  The self-assembly of binary nanoparticle superlattices from colloidal
mixtures is a promising method for the fabrication of complex colloidal
co-crystal structures. However, binary mixtures often form amorphous or
metastable phases instead of the thermodynamically stable phase. Here we show
that in binary mixtures of differently sized spherical particles, an excess of
the smaller component can promote -- and, in some cases, may be necessary for
-- the self-assembly of a binary co-crystal. Using computer simulations, we
identify two mechanisms responsible for this phenomenon. First, excess small
particles act like plasticizers and enable systems to reach a greater
supersaturation before kinetic arrest occurs. Second, they can disfavor
competing structures that may interfere with the growth of the target
structure. We find the phase behavior of simulated mixtures of hard spheres
closely matches published experimental results. We demonstrate the generality
of our findings for mixtures of particles of arbitrary shape by presenting a
binary mixture of hard shapes that only self-assembles with an excess of the
smaller component. mixtures of particles of arbitrary shape by presenting a
binary mixture of hard shapes that only self-assembles with an excess of the
smaller component.
