Complexity in nature, whether in chlorophyll
or in living organisms, often results from self-assembly and is
considered particularly robust. Compact clusters of elemental particles
can be shown to be of practical relevance, and are found in atomic
nuclei, nano particles or viruses. Researchers at
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have decoded the
structure and the process behind the formation of one class of such
highly ordered clusters. Their findings have increased understanding of
how structures are formed in clusters.
In physics, a cluster is defined as an independent material form at
the transition area between isolated atoms and more extensive solid
objects or liquids. Magic number 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
for physics for their research in 1963. 'Until now, scientists have
assumed that the effect is caused purely as a result of the attraction
between atoms,' says Prof. Dr. Nicolas Vogel, Professor for Particle
Synthesis. Our research now proves that particles which don't attract
each other also form structures such as these. Our publication
contributes to a greater understanding of how structures are formed in
clusters in general.'
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 Chair of Multi-scale
Simulation -- both from the Department of Chemical and Biological
Engineering -- have worked closely together with the materials science
expert Prof. Dr. Erdmann Spiecker from the Chair of Materials Science
(research into micro and nanostructures), pooling their expertise from
the various areas. Vogel was responsible for synthesis, Spiecker for
structure analysis and Engel for modelling clusters from colloidal
polymer balls. The term colloidal is derived from the ancient Greek word
for glue and refers to particles or droplets which are finely
distributed in a dispersion medium, either a solid object, a gas, or a
liquid. 'Our three approaches are particularly 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
forming of structures for the first time.'
Structures assemble themselves
The first step for the researchers in a process which covers several
steps was to synthesise minute colloidal clusters, no larger than a
tenth of the diameter of a single hair in total. 'First of all, water
evaporates from an emulsion droplet and the polymer balls are pushed
together. Over time, they assemble increasingly smoother sphere-shaped
clusters and begin to crystallise. It is remarkable how several thousand
individual particles independently find their ideal position in a
precise and highly symmetrical structure in which all particles are
placed in predictable positions,' explains Prof. Vogel.
The researchers discovered more than 25 different magic number
colloidal clusters of various shapes and sizes and were able to define
four different cluster morphologies: where evaporation was fastest,
buckled clusters were formed, as the droplet interface moved faster than
the colloidal particles could consolidate. If the evaporation rate was
lowered, 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 rate of
evaporation decreased further. These clusters have a particularly high
degree of symmetry and have numerous two, three or five fold symmetry
axes.
Using high resolution microscopy to show the surface of the cluster
does not provide sufficient proof of these symmetries. Even if the
surface of a cluster appears highly ordered, that is no guarantee that
the particles inside the cluster are arranged as expected. To verify
this, the researchers used electron tomography, available at the
Erlangen Centre for Nanoanalysis and Electron Microscopy (CENEM).
Individual clusters are bombarded with highly energised electrons from
all directions and the images recorded. From more than 100 projections,
researchers were able to reconstruct the three dimensional structure of
the clusters and therefore the pattern of the particles within the
clusters in a method reminiscent of computer tomography as used in
medicine.
In the next step, the researchers conducted simulations and highly
accurate numerical calculations. The analyses proved that clusters
consisting of numbers of particles corresponding to a 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
ultra-small metal clusters, but it has never been investigated
directly. Now, with these results, a detailed and systematic
understanding of how such magic number clusters are formed in the
investigated model system is possible for the first time, allowing
conclusions to be drawn for other natural systems where clusters tend to
be formed.
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