Tag Archives: letter-materials-tech

Full(erene) potential


February 3, 2017

In what could be called a classic “Eureka” moment, UC Santa Barbara materials researchers have discovered a simple yet effective method for mastering the electrical properties of polymer semiconductors. The elegant technique allows for the efficient design and manufacture of organic circuitry (the type found in flexible displays and solar cells, for instance) of varying complexity while using the same semiconductor material throughout.

“It’s a different strategy by which you can take a material and change its properties,” said Guillermo Bazan, a professor of chemistry and materials at UCSB. With the addition of fullerene or copper tetrabenzoporphyrin (CuBP) molecules in strategic places, the charge carriers in semiconducting materials — negative electrons and positive “holes” — may be controlled and inverted for better device performance as well as economical manufacture. The discovery is published in a pair of papers that appear in the journals Advanced Functional Materials and Advanced Electronic Materials.

In the realm of polymer semiconductors, device functionality depends on the movement of the appropriate charge carriers across the material. There have been many advances in the synthesis of high-mobility, high-performance materials, said lead author Michael Ford, graduate student in materials, but the fine control of the electrons and holes is what will allow these sophisticated polymers to reach their full potential.

“There’s been a large effort to make new materials, but a lot of them may not be appropriate in conventional low-power devices,” said Ford. Many of these materials exhibit “ambipolar” conduction, meaning that they transport both negative and positive charges, he explained. So, in situations where only a certain charge is required, the opposite charge is also conducting, which diminishes the utility of the material.

“They’re always ‘on’ so you always have a current running through the device,” Ford said. Conventional means of controlling the movement of charge carriers often involves more complex measures, such as multiple metal evaporation steps or depositing additional layers that are difficult to manage. These actions often require more complicated processing or fabrication, which could in the end defeat the purpose of low-cost flexible electronics.

This new development was actually a classic accidental scientific discovery, according to Ford, who was investigating, simultaneously, the properties of two materials. He observed that the use of fullerene additives limited conduction of one charge carrier (negative electrons), while allowing the other (positive holes) to remain mobile.

“In one experiment, we were just trying to do some extra measurements for a poster, and while making a measurement I noticed it solved the problem that I was having with my other material, which was this problem of never turning off,” Ford said. He decided to employ the fullerene additive from one experiment to address issues in the other and found it could be used to allow only positive charges to move, while adding operational stability.

From there, he and his collaborators worked to control negative charge conduction in the same way. A different additive — one that “likes” holes, CuBP — was introduced and turned off ambipolar transport in the opposite way from the fullerene. Now negative electrons remain mobile and hole transport is limited.

“We had two devices, both using the same polymer semiconductor but with different additives,” Ford explained. “One was a switch for holes, and the other was a switch for electrons. This enabled us to develop a complementary inverter, which is just like the building blocks that make up circuits in modern cell phones and computers.”

“So we have for the first time this ability to take these ambipolar semiconductors and design through solution processing a circuit where in certain parts only the electrons are moving, or only the holes are moving,” Bazan said, “but keep the same semiconductor material.” The additives create “traps” that can be used to master the properties of the semiconductor in a straightforward way, he added.

The potential uses of this method are many, particularly in situations where low-cost, low-power flexible electronics would be helpful, such as printable packaging labels that function as temperature sensors for foods and other sensitive items being shipped long distances.

“It’s this idea where we can have an additive that can be a small fraction of the total and which will allow us to master the electronic properties of the semiconductor,” said Bazan. “Once you have that under control, you can do all sorts of cool things.”

Most batteries are composed of two solid, electrochemically active layers called electrodes, separated by a polymer membrane infused with a liquid or gel electrolyte. But recent research has explored the possibility of all-solid-state batteries, in which the liquid (and potentially flammable) electrolyte would be replaced by a solid electrolyte, which could enhance the batteries’ energy density and safety.

Now, for the first time, a team at MIT has probed the mechanical properties of a sulfide-based solid electrolyte material, to determine its mechanical performance when incorporated into batteries.

The new findings were published this week in the journal Advanced Energy Materials, in a paper by Frank McGrogan and Tushar Swamy, both MIT graduate students; Krystyn Van Vliet, the Michael (1949) and Sonja Koerner Professor of Materials Science and Engineering; Yet-Ming Chiang, a professor of materials science and engineering; and four others including an undergraduate participant in the National Science Foundation Research Experience for Undergraduate (REU) program administered by MIT’s Center for Materials Science and Engineering and its Materials Processing Center.

