Nanoscale worlds sometimes resemble macroscale roller-coaster style hills, placed at the tip of a series of hexagons. Surprisingly, these nanohills stem from the self-organisation of particles - the very particles that have been eroded and subsequently redeposited following the bombardment of semi-conductors with ion beams. Now, a new theoretical study constitutes the first exhaustive investigation of the redeposition effect on the evolution of the roughening and smoothing of two-dimensional surfaces bombarded by multiple ions. The results demonstrate that the redeposition can indeed act as stabilising factor during the creation of the hexagonally arranged dot patterns observed in experiments. These findings by Christian Diddens from the Eindhoven University of Technology, in the Netherlands, and Stefan Linz, from Munster University, Germany, have been published in a study published in ("Continuum modeling of particle redeposition during ion-beam erosion"). To calculate multiple simulations of redeposition within reasonable computation times, the authors have developed an elaborate new highly efficient algorithm that combines established erosion models with a redeposition model. The latter made it possible to approximate the entire microscopic redeposition dynamics as a function of the relative height and the local slope of a coarse-grained surface. This approach is also supplemented by a new numerical algorithm to calculate precisely how the matter lifted by the ion beams is subsequently redeposited. This led to the realisation that eroded particles predominantly redeposit in the vicinity of the valleys, whereas almost no particles reattach at the hilltops. Overall, they found that the redeposition mechanism can contribute towards the formation of stable hexagonal patterns. They also confirmed that the aspect ratio of the well-ordered structures resulting from numerical simulation is comparable with experimental findings. This means that the reattachment of eroded particles can play an important role in the observed nanostructures formations. At the same, they comprehensively investigated the distribution of redepositing particles on patterned surfaces.
read more "Shaping the hilly landscapes of a semi-conductor nanoworld"
Plasmonic material could bring ultrafast all-optical communications
Researchers have created a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. In optical communications, laser pulses are used to transmit information along fiber-optic cables for telephone service, the Internet and cable television. Researchers at Purdue University have shown how an optical material made of aluminum-doped zinc oxide (AZO) is able to modulate – or change – how much light is reflected by 40 percent while requiring less power than other "all-optical" semiconductor devices. 
This rendering depicts a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. (Image: Nathaniel Kinsey) "Low power is important because if you want to operate very fast - and we show the potential for up to a terahertz or more - then you need low energy dissipation," said doctoral student Nathaniel Kinsey. "Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don't use any electrical signals to control the system. Both the data stream and the control signals are optical pulses." Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission. "We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application," said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. "You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission." Findings were detailed in a research paper appearing in July in the journal ("Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths"), published by the Optical Society of America. The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications. The researchers have proposed creating an "all optical plasmonic modulator using CMOS-compatible materials," or an optical transistor. In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible. The paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva. Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind "holes" in the valance band, and eventually the electrons recombine with these holes. The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine. "So what we would like to do is drastically speed this up," Kinsey said. This cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time. "We were surprised that it was this fast," Kinsey said. The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics. The AZO films are said to be "Epsilon-near-zero," meaning the refractive index is near zero, a quality found normally in metals and new "metamaterials," which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. The pulsing laser light changes the AZO's index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible. "If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change," he said. The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths. A new low-temperature fabrication process is critical to the material's properties and for its CMOS compatibility. "For industrial applications you can't go to really high fabrication temperatures because that damages underlying material on the chip or device," Kinsey said. "An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric." The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
read more "Plasmonic material could bring ultrafast all-optical communications"
This rendering depicts a new "plasmonic oxide material" that could make possible devices for optical communications that are at least 10 times faster than conventional technologies. (Image: Nathaniel Kinsey) "Low power is important because if you want to operate very fast - and we show the potential for up to a terahertz or more - then you need low energy dissipation," said doctoral student Nathaniel Kinsey. "Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don't use any electrical signals to control the system. Both the data stream and the control signals are optical pulses." Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission. "We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application," said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. "You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission." Findings were detailed in a research paper appearing in July in the journal ("Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths"), published by the Optical Society of America. The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications. The researchers have proposed creating an "all optical plasmonic modulator using CMOS-compatible materials," or an optical transistor. In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible. The paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva. Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind "holes" in the valance band, and eventually the electrons recombine with these holes. The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine. "So what we would like to do is drastically speed this up," Kinsey said. This cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time. "We were surprised that it was this fast," Kinsey said. The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics. The AZO films are said to be "Epsilon-near-zero," meaning the refractive index is near zero, a quality found normally in metals and new "metamaterials," which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. The pulsing laser light changes the AZO's index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible. "If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change," he said. The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths. A new low-temperature fabrication process is critical to the material's properties and for its CMOS compatibility. "For industrial applications you can't go to really high fabrication temperatures because that damages underlying material on the chip or device," Kinsey said. "An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric." The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
How to look for a few good catalysts
Two key physical phenomena take place at the surfaces of materials: catalysis and wetting. A catalyst enhances the rate of chemical reactions; wetting refers to how liquids spread across a surface. Now researchers at MIT and other institutions have found that these two processes, which had been considered unrelated, are in fact closely linked. The discovery could make it easier to find new catalysts for particular applications, among other potential benefits. “What’s really exciting is that we’ve been able to connect atomic-level interactions of water and oxides on the surface to macroscopic measurements of wetting, whether a surface is hydrophobic or hydrophilic, and connect that directly with catalytic properties,” says Yang Shao-Horn, the W.M. Keck Professor of Energy at MIT and a senior author of a paper describing the findings in the ("Reactivity of Perovskites with Water: Role of Hydroxylation in Wetting and Implications for Oxygen Electrocatalysis"). The research focused on a class of oxides called perovskites that are of interest for applications such as gas sensing, water purification, batteries, and fuel cells. 
Materials that have good wetting properties, as illustrated on the left, where droplets spread out flat, tend to have hydroxyl groups attached to the surface, which inhibits catalytic activity. Materials that repel water, as shown at right, where droplets form sharp, steep boundaries, are more conducive to catalytic activity, as shown by the reactions among small orange molecules. Since determining a surface’s wettability is “trivially easy,” says senior author Kripa Varanasi, an associate professor of mechanical engineering, that determination can now be used to predict a material’s suitability as a catalyst. Since researchers tend to specialize in either wettability or catalysis, this produces a framework for researchers in both fields to work together to advance understanding, says Varanasi, whose research focuses primarily on wettability; Shao-Horn is an expert on catalytic reactions. “We show how wetting and catalysis, which are both surface phenomena, are related,” Varanasi says, “and how electronic structure forms a link between both.” While both effects are important in a variety of industrial processes and have been the subject of much empirical research, “at the molecular level, we understand very little about what’s happening at the interface,” Shao-Horn says. “This is a step forward, providing a molecular-level understanding.” “It’s primarily an experimental technique” that made the new understanding possible, explains Kelsey Stoerzinger, an MIT graduate student and the paper’s lead author. While most attempts to study such surface science use instruments requiring a vacuum, this team used a device that could study the reactions in humid air, at room temperature, and with varying degrees of water vapor present. Experiments using this system, called ambient pressure X-ray photoelectron spectroscopy, revealed that the reactivity with water is key to the whole process, she says. The water molecules break apart to form hydroxyl groups — an atom of oxygen bound to an atom of hydrogen — bonded to the material’s surface. These reactive compounds, in turn, are responsible for increasing the wetting properties of the surface, while simultaneously inhibiting its ability to catalyze chemical reactions. Therefore, for applications requiring high catalytic activity, the team found, a key requirement is that the surface be hydrophobic, or non-wetting. “Ideally, this understanding helps us design new catalysts,” Stoerzinger says. If a given material “has a lower affinity for water, it has a higher affinity for catalytic activity.” Shao-Horn notes that this is an initial finding, and that “extension of these trends to broader classes of materials and ranges of hydroxyl affinity requires further investigation.” The team has already begun further exploration of these areas. This research, she says, “opens up the space of materials and surfaces we might think about” for both catalysis and wetting.
