Nanoparticle superlattice with a C14 Frank-Kasper structure formed by highly monodisperse one-size gold nanoparticles in suspension.

Synthesizing nanoparticle superlattices (NPSLs) with different symmetries is of great interest due to their impact on the collective emergent properties and potential applications. While several parameters have been identified as determinants for forming different symmetries of NPSLs, the high core dispersity, softness, and ligand interpenetration were proposed to drive the formation of the C14 Frank-Kasper (C14) structure like MgZn-type. Here, we report that the C14 phase can be formed in highly monodisperse one-size spherical nanoparticles (NPs) by controlling the interplay among their softness and ligand grafting density. When the grafting density is high, two coexisting phases of C14/fcc and hcp/fcc are formed for high and low softness AuNPs, respectively. However, reducing the grafting density reveals a gradual transition from the coexisting phases to pure fcc phases. These observations suggest that the formation of the C14 phase may be attributed to the high deformability of the ligand shell of NPs at high softness and high grafting density conditions. Additionally, they suggest that NPs behave like hard spheres at low grafting density, preferring the formation of pure fcc phase.