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Reseachers Create First Integrated Circularly Polarized Light Detector on a Silicon Chip

What do you get when you combine some biomimicry, metamaterials and nanowires? It turns out to be the first integrated circularly polarized light detector on a silicon chip. Its development could usher in a new generation of portable sensors that can use polarized light for applications ranging from drug screening to quantum computing.

Researchers at Vanderbilt University have used silver nanowires to fabricate a metamaterial that is capable of detecting polarized light in a way not unlike the way cuttlefish, bees, or mantis shrimp do it.

“Although it is largely invisible to human vision, the polarization state of light can provide a lot of valuable information,” said assistant professor Jason Valentine in a press release. “However, the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturize. We have managed to get around this limitation by the use of ‘metamaterials’—materials engineered to have properties that are not found in nature.”

Polarized light comes in basically two forms—linear or circular. In contrast to non-polarized light, in which the electric fields of the photons are oriented in random directions, polarized light, whether linear or circular, features electric fields oriented in a single plane. (With circularly polarized light, the plane is continually rotating through 360 degrees.)

One of the distinguishing capabilities of circularly polarized light (CPL) is that it can discern the difference between right-handed and left-handed versions of molecules—a property known as chirality. Chirality is critically important in drugs because whether they are left handed or right handed determines their biological activity. For instance, there is the famous case of thalidomide, which in one chirality alleviates morning sickness in pregnant women and in the other causes birth defects. Having a portable sensor capable of detecting a drug’s chirality could be a game changer.

“Inexpensive CPL detectors could be integrated into the drug production process to provide real time sensing of drugs,” said Vanderbilt University doctoral student Wei Li, in a press release. “Portable detectors could be used to determine drug chirality in hospitals and in the field.”

In research published in the journal Nature Communications, the researchers fabricated the portable CPL sensors by laying down nanowires in a zig-zag pattern over a thin sheet of acrylic affixed to a thick silver plate. This material is affixed to the bottom of a silicon wafer with the nanowire side up.

The nanowires create a sea of electrons that produces “plasmon” density waves, the oscillations in the density of electrons that are generated when photons hit a metal surface. These plasmon density waves absorb energy from the photons that pass through the silicon wafer. The absorption of the energy produces “hot” or energetic electrons, which generate a detectable electrical current.

The researchers found that they could make the zig-zag pattern of nanowires with a right- or left-handed orientation. When they arranged the nanowires in right-handed pattern, the surface absorbed right circularly polarized light and reflected left circularly polarized light. When arranged in a left-handed pattern, the opposite effect occurred. And when they arranged the nanowires to have both left- and right-handed patterns, the sensor could discern between left and right circularly polarized light.

The researchers concede that their current prototype is not efficient enough to be commercially viable. However, they have a few tricks up their sleeves that they believe will improve that efficiency in the next generation of their devices.

Graphene Has a Place on the Hype Cycle, Says European Flagship Director

Two years ago, the European Commission announced the Graphene Flagship, a 10-year, €1 billion effort to help move graphene out of research labs and into commercial applications. The massive effort, which celebrates its second anniversary this week, now includes groups from 23 countries. How has it fared? IEEE Spectrum senior associate editor Rachel Courtland catches up with flagship director Jari Kinaret, a theoretical physicist based at Chalmers University in Sweden, to talk about the program’s progress, graphene hype, and the role of other 2-D materials

To start off, how has the flagship program performed so far?

We are on track to do what we had promised. The first year, we produced more than 300 publications and nearly 600 conference talks, and filed a number of patent applications and some invention disclosures.

One thing that has happened is that we have increased the number of industrial partners. Two years ago, when the flagship started, we were about 75 partners. Something like 20 percent of them were industrial. Then we went through our competitive call, and we added another dozen partners last spring. So, right now we are about 140 partners and the industrial portion is maybe 25 percent. Next April, we will be about 150 partners, and about one-third of them will be industries. This increase in the industrial nature of the program is quite visible, and it’s very much according to plan.

Do you have a sense of where the research community and industry would be without this funding? How do you measure the impact of the program?

