Complexity in nature, either in chlorophyll or in living organisms, is often the result of self-assembly and is considered particularly robust. Compact clusters of elementary particles can be shown to be of practical relevance and found in atomic nuclei, nanoparticles or viruses. Researchers from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have decoded the structure and process behind the formation of a class of such highly ordered clusters. Their findings have led to a better understanding of how structures are formed in clusters.
In physics, a cluster is defined as an independent material form on the transition zone between isolated atoms and more extensive solid objects or fluids. Magical numbers clusters can be traced back to the work of Eugene Wigner, Maria Göppert-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. & # 39; Until now, scientists have assumed that the effect is purely caused by the attraction between atoms, & # 39; Dr. Nicolas Vogel, professor of particle synthesis. Our research now proves that particles that do not attract each other also form structures such as these. Our publication contributes to a better understanding of how structures in clusters in general are formed. & # 39;
The research is based on an interdisciplinary collaboration: Prof. dr. Nicolas Vogel, researcher at the chair of particle technology and prof. Dr. Michael Engel, researcher at the multi-level simulation chair – both from the department of chemical and biological sciences Engineering – worked closely with the materials scientist prof. Erdmann Spiecker of the Materials Science chair (research into micro and nanostructures), in which their expertise from the various fields is combined. Vogel was responsible for the synthesis, Spiecker for structure analysis and Engel for modeling clusters of colloidal polymer balls. The term colloidal is derived from the ancient Greek word for glue and refers to particles or droplets that are finely divided into a dispersion medium, either a solid object, a gas or a liquid. "Our three approaches are closely linked in this project", underlines Prof. Engel, "they complement each other and allow us to gain a deep understanding of the fundamental processes behind the process." formation of structures for the first time. & # 39;
Structures collect themselves
The first step for the researchers in a process involving several steps was to assemble very small colloidal clusters, not more than a tenth of the diameter of a single hair. First of all, water evaporates from an emulsion droplet and the polymer balls are pressed against each other. Over time, they collect ever smoother spherical clusters and begin to crystallize. It is remarkable how a few thousand individual particles independently find their ideal position in a precise and very symmetrical structure in which all particles are placed in predictable positions, "explains Prof. dr. Bird out.
The researchers discovered more than 25 different colloidal clusters with magical numbers of different shapes and sizes and were able to define four different cluster morphologies: where the evaporation was the fastest, bent clusters were formed because the drip interface moved faster than the colloidal particles could consolidate. When the evaporation rate was reduced, the clusters were predominantly spherical. Spherical clusters have a uniformly 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 degree of symmetry and have numerous two, three or five fold symmetry axes.
The use of high-resolution microscopy to show the surface of the cluster provides insufficient evidence of these symmetries. Even if the surface of a cluster seems very ordered, it is no guarantee that the particles in the cluster are arranged in the right way. To verify this, the researchers used electro tomography, available at the Erlangen Center for Nanoanalysis and Electron Microscopy (CENEM). Individual clusters are bombarded with high-energy electrons from all directions and the images are recorded. From more than 100 projections, researchers were able to reconstruct the three-dimensional structure of the clusters and thus the pattern of the particles in the clusters in a method reminiscent of computed tomography as used in medicine.
In the next step, the researchers performed simulations and extremely accurate numerical calculations. The analyzes have shown that clusters consisting of numbers of particles that correspond to a magic number are indeed more stable, as predicted on the basis of the theory. It is known that the observed icosahedral symmetry can be found in viruses and ultra-small metal clusters, but it has never been directly investigated. With these results, for the first time, a detailed and systematic insight into how such magic number clusters are formed in the model system under study is possible, allowing conclusions to be drawn for other natural systems where clusters tend to be formed.
University of Erlangen-Nuremberg. .