Tobias Hanrath1, Kaifu Bian1, Josh J. Choi1,
Zhongwu Wang2, Detlef-M. Smilgies2
1Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY
2Cornell High Synchrotron Source, Cornell University, Ithaca, NY
A Compared to the immense progress made in synthetic control of size, shape and composition of individual colloidal nanocrystals, the structural control over their ordered assemblies is less well developed, but rapidly evolving. We summarize recent experiments at CHESS that provided new fundamental insights into the directed self-assembly of nanocrystal superlattices allotropes. Specifically, we found that identical nanocrystal building blocks can be assembled into oriented superstructures with predefined symmetries, including face-centered cubic (fcc), body-centered cubic (bcc), and a variety of body-centered tetragonal (bct) structures. Simultaneous small- and wide-angle X-ray scattering data from D-1 illustrate the coaxial alignment of the nearly spherical lead salt nanocrystals on their superlattice sites. Importantly, our in situ experiments show that the coherent nanocrystal superlattice symmetry distortion is driven by the orientational ordering of the constituent nanocrystals; this process is analogous to martensitic phase transitions in atomic crystals. The ability to direct the self-assembly into superlattices with predefined symmetries provides a fertile opportunity space for experiments elucidating fundamental structure-property relationships.
More recently, we investigated the structural stability of colloidal PbS nanocrystals superlattice allotropes with fcc or bcc symmetry under pressure within the diamond anvil cell at B2. Small angle-X-ray scattering SAXS analysis showed that the nanocrystal packing density is higher in the bcc than in the fcc SL, which we attribute to the cuboctahedra shape of the constituent nanocrystals. Using the high-pressure rock salt/orthorhombic phase transition as a stability indicator, we discovered that the transition pressure for nanocrystals in a bcc superlattice occurs at 8.5 GPa; which is 1.5 GPa higher than the transition pressure (7.0 GPa) observed for a fcc superlattice. The higher structural stability in the bcc superlattice is attributed primarily to the effective absorption of loading force in specific SL symmetry and to a lesser extent to the surface energy of the nanocrystals. The experimental results provide new insights into the fundamental relationship between the symmetry of the self-assembled SL and the structural stability of the constituent NCs.