Of course as a scientist you would like to do a controlled measurement. You have one sample where you do something and then you have another sample where you don’t do something and you can see the difference. You can’t do that with this kind of one-time event. You can’t have Europe with the flagship and Europe without the flagship and compare those two.

What we can see is that the flagship, through its visibility, has engaged many new industrial branches and many new companies. We cover a very broad range of activities, from say, basic chemical companies like BASF, to component manufacturers—ST Microelectronics would be an example—to systems integrators such as Airbus. Without the flagship, it would be very difficult for actors in different parts of the value chain to find each other. Now we can bring them all together, and they can work together saying, “to make my components better, I need this kind of material” or “to make a new system if would be really great if we had a component like this.”

So people can work along the entire chain vertically, but also horizontally. People working with high-frequency electronics or photonics or sensors see that, while the applications are rather different, the manufacturing techniques that need to be developed to commercialize them are similar. We are now working across work packages to focus more on the manufacturing issues. That would be very difficult, if not impossible, without the flagship.

Then there is the great catalyst effect. Many partners have used their participation [to attract] additional funding from their own governments or private funders. Membership is taken as a token of quality.

Are there any accomplishments so far that you’re particularly proud of?

The one that has had perhaps the most media interest is something called [the] shear exfoliation technique, work that was done by Jonathan Coleman and his group at Trinity College Dublin. It is basically a way that you can make graphene in your own kitchen. This shear exfoliation technique has been commercialized by Thomas Swan & Co, Ltd. in the U.K.

Another example, is that we have a group of [researchers] who got together to make a very fast graphene-based photodetector that functions at 50 GHz. It was slightly embarrassing, because they know it functions at least at 50 GHz, but that’s where their measurement equipment finished. They estimated that it functions probably at 200 GHz. That kind of technology has raised a lot of interest among companies that are interested in optical fiber communication.

The third one I would like to pick shows the benefit of the flagship to organizations that are not partners. In the Graphene Week conference last summer in Manchester, which the Graphene Flagship organizes, [Robert Roelver at Bosch] reported on work that they had done with a flagship partner, the Max Planck Institute in Stuttgart, developing a graphene-based magnetic field sensor that was 100 times more efficient than any other existing magnetic field sensor. These magnetic field sensors may sound esoteric, but we have them in cell phones as a backup to GPS. That’s three examples that I can give you right off the top of my head.

Where do you hope to be when the flagship is complete?

At the end of 10 years, we hope that graphene and related layered materials have left the academic laboratories and entered society as new products in many areas. This is a very ambitious goal if you think of other new materials, like carbon fiber composites. They were developed in the 70’s, roughly speaking, and now they are starting to make an appearance in things like [family] cars. We want to get to that point, and we realize 10 years is too short to make it all the way. But in 10 years we can certainly make it a good part of the way there, so that in some areas, people are no longer surprised or raising eyebrows if they hear, ‘This is made of graphene.’”

There’s already a lot of discussion about graphene’s applications, and it seems like a lot of people are asking whether the material is overly hyped.

Yes, a very good point. One of the things I always show in my presentations these days is something called the Gartner hype cycle.

It’s clear that graphene has probably passed the high peak, the peak of inflated expectations, and we are in the downward-going slope. That downward-going slope of the hype occurs when people realize that this isn’t the solution to all the world’s problems. Typically it’s associated with negative press. People are starting to wonder if everything that has been promised is only hot air or if something is really coming out of it.

What we hope to do is make this dip less dramatic. One way it’s probably going to be less dramatic is that different graphene technologies mature at different rates. If you are in the sports field, you would have to say that graphene has already reached the plateau of productivity. The plateau of productivity is where your reaction if you hear that something contains graphene is, “so what?” It basically has no news value. I can take a ten-minute walk from my office and buy both alpine skis and tennis racquets made of graphene composites. It’s becoming, if not commonplace, at least within reach. Other areas like graphene-based electronics are going to take a much longer time to get to the plateau of productivity.

How can you push a graphene-based technology toward that plateau of productivity?

