Nanoclast iconNanoclast

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.

Read More

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.

Read More

Photonic Circuits Get a Boost from Combination of 2-D Materials

Recently the field of plasmonics—which exploits the surface plasmons generated when photons hit a metal structure—has opened up the real possibility that photonic circuits could duplicate what electronic ICs do.  Previously, photonic circuits were just too large to be functional because of their need to accommodate different wavelengths of light.

Despite several advances, the plasmons still lost energy too quickly, which reduced the distance they could travel.  Now researchers in Spain, Italy, and the United States have developed a solution to this issue by combining graphene and boron nitride.

It has been known that when graphene is encapsulated in boron nitride, electrons can move ballistically for long distances without scattering, even at room temperature.

In this latest research, the international team—representing the Institute of Photonic Sciences (ICFO) in Barcelona, CIC nanoGUNE in San Sebastian, Spain, CNR/Scuola Normale Superiore in Pisa, Italy (all members of the EU Graphene Flagship), plus Columbia University in New York City—discovered, somewhat surprisingly, that the combination of graphene sandwiched between two films of hexagonal ​boron nitride (h-BN) is an excellent host for extremely strongly confined light and quite capable of suppressing of plasmon losses.

"It is remarkable that we make light move more than 150 times slower than the speed of light, and at length scales more than 150 times smaller than the wavelength of light,” said ICFO’s Frank Koppens, in a press release. “In combination with the all-electrical capability to control nanoscale optical circuits, one can envision very exciting opportunities for applications."

Columbia University has been at the forefront of creating these heterostructures by combining graphene and boron nitride since 2013. This latest research is seen as just an initial step in investigating the nano-optoelectronic properties of combining different two-dimensional materials.

"Boron nitride has proven to be the ideal 'partner' for graphene, and this amazing combination of materials continues to surprise us with its outstanding performance in many areas,” said Columbia University professor James Hone, in a press release.

In May of last year, researchers from CIC nanoGUNEand ICFO, along with the company Graphenea, demonstrated that an optical antenna made from graphene can capture infrared light and transform it into plasmons.

Rainer Hillenbrand from CIC nanoGUNE believes this latest research represents a significant advance in optoelectronics.

“Now we can squeeze light and at the same time make it propagate over significant distances through nanoscale materials, said Hillenbrand in the press release. “In the future, low-loss graphene plasmons could make signal processing and computing much faster, and optical sensing more efficient."

Nanobowl Solar Concentrator Boosts Organic Solar Cell Efficiency

Solar-concentrator technology has long held out promise as a way to increase the conversion efficiency of photovoltaics.

Now researchers at the Hong Kong University of Science and Technology (HKUST) have tackled the tricky issue of creating a solar concentrator for an organic photovoltaic (OPV) by developing a novel “nanobowl” optical concentrator fabricated on low-price aluminum foil.

Most OPV devices are based on a design that includes a glass substrate with indium tin oxide (ITO) electrodes. But there are a number of problems with this design, most notably that the solar cells are not flexible and that the ITO electrode compromises the OPV’s performance.

Aluminum foil substrates have the advantages of excellent conductivity, flexibility, inexpensiveness, and roll-to-roll manufacturing. But it is more difficult to get a uniform organic semiconductor “active layer” on aluminum foil’s textured surface than on the smooth surface of a glass substrate.

In work published in the journal Science Bulletin, the Hong Kong researchers were able to overcome this obstacle by incorporating nanobowl optical concentrators on the aluminum foil using a chemical process. The nanobowl optical concentrators improve the optical absorption in the active layer of the OPV despite the textured surface of the aluminum foil. Simulation studies carried out by the researchers revealed that such improvement was the result of the superior photon capturing capability of the nanobowls.

The nanobowl optical concentrator OPV demonstrated a more than 28-percent enhancement in power conversion efficiency versus devices without a nanobowl, the researchers claim. This 28-percent increase still only brings the nanobowl solar cell to an energy conversion efficiency of 3.12 percent, well below the 8.4-percent conversion efficiency record for OPVs reached last year.

Despite this comparatively low conversion efficiency, the promise of this latest research is that it shows a way forward in the development of geometrical light trapping with low-cost, chemical-based processes for manufacturing OPVs.

Graphene Hybrid Resists High Temperatures and Humidity

Almost three years ago, researchers at the University of Exeter in the UK developed a hybrid material that had molecules of ferric chloride sandwiched between two layers of graphene. The new material had better conductivity than graphene on its own. The researchers dubbed their creation GraphExeter.

Now the team at Exeter has announced that GraphExeter is resistant to both high temperatures and humidity.

This resistance to heat came as a surprise to the researchers because the molecules in the material on their own actually melt in air at room temperature.

Read More

Piezoelectricity in a Free-Standing 2-D Material

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated that a free-standing single layer of molybdenum disulfide (MoS2) can exhibit the piezoelectric effect. The result could make possible new nanoscale low-power switches for sensors and other electronics.

The piezoelectric effect, in which compressing or stretching a material produces a voltage or where a voltage can cause a material to expand or contract, has been a mainstay for a range of electronic devices. But it has been limited to bulk crystals. This latest demonstration of the effect in a two-dimensional material promises to expand those applications.

 “The discovery of piezoelectricity at the molecular level not only is fundamentally interesting, but also could lead to tunable piezo-materials and devices for extremely small force generation and sensing,” said Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division, in a press release.

In October 2014, joint research out of Columbia University and Georgia Tech also demonstrated that MoS2 exhibits piezoelectricity and the piezotronic effect, which is the use of the piezoelectric effect as the gate voltage in transistor or similar device.

But in that research, the team at Columbia positioned thin flakes of MoS2 on flexible plastic substrates. In the new work out of Berkeley Lab, however, no substrate was used, and the researchers showed that free-standing MoS2 could demonstrate a piezoelectric effect.

In both efforts, the piezoelectric effect in MoS2 only appeared when an odd number of layers were used (1,3,5, etc.).

“This discovery is interesting from a physics perspective since no other material has shown similar layer-number sensitivity,”  Hanyu Zhu, one of the co-authors of the research published in the journal Nature Nanotechnology,  said in a press release. “The phenomenon might also prove useful for applications in which we want devices consisting of as few as possible material types, where some areas of the device need to be non-piezoelectric.”

In addition to logic switches and bio-sensors, the researchers believe that piezoelectricity in MoS2 could have an impact on the development of so-called “valleytronics.” Valleytronics essentially moves us away from the use of charge as a means for storing information and toward a scheme where we instead we encode data in the wave “quantum number” of an electron in a crystalline material. The term valleytronics refers to the fact that if you plotted the energy of electrons relative to their momentum on a graph, the resulting curve would feature two deep valleys.

The researchers are investigating the possibility of using piezoelectricity to directly control valleytronic properties  in molybdenum disulfide.

Advertisement

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
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
Advertisement
Load More