  Ordering nanoparticles into a desired super-structure is often crucial for
their technological applications. We use molecular dynamics simulations to
study the assembly of nanoparticles in a polymer brush randomly grafted to a
planar surface as the solvent evaporates. Initially, the nanoparticles are
dispersed in a solvent that wets the polymer brush. After the solvent
evaporates, the nanoparticles are either inside the brush or adsorbed at the
surface of the brush, depending on the strength of the nanoparticle-polymer
interaction. For strong nanoparticle-polymer interactions, a 2-dimensional
ordered array is only formed when the brush density is finely tuned to
accommodate a single layer of nanoparticles. When the brush density is higher
or lower than this optimal value, the distribution of nanoparticles shows large
fluctuations in space and the packing order diminishes. For weak
nanoparticle-polymer interactions, the nanoparticles order into a hexagonal
array on top of the polymer brush as long as the grafting density is high
enough to yield a dense brush. An interesting healing effect is observed for a
low-grafting-density polymer brush that can become more uniform in the presence
of weakly adsorbed nanoparticles.
