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Imagine an electronic newspaper that you could roll up and spill your coffee on, even as it updated itself before your eyes.

It’s an example of the technological revolution that has been waiting to happen, except for one major problem that, until now, scientists have not been able to resolve.

Researchers at McMaster University have cleared that obstacle by developing a new way to purify carbon nanotubes – the smaller, nimbler semiconductors that are expected to replace silicon within computer chips and a wide array of electronics.

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the Adronov group. Credit: Alex Adronov, McMaster University

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the Adronov group. Credit: Alex Adronov, McMaster University

“Once we have a reliable source of pure nanotubes that are not very expensive, a lot can happen very quickly,” says Alex Adronov, a professor of Chemistry at McMaster whose research team has developed a new and potentially cost-efficient way to purify carbon nanotubes.

Carbon nanotubes – hair-like structures that are one billionth of a metre in diameter but thousands of times longer – are tiny, flexible conductive nano-scale materials, expected to revolutionize computers and electronics by replacing much larger silicon-based chips.

A major problem standing in the way of the new technology, however, has been untangling metallic and semiconducting carbon nanotubes, since both are created simultaneously in the process of producing the microscopic structures, which typically involves heating carbon-based gases to a point where mixed clusters of nanotubes form spontaneously as black soot.

Only pure semiconducting or metallic carbon nanotubes are effective in device applications, but efficiently isolating them has proven to be a challenging problem to overcome. Even when the nanotube soot is ground down, semiconducting and metallic nanotubes are knotted together within each grain of powder. Both components are valuable, but only when separated.

Researchers around the world have spent years trying to find effective and efficient ways to isolate carbon nanotubes and unleash their value.

While previous researchers had created polymers that could allow semiconducting carbon nanotubes to be dissolved and washed away, leaving metallic nanotubes behind, there was no such process for doing the opposite: dispersing the metallic nanotubes and leaving behind the semiconducting structures.

Now, Adronov’s research group has managed to reverse the electronic characteristics of a polymer known to disperse semiconducting nanotubes – while leaving the rest of the polymer’s structure intact. By so doing, they have reversed the process, leaving the semiconducting nanotubes behind while making it possible to disperse the metallic nanotubes.

The researchers worked closely with experts and equipment from McMaster’s Faculty of Engineering and the Canada Centre for Electron Microscopy, located on the university’s campus.

“There aren’t many places in the world where you can to this type of interdisciplinary work,” Adronov says.

The next step, he explains, is for his team or other researchers to exploit the discovery by finding a way to develop even more efficient polymers and scale up the process for commercial production.

One of the most critical issues the United States faces today is preventing terrorists from smuggling nuclear weapons into its ports. To this end, the U.S. Security and Accountability for Every Port Act mandates that all overseas cargo containers be scanned for possible nuclear materials or weapons.

Detecting neutron signals is an effective method to identify nuclear weapons and special nuclear materials. Helium-3 gas is used within detectors deployed in ports for this purpose.

The catch? While helium-3 gas works well for neutron detection, it’s extremely rare on Earth. Intense demand for helium-3 gas detectors has nearly depleted the supply, most of which was generated during the period of nuclear weapons production during the past 50 years. It isn’t easy to reproduce, and the scarcity of helium-3 gas has caused its cost to skyrocket recently — making it impossible to deploy enough neutron detectors to fulfill the requirement to scan all incoming overseas cargo containers.

Helium-4 is a more abundant form of helium gas, which is much less expensive, but can’t be used for neutron detection because it doesn’t interact with neutrons.

A group of Texas Tech University researchers led by Professors Hongxing Jiang and Jingyu Lin report this week in Applied Physics Letters, from AIP Publishing, that they have developed an alternative material — hexagonal boron nitride semiconductors — for neutron detection. This material fulfills many key requirements for helium gas detector replacements and can serve as a low-cost alternative in the future.

The group’s concept was first proposed to the Department of Homeland Security’s Domestic Nuclear Detection Office and received funding from its Academic Research Initiative program six years ago.

By using a 43-micron-thick hexagonal boron-10 enriched nitride layer, the group created a thermal neutron detector with 51.4 percent detection efficiency, which is a record high for semiconductor thermal neutron detectors.

“Higher detection efficiency is anticipated by further increasing the material thickness and improving materials quality,” explained Professor Jiang, Nanophotonics Center and Electrical & Computer Engineering, Whitacre College of Engineering, Texas Tech University.