read more "How to look for a few good catalysts"
Materials that have good wetting properties, as illustrated on the left, where droplets spread out flat, tend to have hydroxyl groups attached to the surface, which inhibits catalytic activity. Materials that repel water, as shown at right, where droplets form sharp, steep boundaries, are more conducive to catalytic activity, as shown by the reactions among small orange molecules. Since determining a surface’s wettability is “trivially easy,” says senior author Kripa Varanasi, an associate professor of mechanical engineering, that determination can now be used to predict a material’s suitability as a catalyst. Since researchers tend to specialize in either wettability or catalysis, this produces a framework for researchers in both fields to work together to advance understanding, says Varanasi, whose research focuses primarily on wettability; Shao-Horn is an expert on catalytic reactions. “We show how wetting and catalysis, which are both surface phenomena, are related,” Varanasi says, “and how electronic structure forms a link between both.” While both effects are important in a variety of industrial processes and have been the subject of much empirical research, “at the molecular level, we understand very little about what’s happening at the interface,” Shao-Horn says. “This is a step forward, providing a molecular-level understanding.” “It’s primarily an experimental technique” that made the new understanding possible, explains Kelsey Stoerzinger, an MIT graduate student and the paper’s lead author. While most attempts to study such surface science use instruments requiring a vacuum, this team used a device that could study the reactions in humid air, at room temperature, and with varying degrees of water vapor present. Experiments using this system, called ambient pressure X-ray photoelectron spectroscopy, revealed that the reactivity with water is key to the whole process, she says. The water molecules break apart to form hydroxyl groups — an atom of oxygen bound to an atom of hydrogen — bonded to the material’s surface. These reactive compounds, in turn, are responsible for increasing the wetting properties of the surface, while simultaneously inhibiting its ability to catalyze chemical reactions. Therefore, for applications requiring high catalytic activity, the team found, a key requirement is that the surface be hydrophobic, or non-wetting. “Ideally, this understanding helps us design new catalysts,” Stoerzinger says. If a given material “has a lower affinity for water, it has a higher affinity for catalytic activity.” Shao-Horn notes that this is an initial finding, and that “extension of these trends to broader classes of materials and ranges of hydroxyl affinity requires further investigation.” The team has already begun further exploration of these areas. This research, she says, “opens up the space of materials and surfaces we might think about” for both catalysis and wetting.
Black phosphorus could replace silicon computer chips
Silicon Valley in Northern California got its nickname from the multitude of computer chip manufacturers that sprung up in the surrounding area in the 1980’s. Despite its ubiquity as a chip building material, silicon may be facing some competition from a new version of an old substance. Researchers working at the Institute for Basic Science (IBS) Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea, led in part by Director Young Hee Lee, have created a high performance transistor using black phosphorus (BP) which has revealed some fascinating results ("High performance n-type black phosphorus transistors with type control via thickness and contact-metal engineering"). 
Figure 1: Atomic structure of black phosphorus monolayer. Transistors are made up of materials with semiconducting properties, which come in two varieties: n-type (excess electrons) and p-type (excess holes). With the BP crystal, researchers have discovered that they can change its thickness and/or the contact metals and that will determine if it is high performance n-type, p-type, or ambipolar (function as both n- or p-type) material. What does this mean? Silicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip. The BP crystals can operate as both n-type and p-type or something in between, but don’t require extrinsic doping. This means that instead of having to fabricate a silicon-arsenic crystal sandwiched between silicon-boron crystals, a transistor can have a single, lightweight, pure black phosphorus logic chip -- no doping required. Additionally, changing the metals used to connect the chip to the circuit has an influence on whether BP will be n- or p-type. Instead of doping to make an n- and p-type material, both n- and p-type BP can be put all together on one chip just by changing its thickness and the contact metal used. Why is this important? Technology manufacturers are in an arms race to make their devices lighter, smaller and more efficient. By using BP that is only several atomic layers thick, transistors can be made smaller and more energy efficient than what exists now. Silicon chips exist in all of our electronic devices, and as manufacturers make devices smaller and more energy efficient, they begin to approach the threshold for just how small components can be. BP may provide a thinner, more efficient alternative to silicon chips in electrical devices. 
Figure 2: Atomic structure of black phosphorus and n/p-type transistor property of BP transistor. (click on image to enlarge) Another example is tiny autonomous data recording and transmitting devices which will make up the Internet of Things (IoT). A major constraint from preventing IoT from taking off immediately is the inability to scale down the component size and the lack of a long-term power solution. 2 dimensional layered materials (such as black phosphorus) are interesting in this aspect, since both the electrical and mechanical properties are often enhanced compared to their bulk (3 dimensional) counterparts. Is BP a good alternative to current semiconductor materials? It is a great material for transistors since it has a high carrier mobility (how quickly an electron can move through it). This gives BP the ability to operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption. With aluminum as a contact, thicker BP flakes (13 nanometer) show ambipolar properties similar to graphene while thin 3 nm flakes are unipolar n-type with switching on/off ratios greater than 105. The thinner they can make the material, the better the switching performance. Perello explains, “The driving force in back phosphorus is the carrier mobility. Everything centers around that. The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with.” Is it ready to compete with silicon? Unlike other industry standard semiconductor materials, there isn’t a good method for making pure BP on a large scale. Currently, thin layers can be made only from scraping bulk crystalline BP samples, as no other manufacturing method exists yet. Tackling the scaling problem is already underway, with chemical vapor deposition (CVD) and other thin film growth techniques being investigated in labs across the world. The lack of a monolayer fabrication technique isn’t necessarily a problem though. SKKU research fellow David Perello explains, “We can probably operate with 3, 5, or 7 layers and that might actually be better in terms of performance.” When asked if BP was ready to compete with silicon today, Perello said, “I don’t think it can compete with silicon at the moment, that’s a dream everybody has. Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these BP devices.” This doesn’t mean that BP isn’t worth exploring further though. According to Perello, “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication and lithography facilities means that we could make it significantly better. We expect the upper bound for carrier mobility in black phosphorus to be much higher than silicon.” At present, BP isn’t ready for commercial use and its potential has just started to be recognized. If it continues to perform in further tests, it should be strong a contender as a chip material for future technology.
read more "Black phosphorus could replace silicon computer chips"
Figure 1: Atomic structure of black phosphorus monolayer. Transistors are made up of materials with semiconducting properties, which come in two varieties: n-type (excess electrons) and p-type (excess holes). With the BP crystal, researchers have discovered that they can change its thickness and/or the contact metals and that will determine if it is high performance n-type, p-type, or ambipolar (function as both n- or p-type) material. What does this mean? Silicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip. The BP crystals can operate as both n-type and p-type or something in between, but don’t require extrinsic doping. This means that instead of having to fabricate a silicon-arsenic crystal sandwiched between silicon-boron crystals, a transistor can have a single, lightweight, pure black phosphorus logic chip -- no doping required. Additionally, changing the metals used to connect the chip to the circuit has an influence on whether BP will be n- or p-type. Instead of doping to make an n- and p-type material, both n- and p-type BP can be put all together on one chip just by changing its thickness and the contact metal used. Why is this important? Technology manufacturers are in an arms race to make their devices lighter, smaller and more efficient. By using BP that is only several atomic layers thick, transistors can be made smaller and more energy efficient than what exists now. Silicon chips exist in all of our electronic devices, and as manufacturers make devices smaller and more energy efficient, they begin to approach the threshold for just how small components can be. BP may provide a thinner, more efficient alternative to silicon chips in electrical devices. Figure 2: Atomic structure of black phosphorus and n/p-type transistor property of BP transistor. (click on image to enlarge) Another example is tiny autonomous data recording and transmitting devices which will make up the Internet of Things (IoT). A major constraint from preventing IoT from taking off immediately is the inability to scale down the component size and the lack of a long-term power solution. 2 dimensional layered materials (such as black phosphorus) are interesting in this aspect, since both the electrical and mechanical properties are often enhanced compared to their bulk (3 dimensional) counterparts. Is BP a good alternative to current semiconductor materials? It is a great material for transistors since it has a high carrier mobility (how quickly an electron can move through it). This gives BP the ability to operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption. With aluminum as a contact, thicker BP flakes (13 nanometer) show ambipolar properties similar to graphene while thin 3 nm flakes are unipolar n-type with switching on/off ratios greater than 105. The thinner they can make the material, the better the switching performance. Perello explains, “The driving force in back phosphorus is the carrier mobility. Everything centers around that. The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with.” Is it ready to compete with silicon? Unlike other industry standard semiconductor materials, there isn’t a good method for making pure BP on a large scale. Currently, thin layers can be made only from scraping bulk crystalline BP samples, as no other manufacturing method exists yet. Tackling the scaling problem is already underway, with chemical vapor deposition (CVD) and other thin film growth techniques being investigated in labs across the world. The lack of a monolayer fabrication technique isn’t necessarily a problem though. SKKU research fellow David Perello explains, “We can probably operate with 3, 5, or 7 layers and that might actually be better in terms of performance.” When asked if BP was ready to compete with silicon today, Perello said, “I don’t think it can compete with silicon at the moment, that’s a dream everybody has. Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these BP devices.” This doesn’t mean that BP isn’t worth exploring further though. According to Perello, “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication and lithography facilities means that we could make it significantly better. We expect the upper bound for carrier mobility in black phosphorus to be much higher than silicon.” At present, BP isn’t ready for commercial use and its potential has just started to be recognized. If it continues to perform in further tests, it should be strong a contender as a chip material for future technology.
