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Thermoelectric Nanowires Promise Energy Harvesting From Car Exhaust

Researchers at Sandia National Labs have developed a manufacturing process capable of controlling the crystal orientation, crystal size, and alloy uniformity of nanowires so that they could be used in a range of thermoelectric applications. 

Because thermoelectric materials are capable of generating an electrical current as a result of a difference in temperature between one side of the material and the other, the Sandia team believes the new nanowires could make it possible for carmakers to harvest power from the heat wasted by exhaust systems or lead to more efficient devices for cooling computer chips.

Nanowires have been suggested for a range of applications, but in thermoelectric applications, the quality of the nanowires has heretofore been inadequate. The trick for any thermoelectric material is to combine high electrical conductivity and relatively low thermal conductivity—a property known as thermoelectric efficiency.

Researchers have been investigating a number of nanomaterials for thermoelectric applications; traditional materials possess a relatively poor thermoelectric conversion efficiency or they are prohibitively expensive for commercial uses.

The Sandia researchers turned to nanowires despite their previous poor performance, believing that if they could better control the manufacturing process, they could improve the nanowires’ quality enough to make them a useful thermoelectric material.

In research published in the Cambridge Journal of Materials Research, the Sandia team employed a method known as room-temperature electroforming, which is widely used in commercial electroplating. In electroforming, material is deposited at a constant rate, which results in the nanowires growing uniformly.

This uniformity of composition held for the entire length of each nanowire and even across an array of them. The crystals that made up the nanowires were all oriented in one direction, making it easier for electrons to travel along the conduits.

“There are little nuances in the technique that I do to allow the orientation, the crystal growth, and the composition to be maintained within a fairly tight range,” said Graham Yelton of Sandia in a press release. “It’s turning the knobs of the process to get these things to behave.”

The next step in the research will be to make an electrical contact with the nanowire-based material and to measure the resulting thermoelectric behavior.

One hurdle the team has to overcome: “Thermoelectric materials readily form oxides or intermetallics, leading to poor contact connections or higher electrical contact resistance. That reduces the gains achieved in developing the materials,” Yelton said.

So far the team has had some success in getting good contact at the bottom of an array, but making a connection at the top has proved difficult.

At the moment, the researchers are seeking additional funding to solve the problem of making contacts, and then they plan to characterize the thermal electric properties of the arrays.

Is "Valleytronics" the Next Big Thing in Quantum Computing?

Researchers at the Lawrence Berkeley National Laboratory (LBL) have developed a new pathway to achieving “valleytronics” using two-dimensional (2D) semiconductors.  The LBL researchers believe that this new approach could make valleytronics a more stable alternative to “spintronics” as a replacement for traditional electronics.

The term valleytronics is starting to filter into in the lexicon of cutting-edge electronics research. What it actually means is complicated, but it represents a movement away from exploiting the electrical charge of electrons as a means for storing information and instead using the wave quantum number of an electron in a crystalline material to encode data.

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Transistor Made From Silicene for First Time

Ever since silicene—a one-atom-thick layer of silicon—was first predicted in computer models a decade before graphene was first synthesized in 2004, it’s been on a roller coaster ride. It’s gone from being considered the next big thing to being thought of as an impossibility outside of computer models.

Drawing on an increasing amount of recent research demonstrating that silicene can survive—for at least a little while—outside the virtual world of computer models, researchers at the University of Texas at Austin have taken it all a step further by demonstrating a method for fabricating a field-effect transistor out of silicene. The device reportedly lives up to the switching speeds that had been promised in the computer models. Most importantly, this research marks the first time that anyone has been able to fabricate a transistor out of silicene.

In research published in the journal Nature Nanotechnology, the Texas researchers grew their silicene on a thin film of silver and capped it with aluminum oxide. Adding this light coat of protective oxide to create a protective shell—what’s known in the business as passivation—has recently proven effective in protecting graphene devices.

The researchers took the encapsulated silicene and placed it on a silicon dioxide wafer with the silver side up. They then put patterns into the silver side that would allow for contacts to be made so it could operate as a transistor.

The device was not tested in the open air, but it at least remained stable in vacuum conditions. Of course, this situation is not practical for real-world applications, but the researchers feel that this marks an important step towards realizing commercially viable silicene-based transistors.