You need to identify the applications that are worth pursuing. Those could be applications where the replacement of an existing material by graphene is relatively simple or applications where the benefit of using graphene is worth the extra effort.

One thing that seems to be common is the gap between the stage where it works in the lab and the place where it works well enough that you can be pretty sure that it will be a product. External programs such as the flagship, where many partners collaborate and share the risk, are very helpful.

Graphene lacks a natural band gap, the energy barrier in a semiconductor that gives the material a natural “off” and “on” state. Is this a big issue?

This is a non-issue in many areas—take composite materials or using graphene in electrodes in batteries. In photonics, that means you can absorb light regardless of wavelength, so it’s a very positive aspect. In digital electronics it is close to being a game stopper. You can’t just take your silicon MOSFET design and replace the silicon with graphene, because you would find great difficulty in turning your transistor off. For high-frequency applications, the issue is not nearly as bad but it is still a challenge.

For digital electronics, there is still quite a lot of promise, because the electrons are so fast. You need to come up with some other kind of design than the standard MOSFET structure. People are working on that. Tunneling transistors or other vertical structures show a respectable on-off ratio, but have challenges when it comes to manufacturing. So yes, lack of bandgap is an issue. You probably should not take the hexagonal piece of graphene and try to fit it into the circular hole that is digital electronics.

What about other two-dimensional materials?

Indeed, some of them have bandgaps and therefore they would be easier substitutes for silicon. There are challenges regarding the materials’ quality, production, and integration with existing technologies, but as far as existence of a bandgap is concerned, they certainly offer an easier solution than graphene does.

Other materials have been in the flagship from the very beginning, and they still are in the flagship. If one was pedantic, it should be called “Graphene and Related Materials Flagship”, but that just does not work. Your editor would not allow you to have a title like that.

Is there anything else you’d like to add?

When physicists talk about graphene, we think first about the electronics applications, because they tend to be closest to our minds. The real first applications may be in areas we perhaps don’t prioritize so much, things like different kinds of composite materials.

Now people are getting excited about the fact that graphene is an impermeable membrane—it doesn’t let anything to go through. If you have an impermeable membrane, you can make holes in it, and you can choose what goes through and what doesn’t. So you may be able to separate carbon dioxide from other gases, and that would be of great interest for, say, carbon sequestration. The fact that there is more to the world than electronics is, I think, what we are learning to appreciate more and more.

Graphene Keeping It Cool In Electronics

Cooling fans and other system-level solutions are reaching their limits as circuit densities continue to grow. It’s no wonder then that graphene’s remarkable heat conductivity has led to a lot of research into using it to for thermal management in electronics.

Now an international team of researchers, organized by a team at the University of Michigan, has found that layered graphene can be an important tool for thermal management because of its ability to release heat efficiently.

In research published in the journal Nature Communications, the scientists demonstrated that the electrostatic interactions between electrically charged particles—known as Coulomb interactions—in  different layers of multi-layered graphene offers a key mechanism for dispersing heat. This occurs despite the fact that all electronic states are strongly confined within individual 2D layers.

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Thin Is in for Invisibility Cloaks

When Xiang Zhang first invented an invisibility cloak that worked at optical wavelengths, his son complained that it wouldn’t allow him to easily stroll around unseen, like Harry Potter sneaking through the library.

“My son always joked that, ‘Daddy, if I want to make myself disappear I have to carry a huge cylinder around myself,’” says Zhang, a professor of mechanical engineering at the University of California, Berkeley. The boy wondered why he couldn’t have something more like a sunblock, a spray-on material that rendered him invisible. That may not be such a far-fetched vision, Zhang says.

He and his colleagues have created an extremely thin cloak that works in visible light, made of a metasurface, a special type of metamaterial that is thin enough to be considered two-dimensional. They described the device in last week’s Science.

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Carbon Nanotubes Can Outperform Other Carbon Capture Materials

Whether you see carbon capture tools as solutions to environmental remediation or power generation, it has become increasingly clear that nanomaterials are pretty good at it.

