Electron Ping Pong in the Nano-World

The research is reported in the journalNature Physics(April 24, 2011).

When intense laser light interacts with electrons in nanoparticles that consist of many million individual atoms, these electrons can be released and strongly accelerated. Such an effect in nano-spheres of silica was recently observed by an international team of researchers in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics. The researchers report how strong electrical fields (near-fields) build up in the vicinity of the nanoparticles and release electrons. Driven by the near-fields and collective interactions of the charges resulting from ionization by the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that were observed so far for single atoms. The exact movement of the electrons can be precisely controlled via the electric field of the laser light. The new insights into this light-controlled process can help to generate energetic extreme ultraviolet (XUV) radiation. The experiments and their theoretical modeling, which are described by the scientists in the journal"Nature Physics," open up new perspectives for the development of ultrafast, light-controlled nano-electronics, which could potentially operate up to one million times faster than current electronics.

Electron acceleration in a laser field is similar to a short rally in a ping pongmatch: a serve, a return and a smash securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. An international team, led by three German groups including Prof. Matthias Kling from the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics in Garching and the Ludwig-Maximilian University Munich, Prof. Eckart Rühl from the Free University of Berlin and Prof. Thomas Fennel from the University of Rostock, was now successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light-fields.

The researchers illuminated silica nanoparticles with a size of around 100 nm with very intense light pulses, lasting around five femtoseconds (one femtosecond is a millionth of a billionth of a second). Such short laser pulses consist of only a few wave cycles. The nanoparticles contained around 50 million atoms each. The electrons are ionized within a fraction of a femtosecond and accelerated by the electric field of the remaining laser pulse. After travelling less than one nanometer away from the surface of the nano-spheres, some of the electrons can be returned to the surface by the laser field to the surface, where they were smashed right back (such as the ping pong ball being hit by the paddle). The resulting energy gain of the electrons can reach very high values. In the experiment electron energies of ca. 60 times the energy of a 700 nm wavelength laser photon (in the red spectral region of light) have been found.

For the first time, the researchers could observe and record the direct elastic recollision phenomenon from a nanosystem in detail. The scientists used polarized light for their experiments. With polarized light, the light waves are oscillating only along one axis and not, as with normal light, in all directions."Intense radiation pulses can deform or destroy nanoparticles. We have thus prepared the nanoparticles in a beam, such that fresh nanoparticles were used for every laser pulse. This was of paramount importance for the observation of the highly energetic electrons.," explains Eckart Rühl.

The accelerated electrons left the atoms with different directions and different energies. The flight trajectories were recorded by the scientists in a three-dimensional picture, which they used to determine the energies and emission directions of the electrons."The electrons were not only accelerated by the laser-induced near-field, which by itself was already stronger than the laser field, but also by the interactions with other electrons, which were released from the nanoparticles," describes Matthias Kling about the experiment. Finally, the positive charging of the nanoparticle-surface also plays a role. Since all contributions add up, the energy of the electrons can be very high."The process is complex, but shows that there is much to explore in the interaction of nanoparticles with strong laser fields," adds Kling.

The electron movements can also produce pulses of extreme ultraviolet light when electrons that hit the surface do not bounce back, but are absorbed releasing photons with wavelengths in the XUV. XUV light is of particular interest for biological and medical research."According to our findings, the recombination of electrons on the nanoparticles can lead to energies of the generated photons, which are up to seven-times higher than the limit that was so far observed for single atoms," explains Thomas Fennel. The evidence of collective acceleration of electrons with nanoparticles offers great potential. Matthias Kling believes that"From this may arise new, promising applications in future, light-controlled ultrafast electronics, which may work up to one million times faster than conventional electronics."

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Two Graphene Layers May Be Better Than One

Graphene, a single layer of carbon atoms, is prized for its remarkable properties, not the least of which is the way it conducts electrons at high speed. However, the lack of what physicists call a band gap -- an energetic threshold that makes it possible to turn a transistor on and off -- makes graphene ill-suited for digital electronic applications.

Researchers have known that bilayer graphene, consisting of two stacked graphene layers, acts more like a semiconductor when immersed in an electric field.

According to NIST researcher Nikolai Zhitenev, the band gap may also form on its own due to variations in the sheets' electrical potential caused by interactions among the graphene electrons or with the substrate (usually a nonconducting, or insulating material) that the graphene is placed upon.