A cost-effective solution to tuned graphene production
Mario Hofmann is holding an example set up of the electrochemical synthesis. Today (30 July), in the journal ("Controlling the properties of graphene produced by electrochemical exfoliation"), a team of researchers report that they have developed a simple electrochemical approach which allows defects to intentionally be created in the graphene, altering its electrical and mechanical properties and making the material even more useful.
The researchers used a technique called electrochemical synthesis to break graphite flakes into graphene layers. By varying the voltage they could change the resulting graphene's thickness, flake area, and number of defects - all of which alter the properties of graphene.
"Graphene is basically a metal - so it's somewhat boring!" explains Mario Hofmann, a researcher at National Cheng Kung University in Taiwan. "But when you start adding defects you begin to get interesting effects."
First studies on the electronic properties of graphene that brought received a lot of attention and the Physics Nobel prize in 2010 used graphene that was produced using adhesive tape to remove flakes of graphene from graphite. However, its defective counterpart graphene oxide could be first to carve out a significant market share as polymer fillers and battery electrodes.
Sol-gel capacitor dielectric offers record-high energy storage
Posted: Using a hybrid silica sol-gel material and self-assembled monolayers of a common fatty acid, researchers have developed a new capacitor dielectric material that provides an electrical energy storage capacity rivaling certain batteries, with both a high energy density and high power density. 
Samples of the new hybrid sol-gel material are shown placed on a clear plastic substrate for testing. If the material can be scaled up from laboratory samples, devices made from it could surpass traditional electrolytic capacitors for applications in electromagnetic propulsion, electric vehicles and defibrillators. Capacitors often complement batteries in these applications because they can provide large amounts of current quickly. The new material is composed of a silica sol-gel thin film containing polar groups linked to the silicon atoms and a nanoscale self-assembled monolayer of an octylphosphonic acid, which provides insulating properties. The bilayer structure blocks the injection of electrons into the sol-gel material, providing low leakage current, high breakdown strength and high energy extraction efficiency. “Sol-gels with organic groups are well known and fatty acids such as phosphonic acids are well known,” noted Joseph Perry, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology. “But to the best of our knowledge, this is the first time these two types of materials have been combined into high-density energy storage devices.” The research, supported by the Office of Naval Research and the Air Force Office of Scientific Research, was reported July 14 in the journal ("Bilayer Structure with Ultrahigh Energy/Power Density Using Hybrid Sol–Gel Dielectric and Charge-Blocking Monolayer"). The need for efficient, high-performance materials for electrical energy storage has been growing along with the ever-increasing demand for electrical energy in mobile applications. Dielectric materials can provide fast charge and discharge response, high energy storage, and power conditioning for defense, medical and commercial applications. But it has been challenging to find a single dielectric material able to maximize permittivity, breakdown strength, energy density and energy extraction efficiency. Perry and colleagues in Georgia Tech’s Center for Organic Photonics and Electronics (COPE) had been working on other capacitor materials to meet these demands, but were not satisfied with the progress. The hybrid sol-gel materials had shown potential for efficient dielectric energy storage because of their high orientational polarization under an electric field, so the group decided to pursue these materials for the new capacitor applications. Using an aluminized mylar film coated with the hybrid sol-gel capacitor material, they showed that the capacitor could be rolled and re-rolled several times while maintaining high energy density, demonstrating its flexibility. But they were still seeing high current leakage. To address that, they deposited a nanoscale self-assembled monolayer of n-octylphosphonic acid on top of the hybrid sol-gel. Less than a nanometer thick, the monolayer serves as an insulating layer. “Our silica sol-gel is a hybrid material because it has polar organic groups attached to the silica framework that gives the sol-gel a high dielectric constant, and in our bilayer dielectric, the n-octylphosphonic acid groups are inserted between the sol-gel layer and the top aluminum layer to block charge injection into the sol-gel,” Perry explained. “It’s really a bilayer hybrid material that takes the best of both reorientation polarization and approaches for reducing injection and improving energy extraction.” In their structures, the researchers demonstrated maximum extractable energy densities up to 40 joules per cubic centimeter, an energy extraction efficiency of 72 percent at a field strength of 830 volts per micron, and a power density of 520 watts per cubic centimeter. The performance exceeds that of conventional electrolytic capacitors and thin-film lithium ion batteries, though it doesn’t match the lithium ion battery formats commonly used in electronic devices and vehicles. “This is the first time I’ve seen a capacitor beat a battery on energy density,” said Perry. “The combination of high energy density and high power density is uncommon in the capacitor world.” Researchers in Perry’s lab have been making arrays of small sol-gel capacitors in the lab to gather information about the material’s performance. The devices are made on small substrates about an inch square. “What we see when we apply an electric field is that the polarization response – which measures how much the polar groups line up in a stable way with the field – behaves in a linear way,” said Perry. “This is what you want to see in a capacitor dielectric material.” The next step will be to scale up the materials to see if the attractive properties transfer to larger devices. If that is successful, Perry expects to commercialize the material through a startup company or SBIR project. “The simplicity of fully solution-based processes for our dielectric material system provides potential for facile scale-up and fabrication on flexible platforms,” the authors wrote in their paper. “This work emphasizes the importance of controlling the electrode-dielectric interface to maximize the performance of dielectric materials for energy storage application.”
read more "Sol-gel capacitor dielectric offers record-high energy storage"
Samples of the new hybrid sol-gel material are shown placed on a clear plastic substrate for testing. If the material can be scaled up from laboratory samples, devices made from it could surpass traditional electrolytic capacitors for applications in electromagnetic propulsion, electric vehicles and defibrillators. Capacitors often complement batteries in these applications because they can provide large amounts of current quickly. The new material is composed of a silica sol-gel thin film containing polar groups linked to the silicon atoms and a nanoscale self-assembled monolayer of an octylphosphonic acid, which provides insulating properties. The bilayer structure blocks the injection of electrons into the sol-gel material, providing low leakage current, high breakdown strength and high energy extraction efficiency. “Sol-gels with organic groups are well known and fatty acids such as phosphonic acids are well known,” noted Joseph Perry, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology. “But to the best of our knowledge, this is the first time these two types of materials have been combined into high-density energy storage devices.” The research, supported by the Office of Naval Research and the Air Force Office of Scientific Research, was reported July 14 in the journal ("Bilayer Structure with Ultrahigh Energy/Power Density Using Hybrid Sol–Gel Dielectric and Charge-Blocking Monolayer"). The need for efficient, high-performance materials for electrical energy storage has been growing along with the ever-increasing demand for electrical energy in mobile applications. Dielectric materials can provide fast charge and discharge response, high energy storage, and power conditioning for defense, medical and commercial applications. But it has been challenging to find a single dielectric material able to maximize permittivity, breakdown strength, energy density and energy extraction efficiency. Perry and colleagues in Georgia Tech’s Center for Organic Photonics and Electronics (COPE) had been working on other capacitor materials to meet these demands, but were not satisfied with the progress. The hybrid sol-gel materials had shown potential for efficient dielectric energy storage because of their high orientational polarization under an electric field, so the group decided to pursue these materials for the new capacitor applications. Using an aluminized mylar film coated with the hybrid sol-gel capacitor material, they showed that the capacitor could be rolled and re-rolled several times while maintaining high energy density, demonstrating its flexibility. But they were still seeing high current leakage. To address that, they deposited a nanoscale self-assembled monolayer of n-octylphosphonic acid on top of the hybrid sol-gel. Less than a nanometer thick, the monolayer serves as an insulating layer. “Our silica sol-gel is a hybrid material because it has polar organic groups attached to the silica framework that gives the sol-gel a high dielectric constant, and in our bilayer dielectric, the n-octylphosphonic acid groups are inserted between the sol-gel layer and the top aluminum layer to block charge injection into the sol-gel,” Perry explained. “It’s really a bilayer hybrid material that takes the best of both reorientation polarization and approaches for reducing injection and improving energy extraction.” In their structures, the researchers demonstrated maximum extractable energy densities up to 40 joules per cubic centimeter, an energy extraction efficiency of 72 percent at a field strength of 830 volts per micron, and a power density of 520 watts per cubic centimeter. The performance exceeds that of conventional electrolytic capacitors and thin-film lithium ion batteries, though it doesn’t match the lithium ion battery formats commonly used in electronic devices and vehicles. “This is the first time I’ve seen a capacitor beat a battery on energy density,” said Perry. “The combination of high energy density and high power density is uncommon in the capacitor world.” Researchers in Perry’s lab have been making arrays of small sol-gel capacitors in the lab to gather information about the material’s performance. The devices are made on small substrates about an inch square. “What we see when we apply an electric field is that the polarization response – which measures how much the polar groups line up in a stable way with the field – behaves in a linear way,” said Perry. “This is what you want to see in a capacitor dielectric material.” The next step will be to scale up the materials to see if the attractive properties transfer to larger devices. If that is successful, Perry expects to commercialize the material through a startup company or SBIR project. “The simplicity of fully solution-based processes for our dielectric material system provides potential for facile scale-up and fabrication on flexible platforms,” the authors wrote in their paper. “This work emphasizes the importance of controlling the electrode-dielectric interface to maximize the performance of dielectric materials for energy storage application.”