Lithium-ion batteries have provided a lightweight energy-storage solution that has enabled many of today’s high-tech devices, from smartphones to electric cars. But substituting the conventional liquid electrolyte with a solid electrolyte in such batteries could have significant advantages. Such all-solid-state lithium-ion batteries could provide even greater energy storage ability, pound for pound, at the battery pack level. They may also virtually eliminate the risk of tiny, fingerlike metallic projections called dendrites that can grow through the electrolyte layer and lead to short-circuits.

“Batteries with components that are all solid are attractive options for performance and safety, but several challenges remain,” Van Vliet says. In the lithium-ion batteries that dominate the market today, lithium ions pass through a liquid electrolyte to get from one electrode to the other while the battery is being charged, and then flow through in the opposite direction as it is being used. These batteries are very efficient, but “the liquid electrolytes tend to be chemically unstable, and can even be flammable,” she says. “So if the electrolyte was solid, it could be safer, as well as smaller and lighter.”

But the big question regarding the use of such all-solid batteries is what kinds of mechanical stresses might occur within the electrolyte material as the electrodes charge and discharge repeatedly. This cycling causes the electrodes to swell and contract as the lithium ions pass in and out of their crystal structure. In a stiff electrolyte, those dimensional changes can lead to high stresses. If the electrolyte is also brittle, that constant changing of dimensions can lead to cracks that rapidly degrade battery performance, and could even provide channels for damaging dendrites to form, as they do in liquid-electrolyte batteries. But if the material is resistant to fracture, those stresses could be accommodated without rapid cracking.

Until now, though, the sulfide’s extreme sensitivity to normal lab air has posed a challenge to measuring mechanical properties including its fracture toughness. To circumvent this problem, members of the research team conducted the mechanical testing in a bath of mineral oil, protecting the sample from any chemical interactions with air or moisture. Using that technique, they were able to obtain detailed measurements of the mechanical properties of the lithium-conducting sulfide, which is considered a promising candidate for electrolytes in all-solid-state batteries.

“There are a lot of different candidates for solid electrolytes out there,” McGrogan says. Other groups have studied the mechanical properties of lithium-ion conducting oxides, but there had been little work so far on sulfides, even though those are especially promising because of their ability to conduct lithium ions easily and quickly.

Previous researchers used acoustic measurement techniques, passing sound waves through the material to probe its mechanical behavior, but that method does not quantify the resistance to fracture. But the new study, which used a fine-tipped probe to poke into the material and monitor its responses, gives a more complete picture of the important properties, including hardness, fracture toughness, and Young’s modulus (a measure of a material’s capacity to stretch reversibly under an applied stress).

“Research groups have measured the elastic properties of the sulfide-based solid electrolytes, but not fracture properties,” Van Vliet says. The latter are crucial for predicting whether the material might crack or shatter when used in a battery application.

The researchers found that the material has a combination of properties somewhat similar to silly putty or salt water taffy: When subjected to stress, it can deform easily, but at sufficiently high stress it can crack like a brittle piece of glass.

By knowing those properties in detail, “you can calculate how much stress the material can tolerate before it fractures,” and design battery systems with that information in mind, Van Vliet says.

The material turns out to be more brittle than would be ideal for battery use, but as long as its properties are known and systems designed accordingly, it could still have potential for such uses, McGrogan says. “You have to design around that knowledge.”

Transparent window coatings that keep buildings and cars cool on sunny days. Devices that could more than triple solar cell efficiencies. Thin, lightweight shields that block thermal detection. These are potential applications for a thin, flexible, light-absorbing material developed by engineers at the University of California San Diego.

The material, called a near-perfect broadband absorber, absorbs more than 87 percent of near-infrared light (1,200 to 2,200 nanometer wavelengths), with 98 percent absorption at 1,550 nanometers, the wavelength for fiber optic communication. The material is capable of absorbing light from every angle. It also can theoretically be customized to absorb certain wavelengths of light while letting others pass through.

This is a near-perfect broadband absorber that's thin, flexible and transparent in visible light. Credit: UC San Diego Jacobs School of Engineering

This is a near-perfect broadband absorber that’s thin, flexible and transparent in visible light. Credit: UC San Diego Jacobs School of Engineering

Materials that “perfectly” absorb light already exist, but they are bulky and can break when bent. They also cannot be controlled to absorb only a selected range of wavelengths, which is a disadvantage for certain applications. Imagine if a window coating used for cooling not only blocked infrared radiation, but also normal light and radio waves that transmit television and radio programs.

