Molecular dynamics simulation study of femtosecond laser ablation of silver thin films and bulk targets in water environment

Full text: Laser ablation of metal target in liquid environment is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms responsible for the nanoparticle formation and the key processes that control the nanoparticle size distributions are not yet fully understood. In this presentation, we report the results of the first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of liquid (parameterized for water), a fully atomistic description of laser interactions with metal targets, and advanced acoustic impedance matching boundary conditions is developed for the simulations. The model is implemented in a computationally-efficient parallel code, which is used to perform a series of large-scale simulations of laser ablation of thin silver films deposited on a silica substrate and a bulk silver target. In contrast to the laser ablation in vacuum, where the superheated Ag undergoes an explosive decomposition into liquid droplets and vapor, the phase decomposition in the liquid environment is partially suppressed and the hot metal vapor/clusters ejected from the irradiated target are localized in a low-density mixing region where the liquid is brought to the supercritical state. Due to the soft confinement provided by the water environment, a thin Ag molten layer forms between the region of high density Ag vapor and the supercritical water as shown in right frame of the Figure. The thin Ag molten layer acts as a source of Ag vapor for continuous growth of nanoparticles in the supercritical water. The main nanoparticle formation mechanism is the condensation of clusters from the Ag vapor, followed by coalescence and coarsening within the supercritical water region. The results of the simulations support the notion of the important role of the cavitation bubble in the process of nanoparticle formation, often suggested in interpretation of experimental observation. The simulations also predict that larger nanoparticles can be generated at high fluence, when active water motion incurs the instability of the floating molten metal layer. The distinct contributions of the vapor condensation and disintegration of a thin metal layer lead to the formation of a bimodal nanoparticle size distribution commonly observed in experiments performed at high laser fluences. (author)