A thermodynamic investigation of selenium confined in silicalite zeolite

in Molecular Simulation (2004), 30(9), 601-606

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 ... [more ▼]

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 200degreesC while solely on the external surface at 650degreesC. This indicates that adsorption inside the pore network can only proceed below the pseudo-wetting transition temperature that lies between 200 and 650degreesC. The existence of such transition temperature is thus crucial if one aims to produce nanowires from microporous materials by adsorption from a gas phase. [less ▲]

Properties of molten Ge chalcogenides: an ab initio molecular dynamics study

in Journal of Physics-Condensed Matter (2003), 15(1 Sp. Iss. SI), 167-173

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available ... [more ▼]

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available diffraction data and obtain a new view of the molten Ge chalcogenides. We show that the anomalous volume contraction observed in the liquid 30 K above the eutectic temperature corresponds to a significant change of the Ge-Te partial structure factor. The detailed structural analysis shows that volume variations observed upon melting in Ge15Te85, as in liquid, GeSe and GeTe, can be explained in terms of the competition between two types of local environment of the germanium atoms. A symmetrical coordination octahedron is entropically favoured at high temperature, while an asymmetrical octahedron resulting from the local manifestation of the Peierls distortion is electronically favoured at lower temperatures. [less ▲]

Quantum confinement and fullerenelike surface reconstructions in nanodiamonds

in Physical Review Letters (2003), 90(3), 0374011-03740140374014

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap ... [more ▼]

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap, at variance with Si and Ge where quantum confinement effects persist up to 6-7 nm. In addition, our calculations show that the surface of nanodiamond particles larger than sime 1 nm reconstructs in a fullerenelike manner, giving rise to a new family of carbon clusters: bucky diamonds. Signatures of these surface reconstructions are compatible with pre-edge features observed in measured absorption spectra [less ▲]

Local structure of liquid GeTe via neutron scattering and ab initio simulations

in Physical Review B (2002), 65(11),

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare ... [more ▼]

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare these results with ab initio molecular dynamics simulations. The simulations were based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. [less ▲]

First-principles computation of material properties: the ABINIT software project

in Computational Materials Science (2002), 25(3), 478-492492

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a ... [more ▼]

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a DFT software project can benefit enormously from widespread collaboration, if handled properly. Also, modern software engineering concepts can considerably ease its development. The ABINIT project relies upon these ideas: freedom of sources, reliability, portability, and self-documentation are emphasised, in the development of a sophisticated plane-wave pseudopotential code. We describe ABINITv3.0, distributed under the GNU General Public License. The list of ABINITv3.0 capabilities is presented, as well as the different software techniques that have been used until now: PERL scripts and CPP directives treat a unique set of FORTRAN90 source files to generate sequential (or parallel) object code for many different platforms; more than 200 automated tests secure existing capabilities; strict coding rules are followed; the documentation is extensive, including online help files, tutorials, and HTML-formatted sources [less ▲]

First-principles computation of material properties: the ABINIT software project

in Computational Materials Science (2002), 25(3), 478-492

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a ... [more ▼]

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a DFT software project can benefit enormously from widespread collaboration, if handled properly. Also, modern software engineering concepts can considerably ease its development. The ABINIT project relies upon these ideas: freedom of sources, reliability, portability, and self-documentation are emphasised, in the development of a sophisticated plane-wave pseudopotential code. We describe ABINITv3.0, distributed under the GNU General Public License. The list of ABINITv3.0 capabilities is presented, as well as the different software techniques that have been used until now: PERL scripts and CPP directives treat a unique set of FORTRAN90 source files to generate sequential (or parallel) object code for many different platforms; more than 200 automated tests secure existing capabilities; strict coding rules are followed; the documentation is extensive, including online help files, tutorials, and HTML-formatted sources. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Ab initio simulations of liquid semiconductors using the pseudopotential-density functional method

in Journal of Physics : Condensed Matter (2001), 13(41), 817-854854

One of the most difficult problems in condensed matter physics is describing the microscopic nature of the liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss a ... [more ▼]

One of the most difficult problems in condensed matter physics is describing the microscopic nature of the liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss a particular microscopic structure; only ensemble averages can be specified. Such averages can be performed via well crafted molecular dynamics simulations: the length of the simulation, the size of the ensemble and the nature of the interatomic forces must all be carefully analysed. Historically, a problematic issue in doing such simulations is that of how to describe the interatomic forces in the liquid state. This matter is especially challenging for the melt of semiconductors, such as silicon or gallium arsenide, where the chemical bond contains a strong covalent component. It is difficult to use pairwise interatomic potentials in such cases. Although many-body potentials can be utilized for simulations of these materials, one must map quantum phenomena such as hybridization onto classical interatomic potentials. This mapping is complex and difficult. In this review, we illustrate how one can avoid this problem by utilizing quantum forces to simulate liquids. Our focus is on the pseudopotential-density functional method. Within the pseudopotential method, only the valence electrons are explicitly treated and within the density functional theory, exchange and correlation terms are mapped onto an effective one-electron potential [less ▲]

Structural and electronic properties of high-temperature fluid selenium

in Computational Materials Science (2000), 17(2-4), 239-242242

A semi-empirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces. The band structure parameters are ... [more ▼]

A semi-empirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces. The band structure parameters are obtained by fitting ab initio calculations. The simulated liquid structures are in very good agreement with the most recent X-ray scattering and EXAFS measurements. The Monte Carlo simulations performed show that the complex liquid structures observed result from the breaking and branching of the selenium chains. The total coordination number is shown to result from the balance between one-, two- and three-fold coordinated atoms. The role of these defects is discussed in relationship with the electrical conductivity of the liquid, i.e. the semiconductor-metal and metal-non-metal transitions observed at high pressure and temperature [less ▲]

Evidence of a reentrant Peierls distortion in liquid GeTe

in Physical Review Letters (2000), 85(9), 1950-1953

The local atomic order of semiconducting liquid GeTe is studied using first-principles molecular-dynamics simulations. Our work points out a high degree of alternating chemical order in the liquid and ... [more ▼]

The local atomic order of semiconducting liquid GeTe is studied using first-principles molecular-dynamics simulations. Our work points out a high degree of alternating chemical order in the liquid and demonstrates the presence of a Peierls distortion close above the melting temperature. This distortion, absent in the high temperature crystalline structure of NaCl type, is a remnant of the atomic arrangement in the A7 low temperature crystalline phase. It disappears slowly with temperature, as the liquid evolves from a semiconducting to a metallic state. [less ▲]

On the structure of liquid IV-VI semiconductors

in Journal of Non-Crystalline Solids (1999), 250-252

The crystalline structure of most stoichiometric IV-VI semiconductors can be viewed as a distortion of the NaCl structure; the distortion amplitude depends on the components. As a result, corrugated ... [more ▼]

The crystalline structure of most stoichiometric IV-VI semiconductors can be viewed as a distortion of the NaCl structure; the distortion amplitude depends on the components. As a result, corrugated planes of covalently bonded atoms are linked by dispersion (Van der Waals) forces. Neutron scattering experiments were performed on the liquid phases of a series of IV-VI compounds (GeS, GeSe, GeTe, SnS, SnSe and SnTe). We show that the evolution of the local order is correlated with the conductivity measurements [less ▲]