(475j) Self-Limiting Assembly of Hollow Quasispherical Nanostructures

The synthesis of anisotropic nanoparticles has matured tremendously in the past 20 years, resulting in the production of particles with low shape- and size-polydispersity that span a rich design space. As a result, colloidal crystallization has produced a wide array of nanoparticle superlattices with varying degrees of structural complexity. Still, there is ample room for further progress. For example, geometric frustration, which arises when local order is incompatible with long-rage order, is relatively underexplored. In condensed matter physics, geometric frustration gives rise to complex structures, suggesting it may also offer a form of structure control in self-assembling nanoparticle systems.

Here, we computationally study the self-assembly of right triangle bipyramids (rTBPs), whose shape introduces geometric frustration that we hypothesized would hinder typical colloidal crystallization. In dilute, homogeneous states, we find that rTBPs assemble into discrete, highly ordered, quasispherical nanoclusters with 130 particles in each cluster. Intriguingly, these clusters have a hollow core, suggesting potential applications in nanoscale cargo delivery. We identify a stepwise self-assembly pathway whereby distinct, low-energy subclusters hierarchically assemble into the 130-particle cluster. In the presence of an attractive planar surface, dilute systems of rTBPs form large populations of subclusters arranged in islands, and the distribution of specific subclusters present in each island depends on the number layers of particles that comprise the island. In more concentrated systems, rTBPs form the same 130-particle clusters observed in the homogeneous case. Additionally, in the more concentrated systems both with and without the planar substrate, we find dimers, trimers, and higher-order structures of 130-rTBP clusters. These findings highlight how one can leverage geometric frustration in the design of self-assembled nanostructures.