Direct Synthesis of CuPd Icosahedra Supercrystals studied by in situ X-ray Scattering

Nanocrystal self-assembly into supercrystals provides a versatile platform for creating novel materials and devices with tailored properties. While common self-assembly strategies imply the use of purified nanoparticles after synthesis, conversion of chemical precursors directly into nanocrystals and then supercrystals in simple procedures has been rarely reported. Here we study the nucleation and growth of CuPd icosahedra and their consecutive assembly into extended closed-packed face-centered cubic (fcc) supercrystals. To this end, we simultaneously and in situ measure X-ray total scattering with pair-distribution function analysis (TS-PDF) and small-angle X-ray scattering (SAXS). We find that the supercrystals formation is preceded by an intermediate dense phase of nanocrystals displaying short-range order (SRO). We further show that the organization of oleic acid/oleylamine surfactants into lamellar structures likely drives the emergence of the SRO phase and later of the supercrystals by creating an excluded volume to particle diffusion. The supercrystal formation as well as their disassembly are triggered by temperature. Our study demonstrates that depletion effects can be crucial in the direct synthesis of supercrystals. We also provide a general approach to investigate novel preparation routes of supercrystals in situ and across several length scales via X-ray scattering.