Deviatoric Stress-Driven Fusion of Nanoparticle Superlattices

We model the mechanical response
of alkanethiol-passivated gold
nanoparticle superlattice (supercrystal) at ambient and elevated pressures
using large-scale molecular dynamics simulation. Because of the important
roles of soft organic ligands in mechanical response, the supercrystals
exhibit entropic viscoelasticity during compression at ambient pressure.
Applying a hydrostatic pressure of several hundred megapascals on
the superlattice, combined with a critical deviatoric stress of the
same order along the [110] direction of the face-centered-cubic supercrystal,
can drive the room-temperature sintering (“fusion”)
of gold nanoparticles into ordered gold nanowire arrays. We discuss
the molecular-level mechanism of such phenomena and map out a nonequilibrium
stress-driven processing diagram, which reveals a region in stress
space where fusion of nanoparticles can occur, instead of other competing
plasticity or phase transformation processes in the supercrystal.
We further demonstrate that, for silver–gold (Ag–Au)
binary nanoparticle superlattices in sodium chloride-type superstructure,
stress-driven fusion along the [100] direction leads to the ordered
formation of Ag–Au multijunction nanowire arrays.