NIST fellow Joseph Stroscio says that their measurements indicate that interactions with the disordered insulating substrate material causes pools of electrons and electron holes (basically, the absence of electrons) to form in the graphene layers. Both electron and hole"pools" are deeper on the bottom layer because it is closer to the substrate. This difference in"pool" depths, or charge density, between the layers creates the random pattern of alternating charges and the spatially varying band gap.

Manipulating the purity of the substrate could give researchers a way to finely control graphene's band gap and may eventually lead to the fabrication of graphene-based transistors that can be turned on and off like a semiconductor.

Still, as shown in the group's previous work, while these substrate interactions open the door to graphene's use as a practical electronic material, they lower the window on speed. Electrons do not move as well through substrate-mounted bilayer graphene; however, this may likely be compensated for by engineering the graphene/substrate interactions.

Stroscio's team plans to explore further the role that substrates may play in the creation and control of band gaps in graphene by using different substrate materials. If the substrate interactions can be reduced far enough, says Stroscio, the exotic quantum properties of bilayer graphene may be harnessed to create a new quantum field effect transistor.

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Electronics: A Step Toward Valleytronics

The discovery was published inPhysical Review Letterson March 28, 2011 and was also the subject of a separate Viewpoint article inPhysics.

Information in solid-state, either classical or quantum, is generally carried by electrons and holes. The information can be encoded in various degrees of freedom such as charge or spin. Charge representations, for example the absence or presence of an electron in a quantum dot, are attractive as they are easily manipulated and interrogated through electric fields. The advantage of spin representations, used in the field of spintronics, is their superior shielding from undesired electric fluctuations in the environment, making the information in these latter representations more robust. In the future, there might be a third middle-ground alternative in the valley degree of freedom that exists in certain crystals, including graphene.

The valley degree of freedom in graphene gained attention in 2007 when it was proposed that electrons and holes could be filtered according to which valley they occupy. Unfortunately, the structures required for this and subsequent valley filters are difficult to fabricate, and as a result a valley filter has yet to be demonstrated experimentally. The present study from NRL shows that an extended line defect in graphene acts as a natural valley filter."As the structure is already available, we are hopeful that valley-polarized currents could be generated in the near future" said Dr. Daniel Gunlycke who made the discovery together with Dr. Carter White. Both work in NRL's Chemistry Division.

Valley refers to energy depressions in the band structure, which describes the energies of electron waves allowed by the symmetry of the crystal. For graphene, these regions form two pairs of cones that determine its low-bias response. As a large crystal momentum separates the two valleys, the valley degree of freedom is robust against slowly varying potentials, including scattering caused by low-energy acoustic phonons that often require low-bias electronic devices to operate at low temperatures typically only accessible in laboratories.

Valley polarization is achieved when electrons and holes in one valley are separated spatially from those in the other valley, but this is difficult to do as the two valleys have the same energies. It was found, however, that this spatial separation can be obtained in connected graphene structures that possess reflection symmetry along a particular crystallographic direction with no bonds crossing the reflection plane. This property turns out to be present in a recently observed line defect in graphene. The reflection symmetry only permits electron waves that are symmetric to pass through the line defect. Anti-symmetric waves are reflected. By projecting an arbitrary low-energy wave in graphene onto its symmetric component, one gets the transmission amplitude through this defect, which is strongly dependent on the valley. Electron and hole waves approaching the line defect at a high angle of incidence results in a polarization near 100%.

There is a long way to go before valleytronics can become a viable technology, explains Gunlycke. The recent advance, however, provides a realistic way to reach a crucial milestone in its development. This research was supported by the Office of Naval Research, both directly and through the Naval Research Laboratory.

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New Heat Pump Could Last 10,000 Years, Experts Say

The new heat pump consists of many miniature heat pumps as small as one cubic millimeter. To heat a house one needs several thousand of them. They are put together into larger units that can be tall and thin or short and wide.

"The most important advantages of the new heat pump is that you can regulate its size and form and that it is more durable than heat pumps are today. It is also more environmentally friendly," Doctor of Physics Jan Kåre Bording says, who is Chief Engineer at the University of Stavanger in Norway.

Together with his colleague, Professor of Materials Science Vidar Hansen, he is developing a new heat pump that is thermo-electric. They have investigated its disadvantages and advantages compared with the heat pumps we use today.