“Our approach of using hexagonal boron nitride semiconductors for neutron detection centers on the fact that its boron-10 isotope has a very large interaction probability with thermal neutrons,” Jiang continued. “This makes it possible to create high-efficiency neutron detectors with relatively thin hexagonal boron nitride layers. And the very large energy bandgap of this semiconductor — 6.5 eV — gives these detectors inherently low leakage current densities.”

The key significance of the group’s work? This is a completely new material and technology that offers many advantages.

“Compared to helium gas detectors, boron nitride technology improves the performance of neutron detectors in terms of efficiency, sensitivity, ruggedness, versatile form factor, compactness, lightweight, no pressurization … and it’s inexpensive,” Jiang said.

This means that the material has the potential to revolutionize neutron detector technologies.

“Beyond special nuclear materials and weapons detection, solid-state neutron detectors also have medical, health, military, environment, and industrial applications,” he added. “The material also has applications in deep ultraviolet photonics and two-dimensional heterostructures. With the successful demonstration of high-efficiency neutron detectors, we expect it to perform well for other future applications.”

The main innovation behind this new type of neutron detector was developing hexagonal boron nitride with epitaxial layers of sufficient thickness — which previously didn’t exist.

“It took our group six years to find ways to produce this new material with a sufficient thickness and crystalline quality for neutron detection,” Jiang noted.

Based on their experience working with III-nitride wide bandgap semiconductors, the group knew at the outset that producing a material with high crystalline quality would be difficult.

“It’s surprising to us that the detector performs so well, despite the fact that there’s still a little room for improvement in terms of material quality,” he said.

One of the most important impacts of the group’s work is that “this new material and its potential should begin to be recognized by the semiconductor materials and radiation detection communities,” Jiang added.

Now that the group has solved the problem of producing hexagonal boron nitride with sufficient thickness, as well as crystalline quality to enable the demonstration of neutron detectors with high efficiency, the next step is to demonstrate high-sensitivity of large-size detectors.

“These devices must be capable of detecting nuclear weapons from distances tens of meters away, which requires large-size detectors,” Jiang added. “There are technical challenges to overcome, but we’re working toward this goal.”

Scientists at the Energy Department’s National Renewable Energy Laboratory (NREL), in collaboration with researchers at Shanghai Jiao Tong University (SJTU), devised a method to improve perovskite solar cells, making them more efficient and reliable with higher reproducibility.

The research, funded by the U.S. Department of Energy SunShot Initiative, involved hybrid halide perovskite solar cells and revealed treating them with a specific solution of methyl ammonium bromide (MABr) would repair defects, improving efficiency. The scientists converted a low-quality perovskite film with pinholes and small grains into a high-quality film without pinholes and with large grains. Doing so boosted the efficiency of the perovskite film in converting sunlight to 19 percent.

The efficiency of perovskites in converting sunlight into electricity has jumped from slightly less than 4 percent in 2009, when the first tests were done, to more than 22 percent today. However, the efficiency can fluctuate according to the skills of the researchers making perovskites at different laboratories, to somewhere between 15 percent and 20 percent.

Perovskite films are typically grown using a solution of precursor chemicals that form the crystals, which are then exposed to a second anti-solvent that removes the precursor solvent. The fast-crystallization process is almost an art. NREL researchers found that, because of the narrow time window for properly adding the anti-solvent, it is easy to miss that window and perovskite crystals with defects could form. Defects, like noncontinuous crystals and nonuniform crystals with relatively small crystallite sizes and pinholes, can significantly reduce the effectiveness of a perovskite cell.

The scientists from NREL and SJTU came up with a better method, using what’s called the Ostwald ripening process. The process involves small crystals dissolving and then redepositing onto larger crystals. The researchers were able to induce the Ostwald ripening process by treating the perovskite with a MABr solution. The amount of the solution proved key, as the ideal was proven to be about 2 milligrams per milliliter.

“With the Ostwald ripening process, different-sized nanocrystals formed with different film qualities could then grow into pinhole-free perovskite films with similar large crystal sizes,” the researchers noted in the article. “Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication.”

The improved film quality made the cells more stable. The perovskite cells treated with MABr were shown to be more efficient than those without the treatment. Untreated cells had an efficiency of about 14 percent to 17 percent, while cells treated with the MABr solution had an efficiency of more than 19 percent.

Designers of solar cells may soon be setting their sights higher, as a discovery by a team of researchers has revealed a class of materials that could be better at converting sunlight into energy than those currently being used in solar arrays. Their research shows how a material can be used to extract power from a small portion of the sunlight spectrum with a conversion efficiency that is above its theoretical maximum — a value called the Shockley-Queisser limit. This finding, which could lead to more power-efficient solar cells, was seeded in a near-half-century old discovery by Russian physicist Vladimir M. Fridkin, a visiting professor of physics at Drexel, who is also known as one of the innovators behind the photocopier.

