  While evaporating solvent is a widely used technique to assemble nano-sized
objects into desired superstructures, there has been limited work on how the
assembled structures are affected by the physical aspects of the process. We
present large scale molecular dynamics simulations of the evaporation-induced
assembly of nanoparticles suspended in a liquid that evaporates in a controlled
fashion. The quality of the nanoparticle crystal formed just below the
liquid/vapor interface is found to be better at relatively slower evaporation
rates, as less defects and grain boundaries appear. This trend is understood as
the result of the competition between the accumulation and diffusion times of
nanoparticles at the liquid/vapor interface. When the former is smaller,
nanoparticles are deposited so fast at the interface that they do not have
sufficient time to arrange through diffusion, which leads to the prevalence of
defects and grain boundaries. Our results have important implications in
understanding assembly of nanoparticles and colloids in non-equilibrium liquid
environments.