By developing a novel nanoparticle-based design, a team led by professors Zhaowei Liu and Donald Sirbuly at the UC San Diego Jacobs School of Engineering has created a broadband absorber that’s thin, flexible and tunable. The work was published online on Jan. 24 in Proceedings of the National Academy of Sciences.

“This material offers broadband, yet selective absorption that could be tuned to distinct parts of the electromagnetic spectrum,” Liu said.

The absorber relies on optical phenomena known as surface plasmon resonances, which are collective movements of free electrons that occur on the surface of metal nanoparticles upon interaction with certain wavelengths of light. Metal nanoparticles can carry a lot of free electrons, so they exhibit strong surface plasmon resonance — but mainly in visible light, not in the infrared.

UC San Diego engineers reasoned that if they could change the number of free electron carriers, they could tune the material’s surface plasmon resonance to different wavelengths of light. “Make this number lower, and we can push the plasmon resonance to the infrared. Make the number higher, with more electrons, and we can push the plasmon resonance to the ultraviolet region,” Sirbuly said. The problem with this approach is that it is difficult to do in metals.

To address this challenge, engineers designed and built an absorber from materials that could be modified, or doped, to carry a different amount of free electrons: semiconductors. Researchers used a semiconductor called zinc oxide, which has a moderate number of free electrons, and combined it with its metallic version, aluminum-doped zinc oxide, which houses a high number of free electrons — not as much as an actual metal, but enough to give it plasmonic properties in the infrared.

The materials were combined and structured in a precise fashion using advanced nanofabrication technologies in the Nano3 cleanroom facility at the Qualcomm Institute at UC San Diego. The materials were deposited one atomic layer at a time on a silicon substrate to create an array of standing nanotubes, each made of alternating concentric rings of zinc oxide and aluminum-doped zinc oxide. The tubes are 1,730 nanometers tall, 650 to 770 nanometers in diameter, and spaced less than a hundred nanometers apart. The nanotube array was then transferred from the silicon substrate to a thin, elastic polymer. The result is a material that is thin, flexible and transparent in the visible.

“There are different parameters that we can alter in this design to tailor the material’s absorption band: the gap size between tubes, the ratio of the materials, the types of materials, and the electron carrier concentration. Our simulations show that this is possible,” said Conor Riley, a recent nanoengineering Ph.D. graduate from UC San Diego and the first author of this work. Riley is currently a postdoctoral researcher in Sirbuly’s group.

Those are just a few exciting features of this particle-based design, researchers said. It’s also potentially transferrable to any type of substrate and can be scaled up to make large surface area devices, like broadband absorbers for large windows. “Nanomaterials normally aren’t fabricated at scales larger than a couple centimeters, so this would be a big step in that direction,” Sirbuly said.

The technology is still at the developmental stage. Liu and Sirbuly’s teams are continuing to work together to explore different materials, geometries and designs to develop absorbers that work at different wavelengths of light for various applications.

A team of researchers, affiliated with UNIST has recently announced that they have successfully developed a new way to increase energy efficiency of metal-air batteries which are next-generation energy devices by using a conducting polymer.

This breakthrough research, led by Professor Hyun-Kon Song and Professor Guntae Kim of Energy and Chemical Engineering is appeared in the January issue of Energy & Environmental Science (EES), a world renowned academic journal in the field of energy.

In the cathode of metal-air batteries or fuel cells, oxygen is reduced to metal oxide or water. At this time, catalysts are required to accelerate the reaction. As the well known catalyst, platinum has remaining economical issue to commercialize the metal-air betteries or fuel cell due to its high price.

In the study, they reported that catalytic activity of provskite which can be substitute to platinum was dramatically enhanced by simply adding a kind of conducting polymer, polypyrrole and its principle.

When the perovskite or polypyrrole are used alone, their activities can not be reached to that of platinum. However, as a result of physically mixing perovksite with polypyrrole, the activity was dramatically enhanced and it was reached to that of platinum. This is first synergistic effect in oxygen electrocatalysis even though there was any chemical interaction between pervskite and polypyrrole.