The team, which includes scientists from Drexel University, the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, the University of Pennsylvania and the U. S. Naval Research Laboratory recently published its findings in the journal Nature Photonics. Their article “Power conversion efficiency exceeding the Shockley-Queisser limit in a ferroelectric insulator,” explains how they were able to use a barium titanate crystal to convert sunlight into electric power much more efficiently than the Shockley-Queisser limit would dictate for a material that absorbs almost no light in the visible spectrum — only ultraviolet.

A phenomenon that is the foundation for the new findings was observed by Fridkin, who is one of the principal co-authors of the paper, some 47 years ago, when he discovered a physical mechanism for converting light into electrical power — one that differs from the method currently employed in solar cells. The mechanism relies on collecting “hot” electrons, those that carry additional energy in a photovoltaic material when excited by sunlight, before they lose their energy. And though it has received relatively little attention until recently, the so-called “bulk photovoltaic effect,” might now be the key to revolutionizing our use of solar energy.

The limits of solar energy

Solar energy conversion has been limited thus far due to solar cell design and electrochemical characteristics inherent to the materials used to make them.

“In a conventional solar cell — made with a semiconductor — absorption of sunlight occurs at an interface between two regions, one containing an excess of negative-charge carriers, called electrons, and the other containing an excess of positive-charge carriers, called holes,” said Alessia Polemi, a research professor in Drexel’s College of Engineering and one of the co-authors of the paper.

In order to generate electron-hole pairs at the interface, which is necessary to have an electric current, the sunlight’s photons must excite the electrons to a level of energy that enables them to vacate the valence band and move into the conduction band — the difference in energy levels between these two bands is referred to as the “band gap.” This means that in photovoltaic materials, not all of the available solar spectrum can be converted into electrical power. And for sunlight photon energies that are higher than the band gap, the excited electrons will lose it excess energy as heat, rather than converting it to electric current. This process further reduces the amount of power can be extracted from a solar cell.

“The light-induced carriers generate a voltage, and their flow constitutes a current. Practical solar cells produce power, which is the product of current and voltage,” Polemi said. “This voltage, and therefore the power that can be obtained, is also limited by the band gap.”

But, as Fridkin discovered in 1969 — and the team validates with this research — this limitation is not universal, which means solar cells can be improved.

New life for an old theory

When Fridkin and his colleagues at the Institute of Crystallography in Moscow observed an unusually high photovoltage while studying the ferroelectric antimony sulfide iodide — a material that did not have any junction separating the carriers — he posited that crystal symmetry could be the origin for its remarkable photovoltaic properties. He later explained how this “bulk photovoltaic effect,” which is very weak, involves the transport of photo-generated hot electrons in a particular direction without collisions, which cause cooling of the electrons.

This is significant because the limit on solar power conversion from the Shockley-Queisser theory is based on the assumption that all of this excess energy is lost — wasted as heat. But the team’s discovery shows that not all of the excess energy of hot electrons is lost, and that the energy can, in fact, be extracted as power before thermalizing.

“The main result — exceeding [the energy gap-specific] Shockley-Queisser [power efficiency limit] using a small fraction of the solar spectrum — is caused by two mechanisms,” Fridkin said. “The first is the bulk photovoltaic effect involving hot carriers and second is the strong screening field, which leads to impact ionization and multiplication of these carriers, increasing the quantum yield.”

Impact ionization, which leads to carrier multiplication, can be likened to an array of dominoes in which each domino represents a bound electron. When a photon interacts with an electron, it excites the electron, which, when subject to the strong field, accelerates and ‘ionizes’ or liberates other bound electrons in its path, each of which, in turn, also accelerates and triggers the release of others. This process continues successively — like setting off multiple domino cascades with a single tipped tile — amounting to a much greater current.

This second mechanism, the screening field, is an electric field is present in all ferroelectric materials. But with the nanoscale electrode used to collect the current in a solar cell, the field is enhanced, and this has the beneficial effect of promoting impact ionization and carrier multiplication. Following the domino analogy, the field drives the cascade effect, ensuring that it continues from one domino to the next.

“This result is very promising for high efficiency solar cells based on application of ferroelectrics having an energy gap in the higher intensity region of the solar spectrum,” Fridkin said.

