Shape-Directed Self-Assembly of Nanodumbbells into Superstructure Polymorphs

Self-assembly of colloidal nanoparticles into ordered superstructures provides a promising route to create novel/enhanced functional materials. Much progress has been made in self-assembly of anisotropic nanoparticles, but the complexity and tunability of superstructures remain restricted by their available geometries. Here we report the controlled packing of nanodumbbells (NDs) with two spherical lobes connected by one rod-like middle bar into varied superlattice polymorphs. When assembled into two-dimensional (2D) monolayer assemblies, such NDs with specific shape parameters could form orientationally ordered degenerate crystal with a 6-fold symmetry, in which these NDs possess no translational order but three allowed orientations with a rotational symmetry of 120 degrees. Detailed analyses identify the distinct roles of subunits in ND assembly: the spherical lobes direct NDs to closely assemble together into a hexagonal pattern, and the rodlike connection between the lobes endows NDs with this specific orientational order. Such intralayer assembly features are well maintained in the two-layer superstructures of NDs, however, the interlayer stackings could be adjusted to produce stable bilayer superstructures and a series of metastable moire patterns. Moreover, in addition to horizontal alignment, these NDs could gradually stand up to form tilted or even vertical packings based on the delicate control over the liquidliquid interface and ND dimensions. This study provides novel insights into creating superstructures by controlling geometric features of nanoscale building blocks, and may spur their novel applications.