“The reaction which oxygen receive electron is oxygen reduction. The property of polypyrrole which is sensitive to oxygen contribute to accelerate this reaction,” says Professor Song.

“Because the oxide-polypyrrole complex is made by simple operation, the catalyst will be proper to apply next-generation energy devices,” says Dong-Gyu Lee, the first author of the research.

The results of this study has been supported by KEIT, KETEP, BK21Plus and MSIP.

A simple technique for producing oxide nanowires directly from bulk materials could dramatically lower the cost of producing the one-dimensional (1D) nanostructures. That could open the door for a broad range of uses in lightweight structural composites, advanced sensors, electronic devices – and thermally-stable and strong battery membranes able to withstand temperatures of more than 1,000 degrees Celsius.

The technique uses a solvent reaction with a bimetallic alloy – in which one of the metals is reactive – to form bundles of nanowires (nanofibers) upon reactive metal dissolution. The process is conducted at ambient temperature and pressure without the use of catalysts, toxic chemicals or costly processes such as chemical vapor deposition. The produced nanowires can be used to improve the electrical, thermal and mechanical properties of functional materials and composites.

The research, which is scheduled to be reported this week in the journal Science, was supported by the National Science Foundation and California-based Sila Nanotechnologies. The process is believed to be the first to convert bulk powders to nanowires at ambient conditions.

Researchers have developed a new low-cost technique for converting bulk powders directly to oxide nanowires. Shown is a crucible in which an alloy of lithium and aluminum is being formed. Credit: Rob Felt, Georgia Tech

Researchers have developed a new low-cost technique for converting bulk powders directly to oxide nanowires. Shown is a crucible in which an alloy of lithium and aluminum is being formed. Credit: Rob Felt, Georgia Tech

“This technique could open the door for a range of synthesis opportunities to produce low-cost 1D nanomaterials in large quantities,” said Gleb Yushin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “You can essentially put the bulk materials into a bucket, fill it with a suitable solvent and collect nanowires after a few hours, which is way simpler than how many of these structures are produced today.”

Yushin’s research team, which included former graduate students Danni Lei and James Benson, has produced oxide nanowires from lithium-magnesium and lithium-aluminum alloys using a variety of solvents, including simple alcohols. Production of nanowires from other materials is part of ongoing research that was not reported in the paper.

The dimensions of the nanowire structures can be controlled by varying the solvent and the processing conditions. The structures can be produced in diameters ranging from tens of nanometers up to microns.

“Minimization of the interfacial energy at the boundary of the chemical reaction front allows us to form small nuclei and then retain their diameter as the reaction proceeds, thus forming nanowires,” Yushin explained. “By controlling the volume changes, surface energy, reactivity and solubility of the reaction products, along with the temperature and pressure, we can tune conditions to produce nanowires of the dimensions we want.”

One of the attractive applications may be separator membranes for lithium-ion batteries, whose high power density has made them attractive for powering everything from consumer electronics to aircraft and motor vehicles. However, the polymer separation membranes used in these batteries cannot withstand the high temperatures generated by certain failure scenarios. As result, commercial batteries may induce fires and explosions, if not designed very carefully and it’s extremely hard to avoid defects and errors consistently in tens of millions of devices.

Using low-cost paper-like membranes made of ceramic nanowires could help address those concerns because the structures are strong and thermally stable, while also being flexible – unlike many bulk ceramics. The material is also polar, meaning it would more thoroughly wetted by various battery electrolyte solutions.

“Overall, this is a better technology for batteries, but until now, ceramic nanowires have been too expensive to consider seriously,” Yushin said. “In the future, we can improve mechanical properties further and scale up synthesis, making the low-cost ceramic separator technology very attractive to battery designers.”

Fabrication of the nanowires begins with formation of alloys composed of one reactive and one non-reactive metal, such as lithium and aluminum (or magnesium and lithium). The alloy is then placed in a suitable solvent, which could include a range of alcohols, such as ethanol. The reactive metal (lithium) dissolves from the surface into the solvent, initially producing nuclei (nanoparticles) comprising aluminum.

Though bulk aluminum is not reactive with alcohol due to the formation of the passivation layer, the continuous dissolution of lithium prevents the passivation and allows gradual formation of aluminum alkoxide nanowires, which grow perpendicular to the surface of the particles starting from the nuclei until the particles are completely converted. The alkoxide nanowires can then be heated in open air to form aluminum oxide nanowires and may be formed into paper-like sheets.

