Exotic crystal double discovered

Colloidal quasicrystals are formed by self-organization

Diffraction pattern of a colloidal quasicrystal with 12-fold symmetry; including the associated tiling pattern. - B: diffraction pattern of a colloidal quasicrystal with 18-fold symmetry; including the associated tiling pattern. - The tiling pattern always represents the complete structure. © Chair of Physical Chemistry I, University of Bayreuth
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An international research team has discovered so-called colloidal quasicrystals for the first time. In contrast to the previously known quasicrystals, which can only be produced under special laboratory conditions, these are simply structured polymers that are formed by self-assembly.

Due to their structural properties, they are expected to be used in the development of novel components in photonics, reports the journal "Proceedings of the National Academy of Sciences" (PNAS).

Unusual symmetrical structures made visible

Quasicrystals are characterized by a very unusual arrangement of the atoms. In normal crystals, the atoms form ordered periodic structures - that is, they combine to form a gapless overall structure in which a single symmetrical pattern regularly repeats.

For geometrical reasons, only 1-, 2-, 3-, 4-, and 6-fold symmetries are possible. This number indicates how often a structure can be rotated in angles between 0 and 360 degrees so that it coincides with itself.

Ordered aperiodic structures

The situation is different with quasicrystals. Here are ordered aperiodic structures, that is, there are at least two different symmetrical patterns that do not repeat regularly, but still form a complete forest structure. Under this condition, 8, 10, or 12-fold symmetries may occur. display

These structural differences between crystals and quasicrystals can be visualized in diffraction experiments with electromagnetic waves. This results in diffraction patterns that indicate how crystals and quasicrystals are built up. The recognizable symmetrical structures are referred to in research as diffraction symmetries.

Colloidal quasicrystals through self-assembly

The newly discovered colloidal quasicrystals are hydrogels, which are polymers that contain water but are themselves insoluble in water. They have a relatively simple structure and, according to the researchers, come about through the fact that several similar "building blocks" come together through self-organization.

These building blocks are polymeric micelles: small spherical structures with diameters between five and 100 nanometers, which can be produced on a larger scale without laboratory outlay. Therefore colloidal quasicrystals are easily accessible to many scientists and also to the industry.

18-fold symmetry observed for the first time

For some time now, the team of Professor Stephan F rster at the University of Bayreuth has been investigating polymer micelles, which can assemble into lattice structures on length scales of up to 100 nanometers. In joint work at the Institut Laue-Langevin in Grenoble and at the DESY in Hamburg, the researchers recently discovered that such processes of self-assembly may result in quasicrystalline lattice structures. In diffraction experiments, not only was a 12-fold symmetry observed, but for the first time ever an 18-fold symmetry.

No practical "glass pearl games"

According to the scientists, such experiments are by no means impractical "glass bead games" of basic research. Because photonics, a discipline of physics that aims at the development of optical technologies for the transfer and storage of information, is interested in high-grade diffraction symmetries in colloidal quasicrystals.

It has been found in recent years that structures with high diffraction symmetries have the property of transmitting light rays only in certain directions. They are a particularly suitable medium when it comes to passing light rays of a certain wavelength in predefined directions. As a result, structures with high diffraction symmetries are highly interesting for the fabrication of photonic devices.

Building materials for photonics?

So are the newly discovered hydrogels with their high diffraction symmetries suitable as building materials for photonics? According to the researchers, one more hurdle must be overcome: Photonics requires structural features of several hundred nanometers, while colloidal quasicrystals do not extend beyond 100 nanometers.

The scientists in Bayreuth, Hamburg and Grenoble are therefore currently working hard to assemble polymer micelles into quasicrystalline bulk structures that can be used in photonic devices. "I am confident that these endeavors will soon be successful, " explains Förster.

Quasicrystals no longer a laboratory curiosity

Colloidal quasicrystals are therefore likely to be far better suited for photonic applications than the approximately 100 quasicrystalline compounds known to date. These are almost exclusively metal alloys, which can only be produced in small quantities and under special laboratory conditions. In addition, these quasicrystalline structures move on a scale of between 0.1 and one nanometer and are, therefore, far too tiny for practical use in photonics.

In order to produce quasicrystalline structures for photonics, it has been necessary to date to use very complicated electron-lithographic methods. The fact that quasicrystals exist at all, was in 1984 for the first time a research team to prove the US physicist Dan Shechtman. For a long time, quasicrystals were considered a laboratory curiosity until photonics became aware of their unusual structural properties.

(University of Bayreuth, 21.01.2011 - DLO)