Nanocrystals arrange themselves to form new lattices

Tiny structure that conducts electricity anisotropically offers foundation for new electronic components

Electronic components such as light-emitting diodes or solar cells can never be too minute. The smaller they are, the less power they consume and the wider the range of possible applications. In order to explore smaller and smaller worlds, scientists are constantly on the lookout for new materials with interesting properties. A research team from the University of Tübingen, working with colleagues at DESY and from Russia, has now made such a discovery.

Three-dimensional lattice of nanocrystals and semiconducting molecules. The precise arrangement of the nanocrystals allows current in the form of electrons (e-) to flow in certain directions. Illustration: University of Tübingen, Andre Maier.The scientists attached semiconducting organic molecules to inorganic nanocrystals to form ordered, three-dimensional lattices that have a uniform superstructure and are electrical conductors. “For the first time ever, we were able to determine a correlation between the conductivity and the direction of electrical transport in such lattices made up of nanocrystals,” said Marcus Scheele from the University of Tübingen, one of the team’s two leaders, adding that this is hugely significant in terms of their use in electronic components.

Read more on the DESY website

Image: Three-dimensional lattice of nanocrystals and semiconduction molecules. The prcise arrangement of the nanocrystals allows current in the form of electrons (e-) to flow in certain directions. Illustration: University of Tübingen, Andre Maier.

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