The dissolved lithium can be recovered and reused. The dissolution process generates hydrogen gas, which could be captured and used to help fuel the heating step.

Though the process was studied first to make magnesium and aluminum oxide nanowires, Yushin believes it has a broad potential for making other materials. Future work will explore synthesis of new materials and their applications, and develop improved fundamental understanding of the process and predictive models to streamline experimental work.

The researchers have so far produced laboratory amounts of the nanowires, but Yushin believes that the process could be scaled up to produce industrial quantities. Though the ultimate cost will depend on many variables, he expects to see fabrication costs cut by several orders of magnitude over existing techniques.

“With this technique, you could potentially produce nanowires for a cost not much more than that of the raw materials,” he said. Beyond battery membranes, the nanowires could be useful in energy harvesting, catalyst supports, sensors, flexible electronic devices, lightweight structural composites, building materials, electrical and thermal insulation and cutting tools.

The new technique was discovered accidentally while Yushin’s students were attempting to create a new porous membrane material. Instead of the membrane they had hoped to fabricate, the process generated powders composed of elongated particles.

“Though the experiment didn’t produce what we were looking for, I wanted to see if we could learn something from it anyway,” said Yushin. Efforts to understand what had happened ultimately led to the new synthesis technique.

In addition to those already named, the research included Alexandre Magaskinski of Georgia Tech and Gene Berdichevsky of Sila Nanotechnologies.

Research by scientists at Swansea University is helping to meet the challenge of incorporating nanoscale structures into future semiconductor devices that will create new technologies and impact on all aspects of everyday life.

Dr Alex Lord and Professor Steve Wilks from the Centre for Nanohealth led the collaborative research published in Nano Letters. The research team looked at ways to engineer electrical contact technology on minute scales with simple and effective modifications to nanowires that can be used to develop enhanced devices based on the nanomaterials. Well-defined electrical contacts are essential for any electrical circuit and electronic device because they control the flow of electricity that is fundamental to the operational capability.

Specialist research equipment and reseach images. Credit: Scienta Omicron

Specialist research equipment and reseach images. Credit: Scienta Omicron

Everyday materials that are being scaled down to the size of nanometres (one million times smaller than a millimetre on a standard ruler) by scientists on a global scale are seen as the future of electronic devices. The scientific and engineering advances are leading to new technologies such as energy producing clothing to power our personal gadgets and sensors to monitor our health and the surrounding environment.

Over the coming years this will make a massive contribution to the explosion that is the Internet of Things connecting everything from our homes to our cars into a web of communication. All of these new technologies require similar advances in electrical circuits and especially electrical contacts that allow the devices to work correctly with electricity.

Professor Steve Wilks said: “Nanotechnology has delivered new materials and new technologies and the applications of nanotechnology will continue to expand over the coming decades with much of its usefulness stemming from effects that occur at the atomic- or nano-scale. With the advent of nanotechnology, new technologies have emerged such as chemical and biological sensors, quantum computing, energy harvesting, lasers, and environmental and photon-detectors, but there is a pressing need to develop new electrical contact preparation techniques to ensure these devices become an everyday reality.”

“Traditional methods of engineering electrical contacts have been applied to nanomaterials but often neglect the nanoscale effects that nanoscientists have worked so hard to uncover. Currently, there isn’t a design toolbox to make electrical contacts of chosen properties to nanomaterials and in some respects the research is lagging behind our potential application of the enhanced materials.”

The Swansea research team1 used specialist experimental equipment and collaborated with Professor Quentin Ramasse of the SuperSTEM Laboratory, Science and Facilities Technology Council. The scientists were able to physically interact with the nanostructures and measure how the nanoscale modifications affected the electrical performance.2

Their experiments found for the first time, that simple changes to the catalyst edge can turn-on or turn-off the dominant electrical conduction and most importantly reveal a powerful technique that will allow nanoengineers to select the properties of manufacturable nanowire devices.

Dr Lord said: “The experiments had a simple premise but were challenging to optimise and allow atomic-scale imaging of the interfaces. However, it was essential to this study and will allow many more materials to be investigated in a similar way.”

“This research now gives us an understanding of these new effects and will allow engineers in the future to reliably produce electrical contacts to these nanomaterials which is essential for the materials to be used in the technologies of tomorrow.

