Astonishing diversity: Semiconductor nanoparticles form numerous
structures
X-ray study reveals how lead sulphide particles self-organize in real
time

Date:
August 5, 2021
Source:
Deutsches Elektronen-Synchrotron DESY
Summary:
The structure adopted by lead sulphide nanoparticles
changes surprisingly often as they assemble to form ordered
superlattices. This is revealed by an experimental study that has
been conducted at DESY's X-ray source PETRA III.



FULL STORY ==========================================================================
The structure adopted by lead sulphide nanoparticles changes surprisingly
often as they assemble to form ordered superlattices. This is revealed
by an experimental study that has been conducted at DESY's X-ray source
PETRA III. A team led by the DESY scientists Irina Lokteva and Felix Lehmku"hler, from the Coherent X-ray Scattering group headed by Gerhard Gru"bel, has observed the self-organisation of these semiconductor nanoparticles in real time. The results have been published in the
journal Chemistry of Materials. The study helps to better understand
the self-assembly of nanoparticles, which can lead to significantly
different structures.


========================================================================== Among other things, lead sulphide nanoparticles are used in photovoltaic
cells, light-emitting diodes and other electronic devices. In the study,
the team investigated the way in which the particles self-organise to
form a highly ordered film. They did so by placing a drop of liquid
(25 millionths of a litre) containing the nanoparticles inside a small
cell and allowing the solvent to evaporate slowly over the course of
two hours. The scientists then used an X-ray beam at the P10 beamline
to observe in real time what structure the particles formed during
the assembly.

To their surprise, the structure adopted by the particles changed several
times during the process. "First we see the nanoparticles forming
a hexagonal symmetry, which leads to a nanoparticle solid having a
hexagonal lattice structure," Lokteva reports. "But then the superlattice suddenly changes, and displays a cubic symmetry. As it continues to dry,
the structure makes two more transitions, becoming a superlattice with tetragonal symmetry and finally one with a different cubic symmetry." This sequence has been never revealed before in such detail.

The team suggests that the hexagonal structure (hexagonal close-packed,
HCP) persists for as long as the surface of the particles is swollen by
the solvent.

Once the film dries a little bit, its internal structure changes to a
cubic symmetry (body-centred cubic, BCC). However, residues of the solvent still remain between the individual nanoparticles inside the film. As
this evaporates, the structure changes two more times (body-centred
tetragonal BCT and face-centred cubic FCC).

The final structure of the film depends on a number of different factors,
as Lokteva explains. They include the type of solvent and how quickly
it evaporates, size and concentration of the nanoparticles, but also
the nature of the so-called ligands that surround the particles and
their density. Scientists use the term ligand to describe certain
molecules that bind to the nanoparticle surface and prevent them from agglomeration. In the study, the team used oleic acid for this purpose;
its molecules cover the particles, much like the wax that prevents gummy
bears from sticking to each other in a bag. This is a well- established
process in nanotechnology.

"Our research indicates that the final structure of the superlattice also depends on whether the individual nanoparticles are surrounded by many
or few oleic acid molecules," reports Lokteva. "In an earlier study,
we obtained films with a BCC/BCT crystal structure when the ligand
density was high. Here, we specifically looked at nanoparticles with
a low ligand density, and this led to an FCC structure. So when using nanoparticles, the ligand density ought to be determined, which is not
a standard practice at the moment," explains the DESY scientist.

These observations are also important when it comes to other materials,
the team points out. "Lead sulphide is an interesting model system
that helps us to better understand the general mechanisms by which nanoparticles self-assemble," Lokteva explains. "Nature can provide nanostructures with various interesting properties via the phenomenon of self-assembly, and we now have the tools to look over nature's shoulder
as it constructs these structures." DESY is one of the world's leading particle accelerator centres and investigates the structure and function
of matter -- from the interaction of tiny elementary particles and the behaviour of novel nanomaterials and vital biomolecules to the great
mysteries of the universe. The particle accelerators and detectors that
DESY develops and builds at its locations in Hamburg and Zeuthen are
unique research tools. They generate the most intense X-ray radiation
in the world, accelerate particles to record energies and open up new
windows onto the universe. DESY is a member of the Helmholtz Association, Germany's largest scientific association, and receives its funding from
the German Federal Ministry of Education and Research (BMBF) (90 per cent)
and the German federal states of Hamburg and Brandenburg (10 per cent).

========================================================================== Story Source: Materials provided by
Deutsches_Elektronen-Synchrotron_DESY. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Irina Lokteva, Michael Dartsch, Francesco Dallari, Fabian
Westermeier,
Michael Walther, Gerhard Gru"bel, Felix Lehmku"hler. Real-Time
X-ray Scattering Discovers Rich Phase Behavior in PbS Nanocrystal
Superlattices during In Situ Assembly. Chemistry of Materials,
2021; DOI: 10.1021/ acs.chemmater.1c02159 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210805141152.htm

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