Controlling phase changes in solids
Rewritable CDs, DVDs and Blu-Ray discs owe their existence to phase-change materials, those materials that change their internal order when heated and whose structures can be switched back and forth between their crystalline and amorphous phases. Phase-change materials have even more exciting applications on the horizon, but our limited ability to precisely control their phase changes is a hurdle to the development of new technology. One of the most popular and useful phase-change materials is GST, which consists of germanium, antimony, and tellurium. This material is particularly useful because it alternates between its crystalline and amorphous phases more quickly than any other material yet studied. These phase changes result from changes in the bonds between atoms, which also modify the electronic and optical properties of GST as well as its lattice structure. Specifically, resonant bonds, in which electrons participate in several neighboring bonds, influence the material's electro-optical properties, while covalent bonds, in which electrons are shared between two atoms, influence its lattice structure. Most techniques that use GST simultaneously change both the electro-optical and structural properties. This is actually a considerable drawback since in the process of repeating structural transitions, such as heating and cooling the material, the lifetime of any device based on this material is drastically reduced. 
Schematic of the ultrafast transformation pathway. (Image: ICFO/Fritz-Haber-Inst. MPG/SUTD) (click on image to enlarge) In a study recently published in ("Time-domain separation of optical properties from structural transitions in resonantly bonded materials"), researchers from the ICFO groups led by Prof. Simon Wall and ICREA Prof. at ICFO Valerio Pruneri, in collaboration with the Firtz-Haber-Institut der Max-Planck-Gesellschaft, have demonstrated how the material and electro-optical properties of GST change over fractions of a trillionth of a second as the phase of the material changes. Laser light was successfully used to alter the bonds controlling the electro-optical properties without meaningfully altering the bonds controlling the lattice. This new configuration allowed the rapid, reversible changes in the electro-optical properties that are important in device applications without reducing the lifetime of the device by changing its lattice structure. Moreover, the change in the electro-optical properties of GST measured in this study is more than ten times greater than that previously achieved by silicon materials used for the same purpose. This finding suggests that GST may be a good substitute for these commonly used silicon materials. The results of this study may be expected to have far-reaching implications for the development of new technologies, including flexible displays, logic circuits, optical circuits, and universal memory for data storage. These results also indicate the potential of GST for other applications requiring materials with large changes in optical properties that can be achieved rapidly and with high precision.
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Schematic of the ultrafast transformation pathway. (Image: ICFO/Fritz-Haber-Inst. MPG/SUTD) (click on image to enlarge) In a study recently published in ("Time-domain separation of optical properties from structural transitions in resonantly bonded materials"), researchers from the ICFO groups led by Prof. Simon Wall and ICREA Prof. at ICFO Valerio Pruneri, in collaboration with the Firtz-Haber-Institut der Max-Planck-Gesellschaft, have demonstrated how the material and electro-optical properties of GST change over fractions of a trillionth of a second as the phase of the material changes. Laser light was successfully used to alter the bonds controlling the electro-optical properties without meaningfully altering the bonds controlling the lattice. This new configuration allowed the rapid, reversible changes in the electro-optical properties that are important in device applications without reducing the lifetime of the device by changing its lattice structure. Moreover, the change in the electro-optical properties of GST measured in this study is more than ten times greater than that previously achieved by silicon materials used for the same purpose. This finding suggests that GST may be a good substitute for these commonly used silicon materials. The results of this study may be expected to have far-reaching implications for the development of new technologies, including flexible displays, logic circuits, optical circuits, and universal memory for data storage. These results also indicate the potential of GST for other applications requiring materials with large changes in optical properties that can be achieved rapidly and with high precision.
Making the new silicon
An exotic material called gallium nitride (GaN) is poised to become the next semiconductor for power electronics, enabling much higher efficiency than silicon. 
Shown here is a prototype laptop power adapter made by Cambridge Electronics using GaN transistors. At 1.5 cubic inches in diameter, this is the smallest laptop power adapter ever made. In 2013, the Department of Energy (DOE) dedicated approximately half of a $140 million research institute for power electronics to GaN research, citing its potential to reduce worldwide energy consumption. Now MIT spinout Cambridge Electronics Inc. (CEI) has announced a line of GaN transistors and power electronic circuits that promise to cut energy usage in data centers, electric cars, and consumer devices by 10 to 20 percent worldwide by 2025. Power electronics is a ubiquitous technology used to convert electricity to higher or lower voltages and different currents — such as in a laptop’s power adapter, or in electric substations that convert voltages and distribute electricity to consumers. Many of these power-electronics systems rely on silicon transistors that switch on and off to regulate voltage but, due to speed and resistance constraints, waste energy as heat. CEI’s GaN transistors have at least one-tenth the resistance of such silicon-based transistors, according to the company. This allows for much higher energy-efficiency, and orders-of-magnitude faster switching frequency — meaning power-electronics systems with these components can be made much smaller. CEI is using its transistors to enable power electronics that will make data centers less energy-intensive, electric cars cheaper and more powerful, and laptop power adapters one- third the size — or even small enough to fit inside the computer itself. “This is a once-in-a-lifetime opportunity to change electronics and to really make an impact on how energy is used in the world,” says CEI co-founder Tomás Palacios, an MIT associate professor of electrical engineering and computer science who co-invented the technology. Other co-founders and co-inventors are Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, now chair of CEI’s technical advisory board; alumnus Bin Lu SM ’07, PhD ’13, CEI’s vice president for device development; Ling Xia PhD’12, CEI’s director of operations; Mohamed Azize, CEI’s director of epitaxy; and Omair Saadat PhD ’14, CEI’s director of product reliability. Making GaN feasible While GaN transistors have several benefits over silicon, safety drawbacks and expensive manufacturing methods have largely kept them off the market. But Palacios, Lu, Saadat, and other MIT researchers managed to overcome these issues through design innovations made in the late 2000s. Power transistors are designed to flow high currents when on, and to block high voltages when off. Should the circuit break or fail, the transistors must default to the “off” position to cut the current to avoid short circuits and other issues — an important feature of silicon power transistors. But GaN transistors are typically “normally on” — meaning, by default, they’ll always allow a flow of current, which has historically been difficult to correct. Using resources in MIT’s Microsystems Technology Laboratory, the researchers — supported by Department of Defense and DOE grants — developed GaN transistors that were “normally off” by modifying the structure of the material. To make traditional GaN transistors, scientists grow a thin layer of GaN on top of a substrate. The MIT researchers layered different materials with disparate compositions in their GaN transistors. Finding the precise mix allowed a new kind of GaN transistors that go to the off position by default. “We always talk about GaN as gallium and nitrogen, but you can modify the basic GaN material, add impurities and other elements, to change its properties,” Palacios says. But GaN and other nonsilicon semiconductors are also manufactured in special processes, which are expensive. To drop costs, the MIT researchers — at the Institute and, later, with the company — developed new fabrication technologies, or “process recipes,” Lu says. This involved, among other things, switching out gold metals used in manufacturing GaN devices for metals that were compatible with silicon fabrication, and developing ways to deposit GaN on large wafers used by silicon foundries. “Basically, we are fabricating our advanced GaN transistors and circuits in conventional silicon foundries, at the cost of silicon. The cost is the same, but the performance of the new devices is 100 times better,” Lu says. Major applications CEI is currently using its advanced transistors to develop laptop power adaptors that are approximately 1.5 cubic inches in diameter — the smallest ever made.