The project is a collaboration project with the Department of Physics, University of Oslo. According to the researchers the heat pump will be fully developed and ready to be launched on the market in five to ten years.

A heat pump that lasts

Thermo-electric heat pumps make use of materials that produce electricity when they are subjected to differences in temperature.

The new heat pump has a dramatically longer life than today's heat pumps.

"The heat pumps we use today consist of several moveable parts. After some time different parts break down and will have to be changed," Bording says.

"The new heat pump consists of several miniature heat pumps and these have a very simple design. In opposite to today's heat pumps, these miniature heat pumps consist of only one part. Because they consist of only one metal part it's easier to avoid wear and tear. You can compare the heat pump to a golden ring. A golden ring won't be broken. The miniature pumps will just continue to pump. We stick fans on them, and they must be replaced, but the heat pump itself will stay and be equally effective after 10 000 years," Bording continues.

The heat pumps used today begin to deteriorate after one year. Then they need to be inspected, and that costs 1500 NOK for an air-to-air heat pump. After 10 to 20 years, larger parts of the pump will fail, for instance the compressor.

Thousands of pumps in a house

The small heat pumps can be put together and form lager units. The researchers also envisage that it may be possible to place several thousand of the small heat pumps at different places in the house.

"We don't want a large wood-burning stove in the middle of the house as in the old days. It's better with more, smaller heat sources," Hansen says.

Initially, however, researchers will create units that can be placed at one or two locations in the house. The new heat pumps offer great flexibility as to where in the house you want them. It would be an advantage to have them in places where it is especially cold.

"For example, it may be a good idea to put them under the floor, so that the floor will heat the room. When the heat pump has a large surface, it produces more heat," Bording says.

More environmentally friendly

The new heat pumps will be more environmentally friendly than those in use today. One problem with today's heat pumps is that they can leak cooling gas. Cooling gas is usually freon gas, which destroys the ozone layer. There is no risk of losing cooling gas in thermoelectric heat pumps, since gas has been replaced with clean electricity.

"We have seen that several of the gases used in heat pumps, have become illegal, such as freon 12. Gradually gases that are in use today can also become illegal, so that we can no longer use the heat pumps we have today," Bording says.

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Replacing Batteries May Become a Thing of the Past, Thanks to 'Soft Generators'

A class of variable capacitor generators known as"dielectric elastomer generators" (DEGs) shows great potential for wearable energy harvesting. In fact, researchers at the Auckland Bioengineering Institute's Biomimetics Lab believe DEGs may enable light, soft, form-fitting, silent energy harvesters with excellent mechanical properties that match human muscle. They describe their findings in the American Institute of Physics' journalApplied Physics Letters.

"Imagine soft generators that produce energy by flexing and stretching as they ride ocean waves or sway in the breeze like a tree," says Thomas McKay, a Ph.D. candidate working on soft generator research at the Biomimetics Lab."We've developed a low-cost power generator with an unprecedented combination of softness, flexibility, and low mass. These characteristics provide an opportunity to harvest energy from environmental sources with much greater simplicity than previously possible."

Dielectric elastomers, often referred to as artificial muscles, are stretchy materials that are capable of producing energy when deformed. In the past, artificial muscle generators required bulky, rigid, and expensive external electronics.

"Our team eliminated the need for this external circuitry by integrating flexible electronics -- dielectric elastomer switches -- directly onto the artificial muscles themselves. One of the most exciting features of the generator is that it's so simple; it simply consists of rubber membranes and carbon grease mounted in a frame," McKay explains.

McKay and his colleagues at the Biomimetics Lab are working to create soft dexterous machines that comfortably interface with living creatures and nature in general. The soft generator is another step toward fully soft devices; it could potentially be unnoticeably incorporated into clothing and harvest electricity from human movement. When this happens, worrying about the battery powering your cell phone or other portable electronics dying on you will become a thing of the past. And as an added bonus, this should help keep batteries out of landfills.

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Cheaper Hydrogen Fuel Cells: Utility of Non-Precious-Metal Catalysts Documented

In a paper published April 21 inScience, Los Alamos researchers Gang Wu, Christina Johnston, and Piotr Zelenay, joined by researcher Karren More of Oak Ridge National Laboratory, describe the use of a platinum-free catalyst in the cathode of a hydrogen fuel cell. Eliminating platinum -- a precious metal more expensive than gold -- would solve a significant economic challenge that has thwarted widespread use of large-scale hydrogen fuel cell systems.