The experimental research does confirm some of the theories that had been based solely on computer models. It demonstrated that silicene has electrical properties similar to those of graphene, including allowing electrons to travel through the material without any barriers.

While this research provides some confirmation of silicene’s attractive electronic properties, the research falls somewhat short of supporting the notion that because silicene is made from silicon it will be easier for the electronics industry to adopt. Silicene may be a one-atom-thick relative of silicon, which the electronics industry has characterized for the last half-century, but this research doesn’t seem to indicate that it has become any more friendly to large-scale manufacturing than its more mature cousin.

Nanoballs Inflate Voltage Capacity of Power Cables, Save Energy

While nanotechnology gets a lot of attention for enabling news types of energy generation, such as photovoltaics,  or ushering in a new energy storage technology, it may in fact be in the mundane role of energy savings and efficiency where it may hold the most promise.

Along these lines, researchers at the Chalmers University of Technology in Sweden have demonstrated that the addition of carbon nanoballs (also known as C60, Buckyballs, and fullerenes) into the plastic insulation used in high-voltage lines enable the cables handle up to 26 percent more voltage than standard cables.

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Graphene Becomes Magnetic and Electric at Same Time

Combining the electric with the magnetic in one material has become a hot research pursuit lately. First researchers exploited the natural properties of bismuth ferrite. Then researchers engineered both electric and magnetic polarization in perovskite—a material that does not have that property naturally.

Now researchers at the University of California Riverside have brought this combination of the electric and the magnetic to the wonder material, graphene.

Of course, it is possible to induce magnetism in graphene by doping the material with magnetic impurities. Unfortunately, that process comes at the high cost of eliminating all the attractive electrical properties of graphene, such as its high conductivity.

The UC Riverside researchers found a way to make graphene magnetic without sacrificing its attractive electrical properties.

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Perovskite Manipulated To Carry Both Electrical and Magnetic Polarization

Researchers at the University of Liverpool in the UK have demonstrated the ability to manipulate a material so that it has both magnetic and electrical polarization, a feature that could lead to low-power electrical writing of information with non-destructive magnetic reading, and logic devices that can operate without charge current flow.

A fair amount of research has gone into exploiting this property of both electrical and magnetic polarization in bismuth ferrite, which has the rare property of possessing a permanent local magnetic field while always possessing an electric polarization that can be switched by applying an electric field.

However, in research published in the journal Science, the Liverpool team employed crystal chemistry to engineer specific atomic displacements in a layered perovskite to give it properties it did not previously possess. Perovskites are a class of crystals that have become all the rage, especially in the world of photovoltaics, because of their low cost, high charge-carrier mobility, and long diffusion lengths. In real world terms, this means that the electrons in perovskite-based photovoltaics can travel through thicker solar cells, which absorb more light and thereby generate more electricity than thinner cells.

In these most recent experiments, concern for perovskites’ attractive properties in photovoltaics were set aside so that the perovskite crystal could be engineered to have a novel set of properties.

“By designing in the required atomic-level changes using both computation and experiment together, we produced three properties (polarization, magnetization, magnetoelectricity) from a material that initially displayed none of them,” said Liverpool professor Matthew Rosseinsky, in a press release.

Rosseinsky added: “We were able to demonstrate that the magnetization and polarization are coupled by measuring the linear magnetoelectric coefficient, a key physical quantity for the integration of such materials in a device. This coupling arises because both properties are produced by the same single set atomic motions that we built in to the material.”

While these developments could potentially lead to applications for information storage, the researchers concede that a number of challenges still have to be overcome before making that step, including switching the polarization and making the material more electrically insulating.

Graphene Devices Stand the Test of Time

In the decade since graphene was first synthesized, researchers have been preoccupied about overcoming the fact that the material lacks an inherent band gap, which limits its potential in digital logic applications.

While researchers have been reasonably successful at engineering a band gap into graphene in the lab, there is another looming show stopper: How robust is the material when facing real-world environments?

Now researchers from AMO GmbH in Germany and Spain-based Graphenea SE have demonstrated a sophisticated encapsulation technique that they claim is easily reproducible and should allow graphene devices in normal atmosphere to last for several months.

