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.

Links

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

The slow rise of The Andes

The Andes is the world’s longest continental mountain range and the highest outside Asia, with an average elevation of 4000 metres. The question of how quickly the mountains reached such heights has been a contentious one that University of Michigan paleoclimatologist Christopher Poulsen and graduate student Nadja Insel working with Todd Ehlers of the University of Tuebingen in Germany, believe they may now have settled with a new interpretation of isotopic data. Their work suggests that the rise of the Andes was a very gradual process.

Poulsen’s uplifting work suggests that previous interpretations of the evidence have misconstrued changes in oxygen isotope ratios as being due to a rapid rise of the mountain range whereas the more likely explanation is that the changes are due to shifts in ancient climate.

“In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain and elevation,” Poulsen explains.
“As a rain cloud ascends a mountain range, it begins to precipitate.
Because atoms of oxygen-18 are more massive than those of oxygen-16, it is preferentially rained out. Thus, as you go up the mountain, the precipitation becomes more and more depleted in oxygen-18, and the ratio of oxygen-18 to oxygen-16 decreases.” Geologists use the ratio of these isotopes, preserved in rock, to infer past elevations and so the rate of rise of a mountain range.

“If the ratio decreases with time, as the samples get younger, the interpretation would typically be that there has been an increase in elevation at that location,” Poulsen adds. He points out that that is the precise conclusion drawn by a series of papers on the uplift history of The Andes published over the past four years. On the basis of oxygen isotope ratios determined by analysis of carbonate rocks, the authors of those papers suggested that the central Andes rose about 2500 to 3500 metres in a mere three million years, Other geologists had assumed that the rise to those heights took place over tens of millions of years.

Unfortunately, elevation is not the only thing to disturb oxygen isotope ratios in precipitation. “It can also be affected by where the vapour came from and how much it rained,” says Poulsen. “More intense rainfall also causes oxygen-18 to be preferentially precipitated.” He and his colleagues were skeptical of the rapid-rise scenario, and so performed climate modelling experiments to investigate whether something other than altitude might have given rise to the shift in ratio observed in carbonate deposits.

Andes: Credit to http://www.flickr.com/photos/atyt/

“The key result in our modelling study is that we identified an elevation threshold for rainfall,” Poulsen says. “Once The Andes reached an elevation greater than 70 percent of the current elevation, the precipitation rate abruptly increased. In our model, the increased precipitation also caused the ratio of oxygen-18 to oxygen-16 to significantly decrease. Our conclusion, then, is that geologists have misinterpreted the isotopic records in the central Andes. The decrease in the ratio is not recording an abrupt increase in elevation; it is recording an abrupt increase in rainfall.”

This conclusion is backed up by geochemical and sedimentological data, Poulsen said. “There is evidence that the central Andes became less arid at the same time that the isotope records show a decrease in the ratio of oxygen-18 to oxygen-16.”

Links

Christopher Poulsen

Science Express, 2010, online

New battery-boosting recipe

A common problem with portable electronic devices is that their
rechargeable lithium batteries deliver power for only a short time, and
lose their ability to be fully recharged as the battery gets older. Now,
Italian chemists have added tin and sulfur to the rechargeable recipe to
overcome these problems in next generation batteries.

Bruno Scrosati and Jusef Hassoun of the University of Rome point out
that theoretically at least, lithium-sulfur batteries would be the
energy source of choice. They would, after all, have much higher energy
density than lithium-ion batteries. However, the electrodes in such
batteries slowly dissolve and the lithium metal forms branching, or
dendritic, deposits that lead to electrical short circuits. Commercial
“lithium” batteries contain graphite through which lithium ions can
diffuse, which means no disintegrating lithium metal electrodes, but
lower energy density.

Comparison of battery energies (Credit: Wiley)

The Italian team has combined the advantages of lithium metal with the
longevity of lithium ion by developing a new type of lithium-metal-free
cell that has a negative electrode composed of a carbon/lithium sulfide
composite. The battery’s charge carrying electrolyte solution is
replaced by a lithium-ion-containing liquid enclosed in a polymer gel
membrane. The polymer protects the electrodes from corrosion. For the
anode (positive electrode), Scrosati and Hassoun chose nanoscopic tin
particles embedded in a protective carbon matrix.

The electricity generation process involves the lithium sulfide cathode
splitting into elemental sulfur and lithium ions, which releases
electrons. The lithium ions migrate through the electrolyte membrane to
the anode, where they take up electrons to become uncharged lithium
atoms, which are then bound into an alloy by the tin nanoparticles. The
process is reversible by applying a current (from the mains supply) in
the opposite direction, so that the battery can be charged repeatedly.
The new battery surpasses all previous attempts at a lithium-metal-free
battery with its specific energy of about 1100 Watt-hours per kilogram.
Such a high energy could not only be useful for portable music players
and mobile phones but is substantial enough for electric vehicles.

Links

Angew Chem Int Edn, 2010, in press
Scrosati