Building toward a breakthrough

“Who would have expected that an electrical insulator could be used to improve solar energy conversion?” said Jonathan E. Spanier, a professor of materials science, physics and electrical engineering at Drexel and one of the principal authors of the study. “Barium titanate absorbs less than a tenth of the spectrum of the sun. But our device converts incident power 50 percent more efficiently than the theoretical limit for a conventional solar cell constructed using this material or a material of the same energy gap.”

This breakthrough builds on research conducted several years ago by Andrew M. Rappe, Blanchard Professor of Chemistry and of Materials Science & Engineering at the University of Pennsylvania, one of the principal authors, and Steve M. Young, also a co-author on the new report. Rappe and Young showed how bulk photovoltaic currents could be calculated — which led Spanier and collaborators to investigate if higher power conversion efficiency could be attained in ferroelectrics.

“There are many exciting reports utilizing nanoscale materials or phenomena for improving solar energy conversion,” Spanier said. “Professor Fridkin appreciated decades ago that the bulk photovoltaic effect enables free electrons that are generated by light and have excess energy to travel in a particular direction before they cool or ‘thermalize’–and lose their excess energy to vibrations of the crystal lattice.”

Rappe was also responsible for connecting Spanier to Fridkin in 2015, a collaboration that set in motion the research now detailed in Nature Photonics — a validation of Fridkin’s decades-old vision.

“Vladimir is internationally renowned for his pioneering contributions to the field of electroxerography, having built the first working photocopier in the world,” Rappe said. “He then became a leader in ferroelectricity and piezoelectricity, and preeminent in understanding light interactions with ferroelectrics. Fridkin explained how, in crystals that lack inversion symmetry, photo-excited electrons acquire asymmetry in their momenta. This, in turn, causes them to move in one direction instead of the opposite direction. It is amazing that the same person who discovered these bulk photovoltaic effects nearly 50 years ago is now helping to harness them for practical use in nanomaterials.”

Dmitry Fedyanin from the Moscow Institute of Physics and Technology and Mario Agio from the University of Siegen and LENS have predicted that artificial defects in the crystal lattice of diamond can be turned into ultrabright and extremely efficient electrically-driven quantum emitters. Their work published in New Journal of Physics demonstrates the potential for a number of technological breakthroughs, including the development of quantum computers and secure communication lines, which, in contrast to previously proposed schemes, would be able to operate at room temperature.

The research conducted by Dmitry Fedyanin and Mario Agio is focused on the development of efficient electrically-driven single-photon sources — devices that emit single photons when an electrical current is applied. In other words, using such devices, one can generate a photon “on demand” by simply applying a small voltage across the devices, the probability of an output of zero photons is vanishingly low and generation of two or more photons simultaneously is fundamentally impossible.

Until recently, it was thought that quantum dots (nanoscale semiconductor particles) are the most promising candidates for true single-photon sources. However, they operate only at very low temperatures, which is their main drawback – mass application would not be possible if a device has to be cooled with liquid nitrogen or even colder liquid helium, or using refrigeration units, which are even more expensive and power-hungry. At the same time, it was known that certain point defects in the crystal lattice of diamond, which occur when foreign atoms (such as silicon or nitrogen) enter the diamond accidentally or through targeted implantation, can efficiently emit single photons at room temperature. However, this has only been achieved by optical excitation of these defects using external high-power lasers. This method is ideal for research in scientific laboratories, but it is very inefficient in practical devices. Experiments with electrical excitation, on the other hand, did not yield the best results — in terms of brightness, diamond sources lost out significantly (by several orders of magnitude) to quantum dots. As there were no theories describing the photon emission from colour centres in diamonds under electrical excitation, it was not possible to assess the potential of these single-photon sources to see if they could be used as a basis for the quantum devices of the future.

The new publication gives an affirmative answer — defects in the structure of diamond at the atomic level can be used to design highly efficient single-photon sources that are even more promising than their counterparts based on quantum dots.

Operation at the single?photon level will not only increase the energy efficiency of the existing data processing and data transmission devices by more than one thousand times, but will also lay the foundations for the development of novel quantum devices. Building quantum computers is still a prospect of the future, but secure communication lines based on quantum cryptography are already starting to be used. However, today they do not use true single-photon sources; instead, they rely on what are known as attenuated lasers. This means that not only is there a high probability of sending zero photons into a channel, which greatly reduces the speed of data transfer, but there is also a high probability of sending two, three, four, or more light quanta simultaneously. One could intercept these “extra” photons and neither the sender nor the recipient would know about it. This makes the communication channel vulnerable to eavesdropping and quantum cryptography loses its main advantage – fundamental security against all types of attacks.

