Structural control in clusters of soft colloidal particles

The ability to control the structure of colloidal clusters enabled the first robust assembly of colloidal diamond, targeted to produce photonic devices. Many nanocluster properties are highly tunable with cluster shape, but only a limited set of structures are typically stable and synthesizing ideal particles to target a particular geometry is difficult to do reliably. Colloidal clusters are often modeled as assemblies of perfectly hard, spherical particles; however, particles with long ligands, surface charges, or made from deformable materials violate this assumption. Using coarse-grained molecular dynamics simulations, we model the assembly of small clusters composed of soft, repulsive spheres. We find that hard and soft particles assemble clusters with differing geometries. Increasing particle softness favors higher-symmetry clusters, forming a geometrical series that is distinct from previously described hard-sphere-like or second-moment-minimization clusters. By predicting the effects of particle properties on cluster structure, our findings will guide targeted synthesis and growth of novel and tunable colloidal clusters.