Atomic-Scale Simulation

Group members

First-principles simulation: An atomic-scale microscope

The general purpose of this project is to bring the potential of first-principles simulation to the study of atomic-scale phenomena, both from the structural and dynamical point of view. Such approaches, supported by high speed computers, can reliably simulate the interactions between the atoms within a quantum mechanical description. Simulation on the computer permits the visualization of atomic processes, thus acting as a genuine microscope.

The principal aim consists in generating realistic structural models by using tools such as first-principles molecular dynamics. However, special attention is devoted to theoretical tools permitting the simulation of specific measurements (e.g. neutron, XPS, ESR, IR, Raman). Through confrontation of comparable quantities in theory and experiment, the decoding of the full wealth contained in experimental data is made possible.

Research

Vitreous materials - Modeling approaches are developed in order to extract from experimental data detailed information regarding the structure of network-forming glasses (e.g. SiO₂, B₂O₃, GeSe₂). In general, the following procedure is followed. First, model structures are generated by first-principles molecular dynamics. Then, the structural and dynamical properties of these models are characterized by direct comparison to experimental data, such as neutron and X-ray diffraction, inelastic neutron scatering, infrared absorption, and Raman scattering. Other issues, such as the characterization of intermediate range order and of excess low-frequency vibrational modes are also investigated.

Model of vitreous B₂O₃.

Thin dielectric films on silicon - With the continuously reducing dimensions of electronic devices, there is a growing demand for an understanding of the properties of the Si(100)-SiO₂ interface at the atomic scale. The characterization of the structure of this interface relies to a large extent on the understanding of the atomistic mechanisms which govern the oxidation process. One of the main goals of this project consists in providing an atomistic description of the oxidation mechanism. More generally, the potential of first-principles approaches is brought to the research area of electronic devices, covering problems, such as nitrogen incorporation, diffusion of dopants, role of hydrogen, breakdown phenomena, charge trapping, characterization of intrinsic defects, alternative dielectrics.

Model of the Si(100)-SiO₂ interface.

Aqueous systems - Within this project, first-principles molecular dynamics are used to investigate the properties of the water network in the neighborhood of hydrated species. Special attention is devoted to the hydration of charged ions. The structure of aqua-ions has been thoroughly characterized experimentally and provides a reliable database for the assessment of current theoretical approaches in this context. Furthermore, molecular dynamics simulations provide insight in dynamical processes, such as the exchange of water molecules in the first or second hydration shell. The long-term challenge of this project is to prepare the appropriate tools for addressing atomic processes in chemical and biological systems.

Two different structural configurations of the first solvation shell of the Cu(II) aqua ion.

Catalytic processes on surfaces - The goal of this project is to study the catalytic potential of small metallic clusters on insulator surfaces.

Small Fe clusters on a MgO(100) surface.

Illustrative research projects

The Cu (II) Aqua Ion

Cu Video [360x288] (mpeg 8.5 MB)
Cu Video [540x432] (mpeg 15.1 MB)

Durant longtemps, les chimistes ont supposé que l'atome de cuivre à deux électrons manquants (ion Cu2+) attirait six molécules d'eau (H2O) selon un arrangement octaédrique déformé. Si les résultats des mesures des distances équatoriales entre les liaisons Cu-O étaient constants, les résultats pour les distances axiales présentaient d'importantes variations. Ces simulations ont amené à reconsidérer l'arrangement des molécules d'eau autour du ion Cu2+.Les résultats ont montré que le ion Cu2+ était entouré de cinq molécules d'eau au lieu de six. L'arrangement de cette première couche d'hydratation subit des transformations très fréquentes entre des configurations pyramidales à base carrée et bipyramidales trigonales. Le passage entre ces deux états est extrêmement rapide puisqu'il se produit en un temps de quelques picosecondes. Ces résultats ont été publiés le 2 février 2001 par la revue américaine Science sous le titre First Solvation Shell of the Cu(II) Aqua Ion: Evidence for Fivefold Coordination.

Atomic Scale Processes during Oxidation at Si(100) Interfaces

Si Oxidation Video (mpeg 2.4 MB)

By applying an appropriate temperature gradient on an ideal interface and by using quantum molecular dynamics, we simulate atomic processes occuring upon oxidation at the Si(001)-SiO2 interface. A shortliving bonding configuration is identified in which an oxygen atom is bonded to three silicon atoms. This appears to be the dominant mechanism through which topological modifications of the structure occur. The structure resulting from the final quench represents the first example of a computer generated model of the interface.

Neutral oxygen molecule diffusing silica

Silicon oxidation mechanism at the atomic scale

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