the pores at 200°C while solely on the external surface at 650°C. This indicates that adsorption inside the pore network can only proceed below the pseudo-wetting transition temperature that lies between 200 and 650°C. The existence of such transition temperature is thus crucial if one aims to produce nanowires from microporous materials by adsorption from a gas phase.