Polymer-electrolyte hydrogen fuel cells convert hydrogen and oxygen into electricity. The cells can be enlarged and combined in series for high-power applications, including automobiles. Under optimal conditions, the hydrogen fuel cell produces water as a"waste" product and does not emit greenhouse gasses. However, because the use of platinum in catalysts is necessary to facilitate the reactions that produce electricity within a fuel cell, widespread use of fuel cells in common applications has been cost prohibitive. An increase in the demand for platinum-based catalysts could drive up the cost of platinum even higher than its current value of nearly$1,800 an ounce.

The Los Alamos researchers developed non-precious-metal catalysts for the part of the fuel cell that reacts with oxygen. The catalysts -- which use carbon (partially derived from polyaniline in a high-temperature process), and inexpensive iron and cobalt instead of platinum -- yielded high power output, good efficiency, and promising longevity. The researchers found that fuel cells containing the carbon-iron-cobalt catalyst synthesized by Wu not only generated currents comparable to the output of precious-metal-catalyst fuel cells, but held up favorably when cycled on and off -- a condition that can damage inferior catalysts relatively quickly.

Moreover, the carbon-iron-cobalt catalyst fuel cells effectively completed the conversion of hydrogen and oxygen into water, rather than producing large amounts of undesirable hydrogen peroxide. Inefficient conversion of the fuels, which generates hydrogen peroxide, can reduce power output by up to 50 percent, and also has the potential to destroy fuel cell membranes. Fortunately, the carbon- iron-cobalt catalysts synthesized at Los Alamos create extremely small amounts of hydrogen peroxide, even when compared with state-of-the-art platinum-based oxygen-reduction catalysts.

Because of the successful performance of the new catalyst, the Los Alamos researchers have filed a patent for it.

"The encouraging point is that we have found a catalyst with a good durability and life cycle relative to platinum-based catalysts," said Zelenay, corresponding author for the paper."For all intents and purposes, this is a zero-cost catalyst in comparison to platinum, so it directly addresses one of the main barriers to hydrogen fuel cells."

The next step in the team's research will be to better understand the mechanism underlying the carbon-iron-cobalt catalyst. Micrographic images of portions of the catalyst by researcher More have provided some insight into how it functions, but further work must be done to confirm theories by the research team. Such an understanding could lead to improvements in non-precious-metal catalysts, further increasing their efficiency and lifespan.

Project funding for the Los Alamos research came from the U.S. Department of Energy's Energy Efficiency and Renewable Energy (EERE) Office as well as from Los Alamos National Laboratory's Laboratory-Directed Research and Development program. Microscopy research was done at Oak Ridge National Laboratory's SHaRE user facility with support from the DOE's Office of Basic Energy Sciences.

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New Kid on the Plasmonic Block: Researchers Find Plasmonic Resonances in Semiconductor Nanocrystals

"We have demonstrated well-defined localized surface plasmon resonances arising from p-type carriers in vacancy-doped semiconductor quantum dots that should allow for plasmonic sensing and manipulation of solid-state processes in single nanocrystals," says Berkeley Lab director Paul Alivisatos, a nanochemistry authority who led this research."Our doped semiconductor quantum dots also open up the possibility of strongly coupling photonic and electronic properties, with implications for light harvesting, nonlinear optics, and quantum information processing."

Alivisatos is the corresponding author of a paper in the journalNature Materialstitled"Localized surface plasmon resonances arising from free carriers in doped quantum dots." Co-authoring the paper were Joseph Luther and Prashant Jain, along with Trevor Ewers.

The term"plasmonics" describes a phenomenon in which the confinement of light in dimensions smaller than the wavelength of photons in free space make it possible to match the different length-scales associated with photonics and electronics in a single nanoscale device. Scientists believe that through plasmonics it should be possible to design computer chip interconnects that are able to move much larger amounts of data much faster than today's chips. It should also be possible to create microscope lenses that can resolve nanoscale objects with visible light, a new generation of highly efficient light-emitting diodes, and supersensitive chemical and biological detectors. There is even evidence that plasmonic materials can be used to bend light around an object, thereby rendering that object invisible.