For quantum computing it is also essential to have the ability to manipulate individual photons. The quantum of light can be used to represent a qubit – the fundamental unit of quantum information processing, – which is a superposition of two or more quantum states. For example, a qubit can be encoded in the polarization of a single photon. The advantage of the optical quantum computing paradigm is that one can natively combine quantum computations with quantum communication and design high-performance, large and scalable quantum supercomputers, which is not possible to do using other physical systems, such as superconducting circuits or trapped ions.

Dmitry Fedyanin and Mario Agio are the first to successfully reveal the mechanism of electroluminescence of colour centres in diamond and develop a theoretical framework to quantify it. They found that not all states of colour centres can be excited electrically, despite the fact that they may be “accessible” under optical excitation. This is because under optical pumping defects behave like isolated atoms or molecules (such as hydrogen or helium), with virtually no interaction with the diamond crystal. Electrical excitation, on the other hand, is based on the exchange of electrons between the defect and the diamond crystal. This not only brings limitations, but also opens up new possibilities. For example, according to the researchers, certain defects can emit serially two photons at two different wavelengths from two different charge states in a single act of the electroluminescence process. This feature could lead to the development of a fundamentally new class of quantum devices that had simply been disregarded before because these processes are not possible with optical excitation of colour centers. But the most important result of the study is that the researchers found out why high-intensity single-photon emission from colour centers was not observed under electrical pumping. The reason for this was the technologically complex process of doping of diamond by phosphorus, which cannot provide sufficiently high density of conduction electrons in diamond.

The calculations show that using modern doping technologies it is possible to create a bright single-photon source with an emission rate of more than 100,000 photons per second at room temperature. It is truly remarkable that the emission rate only increases as the device temperature increases achieving more than 100 million photons per second at 200 degrees Celsius. “Our single-photon source is one of few, if not the only optoelectronic device that should be heated in order to improve its performance, and the effect of improvement is as high as three orders of magnitude. Normally, both electronic and optical devices need to be cooled by attaching heat sinks with fans, or by placing them in liquid nitrogen,” says Dmitry Fedyanin from the Laboratory of Nanooptics and Plasmonics at MIPT. According to him, the technological improvement of diamond doping will further increase the brightness 10-100 times.

One hundred million photons is very low compared to household light sources (e.g. a normal light bulb emits more than 10^18 photons per second), but it should be emphasized that the entire flow of photons is created by a tiny (~10^-10 metres in size) defect in the crystal lattice of diamond and, unlike a light bulb, photons follow strictly one after the other. For the quantum computers mentioned above, around ten thousand photons per second would be enough — the possibility of developing a quantum computer is currently limited by entirely different factors. In quantum communication lines, however, the use of electrically-driven diamond single-photon sources will not only guarantee complete security, but will also greatly increase the speed of information transfer compared to the pseudo single-photon sources based on attenuated lasers used today.

The newest Airbus and Boeing passenger jets flying today are made primarily from advanced composite materials such as carbon fiber reinforced plastic — extremely light, durable materials that reduce the overall weight of the plane by as much as 20 percent compared to aluminum-bodied planes. Such lightweight airframes translate directly to fuel savings, which is a major point in advanced composites’ favor.

But composite materials are also surprisingly vulnerable: While aluminum can withstand relatively large impacts before cracking, the many layers in composites can break apart due to relatively small impacts — a drawback that is considered the material’s Achilles’ heel.

Now MIT aerospace engineers have found a way to bond composite layers in such a way that the resulting material is substantially stronger and more resistant to damage than other advanced composites. Their results are published this week in the journal Composites Science and Technology.

The researchers fastened the layers of composite materials together using carbon nanotubes — atom-thin rolls of carbon that, despite their microscopic stature, are incredibly strong. They embedded tiny “forests” of carbon nanotubes within a glue-like polymer matrix, then pressed the matrix between layers of carbon fiber composites. The nanotubes, resembling tiny, vertically-aligned stitches, worked themselves within the crevices of each composite layer, serving as a scaffold to hold the layers together.

In experiments to test the material’s strength, the team found that, compared with existing composite materials, the stitched composites were 30 percent stronger, withstanding greater forces before breaking apart.

Roberto Guzman, who led the work as an MIT postdoc in the Department of Aeronautics and Astronautics (AeroAstro), says the improvement may lead to stronger, lighter airplane parts — particularly those that require nails or bolts, which can crack conventional composites.

“More work needs to be done, but we are really positive that this will lead to stronger, lighter planes,” says Guzman, who is now a researcher at the IMDEA Materials Institute, in Spain. “That means a lot of fuel saved, which is great for the environment and for our pockets.”

The study’s co-authors include AeroAstro professor Brian Wardle and researchers from the Swedish aerospace and defense company Saab AB.