Now researchers at Technische Universität Darmstadt in Germany and the Indian Institute of Technology Kanpur have found that they can tailor the gas adsorption properties of vertically aligned carbon nanotubes (VACNTs) by altering their thickness, height, and the distance between them.

“These parameters are fundamental for 'tuning' the hierarchical pore structure of the VACNTs,” explained Mahshid Rahimi and Deepu Babu, doctoral students at the Technische Universität Darmstadt who were the paper's lead authors, in a press release. “This hierarchy effect is a crucial factor for getting high-adsorption capacities as well as mass transport into the nanostructure. Surprisingly, from theory and by experiment, we found that the distance between nanotubes plays a much larger role in gas adsorption than the tube diameter does.”

Previously, carbon materials in gas adsorption-desorption suffered from hysteresis effects in which the property of the material was somewhat behind the factors that were changing it. In the case of carbon nanotubes, the sizes, structure, and distribution of the pores in the material were slow to react to these changes.

In research published in The Journal of Chemical Physics, the researchers first set out through computer modeling to gain a better theoretical understanding of adsorption and selectivity in carbon materials.

Then through experimentation, the researchers validated their models and demonstrated that VACNTs adsorbed the gases of carbon dioxide (CO2) and sulfur dioxide (SO2) better than traditional adsorption materials, such as porous carbon, zeolites, and metal organic frameworks within the mid-pressure (30 bars) regime. “This adsorption range is important for technologically relevant processes like gas storage for automotive purposes,” noted Rahimi in the release.

In future research, the plan is to introduce specific atoms to the carbon nanotubes for elemental doping.

Rahimi added: “This will allow us to further tune the gas selectivity. Another area we'll also explore is ‘controlled carbon nanotube openings’ in such VACNTs to increase the gas adsorption.”

New Avenue Proposed for Bringing Carbyne Into the Real World

A couple of years ago, a material dubbed carbyne—which is a chain of carbon atoms held together by either double or alternating single and triple atomic bonds—was awarded the title of the world’s strongest material. Later, scientists also demonstrated that it has the unusual property of being able to change from being a conductor to an insulator when it’s stretched by as little as 3 percent.

Both are great properties to have. But thus far carbyne has proven nearly impossible to make in the real world. As a result, its exploits have remained firmly planted in the realm of computer models. While carbyne has been found in highly compressed graphite, and it has even been synthesized at room temperature, no one has devised a way to produce the material in bulk.

While researchers at Lawrence Livermore National Laboratory (LLNL) looking at carbyne’s properties in computer models and discovering some new ones that should benefit nanoelectronics, they discovered a potential avenue for producing the material.

In research published in the Journal of Physical Chemistry, the LLNL researchers demonstrated through computer models that it was possible to form carbyne fiber bundles by melting graphite with a laser.

“There’s been a lot of speculation about how to make carbyne and how stable it is,” said Nir Goldman, an LLNL scientist, in the press release. “We showed that laser melting of graphite is one viable avenue for its synthesis.” And depending on how you cook the graphite, the resulting carbyne could have applications, including tunable semiconductors or even hydrogen storage materials. Says Goldman:

Our method shows that carbyne can be formed easily in the laboratory or otherwise. The process also could occur in astrophysical bodies or in the interstellar medium, where carbon-containing material can be exposed to relatively high temperatures and carbon can liquefy.

Whether this technique can be scaled up to producing carbyne in bulk remains to be seen. But if it proves to be a viable method for carbyne synthesis, it does buoy hope for the material’s use in nanoelectronics, where it could perform some amazing feats such as adjusting the amount of electrical current traveling through a circuit, according to the user’s need.

Graphene Filament Enables Fabrication of Electronic Devices with 3-D Printing

Earlier this month, we reported on research that was bringing the attractive qualities of graphene in its 2-D form to the fabrication of 3-D objects.

Now a start-up based in Calverton, NY, Graphene 3D Lab, Inc., has made commercially available a graphene-based conductive polymer filament for use in 3-D printing to fabricate electronic devices. The graphene-based filament, which is targeted for both industry and hobbyists, has been dubbed Black Magic 3D.

