Complexity in nature, whether in chlorophyll or in living organisms, often results from self-assembly and is considered particularly strong. Compact clusters of elementary particles can have practical significance, and they are found in atomic nuclei, in our son or in viruses. Researchers at the University of Friedrich-Alexander-Erlangen-Nuremberg (FAU) directed the structure and process behind the formation of one set of such ordered clusters. The findings increased the way clusters are formed.
In physics, a cluster is defined as an independent material form in the transition zone between isolated atoms and solid objects or larger liquids. The number of magic clusters can be attributed to the work of Eugene Wigner, Maria Gopert-Mayer and Hans Jensen, who used this theory to explain the stability of atomic nuclei and won the Nobel Prize in Physics for their research in 1963. "So far, scientists have assumed that the effect is caused solely by the attraction between the atoms," said Professor Nicholas Fogel, a professor of particle synthesis. Our research now demonstrates that nanoparticles which do not attract each other also create structures such as these. Our publication contributes to a greater understanding of how clusters are formed in general. "
The study is based on interdisciplinary cooperation: Prof. Nicolas Fogel, a researcher in the Chair of Particle Technology and Prof. Dr. Michael Engel, a multi-dimensional simulation researcher at the Department of Chemistry and Biological Engineering – worked closely with the expert on materials science professor Dr. Erdmann Spiecker from the Chair of Materials Science (Research into Microstructure Nano), pooling their expertise in various fields. Vogel was responsible for the Spiecker-Synthesis analysis and Engel structure for clustering models from colloidal polymer spheres. The colloidal term derived from the ancient Greek word for glue refers to particles or droplets which are distributed well in dispersion medium, or solid object, gas, or liquid. "Our three approaches are closely related to this project," emphasizes Prof. Engel. "They complement each other and allow us to understand the underlying processes behind building structures for the first time."
Buildings assemble themselves
The first step for researchers in the process covering a number of steps was to synthesize tiny colloid clusters, no more than one-tenth the diameter of one hair in total. "It's amazing how a few thousand individual particles independently find their ideal position in a very symmetrical structure where all the particles Are located in predicted positions, "explains Prof. Fogel.
The researchers found more than 25 different colloid clusters of different shapes and sizes, and managed to define four different cluster morphologists: where the evaporation was the fastest, clusters of buckles were created when the droplet interface moved faster than the colloidal particles. If the evaporation rate was lowered, the clusters were mostly spherical. Spherical clusters have a uniform curved surface with only a weak pattern of crystals. Clusters with icosahedral symmetry were formed as the evaporation rate decreased further. These clusters have a particularly high level of symmetry and have a large number of two, three or five fold axial symmetry.
Using a high-resolution microscope to show the surface of the cluster does not provide sufficient proof of these symmetries. Even if the surface of a cluster looks very neat, it does not guarantee that the particles within the cluster are arranged as expected. To verify this, the researchers used electron tomography, available at the Erlangen Center for Nanoscale Electron Microscopy (CENEM). Individual clusters are bombarded with powerful electrons from all directions and the images are recorded. From more than 100 projections, the researchers have succeeded in reconstructing the three-dimensional structure of the clusters, thus molding the particles within the clusters by a method reminiscent of computer tomography, as it is used in medicine.
Next, the researchers conducted simulations and precise numerical calculations. The analyzes have shown that clusters consisting of a number of particles corresponding to the magic number are indeed more stable, as predicted on the basis of the theory. It is well known that the observed icosahedral symmetry can be found in viruses and small metal clusters especially, but it has never been directly studied. Now, with these results, a detailed and systematic understanding of how many magic number clusters have been created in the model system has been explored possible for the first time, allowing conclusions to be drawn for other natural systems where clusters tend to form.
University of Erlangen-Nuremberg. .