Among the other feasible applications for the transistors, Palacios says, is better power electronics for data centers run by Google, Amazon, Facebook, and other companies, to power the cloud.
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Shown here is a prototype laptop power adapter made by Cambridge Electronics using GaN transistors. At 1.5 cubic inches in diameter, this is the smallest laptop power adapter ever made. In 2013, the Department of Energy (DOE) dedicated approximately half of a $140 million research institute for power electronics to GaN research, citing its potential to reduce worldwide energy consumption. Now MIT spinout Cambridge Electronics Inc. (CEI) has announced a line of GaN transistors and power electronic circuits that promise to cut energy usage in data centers, electric cars, and consumer devices by 10 to 20 percent worldwide by 2025. Power electronics is a ubiquitous technology used to convert electricity to higher or lower voltages and different currents — such as in a laptop’s power adapter, or in electric substations that convert voltages and distribute electricity to consumers. Many of these power-electronics systems rely on silicon transistors that switch on and off to regulate voltage but, due to speed and resistance constraints, waste energy as heat. CEI’s GaN transistors have at least one-tenth the resistance of such silicon-based transistors, according to the company. This allows for much higher energy-efficiency, and orders-of-magnitude faster switching frequency — meaning power-electronics systems with these components can be made much smaller. CEI is using its transistors to enable power electronics that will make data centers less energy-intensive, electric cars cheaper and more powerful, and laptop power adapters one- third the size — or even small enough to fit inside the computer itself. “This is a once-in-a-lifetime opportunity to change electronics and to really make an impact on how energy is used in the world,” says CEI co-founder Tomás Palacios, an MIT associate professor of electrical engineering and computer science who co-invented the technology. Other co-founders and co-inventors are Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, now chair of CEI’s technical advisory board; alumnus Bin Lu SM ’07, PhD ’13, CEI’s vice president for device development; Ling Xia PhD’12, CEI’s director of operations; Mohamed Azize, CEI’s director of epitaxy; and Omair Saadat PhD ’14, CEI’s director of product reliability. Making GaN feasible While GaN transistors have several benefits over silicon, safety drawbacks and expensive manufacturing methods have largely kept them off the market. But Palacios, Lu, Saadat, and other MIT researchers managed to overcome these issues through design innovations made in the late 2000s. Power transistors are designed to flow high currents when on, and to block high voltages when off. Should the circuit break or fail, the transistors must default to the “off” position to cut the current to avoid short circuits and other issues — an important feature of silicon power transistors. But GaN transistors are typically “normally on” — meaning, by default, they’ll always allow a flow of current, which has historically been difficult to correct. Using resources in MIT’s Microsystems Technology Laboratory, the researchers — supported by Department of Defense and DOE grants — developed GaN transistors that were “normally off” by modifying the structure of the material. To make traditional GaN transistors, scientists grow a thin layer of GaN on top of a substrate. The MIT researchers layered different materials with disparate compositions in their GaN transistors. Finding the precise mix allowed a new kind of GaN transistors that go to the off position by default. “We always talk about GaN as gallium and nitrogen, but you can modify the basic GaN material, add impurities and other elements, to change its properties,” Palacios says. But GaN and other nonsilicon semiconductors are also manufactured in special processes, which are expensive. To drop costs, the MIT researchers — at the Institute and, later, with the company — developed new fabrication technologies, or “process recipes,” Lu says. This involved, among other things, switching out gold metals used in manufacturing GaN devices for metals that were compatible with silicon fabrication, and developing ways to deposit GaN on large wafers used by silicon foundries. “Basically, we are fabricating our advanced GaN transistors and circuits in conventional silicon foundries, at the cost of silicon. The cost is the same, but the performance of the new devices is 100 times better,” Lu says. Major applications CEI is currently using its advanced transistors to develop laptop power adaptors that are approximately 1.5 cubic inches in diameter — the smallest ever made. Among the other feasible applications for the transistors, Palacios says, is better power electronics for data centers run by Google, Amazon, Facebook, and other companies, to power the cloud.
Currently, these data centers eat up about 2 percent of electricity in the United States. But GaN-based power electronics, Palacios says, could save a very significant fraction of that.
Another major future application, Palacios adds, will be replacing the silicon-based power electronics in electric cars. These are in the chargers that charge the battery, and the inverters that convert the battery power to drive the electric motors. The silicon transistors used today have a constrained power capability that limits how much power the car can handle. This is one of the main reasons why there are few large electric vehicles.
GaN-based power electronics, on the other hand, could boost power output for electric cars, while making them more energy-efficient and lighter — and, therefore, cheaper and capable of driving longer distances. “Electric vehicles are popular, but still a niche product. GaN power electronics will be key to make them mainstream,” Palacios says.
Innovative ideas In launching CEI, the MIT founders turned to the Institute’s entrepreneurial programs, which contributed to the startup’s progress. “MIT's innovation and entrepreneurial ecosystem has been key to get things moving and to the point where we are now,” Palacios says. Palacios first earned a grant from the Deshpande Center for Technological Innovation to launch CEI. Afterward, he took his idea for GaN-based power electronics to Innovation Teams (i-Teams), which brings together MIT students from across disciplines to evaluate the commercial feasibility of new technologies. That program, he says, showed him the huge market pull for GaN power electronics, and helped CEI settle on its first products. “Many times, it’s the other way around: You come out with an amazing technology looking for an application. In this case, thanks to i-Teams, we found there were many applications looking for this technology,” Palacios says. For Lu, a key element for growing CEI was auditing Start6, a workshop hosted by the Department of Electrical Engineering and Computer Science, where entrepreneurial engineering students are guided through the startup process with group discussions and talks from seasoned entrepreneurs. Among other things, Lu gained perspective on dividing equity, funding, building a team, and other early startup challenges. “It’s a great class for a student who has an idea, but doesn’t know exactly what’s going on in business,” Lu says. “It’s kind of an overview of what the process is going to be like, so when you start your own company you are ready.”Graphene supercurrents go ballistic
Researchers with Europe's Graphene Flagship have demonstrated superconducting electric currents in the two-dimensional material graphene that bounce between sheet edges without scattering. This first direct observation of the ballistic mirroring of electron waves in a 2D system with supercurrents could lead to the use of graphene-based Josephson junctions in applications such as advanced digital logic circuits, ultrasensitive magnetometers and voltmeters. A Josephson junction is made by sandwiching a thin layer of non-superconducting material between two superconducting layers. Entwined pairs of superconducting electrons known as Cooper pairs are in certain circumstances able to travel without resistance through the insulating or partially-insulating middle layer. The resistance-free current occurs up to a critical current, above which a time-varying (alternating) voltage is set up across the junction. Detecting and measuring the change between current states is the basis of many applications that exploit Josephson junctions. Electronic logic circuits can be constructed from arrays of Josephson junctions, which are also used in superconducting quantum interference devices. SQUIDs are extremely sensitive to electromagnetic fields, and form the basis of magnetometers that can measure fields as low as a few attoteslas (10-18 T), and voltmeters responsive to potential differences of picovolts (10-12 V). Practical uses of such ultra-sensitive devices include the measurement of neurological currents in the brain or heart, and geophysical research. Military applications include remote submarine detection. 