“In the near future this work can help enhance current nanotechnology devices such as biosensors and also lead to new technologies such as Transient Electronics that are devices that diminish and vanish without a trace which is an essential property when they are applied as diagnostic tools inside the human body.”

Silicon crystals are the semiconductors most commonly used to make transistors, which are critical electronic components used to carry out logic operations in computing. However, as faster and more powerful processors are created, silicon has reached a performance limit: the faster it conducts electricity, the hotter it gets, leading to overheating.

Graphene, made of a single-atom-thick sheet of carbon, stays much cooler and can conduct much faster, but it must be into smaller pieces, called nanoribbons, in order to act as a semiconductor. Despite much progress in the fabrication and characterization of nanoribbons, cleanly transferring them onto surfaces used for chip manufacturing has been a significant challenge.

A recent study conducted by researchers at the Beckman Institute for Advanced Science and Technology at the University of Illinois and the Department of Chemistry at the University of Nebraska-Lincoln has demonstrated the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. The paper, “Solution-Synthesized Chevron Graphene Nanoribbons Exfoliated onto H:Si(100),” was published in Nano Letters.

Researchers have made the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. Credit: Adrian Radocea, Beckman Institute for Advanced Science and Technology

Researchers have made the first important step toward integrating atomically precise graphene nanoribbons (APGNRs) onto nonmetallic substrates. Credit: Adrian Radocea, Beckman Institute for Advanced Science and Technology

Graphene nanoribbons measure only several nanometers across, beyond the limits of conventional chip top-down patterning used in chip manufacturing. As a result, when carved from larger pieces of graphene by various nanofabrication approaches, graphene nanoribbons are neither uniform nor narrow enough to exhibit the desired semiconductor properties.

“When you’re going from the top-down, it’s very hard to get control over the width. It turns out that if the width modulates by just an atom or two, the properties change significantly,” said Adrian Radocea, a doctoral student in Beckman’s Nanoelectronics and Nanomaterials Group.

As a result, the nanoribbons must be made from “the bottom up,” from smaller molecules to create atomically precise nanoribbons with highly uniform electronic properties.

“It’s like molecular building blocks: kind of like snapping Legos together to building something,” said Radocea. “They lock in place, and you end up with the exact control over the ribbon width.”

The “bottom-up” approach was first shown for graphene nanoribbons by Cai et al. in a 2010 Nature paper demonstrating the growth of atomically precise graphene nanoribbons on metallic substrates. In 2014, the research group of Alexander Sinitskii at the University of Nebraska-Lincoln developed an alternative approach for making atomically precise graphene nanoribbons in solution.

“The previously demonstrated synthesis on metallic substrates yields graphene nanoribbons of very high quality, but their number is rather small, as the growth it limited to the precious metal’s surface,” said Sinitskii, associate professor of chemistry at University of Nebraska-Lincoln and an author of the study. “It is difficult to scale this synthesis up. In contrast, when nanoribbons are synthesized in the unrestricted three-dimensional solution environment, they can be produced in large quantities.”

The difficulty in cleanly transferring nanoribbons stems from the high sensitivity to environmental contaminants. Both solution-synthesized and surface-grown nanoribbons are exposed to chemicals during the transfer process that can affect the performance of graphene nanoribbon devices. To overcome this challenge, the interdisciplinary team used a dry transfer in an ultra-high vacuum environment.

A fiberglass applicator coated in graphene nanoribbon powder was heated to remove contaminants and solvent residue and then pressed onto a freshly prepared hydrogen-passivated silicon surface. The nanoribbons were studied in great detail with ultra-high vacuum scanning tunneling microscope developed by Joseph Lyding, professor of electrical and computer engineering at Illinois and an author of the study. The researchers obtained atomic-scale images and electronic measurements of the graphene nanoribbons that were critical for confirming their electronic properties and understanding the influence of the substrate.

Computational expertise available at Beckman, Radocea explained, was instrumental in understanding the experimental results. “I was still collecting more data trying to figure out what was going on. Once the modeling results came in and we started looking at the data differently, it all made sense.”

Members of Beckman’s Computational Multiscale Nanosystems Group, Tao Sun, a doctoral student, and Narayana Aluru, professor of mechanical science and engineering, provided expertise in computational modeling via density functional theory to investigate the properties of the nanoribbons.