“Our material is the most electrically conductive material on the market right now and is the best option for 3-D printing of electronics,” claimed Daniel Stolyarov, who along with Elena Polyakova, are Co-CEOs, in an e-mail interview. “Even though our material is more expensive, you only need a very small amount (a few grams), which would cost as low as $1, along with regular material to make 3-D printed electronics. Without graphene this is not possible.”

Stolyarov believes that this graphene-enabled polymer filament is unique on the market in its ability to impart electrical conductivity. Stolyarov argues that their product compares favorably to other 3-D printing filaments that have at best a volume resistivity of 15 Ohms-centimeter (Ohms-cm), whereas Black Magic 3D’s volume resistivity measures at 0.6 Ohms-cm—25 times better. According to Stolyarov, 15 Ohms-cm is just not good enough for most of electronic applications. If electrical properties are poor, the device will not work properly.

Stolyarov has pointed to the emerging trend of 3-D printed electronics, which he believes may soon show explosive growth. An indication of this potential was the recent launch of a new 3-D printer from a company called Voxel8 that specifically targets the printing of electronics and circuitry.

However, Stolyarov is quick to note that his company’s graphene-based filament can be used with just about any 3-D printer on the market now, from hobbyist to industrial.

To demonstrate how the graphene-based filament can fabricate devices requiring high thermal and electrical conductivity, the company produced a battery. It seems that this battery design remains primarily to demonstrate the capabilities of the graphene 3-D printing filament.

“The 3-D printed graphene battery project is still being developed and we are very much looking forward to offering more details on the technology in the future,” said Stolyarov.

Silicon Nanoparticle Could be Heart of New Optical Transistor

The quest to transform the basis of computing from electrons to photons has been full of challenges. The aim has been to get photonic circuits to do what electronic ICs do but do it much faster—at the speed of light, achieving it has remained elusive. 

Researchers at ITMO University in St. Petersburg, Russia suggest that a new technique could be a big step toward photonic ICs and optical computing. It uses a single silicon nanoparticle as an optical transistor.

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Researchers Create Quantum Dots that Transmit Identical Single Photons

Single photons will play an important role in quantum communication. This will require quantum repeaters, in much the same way that optical amplifiers are required for the transmission of digital data through optical fibers. However, these quantum repeaters will work only if all the single photons that they receive have the exact same wavelength. A team of researchers at universities in Switzerland, Germany, and France reported on 8 September in Nature Communications that they’ve developed quantum dots that can produce streams of photons with identical wavelengths. 

To create the photons, the researchers used self-assembled indium gallium arsenide (InGaAs) quantum dots embedded in gallium arsenide. Each quantum dot, although it consists of about a hundred thousand atoms, traps a single electron that can occupy two energy levels. By illuminating the quantum dot with laser light, the electron is moved into the higher energy state. When the electron drops down to its lower energy level, it emits a single photon whose wavelength is determined by the difference in the energy of the two levels. “In many ways it behaves like a single atom, and this is why it is often called an 'artificial atom,’” says Andreas Kuhlmann a post-doctoral researcher at the University of Basel who was the paper’s lead author. “But because it is inside a semiconductor it is quite robust, and that is, of course, nice if you want at one point to develop a product.”  

Still, unlike a single atom, the atoms in the quantum dot are an unruly bunch.  The fluctuating nuclear spins of the atoms in the quantum dot interact with the electron spin, and fluctuating electric fields created by electrons hopping around cause the two energy levels of the quantum dot to wobble. This results in the emission of photons with wavelengths that vary, if ever so slightly. “In order to get indistinguishable single photons, which all have exactly the same color, we needed to find a way to suppress the noise,” explains Kuhlmann.

How did they do it? They cooled the quantum dot to 4.2 Kelvin. They could reduce the noise even further by cooling it even more, an option that is not that attractive.  Despite what one might think at first blush, “4.2 Kelvin is in some ways quite warm; at some point you want to develop a product, and it makes sense to stay as warm as possible,” says Kuhlmann.