(Image: Technical University of Delft) In the latest issue of the journal ("Ballistic Josephson junctions in edge-contacted graphene"), an international team of physicists led by Graphene Flagship member Lieven Vandersypen, who is based at the Kavli Institute of Nanoscience in Delft, demonstrate unambiguous signatures of Josephson junctions in graphene, a two-dimensional allotrope of carbon atoms arranged in a hexagonal lattice. In the paper, the lead authors of which are Victor Calado and Srijit Goswami, the researchers look at ballistic supercurrents in graphene, with the electrons mirroring between one-dimensional edge contacts made of molybdenum-rhenium. The ultraclean graphene used in the experiment – required in order to preserve the material's unique electrical properties – is shielded from environmental contamination by being encapsulated between sheets of the insulating 2D material hexagonal boron nitride. This three-layer stack is then cut into the desired shape, and the graphene placed in contact with the superconducting alloy. Just as with light bouncing back and forth between two mirrors, leading to an interference pattern set up by the superposition of incident and reflected electromagnetic waves, electrons can reflect from the edges of a superconductor. The difference is that electron interference is only observed in ultraclean samples, in which it is possible for the charged particles to move in ballistic trajectories with minimal scattering from impurities in the material. This is what Calado, Goswami and colleagues observed in their setup, with a striking modulation of the supercurrent. In their paper, the researchers refer to the critical current oscillating as a result of phase-coherent interference of the electrons and electron holes that carry the current. This is caused by the formation of a resonant (Fabry-Pérot) cavity between the mirror points. Furthermore, relatively large supercurrents are seen, travelling over distances of up to 1.5 micrometres. The researchers believe this to be the first direct observation of the ballistic mirroring of supercurrents in graphene. "This work allows us to unravel new physics related to the interplay between superconductivity and the relativistic behaviour of electrons in graphene," said Goswami. "With this technology, we can study and exploit graphene Josephson junctions in a new, exciting regime."
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(Image: Technical University of Delft) In the latest issue of the journal ("Ballistic Josephson junctions in edge-contacted graphene"), an international team of physicists led by Graphene Flagship member Lieven Vandersypen, who is based at the Kavli Institute of Nanoscience in Delft, demonstrate unambiguous signatures of Josephson junctions in graphene, a two-dimensional allotrope of carbon atoms arranged in a hexagonal lattice. In the paper, the lead authors of which are Victor Calado and Srijit Goswami, the researchers look at ballistic supercurrents in graphene, with the electrons mirroring between one-dimensional edge contacts made of molybdenum-rhenium. The ultraclean graphene used in the experiment – required in order to preserve the material's unique electrical properties – is shielded from environmental contamination by being encapsulated between sheets of the insulating 2D material hexagonal boron nitride. This three-layer stack is then cut into the desired shape, and the graphene placed in contact with the superconducting alloy. Just as with light bouncing back and forth between two mirrors, leading to an interference pattern set up by the superposition of incident and reflected electromagnetic waves, electrons can reflect from the edges of a superconductor. The difference is that electron interference is only observed in ultraclean samples, in which it is possible for the charged particles to move in ballistic trajectories with minimal scattering from impurities in the material. This is what Calado, Goswami and colleagues observed in their setup, with a striking modulation of the supercurrent. In their paper, the researchers refer to the critical current oscillating as a result of phase-coherent interference of the electrons and electron holes that carry the current. This is caused by the formation of a resonant (Fabry-Pérot) cavity between the mirror points. Furthermore, relatively large supercurrents are seen, travelling over distances of up to 1.5 micrometres. The researchers believe this to be the first direct observation of the ballistic mirroring of supercurrents in graphene. "This work allows us to unravel new physics related to the interplay between superconductivity and the relativistic behaviour of electrons in graphene," said Goswami. "With this technology, we can study and exploit graphene Josephson junctions in a new, exciting regime."
Spins in graphene with a hedgehog texture
At a surface or interface the electron spin can form specific patterns but it remains in the surface plane. Helmholtz Zentrum Berlin (HZB) researchers have now succeeded in turning the spin out of the plane, and they explain why this is a principle property. The results were published on 27. July 2015 in ("Tunable Fermi level and hedgehog spin texture in gapped graphene"). They are building on previous work published earlier in 2011 in ("Effect of sublattice asymmetry and spin-orbit interaction on out-of-plane spin polarization of photoelectrons"). 
The hedgehog-configuration of the spins and the Fermi-Level is shown. (Illustration: Thomas Splettstößer/HZB) If an electron bounces back from an obstruction it runs, as one should think, exactly back the way it came from. Quantum mechanics, however, has its own rules when it comes to electrons and particularly when it comes to electrons in graphene. When an electron in graphene runs head on against an obstruction and is scattered back, it does change it course by 180°. Its spin, however, should also turn by 180° but it rotates only be 90°. Indeed, an electron has to be rotated by 720° to get it back into its original states. High spin-orbit interaction plus bandgap = Hedgehog texture To do this experiment, several preconditions have to be met. First of all, the electron spin property has to be imparted on the graphen. Varykhalov and coworkers have much experience since they succeeded in this in a remarkable experiment in 2008. They squeezed gold atoms underneath the graphene und thereby enhanced the spin-orbit interaction in the graphene by a factor of 10,000. Precondition no. 2 is to allow for the 180° backscattering. This is challenging since graphene is first and foremost famous the absence of backscattering. To this end, Varykhalov et al. created a band gap in the graphene. This means nothing else than sending electrons back by 180°. If both is fulfilled, the spins in this band gap have to be oriented perpendicular to the graphene plane, more far away, however, in the plane. The continuous transition between the two has the appearance of the prickles of a hedgehog. Model calculations have been performed by theoreticians from Budapest and confirm the experimental results. Construction of a spin filter For symmetry reasons the hedgehog structure has to be reversed elsewhere in the graphene. This does not mean that the hedgehog had no influence on the graphene. On the contrary, the so-called valley Hall effect can be used to realize a spin filter. This effect means that the electrons in the graphene are deflected to the right or left depending on which valley they are in. Because according to the results by Varykhalov et al. the two valleys correspond to two spin orientations, the two spins assemble at opposite sides of the graphene sample.
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The hedgehog-configuration of the spins and the Fermi-Level is shown. (Illustration: Thomas Splettstößer/HZB) If an electron bounces back from an obstruction it runs, as one should think, exactly back the way it came from. Quantum mechanics, however, has its own rules when it comes to electrons and particularly when it comes to electrons in graphene. When an electron in graphene runs head on against an obstruction and is scattered back, it does change it course by 180°. Its spin, however, should also turn by 180° but it rotates only be 90°. Indeed, an electron has to be rotated by 720° to get it back into its original states. High spin-orbit interaction plus bandgap = Hedgehog texture To do this experiment, several preconditions have to be met. First of all, the electron spin property has to be imparted on the graphen. Varykhalov and coworkers have much experience since they succeeded in this in a remarkable experiment in 2008. They squeezed gold atoms underneath the graphene und thereby enhanced the spin-orbit interaction in the graphene by a factor of 10,000. Precondition no. 2 is to allow for the 180° backscattering. This is challenging since graphene is first and foremost famous the absence of backscattering. To this end, Varykhalov et al. created a band gap in the graphene. This means nothing else than sending electrons back by 180°. If both is fulfilled, the spins in this band gap have to be oriented perpendicular to the graphene plane, more far away, however, in the plane. The continuous transition between the two has the appearance of the prickles of a hedgehog. Model calculations have been performed by theoreticians from Budapest and confirm the experimental results. Construction of a spin filter For symmetry reasons the hedgehog structure has to be reversed elsewhere in the graphene. This does not mean that the hedgehog had no influence on the graphene. On the contrary, the so-called valley Hall effect can be used to realize a spin filter. This effect means that the electrons in the graphene are deflected to the right or left depending on which valley they are in. Because according to the results by Varykhalov et al. the two valleys correspond to two spin orientations, the two spins assemble at opposite sides of the graphene sample.
Asymmetric optical-invisibility camouflage
A joint research team from RIKEN and Tokyo Institute of Technology has constructed the design theory of asymmetric invisibility camouflage devices ("Optical Lattice Model Toward Nonreciprocal Invisibility Cloaking"). Optical invisibility camouflage (or invisibility cloaking) is a technology to make an object seem invisible by causing incident light to avoid the object, flow around the object, and return undisturbed to its original trajectory. Such sophisticated manipulation of light will probably be realistic thanks to the recent progress in the research on metamaterials1. To date several research institutes have carried out the theoretical and experimental study of invisibility camouflage devices, using the extraordinary optical properties of metamaterials and the technique of transformation optics2. 