“Density functional theory calculations provided a deeper understanding of the electronic properties of the integrated system and the interactions between graphene nanoribbons and the silicon substrate,” said Sun. “It was exciting that the computational results could help explain and confirm the experimental results and provided a coherent story.”

“Atomically precise graphene nanoribbons (APGNRs) are serious candidates for the post-silicon era when conventional silicon transistor scaling fails,” said Lyding. “This demonstrates the first important step toward integrating APGNRs with technologically relevant silicon substrates.”

“I find the project very exciting because you are building things with atomic level control, so you try to put every atom exactly where you want it to go,” said Radocea. “There aren’t many materials out there where you can say you have that ability. Nanoribbons are exciting because there is a real need and a real application.”

Based on a study of the optical properties of novel ultrathin semiconductors, researchers of Ludwig-Maximilians-Universitaet (LMU) in Munich have developed a method for rapid and efficient characterization of these materials.

Chemical compounds based on elements that belong to the so-called transition metals can be processed to yield atomically thin two-dimensional crystals consisting of a monolayer of the composite in question. The resulting materials are semiconductors with surprising optical properties. In cooperation with American colleagues, a team of LMU physicists led by Alexander Högele has now explored the properties of thin-film semiconductors made up of transition metal dichalcogenides (TMDs). The researchers report their findings in the journal Nature Nanotechnology.

These semiconductors exhibit remarkably strong interaction with light and therefore have great potential for applications in the field of opto-electronics. In particular, the electrons in these materials can be excited with polarized light. “Circularly polarized light generates charge carriers that exhibit either left- or right-handed circular motion. The associated angular momentum is quantized and described by the so-called valley index which can be detected as valley polarization,” Högele explains. In accord with the laws of quantum mechanics, the valley index can be used just like quantum mechanical spin to encode information for many applications including quantum computing.

However, recent studies of the valley index in TMD semiconductors have led to controversial results. Different groups worldwide have reported inconsistent values for the degree of valley polarization. With the aid of their newly developed polarimetric method and using monolayers of the semiconducting TMD molybdenum disulfide as a model system, the LMU researchers have now clarified the reasons for these discrepancies: “Response to polarized light turns out to be very sensitive to the quality of the crystals, and can thus vary significantly within the same crystal,” Högele says. “The interplay between crystal quality and valley polarization will allow us to measure rapidly and efficiently those properties of the sample that are relevant for applications based on the valley quantum degree of freedom.”

Moreover, the new method can be applied to other monolayer semiconductors and systems made up of several different materials. In the future, this will enable the functionalities of devices based on atomically thin semiconductors — such as novel types of LEDs — to be characterized swiftly and economically.

Physicists at the National Institute of Standards and Technology (NIST) have cooled a mechanical object to a temperature lower than previously thought possible, below the so-called “quantum limit.”

The new NIST theory and experiments, described in the Jan. 12, 2017, issue of Nature, showed that a microscopic mechanical drum–a vibrating aluminum membrane–could be cooled to less than one-fifth of a single quantum, or packet of energy, lower than ordinarily predicted by quantum physics. The new technique theoretically could be used to cool objects to absolute zero, the temperature at which matter is devoid of nearly all energy and motion, NIST scientists said.

“The colder you can get the drum, the better it is for any application,” said NIST physicist John Teufel, who led the experiment. “Sensors would become more sensitive. You can store information longer. If you were using it in a quantum computer, then you would compute without distortion, and you would actually get the answer you want.”

“The results were a complete surprise to experts in the field,” Teufel’s group leader and co-author José Aumentado said. “It’s a very elegant experiment that will certainly have a lot of impact.”

The drum, 20 micrometers in diameter and 100 nanometers thick, is embedded in a superconducting circuit designed so that the drum motion influences the microwaves bouncing inside a hollow enclosure known as an electromagnetic cavity. Microwaves are a form of electromagnetic radiation, so they are in effect a form of invisible light, with a longer wavelength and lower frequency than visible light.

The microwave light inside the cavity changes its frequency as needed to match the frequency at which the cavity naturally resonates, or vibrates. This is the cavity’s natural “tone,” analogous to the musical pitch that a water-filled glass will sound when its rim is rubbed with a finger or its side is struck with a spoon.

