Computational Studies of Metallic Glasses and Nanoparticles

This dissertation presents research using classically based Molecular Dynamics techniques to study the structure and dynamics of phases exhibited by unique metallic systems. These systems include metallic glasses, nanoparticles, and lastly glassy nanoparticles. It is arranged in the order the research was conducted since later material builds on formerly presented materials. Introductory material common to all chapters in this dissertation relating to Molecular Dynamics techniques is presented in the opening chapter. This includes an introduction to metallic force fields, integration of the classical equations of motion and Langevin Dynamics.

Chapter 2 explores transport dynamics in a known glass former (a mixture of silver and copper). This system presents an interesting target for computational study because it is a real glass forming system that closely resembles model binary Lennard-Jones systems that have been previously studied. Lennard-Jones glasses are interesting because they have decay functions that obey the Kohlrausch-Williams-Watts (KWW) law. Comparisons will be made between dynamics (mean squared displacement, cage correlation funcion) in model systems and models of real glass formers. Additionally, a model for fractal distributions of waiting times in glassy materials will be examined and compared to the waiting times in this metallic glass.

It has been experimentally observed that spontaneous alloying of bimetallic core-shell Au-Ag nanoparticles (NPs) can occur shortly after synthesis. Chapter 3 will use computational techniques to explore a possible mechanism for such alloying. Nanoparticles differ in many ways from their bulk counterparts in both physical and chemical properties. Some of these differences are attributed to the large surface area to volume ration present in nanoparticles. Computational techniques will be used to explore whether the hypothesis that a small fraction of vacancies formed at the Au-Ag core-shell interface, during synthesis, can result in the alloying of the nanoparticle. And, if this alloying occurs on a time scale consistent with experimental observations.

Chapter 4 computationally explores experimental observations involving the transient response of metallic nanoparticles to the nearly instantaneous heating undergone when photons are absorbed during ultrafast laser excitation experiments. Because the time scale for heating is faster than a single period of the breathing mode for spherical nanoparticles, hot-electron pressure and lattice heating contribute to thermal excitation of the lattice. Both mechanism are rapid enough to coherently excite the breathing mode ofthe spherical particles. Molecular Dynamics simulations are used to replicate the laser-excitation event allowing the nanoparticle dynamics to be probed after excitation.

It was observed during the studies of metallic nanoparticles dissused on in Chapter 4 that the time scale for the cooling of these particle is very short (on the order of tens of picoseconds). Since these experiments are carried out in condensed phase surroundings, the large surface area to volume ratio makes the heat transfer to the surrounding solvent a relatively rapid process. This leads to cooling rates commensurate with those experimentally observed for glass formation. Molecular Dynamics will be used to determine if it is plausible to construct a glassy nanobead from the metallic glass forming system discussed in Chapter 5. It may be feasible to use laser excitation to melt, alloy and quench copper-silver nanoparticles in order to form a glassy system.