Figure 1. Path of incident light around invisibility camouflage device. (a) Existing camouflage device with optical path independent of light direction. No light can enter into device, and therefore hiding person cannot see outside. (b) Asymmetric camouflage device. Rightward-propagating light avoids hiding person, whereas leftward-propagating light travels straight to enter hiding person's eyes. Hiding person cannot be seen from onlookers on right side but can see them. Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers (Fig. 1a). However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside. To overcome this problem, the research team has formulated a theory of asymmetric (or nonreciprocal) camouflage that can achieve unidirectional transparency in which "they cannot see us, but we can see them." This theory is unrelated to transformation optics but instead based on the concept of 'Lorentz/Coulomb-like forces for photons.' Unidirectional transparency needs a high-level nonreciprocity in the propagation of light. For example, as shown in Fig. 1b), rightward-propagating light have to avoid and circumvent the hiding person, whereas leftward-propagating light have to travel straight to enter the eyes of the hiding person. Such nonreciprocity can be achieved by controlling the movement of photons with two forces that are analogous to Lorentz force3 and Coulomb force4 for moving charged particles. These Lorentz-like and Coulomb-like forces can be generated with an optical resonator lattice consisting of metamaterials. Asymmetric camouflage can be achieved by surrounding a hiding person with the optical resonator lattice. Explanations of Technical Terms 1. Metamaterial Artificial material consisting of multiple nanostructural elements such as minute resonators, arranged periodically with a pitch smaller than the wavelength of light. It can exhibit extraordinary permittivity and permeability values that are not found in nature. Using metamaterials enables to create a unique electromagnetic field surrounding an object we wish to hide, and it should therefore be possible to control the optical path around the object to make it appear invisible. 2. Transformation optics A mathematical technique to design optical systems on the basis of the idea that a distorted space is equivalent in terms of the propagation of light to a flat space filled with a medium having an appropriate spatial distribution of refractive index. (Not used in this research of asymmetric invisibility camouflage.) 3. Lorentz force A force that a moving charged particle experiences in a magnetic field. According to Fleming's rule, when the middle finger, index finger, and thumb of the left hand are stretched perpendicular to each other, if the direction of the middle finger represents the moving direction of the electric current due to the moving particle and the index finger represents the direction of the magnetic field, then the particle experiences the Lorentz force in the direction of the thumb. The direction of the Lorentz force is reversed if the moving direction of the particle is reversed. 4. Coulomb force A force that a charged particle experiences in an electric field. The direction of the Coulomb force depends on the charge: it is in the direction of the electric field for a positive charge and in the opposite direction for a negative charge. The force is not dependent on the direction of particle movement.
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Figure 1. Path of incident light around invisibility camouflage device. (a) Existing camouflage device with optical path independent of light direction. No light can enter into device, and therefore hiding person cannot see outside. (b) Asymmetric camouflage device. Rightward-propagating light avoids hiding person, whereas leftward-propagating light travels straight to enter hiding person's eyes. Hiding person cannot be seen from onlookers on right side but can see them. Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers (Fig. 1a). However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside. To overcome this problem, the research team has formulated a theory of asymmetric (or nonreciprocal) camouflage that can achieve unidirectional transparency in which "they cannot see us, but we can see them." This theory is unrelated to transformation optics but instead based on the concept of 'Lorentz/Coulomb-like forces for photons.' Unidirectional transparency needs a high-level nonreciprocity in the propagation of light. For example, as shown in Fig. 1b), rightward-propagating light have to avoid and circumvent the hiding person, whereas leftward-propagating light have to travel straight to enter the eyes of the hiding person. Such nonreciprocity can be achieved by controlling the movement of photons with two forces that are analogous to Lorentz force3 and Coulomb force4 for moving charged particles. These Lorentz-like and Coulomb-like forces can be generated with an optical resonator lattice consisting of metamaterials. Asymmetric camouflage can be achieved by surrounding a hiding person with the optical resonator lattice. Explanations of Technical Terms 1. Metamaterial Artificial material consisting of multiple nanostructural elements such as minute resonators, arranged periodically with a pitch smaller than the wavelength of light. It can exhibit extraordinary permittivity and permeability values that are not found in nature. Using metamaterials enables to create a unique electromagnetic field surrounding an object we wish to hide, and it should therefore be possible to control the optical path around the object to make it appear invisible. 2. Transformation optics A mathematical technique to design optical systems on the basis of the idea that a distorted space is equivalent in terms of the propagation of light to a flat space filled with a medium having an appropriate spatial distribution of refractive index. (Not used in this research of asymmetric invisibility camouflage.) 3. Lorentz force A force that a moving charged particle experiences in a magnetic field. According to Fleming's rule, when the middle finger, index finger, and thumb of the left hand are stretched perpendicular to each other, if the direction of the middle finger represents the moving direction of the electric current due to the moving particle and the index finger represents the direction of the magnetic field, then the particle experiences the Lorentz force in the direction of the thumb. The direction of the Lorentz force is reversed if the moving direction of the particle is reversed. 4. Coulomb force A force that a charged particle experiences in an electric field. The direction of the Coulomb force depends on the charge: it is in the direction of the electric field for a positive charge and in the opposite direction for a negative charge. The force is not dependent on the direction of particle movement.
Reshaping the solar spectrum to turn light to electricity
When it comes to installing solar cells, labor cost and the cost of the land to house them constitute the bulk of the expense. The solar cells -- made often of silicon or cadmium telluride -- rarely cost more than 20 percent of the total cost. Solar energy could be made cheaper if less land had to be purchased to accommodate solar panels, best achieved if each solar cell could be coaxed to generate more power. A huge gain in this direction has now been made by a team of chemists at the University of California, Riverside that has found an ingenious way to make solar energy conversion more efficient. The researchers report in ("Hybrid Molecule–Nanocrystal Photon Upconversion Across the Visible and Near-Infrared") that by combining inorganic semiconductor nanocrystals with organic molecules, they have succeeded in "upconverting" photons in the visible and near-infrared regions of the solar spectrum. 
Photographs of upconversion in a cuvette containing cadmium selenide/rubrene mixture. The yellow spot is emission from the rubrene originating from (a) an unfocused continuous wave 800 nm laser with an intensity of 300 W/cm2. (b) a focused continuous wave 980 nm laser with an intensity of 2000 W/cm2. The photographs, taken with an iPhone 5, were not modified in any way. (Image: Zhiyuan Huang, UC Riverside) "The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today's solar cells," explained Christopher Bardeen, a professor of chemistry. The research was a collaborative effort between him and Ming Lee Tang, an assistant professor of chemistry. "This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted." Bardeen added that these materials are essentially "reshaping the solar spectrum" so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more. In their experiments, Bardeen and Tang worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons. In lab experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies. "This 550 -- nanometer light can be absorbed by any solar cell material," Bardeen said. "The key to this research is the hybrid composite material -- combining inorganic semiconductor nanoparticles with organic compounds. Organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out." Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes. Bardeen emphasized that the research could have wide-ranging implications. "The ability to move light energy from one wavelength to another, more useful region, for example, from red to blue, can impact any technology that involves photons as inputs or outputs," he said.
read more "Reshaping the solar spectrum to turn light to electricity"
Photographs of upconversion in a cuvette containing cadmium selenide/rubrene mixture. The yellow spot is emission from the rubrene originating from (a) an unfocused continuous wave 800 nm laser with an intensity of 300 W/cm2. (b) a focused continuous wave 980 nm laser with an intensity of 2000 W/cm2. The photographs, taken with an iPhone 5, were not modified in any way. (Image: Zhiyuan Huang, UC Riverside) "The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today's solar cells," explained Christopher Bardeen, a professor of chemistry. The research was a collaborative effort between him and Ming Lee Tang, an assistant professor of chemistry. "This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted." Bardeen added that these materials are essentially "reshaping the solar spectrum" so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more. In their experiments, Bardeen and Tang worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons. In lab experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies. "This 550 -- nanometer light can be absorbed by any solar cell material," Bardeen said. "The key to this research is the hybrid composite material -- combining inorganic semiconductor nanoparticles with organic compounds. Organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out." Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes. Bardeen emphasized that the research could have wide-ranging implications. "The ability to move light energy from one wavelength to another, more useful region, for example, from red to blue, can impact any technology that involves photons as inputs or outputs," he said.
