Classical molecular dynamics (MD) simulations were used to investigate how free surfaces, as well as supporting substrates, affect phase separation in a NiAg alloy. Bulk samples, droplets, and droplets deposited on a graphene substrate were investigated at temperatures that spanned regions of interest in the bulk NiAg phase diagram, i.e., miscible and immiscible liquid, liquid-crystal, and crystal-crystal regions. Using MD simulations to cool down a bulk sample from 3000 K to 800 K, it was found that phase separation below 2400 K takes place in agreement with the phase diagram. When free surface effects were introduced, phase separation was accompanied by a core-shell transformation: spherical droplets created from the bulk samples became core-shell nanoparticles with a shell made mostly of Ag atoms and a core made of Ni atoms. When such droplets were deposited on a graphene substrate, the phase separation was accompanied by Ni layering at the graphene interface and Ag at the vacuum interface. Thus, it should be possible to create NiAg core-shell and layer-like nanostructures by quenching liquid NiAg samples on tailored substrates. Furthermore, interesting bimetallic nanoparticle morphologies might be tuned via control of the surface and interface energies and chemical instabilities of the system.
The
chemical composition and morphology of Au
x
Co1–x
thin films and
nanoparticles are controlled via a combination of cosputtering, pulsed
laser-induced dewetting (PLiD), and annealing, leading to tunable
magnetic and optical properties. Regardless of chemical composition,
the as-deposited thin films and as-PLiD nanoparticles are found to
possess a face-centered cubic (FCC) Au
x
Co1–x
solid-solution crystal structure.
Annealing results in large phase-separated grains of Au and Co in
both the thin films and nanostructures for all chemical compositions.
The magnetic and optical properties are characterized via vibrating
sample magnetometry (VSM), ellipsometry, optical transmission spectroscopy,
and electron energy loss spectroscopy (EELS). Despite the exceptionally
high magnetic anisotropy inherent to Co, the presence of sufficient
Au (72 atom %) in the Au
x
Co1–x
solid solution results in superparamagnetic thin
films. Among the as-PLiD nanoparticle samples, an increased Co composition
leads to a departure from traditional ferromagnetism in favor of wasp-waisted
hysteresis caused by magnetic vortices. Phase separation resulting
from annealing leads to ferromagnetism for all compositions in both
the thin films and nanoparticles. The optical properties of Au
x
Co1–x
nanostructures
are also largely influenced by the chemical morphology, where the
Au
x
Co1–x
intermixed solid solution has significantly damped plasmonic performance
relative to pure Au and comparable to pure Co. Phase separation greatly
enhances the quality factor, optical absorption, and electron energy
loss spectroscopy (EELS) signatures. The enhancement of the localized
surface plasmon resonances (LSPRs) scales with the reduction in Co
composition, despite EELS evidence that excitation of the Co portions
of a nanoparticle can provide a similar, and in some instances enhanced,
LSPR resonance compared to Au. This behavior, however, is seemingly
limited to the LSPR dipole mode, while higher-order modes are greatly
damped by a Co aloof position. This observed magneto-plasmonic functionality
and tunability could be applicable in biomedicine, namely, cancer
therapeutics.
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