Generation of Subsurface Voids, Incubation Effect, and Formation of Nanoparticles in Short Pulse Laser Interactions with Bulk Metal Targets in Liquid: Molecular Dynamics Study

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.