Controllable self-assembly of atomically precise Au31Cu32 nanoclusters into superstructures

Assembly of nanoparticles (NPs) is of crucial importance for the construction of functional materials and devices. Although tremendous progress has been made in constructing various types of superlattices, it is still difficult to elucidate the assembly mechanism at the molecular/atomic level, because the surface ligands and interactions cannot be seen by electron microscopy. To tackle this, we adopt an atomically precise nanochemistry approach and demonstrate with oblate [Au31Cu32(SPhtBu)34(P(SC4H3)3)8]+ nanoclusters (NCs) which can self-assemble into both template-free spherical superstructures upon fast evaporation and single crystals via slow nonsolvent diffusion. The size of the assembled solid nanospheres (-300 nm) can be controlled by the amount of n-hexane (non-solvent) added to the Au31Cu32 solution. On the other hand, adding a second solvent of lower vapor pressure leads to hollow nanospheres (-380 nm) instead of solid ones. These self-assembly processes are reversible. By contrast, single crystals can be grown by slowly diffusing n-hexane into the Au31Cu32 solution, and subsequent X-ray analysis reveals the details of ligand interactions, that is, each oblate Au31Cu32 coordinates with the closest ones via two types of ligand interactions: (1) between the thiolates along the polar axis, and (2) between the phosphines in the equatorial plane. This indicates that anisotropic van der Waals forces can drive the formation of template-free nanospheres of Au31Cu32 NCs. The results from this work also sheds light on the superstructure formation for conventional NPs.