Abstract
The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology has been employed in a broad range of practical applications. In this paper, we report the results of large-scale molecular dynamics simulations aimed at revealing the key processes that control the surface morphology and nanoparticle size distributions by pulsed laser ablation in liquids. The simulations of bulk Ag targets irradiated in water are performed with an advanced computational model combining a coarse-grained representation of liquid environment and an atomistic description of laser interaction with metal targets. For the irradiation conditions that correspond to the spallation regime in vacuum, the simulations predict that the water environment can prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. The simulations performed at higher laser fluences that correspond to the phase explosion regime in vacuum reveal the accumulation of the ablation plume at the interface with the water environment and the formation of a hot metal layer. The water in contact with the metal layer is brought to the supercritical state and provides an environment suitable for nucleation and growth of small metal nanoparticles from metal atoms emitted from the hot metal layer. The metal layer itself has limited stability and can readily disintegrate into large (tens of nanometers) nanoparticles. The layer disintegration is facilitated by the Rayleigh–Taylor instability of the interface between the higher density metal layer decelerated by the pressure from the lighter supercritical water. The nanoparticles emerging from the layer disintegration are rapidly cooled and solidified due to the interaction with water environment, with a cooling rate of ∼2 × 1012 K/s observed in the simulations. The computational prediction of two distinct mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes provides a plausible explanation for the experimental observations of bimodal nanoparticle size distributions in laser ablation in liquids. The ultrahigh cooling and solidification rates suggest the possibility for generation of nanoparticles featuring metastable phases and highly nonequilibrium structures.
Highlights
Short pulse laser processing and ablation of metal targets in liquids are gaining increasing attention due to the demonstrated ability of the liquid environment to strongly affect the morphology of the laser-treated surfaces[1−11] and to enable production of clean colloidal solutions of nanoparticles.[12−17] Pulsed laser ablation in liquids (PLAL), in particular, has emerged as a promising technique featuring a number of advantages with respect to traditional chemical methods of nanoparticle generation.[18]
In the simulations reported in this paper, we extend the investigation of laser interactions with bulk metal targets to higher laser fluences, where spallation or phase explosion of a surface region of the irradiated target in vacuum takes place
The simulations discussed are performed for irradiation conditions that correspond to the regime of photomechanical spallation, when the dynamic relaxation of laser-induced stresses results in subsurface cavitation and ejection of molten layers or large droplets from the irradiated target.[79,119−121] In particular, for the pulse duration of τL = 100 fs, the absorbed fluence applied in the simulations, Fabs = 150 mJ/cm[2], is about 50% above the spallation threshold in vacuum.[75,80,81]
Summary
Short pulse laser processing and ablation of metal targets in liquids are gaining increasing attention due to the demonstrated ability of the liquid environment to strongly affect the morphology of the laser-treated surfaces[1−11] and to enable production of clean colloidal solutions of nanoparticles.[12−17] Pulsed laser ablation in liquids (PLAL), in particular, has emerged as a promising technique featuring a number of advantages with respect to traditional chemical methods of nanoparticle generation.[18]. In order to fully utilize the potential of pulsed laser irradiation in liquids for both surface nanostructuring and generation of nanoparticles with well-controlled structure, composition, and size distribution, one needs to improve the understanding of the laser-induced processes responsible for the generation of frozen surface features and colloidal nanoparticles Such an understanding can only emerge from simultaneous progress in time-resolved experimental probing, theoretical description, and computational modeling of laserinduced processes. The simulations reported in this paper are performed with a hybrid computational model combining a coarse-grained representation of liquid, a fully atomistic description of laser interaction with metal targets, and acoustic impedance matching boundary conditions designed to mimic the nonreflecting propagation of the laser-induced pressure waves through the boundaries of the computational domain. The energy carried away by the stress wave is calculated, so that the total energy conservation in the combined model could be monitored in the course of the simulation.[92]
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