The assembly of colloidal nanocrystal building blocks into ordered superlattices presents many scientifically interesting and technologically important research challenges to create programmable matter from “crystals-of-crystals”.
The formation of superlattices is a fascinating mesoscale phenomenon governed by the interplay of a range of thermodynamic and kinetic factors. Long-time collaborators Detlef Smilgies, CHESS, and Tobias Hanrath, Chemical and Biomolecular Engineering, have recently summarized the role of time-resolved X-ray scattering techniques in combination with in-situ sample environments to gain unique insights into the relevant processes. Their EPL Focus Article was recently published in a special issue on superlattice formation, edited by Marie-Paule Pileni .
A variety of factors influence the assembly. First of all there are the nanoparticles themselves: their size variation, their shape, and their ligand coverage influence which superlattice symmetries are formed. A spectacular example has been the self-assembly of lead sulfide and lead selenide nanocrystals: These spheroidal nanocrystals have well defined facets formed by (100) and (111) crystallographic planes of the inorganic cores which form cuboctrahedra. Initially these nanocrystals form the expected FCC superlattice, but as solvent further evaporates and particles move closer together, the lattice symmetry changes to body-centered tetragonal and finally to BCC [2,3]. This transition is accompanied by increasing orientational ordering of particles relative to each other. The reason for this peculiar behavior seems to lie in the ligand-ligand and solvent-ligand interactions as superlattices dry. Due to the facetting of the particles the ligand density around the particle is inhomogeneous; in particular at edges and corners there is sterically not enough space to anchor ligands at the same density as on the facets. As particles move closer to each other this anisotropy becomes more pronounced and leads to orientational ordering and superlattice symmetry change.
Image Caption: The “periodic table” of nanocrystal superlattices. Nanocrystals can be made from most elements in the periodic table. In addition, their size, shape and dimensionality is controlled by the synthesis. Finally superlattices with different symmetries can be made by exploiting shape and dimensionality as well as processing parameters.
Credit:Tobias Hanrath, Cornell