Flat-packed particles

Graphene, a Manchester University discovery, is a material comprising sheets of carbon just one atom thick; graphene is like a single layer of graphite. However, it was the discovery that it has some peculiar electronic properties because of the existence of massless quasiparticles that has led to an explosion of interest in this material. Some researchers suggest that ultimately it will become the material that gives us a post-silicon world in computing.

Now, US scientists have made the first observation of the energy bands of complex particles within graphene known as plasmarons. This small step is an important one in understanding graphene and using it to develop devices for that future of ultrafast chemical computers.

At Berkeley Lab’s Advanced Light Source, an international team led by Aaron Bostwick and Eli Rotenberg have shown that these composite plasmaron particles are vital in generating graphene’s unique properties. “Graphene’s true electronic structure can’t be understood without understanding the many complex interactions of electrons with other particles.”

The electric charge carriers in graphene are negative electrons and positive holes, which in turn are affected by plasmons, oscillations in the density of the material that travel like sound waves through a sea of electrons. A plasmaron is “simply” a charge carrier coupled to a plasmon. “Although plasmarons were proposed theoretically in the late 1960s, and indirect evidence for them has been found, our work is the first observation of their distinct energy bands in graphene, or indeed in any material,” Rotenberg says. The team reported details of their findings in the journal Science in May.

Top: graphene structure. Bottom: a theoretical model of plasmaron interactions in graphene, sheets of carbon one atom thick.

The relationships between charge carriers, plasmons, and plasmarons will be important in the development of plasmonics, the architecture analogous to electronics in conventional silicon semiconductor circuitry. An important aspect of studying these relationships is to produce flat graphene sheets; graphene is usually rumpled like unmade bed linen. “One of the best ways to grow a flat sheet of graphene is by heating a crystal of silicon carbide,” Rotenberg explains, “and it happens that our German colleagues Thomas Seyller from the University of Erlangen and Karsten Horn from the Fritz Haber Institute in Berlin are experts at working with silicon carbide. As the silicon recedes from the surface it leaves a single carbon layer.”

With flat graphene sheets in hand, the team used a beam of low-energy, or soft, X-rays to analyse the materials. The resulting data provided them with an image of the electronic bands created by the electrons themselves. Even from the initial experiments, the team suspected graphene’s behaviour was more complicated than simple theory would suggest and seemed to hint at the existence of bare electrons. Since bare electrons cannot exist, the researchers postulated the fuzziness in their image was due to charge carriers emitting plasmons. Additional experiments with graphene sheets isolated from their support material revealed that electrons detached by the X-rays can leave behind either an ordinary hole or a hole bound to a plasmon – a plasmaron, explains Rotenberg.

“By their nature, plasmons couple strongly to photons, which promises new ways for manipulating light in nanostructures, giving rise to the field of plasmonics,” Rotenberg says. “Now we know that plasmons couple strongly to the charge carriers in graphene, which suggests that graphene may have an important role to play in the merging fields of electronics, photonics, and plasmonics on the nanoscale.”

Links

Science, 2010, 328, 999-1002
Eli Rotenberg homepage

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

Cool for cats

Chemists in the US have demonstrated a definitive link between the size of catalyst particles on a solid surface, their electronic properties and their ability to accelerate a chemical reaction. The study could help improve the design of yet more-efficient catalysts to reduce energy requirements for countless industrial processes and cut greenhouse gas emissions.

Ideally, catalysts are substances that speed chemical reactions without themselves being consumed in the reaction. In reality, they are never 100 percent efficient, can be degraded by repeated reaction cycles, and often become poisoned by by-products. Nevertheless, they are at the heart of thousands of chemical reactions used to make everything from pharmaceuticals to plastics.

“One of the big uncertainties in catalysis is that no one really understands what size particles of the catalyst actually make a chemical reaction happen,” says Scott Anderson of the University of Utah. “If we could understand what factors control activity in catalysts, then we could make better and less expensive catalysts.”

Catalysts are commonly made from rare metals including, gold, rhodium, palladium, and platinum, and there is typically a range of catalyst particle sizes present. In almost all cases, the size of the most active particles is unknown. In gold catalysts, which have been intensively studied recently, it has been shown that the bulk of the metal in a catalyst powder exists in the form of particles that are too big to do any catalysis, and only a small fraction of the metal is active.

“If you could make a catalyst with only the right size particles, you could save 90 percent of the cost or more,” asserts Anderson. He also points out that switching to cheaper and more common metals, such as zinc, nickel, and copper, and “tuning” their properties would also let chemists reduce costs significantly. The process of tuning such base metals would involve reducing the particle size until it reaches a catalytic optimum, which is the focus of the Utah team’s work.

Previous work showed how to alter electronic and chemical properties of a catalyst in a gas, but things are different once the particles are mounted on a metal oxide surface for real-life industrial processes.

Anderson and Kaden working to accelerate chemical reactions efficiently (Credit: William Kunkel, University of Utah)
Anderson and Kaden working to accelerate chemical reactions efficiently (Credit: William Kunkel, University of Utah)

In the new study, Anderson and his students took a step toward tuning catalysts to have desired properties. In work with Bill Kaden and William Kunkel, and Tianpin Wu, the team has demonstrated, for the first time, that the size of palladium metal catalyst “nanoparticles” deposited on a titanium dioxide surface affects not only the catalyst’s level of activity in converting carbon monoxide to carbon dioxide, but also the particles’ electronic properties.

As the size of a catalyst metal particle is reduced to the nanoscale, its properties initially remain the same as bulk metal. However, when the particles are just 10 nanometres across (containing 10,000 atoms or so) the movements of electrons in the metal become confined, so boosting their energy.

When there are fewer than about 100 atoms in catalyst particles, the size variations also result in fluctuations in the electronic structure of the catalyst atoms. Those fluctuations strongly affect the particles’ ability to act as a catalyst, Anderson says.

The study not only showed how catalytic activity varies with catalyst particle size, “but we have been able to correlate that size dependence with observed electronic differences in the catalyst particles,” Kaden adds. “People had speculated this should be happening, but no one has ever seen it.”

Links:

Science, 2009, in press
Scott L. Anderson homepage