NIST scientists previously cooled the quantum drum to its lowest-energy “ground state,” or one-third of one quantum. They used a technique called sideband cooling, which involves applying a microwave tone to the circuit at a frequency below the cavity’s resonance. This tone drives electrical charge in the circuit to make the drum beat. The drumbeats generate light particles, or photons, which naturally match the higher resonance frequency of the cavity. These photons leak out of the cavity as it fills up. Each departing photon takes with it one mechanical unit of energy–one phonon–from the drum’s motion. This is the same idea as laser cooling of individual atoms, first demonstrated at NIST in 1978 and now widely used in applications such atomic clocks.

The latest NIST experiment adds a novel twist–the use of “squeezed light” to drive the drum circuit. Squeezing is a quantum mechanical concept in which noise, or unwanted fluctuations, is moved from a useful property of the light to another aspect that doesn’t affect the experiment. These quantum fluctuations limit the lowest temperatures that can be reached with conventional cooling techniques. The NIST team used a special circuit to generate microwave photons that were purified or stripped of intensity fluctuations, which reduced inadvertent heating of the drum.

“Noise gives random kicks or heating to the thing you’re trying to cool,” Teufel said. “We are squeezing the light at a ‘magic’ level–in a very specific direction and amount–to make perfectly correlated photons with more stable intensity. These photons are both fragile and powerful.”

The NIST theory and experiments indicate that squeezed light removes the generally accepted cooling limit, Teufel said. This includes objects that are large or operate at low frequencies, which are the most difficult to cool.

The drum might be used in applications such as hybrid quantum computers combining both quantum and mechanical elements, Teufel said. A hot topic in physics research around the world, quantum computers could theoretically solve certain problems considered intractable today.

In the past decade, two-dimensional, 2D, materials have captured the fascination of a steadily increasing number of scientists. These materials, whose defining feature is having a thickness of only one to very few atoms, can be made of a variety of different elements or combinations thereof. Scientists’ enchantment with 2D materials began with Andre Geim and Konstantin Novoselov’s Nobel Prize winning experiment: creating a 2D material using a lump of graphite and common adhesive tape. This ingeniously simple experiment yielded an incredible material: graphene. This ultra-light material is roughly 200 times stronger than steel and is a superb conductor. Once scientists discovered that graphene had more impressive properties than its bulk component graphite, they decided to investigate other 2D materials to see if this was a universal property.

Christopher Petoukhoff, a Rutgers University graduate student working in the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), studies a 2D material, made of molybdenum disulfide (MoS2). His research focuses on the 2D material’s optoelectronic applications, or how the material can detect and absorb light. Optoelectronics are ubiquitous in today’s world, from the photodetectors in automatic doors and hand dryers, to solar cells, to LED lights, but as anyone who has stood in front of an automatic sink desperately waving their hands around to get it to work will tell you, there is plenty of room for improvement. The 2D MoS2 is particularly interesting for use in photodetectors because of its capability of absorbing the same amount of light as 50nm of the currently used silicon-based technologies, while being 70 times thinner.

Petoukhoff, under the supervision of Professor Keshav Dani, seeks to improve optoelectronic devices by adding a 2D layer of MoS2 to an organic semiconductor, which has similar absorption strengths as MoS2. The theory behind using both materials is that the interaction between the MoS2 layer and the organic semiconductor should lead to efficient charge transfer. Petoukhoff’s research, published in ACS Nano, demonstrates for the first time that charge transfer between these two layers occurs at an ultra-fast timescale, on the order of less than 100 femtoseconds, or one tenth of one millionth of one millionth of a second.

The thinness of these materials, however, becomes a limiting factor in their efficiency as photovoltaics, or light-energy conversion devices. Light absorbing devices, such as solar cells and photodetectors, require a certain amount of optical thickness in order to absorb photons, rather than allowing them to pass through. To overcome this, researchers from the Femtosecond Spectroscopy Unit added an array of silver nanoparticles, or a plasmonic metasurface, to the organic semiconductor-MoS2 hybrid to focus and localize the light in the device. The addition of the metasurface increases the optical thickness of the material while capitalizing on the unique properties of the ultra-thin active layer, which ultimately increase the total absorption.

While this research is still in its infancy, its implications for the future are huge. Combinations with 2D materials have the potential to revolutionize the marketability of optoelectronic devices. Conventional optoelectronic devices are expensive to manufacture and are often made from scarce or toxic elements, such as indium or arsenic. Organic semiconductors have low manufacturing costs, and are made of earth-abundant and non-toxic elements. This research can potentially improve the cost and efficiency of optoelectronics, leading to better products in the future.