“Size matters”

Today’s composite materials are composed of layers, or plies, of horizontal carbon fibers, held together by a polymer glue, which Wardle describes as “a very, very weak, problematic area.” Attempts to strengthen this glue region include Z-pinning and 3-D weaving — methods that involve pinning or weaving bundles of carbon fibers through composite layers, similar to pushing nails through plywood, or thread through fabric.

“A stitch or nail is thousands of times bigger than carbon fibers,” Wardle says. “So when you drive them through the composite, you break thousands of carbon fibers and damage the composite.”

Carbon nanotubes, by contrast, are about 10 nanometers in diameter — nearly a million times smaller than the carbon fibers.

“Size matters, because we’re able to put these nanotubes in without disturbing the larger carbon fibers, and that’s what maintains the composite’s strength,” Wardle says. “What helps us enhance strength is that carbon nanotubes have 1,000 times more surface area than carbon fibers, which lets them bond better with the polymer matrix.”

Stacking up the competition

Guzman and Wardle came up with a technique to integrate a scaffold of carbon nanotubes within the polymer glue. They first grew a forest of vertically-aligned carbon nanotubes, following a procedure that Wardle’s group previously developed. They then transferred the forest onto a sticky, uncured composite layer and repeated the process to generate a stack of 16 composite plies — a typical composite laminate makeup — with carbon nanotubes glued between each layer.

To test the material’s strength, the team performed a tension-bearing test — a standard test used to size aerospace parts — where the researchers put a bolt through a hole in the composite, then ripped it out. While existing composites typically break under such tension, the team found the stitched composites were stronger, able to withstand 30 percent more force before cracking.

The researchers also performed an open-hole compression test, applying force to squeeze the bolt hole shut. In that case, the stitched composite withstood 14 percent more force before breaking, compared to existing composites.

“The strength enhancements suggest this material will be more resistant to any type of damaging events or features,” Wardle says. “And since the majority of the newest planes are more than 50 percent composite by weight, improving these state-of-the art composites has very positive implications for aircraft structural performance.”

It is now feasible to make a prized material for spintronic devices and semiconductors — monolayer graphene nanoribbons with zigzag edges.

Miniscule ribbons of graphene are highly sought-after building blocks for semiconductor devices because of their predicted electronic properties. But making these nanostructures has remained a challenge. Now, a team of researchers from China and Japan have devised a new method to make the structures in the lab. Their findings appear in the current issue of Applied Physics Letters, from AIP Publishing.

“Many studies have predicted the properties of graphene nanoribbons with zigzag edges,” said Guangyu Zhang, senior author on the study. “But in experiments it’s very hard to actually make this material.”

Previously, researchers have tried to make graphene nanoribbons by placing sheets of graphene over a layer of silica and using atomic hydrogen to etch strips with zigzag edges, a process known as anisotropic etching. These edges are crucial to modulate the nanoribbon’s properties.

But this method only worked well to make ribbons that had two or more graphene layers. Irregularities in silica created by electronic peaks and valleys roughen its surface, so creating precise zigzag edges on graphene monolayers was a challenge. Zhang and his colleagues from the Chinese Academy of Sciences, Beijing Key Laboratory for Nanomaterials and Nanodevices, and the Collaborative Innovation Center of Quantum Matter teamed up with Japanese collaborators from the National Institute for Materials Science to solve the problem.

They replaced the underlying silica with boron nitride, a crystalline material that’s chemically sluggish and has a smooth surface devoid of electronic bumps and pits. By using this substrate and the anisotropic etching technique, the group successfully made graphene nanoribbons that were only one-layer thick, and had well-defined zigzag edges.

“This is the first time we have ever seen that graphene on a boron nitride surface can be fabricated in such a controllable way,” Zhang explained.

The zigzag-edged nanoribbons showed high electron mobility in the range of 2000 cm2/Vs even at widths of less than 10nm — the highest value ever reported for these structures — and created clean, narrow energy band gaps, which makes them promising materials for spintronic and nano-electronic devices.

“When you decrease the width of the nanoribbons, the mobility decreases drastically because of edge defects,” said Zhang. “Using standard lithography fabrication techniques, studies have seen mobility of 100 cm2/Vs or even lower, but our material still exceeds 2000 cm2/Vs even at the sub-10 nanometer scale, demonstrating that these nanoribbons are of very high quality.”

In future studies, extending this method to other kinds of substrates could enable the quick large scale processing of monolayers of graphene to make high-quality nanoribbons with zigzag edges.