The plasmonic phenomenon was discovered in nanostructures at the interfaces between a noble metal, such as gold or silver, and a dielectric, such as air or glass. Directing an electromagnetic field at such an interface generates electronic surface waves that roll through the conduction electrons on a metal, like ripples spreading across the surface of a pond that has been plunked with a stone. Just as the energy in an electromagnetic field is carried in a quantized particle-like unit called a photon, the energy in such an electronic surface wave is carried in a quantized particle-like unit called a plasmon. The key to plasmonic properties is when the oscillation frequency between the plasmons and the incident photons matches, a phenomenon known as localized surface plasmon resonance (LSPR). Conventional scientific wisdom has held that LSPRs require a metal nanostructure , where the conduction electrons are not strongly attached to individual atoms or molecules. This has proved not to be the case as Prashant Jain, a member of the Alivisatos research group and one of the lead authors of the Nature Materials paper, explains.

"Our study represents a paradigm shift from metal nanoplasmonics as we've shown that, in principle, any nanostructure can exhibit LSPRs so long as the interface has an appreciable number of free charge carriers, either electrons or holes," Jain says."By demonstrating LSPRs in doped quantum dots, we've extended the range of candidate materials for plasmonics to include semiconductors, and we've also merged the field of plasmonic nanostructures, which exhibit tunable photonic properties, with the field of quantum dots, which exhibit tunable electronic properties."

Jain and his co-authors made their quantum dots from the semiconductor copper sulfide, a material that is known to support numerous copper-deficient stoichiometries. Initially, the copper sulfide nanocrystals were synthesized using a common hot injection method. While this yielded nanocrystals that were intrinsically self-doped with p-type charge carriers, there was no control over the amount of charge vacancies or carriers.

"We were able to overcome this limitation by using a room-temperature ion exchange method to synthesize the copper sulfide nanocrystals," Jain says."This freezes the nanocrystals into a relatively vacancy-free state, which we can then dope in a controlled manner using common chemical oxidants."

By introducing enough free electrical charge carriers via dopants and vacancies, Jain and his colleagues were able to achieve LSPRs in the near-infrared range of the electromagnetic spectrum. The extension of plasmonics to include semiconductors as well as metals offers a number of significant advantages, as Jain explains.

"Unlike a metal, the concentration of free charge carriers in a semiconductor can be actively controlled by doping, temperature, and/or phase transitions," he says."Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be dynamically tuned. The LSPRs of a metal, on the other hand, once engineered through a choice of nanostructure parameters, such as shape and size, is permanently locked-in."

Jain envisions quantum dots as being integrated into a variety of future film and chip-based photonic devices that can be actively switched or controlled, and also being applied to such optical applications as in vivo imaging. In addition, the strong coupling that is possible between photonic and electronic modes in such doped quantum dots holds exciting potential for applications in solar photovoltaics and artificial photosynthesis

"In photovoltaic and artificial photosynthetic systems, light needs to be absorbed and channeled to generate energetic electrons and holes, which can then be used to make electricity or fuel," Jain says."To be efficient, it is highly desirable that such systems exhibit an enhanced interaction of light with excitons. This is what a doped quantum dot with an LSPR mode could achieve."

The potential for strongly coupled electronic and photonic modes in doped quantum dots arises from the fact that semiconductor quantum dots allow for quantized electronic excitations (excitons), while LSPRs serve to strongly localize or confine light of specific frequencies within the quantum dot. The result is an enhanced exciton-light interaction. Since the LSPR frequency can be controlled by changing the doping level, and excitons can be tuned by quantum confinement, it should be possible to engineer doped quantum dots for harvesting the richest frequencies of light in the solar spectrum.

Quantum dot plasmonics also hold intriguing possibilities for future quantum communication and computation devices.

"The use of single photons, in the form of quantized plasmons, would allow quantum systems to send information at nearly the speed of light, compared with the electron speed and resistance in classical systems," Jain says."Doped quantum dots by providing strongly coupled quantized excitons and LSPRs and within the same nanostructure could serve as a source of single plasmons."

Jain and others in Alivsatos' research group are now investigating the potential of doped quantum dots made from other semiconductors, such as copper selenide and germanium telluride, which also display tunable plasmonic or photonic resonances. Germanium telluride is of particular interest because it has phase change properties that are useful for memory storage devices.

"A long term goal is to generalize plasmonic phenomena to all doped quantum dots, whether heavily self-doped or extrinsically doped with relatively few impurities or vacancies," Jain says.

This research was supported by the DOE Office of Science.

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