Metallic liquid crystals

A new class of materials formed by combining liquid crystals and metal clusters glow intensely red in the infra-red region of the electromagnetic spectrum when irradiated over a broad range of wavelengths. The materials, dubbed clustomesogens, could be used in analytical instrumentation and potentially in display technologies.

Liquid crystals are well known in display technologies from digital watches to flat panel televisions. As their name suggests, they are at once liquid and can flow, but their molecules can also be oriented into something akin to a crystal state, usually under the influence of an electric field.

A second class of materials of interest to the optoelectronics field is metal clusters. Clusters are aggregates of just a few atoms, and so their properties are not those of individual atoms nor of the bulk metal, but somewhere in between. Indeed, metal clusters show some rather unusual electronic, magnetic, and optical properties because of the presence of the particular types of bonds that form between metals when just a few are present.

Now, Yann Molard, of the University of Rennes, in France, and colleagues there and at the University of Bucharest have united the two classes in clustomesogens to create metal clusters that exist in a liquid-crystalline phase.

Liquid crystals containing bonds between metal atoms are rare and usually limited to compounds in which just two metal atoms are connected in each unit. Molard and colleagues have produced liquid crystals that contains octahedral clusters made of six molybdenum atoms. Eight bromide ions sit on the eight surfaces of the octahedron, six fluorides and an aromatic organic group, or ligand, is at each vertex of the octahedron. These aromatic ligands each have three long hydrocarbon chains also ending in a pair of aromatic rings.

Yann Molard
Yann Molard

Simple warming these materials initiates a process of self-organization in which the clusters stretch out to form long, narrow units arranged in what is known as a lamellar, plate-like, structure. The flat rings at the ends of the ligands of neighbouring layers are interleaved and the structure has liquid-crystalline properties.

“The association of mesomorphism with the peculiar properties of metallic clusters should lead to clustomesogens that offer great potential in the design of new electricity-to-light energy conversion systems, optically based sensors, and displays,” the team says.


Angew Chem Int Edn, 2010, 49, 3351-3355
Yann Molard homepage

Want optical chips with that?

Ever-smaller and ever-faster microelectronics devices with increased storage space, more communications and other functions, and much-reduced battery usage, are part of the incentive behind research into photonic crystals. These materials are bringing us closer to a technologically viable optical transistor that will form the building blocks of future optoelectronics that use photons instead of electrons to process information.

The optical properties of photonic crystals vary in a regular pattern on a scale of hundreds of nanometres. This physical structure means that light entering a photonic crystal can be controlled. For instance, a photonic crystal can transmit light of one particular wavelength, and block all others.

Rana Biswas

Rana Biswas

The simplest material of this kind has a layered structure, like a film of oil on water. Such one-dimensional structures are used as mirrors, non-reflective coatings, and paints whose colours change with the viewing angle. While nature does not exactly abound with photonic crystals, the natural gemstone, opal, is a photonic crystal, which is what gives it its unique shifting and shimmering colours. Synthetic photonic crystals have been on the science agenda since the nineteenth century, but it is only with the advent of modern fabrication techniques that designer 3D photonic crystals have become attainable.

Optical computers aside, there is a second thread woven into the fabric of photonic crystal research – telecommunications.

Now, researchers at the US Department of Energy’s Ames Laboratory have come up with what might be the perfect way to sort and distribute vast quantities of data through optical fibres. The new technology is based on a filter constructed from a three-dimensional photonic crystal and could allow multiple wavelength channels to be carried along the same stretch of optical fibre without loss and without error. The so-called add-drop filter could ultimately give us the all-optical transmission links that require no electronic components along the route.

There are up to 160 wavelength channels travelling through an optical fibre at the same time, explains Ames physicist Rana Biswas, That means a lot of dialogue is going on simultaneously. He adds that as data is carried along the fibre it is necessary to drop off individual wavelength channels at different points.

When the data being transported in multiple frequency channels over an optical fibre comes to a receiving station, you want to be able to pick off just one of those frequencies and send it to an individual end user, explains Biswas, That’s where these 3D photonic crystals come into play. The same filter technology will also allow optimal use of the fibre’s bandwidth.

The idea of add-drop filters was first conceived in the 1990s, but work focused on 2D photonic crystals until now. The Ames team created a 3D photonic crystal device that contains an entrance waveguide and an exit waveguide for channelling light, which means there is none of the light intensity loss seen with 2D photonics.

There is still at least one hurdle to jump before the 3D add-drop filter can be used in fibre optic communications and that is to scale down the device to the wavelengths of light used in Internet communications – 1.5 micrometres. That remains a big challenge confesses Biswas.

Further reading

Rana Biswas homepage

Feature on photonic crystals from ICT Results

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Waste bacteria build new catalysts

Bacteria could be the key to improving metal catalysts for the chemical industry, according to research in Germany. Scientists from the Forschungszentrum Rossendorf in Dresden have exploited the survival skills of bacteria that live in uranium mining waste to make tiny clusters of the precious metal palladium. These tiny bullets, just a few billionths of a millimetre across, are much better catalysts than normal palladium, which is used in speeding up chemical reactions and in a car’s catalytic converter

The bacterium, Bacillus sphaericus JG-A12 has a protective protein layer that allows it to survive in the extreme environment of a uranium mining waste pile. This protein layer comprises a grid of nanoscopic pockets of identical size that can trap toxic metal ions and prevent them from harming the bacterium by converting them into tiny clumps of insoluble metal.

A matrix of nanoclusters made from palladium (Credit: Pollmann et al/Forschungszentrum Rossendorf)

A matrix of nanoclusters made from palladium (Credit: Pollmann et al/Forschungszentrum Rossendorf)

Katrin Pollmann and her FZR colleagues reasoned that if they applied a dissolved metal of interest, such as palladium, to the protein layer the metal might form a tiny cluster of the metal in each pocket.

Within the pores of the protein layer, palladium ions are transformed into the noble metal palladium by hydrogen. The resulting nanoclusters of metallic palladium, composed of just 50 to 80 atoms, are regularly arranged on the surface layer. This combined metal-protein layer has some unusual physical and chemical properties. Because the metal stabilizes the protein and vice versa the protein layer does not break down at higher temperatures or even in acid. It is the enormous surface area to volume ratio that makes the nanoclusters interesting to chemists. The larger the surface area of a metal catalyst the more exposed metal atoms there are to speed up a chemical reaction. A palladium nanocatalyst could accelerate chemical reactions even at low temperatures.

3D schematic of the bonding of the metallic salt on the protein layer (Credit: Pollmann et al/ Forschungszentrum Rossendorf)

3D schematic of the bonding of the metallic salt on the protein layer (Credit: Pollmann et al/ Forschungszentrum Rossendorf)

The FZR team hopes to take their discovery a step further, however. They are now investigating whether or not they can make a gold nanocatalyst. They also hope to engineer Bacillus sphaericus JG-A12 to manipulate its protein layer so that it could make different types of metal cluster. These designer nanoclusters might have applications in optoelectronics, or as catalysts for specific reactions, as well as new physical properties researchers have so far not seen.

Further reading

Biophys J, 2006, in press;

Rossendorf home page

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