What typically is the death knell for graphene is partly the result of the environment and also the impurities in the production process. As far as the environment is concerned, moisture or oxygen causes the problem. From the production side, the residue from lithography processes adhere to the graphene and change its doping level unintentionally.

The doping levels change the properties that make graphene so attractive in the first place—such as its conductivity and its optical properties. As a result, all the great capabilities that graphene is heralded for are largely lost.

In research published in the Royal Society of Chemistry journal Nanoscale,  the team first identified this problem and then devised their encapsulation technique to overcome it.

The researchers applied their encapsulation technique to field-effect devices using aluminum oxide, an encapsulation material that is often used in making organic light-emitting diodes (OLEDs).

To a technique known in the business as passivation, in which a light coat of a protective oxide is used to create a shell against corrosion, the researchers added a bit of a wrinkle. They were able to grow the oxide layer using an oxidized layer of aluminum that served as as the seed for further growth. This new twist to the passivation technique managed to stabilize the device characteristics over several months when stored and measured in ambient atmosphere.

The researchers believe that this development serves as a major step torward getting graphene devices into real-world applications.

Proven: Graphene Makes Multiple Electrons From Light

Researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have for the first time observed and measured graphene converting a single photon into multiple electrons in a photovoltaic device.  This work should buoy hopes that graphene can serve as a material for photovoltaics with very high energy-conversion efficiencies.

The discovery builds on work conducted last year by the Barcelona-based Institute of Photonic Science (ICFO). ICFO scientists were able to indirectly show that graphene is capable of converting one photon into multiple electrons. In that research, the team excited the graphene by exposing it to photons of different energies (colors). They then used a pulse of terahertz radiation to measure the resulting hot-electron distribution. They determined that a higher photon energy (violet) resulted in higher numbers of hot electrons than a lower photon energy (infrared).

In this most recent EPFL work, the researchers had to devise a way to measure the conversion process, which occurs on a femto-second scale (10-15 seconds). That’s far faster than any conventional method for detecting electron movement.

The team turned to a new technique called ultrafast time- and angle-resolved photoemission spectroscopy” (trARPES). The measurements themselves, the results of which were published in the journal Nano Letters, took place at the Rutherford Appleton Laboratory at Oxford University.

The graphene was placed in an ultra-high vacuum chamber where the material was then hit with an ultrafast “pump” pulse of laser light. The laser light excites the electrons in the graphene bringing them to a higher energy state . In this heightened state, the graphene is then hit with a time-delayed, “probe” pulse that serves to take a snapshot of the energy each electron has at that moment. By doing this numerous times, the researchers create a kind of stop-motion movie of the conversion process.

“This indicates that a photovoltaic device using doped graphene could show significant efficiency in converting light to electricity,” said Marco Grioni of EPFL in a press release.

While nanomaterials in photovoltaics have held out the promise of converting a single photon into multiple electrons in research dating back to 2004, there have been skeptics as to whether this ability will actually lead to higher conversion efficiencies.

Eran Rabani, a researcher at Tel Aviv University, back in 2011 declared that he was not so convinced by the research on electron multiplication.

“Our theory shows that current predictions to increase efficiencies won't work,” Rabani said in a press release at the time. “The increase in efficiencies cannot be achieved yet through Multiexciton Generation, a process by which several charge carriers (electrons and holes) are generated from one photon.”

This skepticism may account for why so much energy has been devoted to measuring and characterizing the generation of multiple electrons from a single photon.

But if multiple electron generation can—as some hope—boost conversion efficiency to 60 percent from what was thought to be a 32 percent limit, then proving that the event indeed occurs is well worth it.

Zinc-oxide Nanostructures Could Help Power Wearables

Korean-based researchers have been pushing the potential of zinc oxide (ZnO) nanomaterials as a platform for piezoelectric power generators for at least the last five years.

Once again researchers from Korea, this time out of the Korea Advanced Institute of Science and Technology (KAIST), have focused on the potential of ZnO nanomaterials to tap into their piezoelectric capability of converting mechanical energy into electrical energy to power micro devices.

In this latest research, published in Applied Physics Letters, the aim was to exploit the piezoelectric capabilities of ZnO-based nanostructures to serve as nanogenerators in wearable electronics.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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