Simulation of the behavior of Au-SiO₂ nano-composite under heavy ion irradiation

Metal-glass nano-composites are interesting owing to their particular optical-properties. Ion irradiation can be used as a tool to fabricate nano-composites. The aim of this thesis was to study, from a fundamental point of view, the behavior of gold nanoparticles (NPs) embedded in an amorphous silica matrix under irradiation. My contribution consists in the interpretation, by modeling and numerical simulation, of experimental results previously obtained by researchers of the Laboratoire des Solides Irradies. In a first type of experiments, the system was irradiated with 4 MeV gold ions while in a second type the irradiation was performed by using 74 MeV krypton ions. For the study of the first type of experiments, the simulation by kinetic Monte Carlo (KMC) on rigid lattice was chosen and developed specifically to study the behavior of NPs irradiated at different temperatures. The simulations allows us to reproduce qualitatively the experimental results at any temperature. We found that the system evolves within a direct coarsening regime at high temperature (T≥900 K) whereas at low temperatures (T≤600 K) it becomes resistant to coarsening and the NPs dissolve. A transition regime is observed between 600 K and 900 K. The KMC simulation highlights and explains an effect of the size of the NPs on their dissolution under irradiation. It also demonstrated that unidirectional ballistic models could account for the law of dissolution. A model based on the theoretical approach of Frost and Russel including a size-dependent recoil generation rate fully describes the dissolution for any NP size. In the second type, the system evolves in the electronic energy deposition regime. Depending on their size, the NP can be vaporized (diameter≤10 nm), transformed into nano-rod (10-30 nm) or faceted NP (30-60 nm) or slightly deformed (≥60 nm) by the incident ions. We imagined a scenario for the mechanism of deformation which is based on the thermal expansion of the melted NP into the track produced by the incident ion in the matrix. Its implementation using a new simulation technique helped to refine certain elements of the scenario and to improve the overall understanding of the process. The qualitative agreement between the experimental data and the results obtained by the simulation validate the proposed mechanism. (author)