A team of scientists led by the Department of Energy’s Oak Ridge National Laboratory has developed a novel way to produce two-dimensional nanosheets by separating bulk materials with nontoxic liquid nitrogen. The environmentally friendly process generates a 20-fold increase in surface area per sheet, which could expand the nanomaterials’ commercial applications.

ORNL's Huiyuan Zhu places a sample of boron nitride, or "white graphene," into a furnace as part of a novel, nontoxic gas exfoliation process to separate 2-D nano materials. Credit: ORNL

ORNL’s Huiyuan Zhu places a sample of boron nitride, or “white graphene,” into a furnace as part of a novel, nontoxic gas exfoliation process to separate 2-D nano materials. Credit: ORNL

“It’s actually a very simple procedure,” said ORNL chemist Huiyuan Zhu, who co-authored a study published in Angewandte Chemie International Edition. “We heated commercially available boron nitride in a furnace to 800 degrees Celsius to expand the material’s 2D layers. Then, we immediately dipped the material into liquid nitrogen, which penetrates through the interlayers, gasifies into nitrogen, and exfoliates, or separates, the material into ultrathin layers.”

Nanosheets of boron nitride could be used in separation and catalysis, such as transforming carbon monoxide to carbon dioxide in gasoline-powered engines. They also may act as an absorbent to mop up hazardous waste. Zhu said the team’s controlled gas exfoliation process could be used to synthesize other 2D nanomaterials such as graphene, which has potential applications in semiconductors, photovoltaics, electrodes and water purification.

Because of the versatility and commercial potential of one-atom-thick 2D nanomaterials, scientists are seeking more efficient ways to produce larger sheets. Current exfoliation procedures use harsh chemicals that produce hazardous byproducts and reduce the amount of surface area per nanosheet, Zhu said.

“In this particular case, the surface area of the boron nitride nanosheets is 278 square meters per gram, and the commercially available boron nitride material has a surface area of only 10 square meters per gram,” Zhu said. “With 20 times more surface area, boron nitride can be used as a great support for catalysis.”

Further research is planned to expand the surface area of boron nitride nanosheets and also test their feasibility in cleaning up engine exhaust and improving the efficiency of hydrogen fuel cells.

Smaller and faster has been the trend for electronic devices since the inception of the computer chip, but flat transistors have gotten about as small as physically possible. For researchers pushing for even faster speeds and higher performance, the only way to go is up.

An array fin transistors made by the MacEtch method. The fins are tall and thin, with a higher aspect ratio and smoother sides than other methods can produce. Credit: Yi Song, University of Illinois

An array fin transistors made by the MacEtch method. The fins are tall and thin, with a higher aspect ratio and smoother sides than other methods can produce. Credit: Yi Song, University of Illinois

University of Illinois researchers have developed a way to etch very tall, narrow finFETs, a type of transistor that forms a tall semiconductor “fin” for the current to travel over. The etching technique addresses many problems in trying to create 3-D devices, typically done now by stacking layers or carving out structures from a thicker semiconductor wafer.

“We are exploring the electronic device roadmap beyond silicon,” said Xiuling Li, a U. of I. professor of electrical and computer engineering and the leader of the study. “With this technology, we are pushing the limit of the vertical space, so we can put more transistors on a chip and get faster speeds. We are making the structures very tall and smooth, with aspect ratios that are impossible for other existing methods to reach, and using a material with better performance than silicon.”

The team published the results in the journal Electron Device Letters.

Typically, finFETs are made by bombarding a semiconductor wafer with beams of high-energy ions. This technique has a number of challenges, Li said. For one, the sides of the fins are sloped instead of straight up and down, making them look more like tiny mountain ranges than fins. This shape means that only the tops of the fins can perform reliably. But an even bigger problem for high-performance applications is how the ion beam damages the surface of the semiconductor, which can lead to current leakage.

The Illinois technique, called metal-assisted chemical etching or MacEtch, is a liquid-based method, which is simpler and lower-cost than using ion beams, Li said. A metal template is applied to the surface, then a chemical bath etches away the areas around the template, leaving the sides of the fins vertical and smooth.

“We use a MacEtch technique that gives a much higher aspect ratio, and the sidewalls are nearly 90 degrees, so we can use the whole volume as the conducting channel,” said graduate student Yi Song, the first author of the paper. “One very tall fin channel can achieve the same conduction as several short fin channels, so we save a lot of area by improving the aspect ratio.”

The smoothness of the sides is important, since the semiconductor fins must be overlaid with insulators and metals that touch the tiny wires that interconnect the transistors on a chip. To have consistently high performance, the interface between the semiconductor and the insulator needs to be smooth and even, Song said.

