Analyzing the structural disorder of nanocrystals

Research applies unprecedented technique in Brazil for the investigation of crystalline nanoparticles

The development of faster and more efficient electronic devices, better catalysts for the chemical industry, alternative energy sources, and so many other technologies depends increasingly on the in-depth understanding of the behavior of materials at the nanometer scale.
The properties of particles on this scale may be completely different from the properties of the same material in its macroscopic version. In addition, nanoparticles of different sizes and shapes can have completely different characteristics, even though they are formed by the same material.
The possibility of regulating the optical and electrical properties of nanoparticles by controlling their composition, shapes and sizes opens the door to an immense variety of applications. In this context, nanocrystals – nanometric particles that have a crystalline structure – have attracted great interest.
A crystal is a type of solid whose atoms or molecules are arranged in a well-defined three-dimensional pattern that repeats itself in space on a regular basis. The optical and electrical properties of crystalline materials depend not only on the atoms or molecules that constitute them but also on the way they are distributed. Therefore, defects or impurities present during crystal formation cause a disorder in the crystal structure. The consequent modification in the electronic structure of the crystal can lead to changes in its properties.

>Read more on the Brazilian Light Source Laboratory website
Image: PDF analysis obtained from electron diffraction data for nanocrystals before (ZrNC-Benz) and after ligand exchange (ZrNC-OLA).
Credit: Reprinted with permission from J. Phys. Chem. Lett. 2019, 10, 7, 1471-1476. Copyright 2019 American Chemical Society.

Watching nanocrystals in action

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 [1].

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.

>Read more on the CHESS website

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