Complexity in nature, either in chlorophyll or in living organisms, often results from self-study and is considered to be particularly stable. Compact clusters of elemental particles can be practically significant, and they are found in atomic nuclei, nanoparticles, or viruses. Researchers from the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have deciphered the structure and process that has shielded the formation of such classificated clusters. Their findings have raised awareness of how structures are formed in clusters.
In physics, the cluster is defined as the independent material form in the transition zone between isolated atoms and wider solid objects or liquids. Magic numbers can be tracked by Eugene Wigner, Maria Göppert-Mayer and Hans Jensen, who used this theory in 1963 to explain the stability of atomic nuclei and in 1963 won the Nobel Prize for Physics. "Scientists have so far accepted that the effect is due to the pure attraction between atoms," says prof. Dr. Nicolas Vogel, Particle Synthesis Professor. Our studies now prove that particles that do not bind together also form structures such as these. Our publication promotes a better understanding of how structures in general form in clusters. "
The research is based on interdisciplinary cooperation: prof. Dr. Nicolas Vogel, researcher of the Department of Particle Technology and Researcher, Department of Multi-level Simulation, prof. Dr. Michael Engel – both from the Department of Chemistry and Biology Engineering – worked closely with the Professor of Material Science Prof. Dr. Erdmann Spiecker from the Department of Materials Science (micro and nanoscale research) combining their knowledge from a variety of fields. Vogel was responsible for synthesis, Spiecker structure analysis, and Engel modeling clusters from colloidal polymer bombs. The colloidal term is derived from an ancient Greek word for glue and refers to particles or droplets that are finely dispersed in a dispersion medium, either as a solid object, gas or liquid. "Our three approaches to this project are particularly closely linked," emphasizes Professor Engels, "they complement each other and allow us to gain a deep understanding of the essential processes for building structures for the first time."
Build assemblies yourself
The first step for researchers in a process involving several degrees was to synthesize minutes of colloidal clusters, which in total is not more than a tenth of one hair diameter. "First, the water evaporates from the emulsion droplet and the polymer balls are pressed together. Over time, they assemble increasingly smooth sphere clusters and begin to crystallize. It is remarkable how several thousands of individual particles independently find their ideal position in a precise and highly symmetric structure in which all particles are positioned in predictable positions, "explains prof. Vogel
The researchers discovered more than 25 different magic numbers of different shapes and sizes of colloidal clusters and could identify four different cluster morphologies: where evaporation was the fastest, necklaces formed, as the droplet interface shifted faster than colloidal particles could be consolidated. When the evaporation rate was lowered, the clusters were mostly spherical. Spherical sets have a uniformly convex surface with a weak crystal structure. As the evaporation rate continued to decrease, a cluster with icosahedral symmetry is formed. These clusters have a particularly high level of symmetry and have several two, three or five times the symmetry axes.
Using high-resolution microscopy to show the surface of the cluster does not provide sufficient evidence of this symmetry. Even if the surface of the cluster appears very well, that does not mean that the particles inside the group are arranged as intended. To test this, researchers used electron tomography, available at the Erlangen Center for Nanosciences and Electron Microscopy (CENEM). Individual sets are bombarded with high energy electrons from all directions and recorded images. From more than 100 projections, researchers were able to reconstruct the three-dimensional structure of clusters and, hence, particle model pools in a method resembling a computer tomography of medicine.
In the next step, the researchers conducted simulations and very precise numerical calculations. The analysis showed that the cluster, which consists of a particle number that corresponds to the number of magicians, is indeed much more stable than predicted on the basis of theory. It is well-known that the observed icosahedral symmetry can be found in viruses and especially in small metal clusters, but it has never been explored directly. Now with these results, it is possible for the first time to comprehend in detail and systematically how such sets of magic numbers are generated in the model system studied, allowing them to draw conclusions about other natural systems in which sets are formed.
University of Erlangen, Nuremberg. .