Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments

Pulsed
laser ablation in liquids (PLAL) is a promising technique
for the generation of colloidal alloy nanoparticles that are of high
demand in a broad range of fields, including catalysis, additive manufacturing,
and biomedicine. Many of the applications have stringent requirements
on the nanoparticle composition and size distributions, which can
only be met through innovations in the PLAL technique guided by a
clear understanding of the nanoparticle formation mechanisms. In this
work, we undertake a combined computational and experimental study
of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu
and Cu/Ag bilayer thin films. Experimental probing of the composition
of individual nanoparticles and predictions from large-scale atomistic
simulations provide consistent evidence of limited mixing between
the two components from bilayer films by PLAL. The simulated and experimental
distributions of nanoparticle compositions exhibit an enhanced abundance
of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed
population of well-mixed alloy nanoparticles. The surprising observation
that the nanoscale phase separation of the two components in the bilayer
films manifests itself in the sharp departure from the complete quantitative
mixing in the colloidal nanoparticles is explained by the complex
dynamic interaction between the ablation plume and liquid environment
revealed in the simulations of the initial stage of the ablation process.
The simulations predict that rapid deceleration of the ablation plume
by the liquid environment results in the formation of a transient
hot and dense metal region at the front of the plume, which hampers
the mixing of the two components and, at the same time, contributes
to the stratification of the plume in the emerging cavitation bubble.
As a result, nanoparticles of different sizes and compositions are
produced in different parts of the emerging cavitation bubble during
the first nanoseconds of the ablation process. Notably, the diameters
of the largest nanoparticles generated in the simulations of the initial
stage of the ablation process are more than twice larger than the
thickness of the original bilayer films. This observation provides
a plausible scenario for the formation of large nanoparticles observed
in the experiments. The conclusion on limited elemental mixing in
the nanoparticles is validated in simulations of bilayers with different
spatial order of Cu and Ag layers, even though the two systems exhibit
some notable quantitative differences mainly related to the different
strength of electron–phonon coupling in Cu and Ag. Overall,
the results of this study provide new insights into the formation
mechanism of bimetallic nanoparticles in ultrashort PLAL from thin
bilayer targets and suggest that the formation of alloy nanoparticles
from immiscible elements may be hampered for targets featuring distinctive
elemental segregation.