  The formation process of core-shell bimetallic nanoparticles synthesized by
sputtering onto a substrate is observed in real time using an originally
developed acoustic technique. The technique enables us to evaluate the
structural change of nanoparticles at room temperature without contacting the
nanoparticles or substrate. In the experiments, the sputtering of metal A
followed by metal B tended to form B-shell/A-core nanoparticles. However, in
Pd-Au alloy system, notable restructuring occurred during synthesis, resulting
in the formation of A-shell/B-core nanoparticles. The formation process is
analyzed using the molecular dynamics simulation, revealing that this
restructuring occurs on a short timescale, and high diffusivity of Au plays an
important role.
