Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (1), 102 - 104
Opto-electronic Components in Gel
Fixed colloids as matrices
for structured materials:
A new synthetic approach to photonic crystals?
Materials with very regular structures, whose size is on the order of a few hundred nanometers, are the subject of intensive research. These materials, which are not easy to produce, are known to the experts as mesostructures, from the Greek word meso = between. These structures thus belong in the range between macro- and atomic microstructures. Because the size range of mesostructures is on the order of the wavelengths of visible light, mesostructure materials have very special optical properties. In the form of photonic crystals they could play an important role in new laser diodes, or other optoelectronic components. They are also under consideration as catalysts.
German researchers working with Rolf Hempelmann have now developed a new method for the synthesis of three-dimensional mesostructures out of metals. They start with tiny spherical latex clumps. In aqueous solution these are very finely disperesed - colloidal. The advantage of these colloidal latex particles is that their size distribution is very narrow, meaning that they are all nearly the same size - about 200 nm on average. The surface of each particle is negatively charged, which makes the spheres repel each other. The clincher is this: once the concentration of the spheres exceeds a certain value, the repulsive forces cause them organize themselves into a regular pattern, a sort of fluid crystal lattice. This could make a perfect "template" for lining up metal atoms. However, the lattice is not stable. A small vibration or contaminant is enough to disrupt the order. Hempelmann and his co-workers have solved this problem by making a gel out of the liquid, in order to fix the particles in their positions. The "gelling agent" is acrylamide, which cross-links to form polyacrylamide, locking in the spheres of latex.
This gel is applied to an electrode, which is then dunked into a solution of silver nitrate. When a current is applied, silver is deposited at the electrode - between the latex spheres. This forms a regular silver mesostructure. "For applications in optics we would like to put semiconductors into the gel in place of the silver," explains Hempelmann.