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Ge-Sb-Te alloys (GST) have been successfully used as Phase Change Materials for data recording applications. They exhibit a unique collection of properties that make them adequate for high performance non-volatile RAM memories. Indeed, their crystalline phase(s) are extremely contrasted, both optically and electrically, against their easily generated amorphous phase(s). Understanding the fast switching mechanisms and the amorphous phase properties remains however quite challenging. In particular, the stability of the amorphous phase against phase separation, recrystallization, and drift of the electronic properties with time and cycling remain problematic. In this work, we generate models of a series of amorphous GST alloys using DFT molecular dynamics and simulated annealing. The obtained structures are compared and generic patterns ...
The nucleation of carbon nanotubes on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. This technique closely follows the conditions of the synthesis of carbon nanotubes by chemical vapor deposition. The possible formation of a carbon cap on the catalyst particle is studied as a function of the carbon chemical potential, for particles of different size, either crystalline or disordered. We show that these parameters strongly influence the structure of the cap/particle interface which in turn will have a strong effect on the control of the structure of the nanotube. In particular, we discuss the presence of carbon on surface or in subsurface layers.
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 (100) 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.
The nucleation of carbon caps on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. It takes place in a well defined carbon chemical potential range, when a critical concentration of surface carbon atoms is reached. The solubility of carbon in the outermost Ni layers, that depends on the initial, crystalline or disordered, state of the catalyst and on the thermodynamic conditions, is therefore a key quantity to control the nucleation.
The healing of graphene grown from a metallic substrate is investigated using tight-binding Monte Carlo simulations. At temperatures (ranging from 1000 to 2500 K), an isolated graphene sheet can anneal a large number of defects suggesting that their healing are thermally activated. We show that in presence of a nickel substrate we obtain a perfect graphene layer. The nickel-carbon chemical bonds keep breaking and reforming around defected carbon zones, providing a direct interaction, necessary for the healing. Thus, the action of Ni atoms is found to play a key role in the reconstruction of the graphene sheet by annealing defects.
Although significant efforts have been directed towards a selective single wall carbon nanotube synthesis, the resulting diameter and chirality distributions are still too broad and their control remains a challenge. Progress in this direction requires an understanding of the mechanisms leading to the chiral selectivity reported by some authors. Here, we focus on one possible such mechanism and investigate the healing processes of defective tubes, at the atomic scale. We use tight-binding Monte Carlo simulations to perform a statistical analysis of the healing of a number of defective tubes. We study the role of temperature as a primary factor to overcome the energy barriers involved by healing, as well as the role of the metal catalyst. Using both electron diffraction patterns and local characterizations, we show that the healing pr...
Optimized growth of Single Wall Carbon Nanotubes requires a full knowledge of the actual state of the catalyst nanoparticle and its interface with the tube. Using Tight Binding based atomistic computer simulations, we calculate carbon adsorption isotherms on nanoparticles of nickel, a typical catalyst, and show that carbon solubility increases for smaller nanoparticles that are either molten or surface molten under experimental conditions. Increasing carbon content favors the dewetting of Ni nanoparticles with respect to sp2 carbon walls, a necessary property to limit catalyst encapsulation and deactivation. Grand Canonical Monte Carlo simulations of the growth of tube embryos show that wetting properties of the nanoparticles, controlled by carbon solubility, are of fundamental importance to enable the growth, shedding a new light on...
Contrairement à la plupart des éléments, les phases liquide et amorphe du tellure se sont avérées être assez mal reproduites par la méthode, pourtant éprouvée, de la dynamique moléculaire ab initio . Les calculs ‘standart’ utilisant la théorie de la fonctionnelle de la densité (DFT) produisent en effet des structures en relatif désaccord avec les expériences de diffraction, en particulier pour ce qui concerne le nombre de premiers voisins et les distances interatomiques, quantités qui apparaissent fortement surestimées dans les simulations. Le tellure étant composant essentiel de la plupart des matériaux à changement de phase (PCMs), cette mauvaise description de ses phases désordonnées soulève d’importantes questions au sujet de la capacité de la dynamique moléculaire ab initio à générer des structures d’amorphe réalistes pour les PCM...
Contrary to almost all other elements, liquid and amorphous phases of pure tellurium have proven difficult to simulate using ab initio molecular dynamics. Standard density functional theory calculations yield structures in relatively poor agreement with available diffraction experiments at low temperature, especially regarding first neighbor distance and coordination number, which are strongly overestimated in the simulations. Tellurium being a key component of many phase change materials, this poor structural description of its disordered phases raises important issues about the ability of ab initio molecular dynamics to generate accurate structural models of amorphous phases. In this work, we use ab initio molecular dynamics performed under constant volume (experimental values) conditions to simulate liquid Tellurium structure and dy...
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, 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.
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