Institut Laue-Langevin

Colloidal Quasi-Crystals discovered

Micelles, as many other colloids, can self-assemble in aqueous solution to form ordered periodic structures. So far these structures exhibited classical crystallographic symmetries. In recent experiments at D11 with its new detector, and for the first time, colloidal water-based quasi-crystals with 12-fold diffraction symmetry have been observed. Results have just been published in PNAS. Due to their structural characteristics, these quasi-crystal systems could probably be used in the development of innovative devices in photonics.

Quasi-crystals are characterised by a very unusual alignment of atoms. Their existence was first proven in 1984 by a research team fronted by the American physicist Dan Shechtman. Afterwards, quasi-crystals were considered a laboratory curiosity for a long period, until photonics researchers were alerted to their unusual structural characteristics.
In normal crystals, atoms form ordered periodical structures; i.e. they arrange themselves into a gap-free integral overall structure, whereby one single symmetrical pattern is regularly repeated. For geometrical reasons, only 1-, 2-, 3-, 4- and 6-fold symmetries are possible. This figure indicates how frequently a structure may be rotated between 0 and 360 degrees, until it is congruent with itself.
Quasi-crystals behave differently. They contain ordered aperiodic structures, i.e. there are at least two different symmetrical patterns, which form a gap-free integral overall structure, despite not repeating themselves regularly. Under these circumstances, 8-, 10- or 12-fold symmetries may originate. A 2-d representation of a 12-fold quasi-crystal structure is shown in Fig. 2

An international research group led by Professor Stephan Förster of the University of Bayreuth has recently discovered colloidal quasi-crystals for the first time. In contrast to the quasi-crystals previously documented, which can only be produced under special laboratory conditions, they are simply structured polymers that evolve through self-assembly. In recent rheo-SANS experiments at D11 at ILL, sheared block-copolymer systems crystallized and formed not only characteristic fcc-like structures but also micellar quasi-crystals with 12-fold symmetry. An example of the corresponding scattering pattern is shown in Fig. 1.

Due to their structural characteristics, these quasi-crystal systems could probably be used in the development of innovative devices in photonics.

In the “Proceedings of the National Academy of Sciences of the United States of America (PNAS)“, the participating scientists from Bayreuth, Zürich, Hamburg and Grenoble report this discovery.

                            Peter Lindner

 Fig. 1, left: D11 diffraction pattern of shear-induced quasi-crystalline colloidal structures,
obtained with the new high-resolution detector of D11 after the Millennium project completion  in 2009 

Fig. 2, right : 12-fold symmetry tiling

Publication:

Steffen Fischer, Alexander Exner, Kathrin Zielske, Jan Perlich, Sofia Deloudi, Walter Steurer, Peter Lindner, Stephan Förster
Colloidal quasicrystals with 12-fold and 18-fold diffraction symmetry,
in: Proceedings of the National Academy of Sciences of the United States of America PNAS published ahead of print January 11, 2011, DOI-Bookmark: 10.1073/pnas.1008695108

This publication has been highlighted in Nature in March 2011. (Nature vol 471, p 309).

In October 2011, the Nobel Prize committee quoted this publication in the justification for the Chemistry Prize (see scientific background page 4).

Contact for further information:
Professor Dr. Stephan Förster
Department of Physical Chemistry I University of Bayreuth