Right now, the researchers use the compound semiconductor indium phosphide with gold as the metal template. However, they are working to develop a MacEtch method that does not use gold, which is incompatible with silicon.

“Compound semiconductors are the future beyond silicon, but silicon is still the industry standard. So it is important to make it compatible with silicon and existing manufacturing processes,” Li said.

The researchers said the MacEtch technique could apply to many types of devices or applications that use 3-D semiconductor structures, such as computing memory, batteries, solar cells and LEDs.

The old rules don’t necessarily apply when building electronic components out of two-dimensional materials, according to scientists at Rice University.

The Rice lab of theoretical physicist Boris Yakobson analyzed hybrids that put 2-D materials like graphene and boron nitride side by side to see what happens at the border. They found that the electronic characteristics of such “co-planar” hybrids differ from bulkier components.

Hybrids of two-dimensional materials like the graphene-molybdenum disulfide illustrated here have electronic properties that don't follow the same rules as their 3-D cousins, according to Rice University researchers. The limited direct contact between the two materials creates an electric field that greatly increases the size of the p/n junction. Credit: Henry Yu/Rice University

Hybrids of two-dimensional materials like the graphene-molybdenum disulfide illustrated here have electronic properties that don’t follow the same rules as their 3-D cousins, according to Rice University researchers. The limited direct contact between the two materials creates an electric field that greatly increases the size of the p/n junction. Credit: Henry Yu/Rice University

Their results appear this month in the American Chemical Society journal Nano Letters.

Shrinking electronics means shrinking their components. Academic labs and industries are studying how materials like graphene may enable the ultimate in thin devices by building all the necessary circuits into an atom-thick layer.

“Our work is important because semiconductor junctions are a big field,” Yakobson said. “There are books with iconic models of electronic behavior that are extremely well-developed and have become the established pillars of industry.

“But these are all for bulk-to-bulk interfaces between three-dimensional metals,” he said. “Now that people are actively working to make two-dimensional devices, especially with co-planar electronics, we realized that the rules have to be reconsidered. Many of the established models utilized in industry just don’t apply.”

The researchers led by Rice graduate student Henry Yu built computer simulations that analyze charge transfer between atom-thick materials.

“It was a logical step to test our theory on both metals and semiconductors, which have very different electronic properties,” Yu said. “This makes graphene, which is a metal — or a semimetal, to be precise — molybdenum disulfide and boron nitride, which are semiconductors, or even their hybrids ideal systems to study.

“In fact, these materials have been widely fabricated and used in the community for almost a decade, which makes analysis of them more appreciable in the field. Furthermore, both hybrids of graphene-molybdenum disulfide and graphene-boron nitride have been successfully synthesized recently, which means our study has practical meaning and can be tested in the lab now,” he said.

Yakobson said 3-D materials have a narrow region for charge transfer at the positive and negative (or p/n) junction. But the researchers found that 2-D interfaces created “a highly nonlocalized charge transfer” — and an electric field along with it — that greatly increased the junction size. That could give them an advantage in photovoltaic applications like solar cells, the researchers said.

The lab built a simulation of a hybrid of graphene and molybdenum disulfide and also considered graphene-boron nitride and graphene in which half was doped to create a p/n junction. Their calculations predicted the presence of an electric field should make 2-D Schottky (one-way) devices like transistors and diodes more tunable based on the size of the device itself.

How the atoms line up with each other is also important, Yakobson said. Graphene and boron nitride both feature hexagonal lattices, so they mesh perfectly. But molybdenum disulfide, another promising material, isn’t exactly flat, though it’s still considered 2-D.

“If the atomic structures don’t match, you get dangling bonds or defects along the borderline,” he said. “The structure has consequences for electronic behavior, especially for what is called Fermi level pinning.”

Pinning can degrade electrical performance by creating an energy barrier at the interface, Yakobson explained. “But your Schottky barrier (in which current moves in only one direction) doesn’t change as expected. This is a well-known phenomenon for semiconductors; it’s just that in two dimensions, it’s different, and in this case may favor 2-D over 3-D systems.”

Yakobson said the principles put forth by the new paper will apply to patterned hybrids of two or more 2-D patches. “You can make something special, but the basic effects are always at the interfaces. If you want to have many transistors in the same plane, it’s fine, but you still have to consider effects at the junctions.

“There’s no reason we can’t build 2-D rectifiers, transistors or memory elements,” he said. “They’ll be the same as we use routinely in devices now. But unless we develop a proper fundamental knowledge of the physics, they may fail to do what we design or plan.”