That wasn’t the only trick they employed to reduce the noise, says Kuhlmann. “The samples are grown layer by layer using molecular beam epitaxy, and when you grow these samples you will get some defects.” Crucial to limiting noise, he says, is limiting these defects because they become populated by electric charges that fluctuate. “It is extremely important that you have high-quality material and that the people who grow these samples know what they are doing,” says Kuhlmann.

The researchers overcame several hurdles. Though nuclear spins are an intrinsic property of the InGaAs semiconductor, the researchers found a way to reduce the influence of the nuclear spins.  Because of its indirect band gap, it can’t emit photons when irradiated by light, explains Kuhlmann. But he and his collaborators created a quantum dot that transmits photons. And because they were able to precisely control the number of electrons trapped in the quantum dot by applying a voltage to a gate, they were able to reduce this number to one. Over a broad voltage range, the quantum dot is empty. However, at a particular voltage, the dot’s sweet-spot, the noise from the nuclear spins is strongly suppressed. “We don’t know why yet”, says Kuhlmann, “but the improvement in the photons is immediately noticeable.” 

The result, says Kuhlmann:

If we switch on our laser, and there is a single electron inside this quantum dot, then this permanent excitation suppresses the fluctuations of the nuclear spins in the sample. When no electron is trapped in the quantum dot, we found that by changing the electrical field that we apply to the device, we also can suppress the spin noise. 

One of the problems that still has to resolved is that a number of photons are lost when the photons exit the device via the gallium arsenide layer that covers the quantum dot. “There are ways to engineer these devices; for example, you put a quantum dot inside a nanowire, and you can send almost all the photons along the waveguide,” says Kuhlmann.  There should be applications galore for the device in quantum communication and computing.  “A single photon is an ideal flying qubit,” says Kuhlmann.

Peering Into Nanoparticles One at a Time Reveals Hidden World

Imagine you could single out individuals in a large group and see what each was doing instead of observing a large number of them all grouped together. You could detect how each is distinct within the group.

This is essentially what researchers at Chalmers University in Sweden have been able to achieve with a new microscopy technique that is capable of looking at a single nanoparticle rather than just a mass of them all clumped together.

“We were able to show that you gain deeper insights into the physics of how nanomaterials interact with molecules in their environment by looking at the individual nanoparticle as opposed to looking at many of them at the same time, which is what is usually done,” said Associate Professor Christoph Langhammer, who led the project, in a press release.

In their findings, published in the journal Nature Materials, the researchers leveraged an imaging technique known as plasmonic  nanospectroscopy, which involves exploiting the oscillations in the density of electrons that are generated when photons hit a metal surface.

The researchers applied the experimental spectroscopy technique to examine hydrogen absorption in single palladium nanoparticles. The observations were surprising. They discovered that despite various nanoparticles having the same size and shape, they would absorb hydrogen at pressures as different as 40 millibars.

In real world applications, this observation could help lead to more sensitive hydrogen sensors for detecting leaks in fuel-cell-powered vehicles.

“One main challenge when working on hydrogen sensors is to design materials whose response to hydrogen is as linear and reversible as possible. In that way, the gained fundamental understanding of the reasons underlying the differences between seemingly identical individual particles and how this makes the response irreversible in a certain hydrogen concentration range can be helpful,” added Langhammer in the release.

While others have been able to image single nanoparticles previously, those efforts came at a rather high cost of heating the nanoparticles up, or impacting them in some other way that eliminates the ability to observe them accurately.

“When studying individual nanoparticles you have to send some kind of probe to ask the particle ‘what are you doing?’,” said Langhammer. “This usually means focusing a beam of high-energy electrons or photons or a mechanical probe onto a very tiny volume. You then quickly get very high energy densities, which might perturb the process you want to look at.”

Not only is this effect is minimized in their new approach, according to Langhammer, but it is also compatible with ambient conditions, so that it is possible to study nanoparticles one at a time in their actual environments. This ability to observe nanoparticles outside the lab could prove to be a key development for studies on the impact of nanoparticles in the environment.



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

Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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