Wafer-thin material heralds future of wearable technology
UOW’s Institute for Superconducting and Electronic Materials (ISEM) has successfully pioneered a way to construct a flexible, foldable and lightweight energy storage device that provides the building blocks for next-generation batteries needed to power wearable electronics and implantable medical devices (, "Self-Assembled Multifunctional Hybrids: Toward Developing High-Performance Graphene-Based Architectures for Energy Storage Devices"). The conundrum researchers have faced in developing miniature energy storage devices, such as batteries and supercapacitors, has been figuring out how to increase the surface area of the device, to store more charge, without making it larger. “Among all modern electronic devices, portable electronics are some of the most exciting,” ISEM PhD student Monirul Islam said. “But the biggest challenge is to charge storage in a small volume as well as being able to deliver that charge quickly on demand.” To solve this problem, a team of PhD students, led by Dr Konstantin Konstantinov under the patronage of ISEM Director Professor Shi Xue Dou and with the support of Professor Hua Kun Liu, the head of ISEM Energy Storage Division, have developed a three-dimensional structure using a flat-pack self-assembly of three components: graphene, a conductive polymer and carbon nanotubes, which are atom-thick lattice-like networks of carbon formed into cylinders. Graphene, made from single atom-thick layers of graphite, was a suitable candidate due its electronic performance and mechanical strength. “We knew in theory that if you can make a sort of carbon skeleton you have a greater surface area and greater surface area means more charge,” Dr Konstantinov said. “If we could efficiently separate the layers of carbon we could then use both surfaces of each layer for charge accumulation. The problem we faced was that fabricating these 3D shapes in practice, not just theory, is a challenging, if not impossible task.” The solution was to flat-pack the components by building the 3D shape layer-by-layer, much like a miniature exercise in cake decoration. The graphene in liquid form was mixed with the conductive polymer and reduced to solid and the carbon nanotubes carefully inserted between the graphene layers to form a self-assembled flat-packed, wafer-thin supercapacitor material. “The real challenge was how to assemble these three components into a single structure with the best use of the space available,” PhD student Monirul Islam said. “Getting the proportions or ratios of the components appropriately in order to obtain a composite material with maximum energy storage performance was another challenge.” Wrong proportions of either ingredient result in a lumpy mess, or a 3D shape that isn’t strong enough to retain the needed flexibility as well as the charge storage ability. There’s also elegance in the simplicity of the team’s design: the researchers dispersed the components in liquid crystalline, which enabled natural chemical interactions to prevent the graphene layers clumping together. The result was a 3D shape with, thanks to the carbon nanotubes, a massive surface area, excellent charge capacity that is also foldable. It can also be cheaply and easily fabricated without the need for expensive vacuum chambers or sophisticated equipment. “Our graphene-based flexible composite is highly conductive, lightweight, is able to fold like a roll or stack like a paper in electronic devices to store a huge amount of charge,” Monirul said. “This material can store charge in a second and deliver the charge in superfast speed and will be more lightweight than traditional batteries used in present day’s electronics.” The ISEM study has been financially supported by the Automotive Australia 2020 CRC as part of its research into electric vehicles. ISEM is the program leader for electrification and plays crucial role for design of next generation electric vehicles A key to unlocking the electric vehicle’s capability is a lightweight and powerful battery pack. “Our simple fabrication method of eco-friendly materials with increased performance has great potential to be scaled up for use supercapacitor and battery technology. Our next step is to use this material to fabricate flexible wearable supercapacitors with high power density and energy density as well as large scale supercapacitors for electric vehicles.”
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Superconducting qubit and magnetic sphere hybrid
Researchers at the University of Tokyo have demonstrated that it is possible to exchange a quantum bit, the minimum unit of information used by quantum computers, between a superconducting quantum-bit circuit and a quantum in a magnet called a magnon ("Coherent coupling between a ferromagnetic magnon and a superconducting qubit"). This result is expected to contribute to the development of quantum interfaces and quantum repeaters. Magnets, often used in our daily life, exert a magnetic force produced by a large number of microscopic magnets – the spins of individual electrons – that are aligned in the same orientation. The collective motions of the ensemble of spins are called spin waves. A magnon is a quantum of such excitations, similar to a photon as a quantum of light, i.e., the electromagnetic wave. At room temperature the motions of electron spins can be largely affected by heat. The properties of individual magnons have not been studied at low temperatures corresponding to the “quantum limit” where all thermally-induced spin fluctuations vanish. 
Illustration of magnet-qubit coupled system. A magnet (ytterium iron garnet; YIG) and a superconducting qubit are placed with a separation of 4 cm. The electric field in the cavity interacts with the qubit, while the magnetic field interacts with the magnet. At an extremely low temperature of around -273 degrees centigrade, magnons, i.e., quanta of the fluctuations in the magnet, coherently couple with the qubit through the electromagnetic field of the cavity. (Image: Yutaka Tabuchi) The research group of Professor Yasunobu Nakamura at the University of Tokyo Research Center for Advanced Science and Technology has succeeded for the first time to couple a magnon in a magnet to a photon in a microwave cavity at an ultralow temperature near absolute zero (-273.14 degrees centigrade). They observed coherent interaction between a magnon and a microwave photon by placing a millimeter-sized ferromagnetic sphere made of yttrium iron garnet in a centimeter-scale microwave cavity. The research group furthermore demonstrated coherent coupling of a magnon to a superconducting quantum-bit circuit. The latter is known as a well-controllable quantum system and as one of the most promising building blocks for quantum processors. The group placed the magnet together with the superconducting qubit in a cavity and demonstrated exchange of information between the magnon and superconducting qubit mediated by the microwave cavity. The results will stimulate research on the quantum behavior of magnons in spintronics devices and open a path toward realization of quantum interfaces and quantum repeaters.
read more "Superconducting qubit and magnetic sphere hybrid"
Illustration of magnet-qubit coupled system. A magnet (ytterium iron garnet; YIG) and a superconducting qubit are placed with a separation of 4 cm. The electric field in the cavity interacts with the qubit, while the magnetic field interacts with the magnet. At an extremely low temperature of around -273 degrees centigrade, magnons, i.e., quanta of the fluctuations in the magnet, coherently couple with the qubit through the electromagnetic field of the cavity. (Image: Yutaka Tabuchi) The research group of Professor Yasunobu Nakamura at the University of Tokyo Research Center for Advanced Science and Technology has succeeded for the first time to couple a magnon in a magnet to a photon in a microwave cavity at an ultralow temperature near absolute zero (-273.14 degrees centigrade). They observed coherent interaction between a magnon and a microwave photon by placing a millimeter-sized ferromagnetic sphere made of yttrium iron garnet in a centimeter-scale microwave cavity. The research group furthermore demonstrated coherent coupling of a magnon to a superconducting quantum-bit circuit. The latter is known as a well-controllable quantum system and as one of the most promising building blocks for quantum processors. The group placed the magnet together with the superconducting qubit in a cavity and demonstrated exchange of information between the magnon and superconducting qubit mediated by the microwave cavity. The results will stimulate research on the quantum behavior of magnons in spintronics devices and open a path toward realization of quantum interfaces and quantum repeaters.
Superfast fluorescence sets new speed record
Researchers have developed an ultrafast light-emitting device that can flip on and off 90 billion times a second and could form the basis of optical computing. At its most basic level, your smart phone's battery is powering billions of transistors using electrons to flip on and off billions of times per second. But if microchips could use photons instead of electrons to process and transmit data, computers could operate even faster. But first engineers must build a light source that can be turned on and off that rapidly. While lasers can fit this requirement, they are too energy-hungry and unwieldy to integrate into computer chips. Duke University researchers are now one step closer to such a light source. In a new study, a team from the Pratt School of Engineering pushed semiconductor quantum dots to emit light at more than 90 billion gigahertz. This so-called plasmonic device could one day be used in optical computing chips or for optical communication between traditional electronic microchips. 
A nanoscale view of the new superfast fluorescent system using a transmission electron microscope. The silver cube is just 75-nanometers wide. The quantum dots (red) are sandwiched between the silver cube and a thin gold foil. (Image: Maiken Mikkelsen, Duke University) The study was published online on July 27 in ("Ultrafast Spontaneous Emission Source Using Plasmonic Nanoantennas"). "This is something that the scientific community has wanted to do for a long time," said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. "We can now start to think about making fast-switching devices based on this research, so there's a lot of excitement about this demonstration." The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon. The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots -- spheres of semiconducting material just six nanometers wide -- that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz. "There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons," said Gleb Akselrod, a postdoctoral research in Mikkelsen's laboratory. "Now we have made an important step towards solving these problems." "The eventual goal is to integrate our technology into a device that can be excited either optically or electrically," said Thang Hoang, also a postdoctoral researcher in Mikkelsen's laboratory. "That's something that I think everyone, including funding agencies, is pushing pretty hard for." The group is now working to use the plasmonic structure to create a single photon source -- a necessity for extremely secure quantum communications -- by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible. Aside from its potential technological impacts, the research demonstrates that well-known materials need not be limited by their intrinsic properties. "By tailoring the environment around a material, like we've done here with semiconductors, we can create new designer materials with almost any optical properties we desire," said Mikkelsen. "And that's an emerging area that's fascinating to think about."
read more "Superfast fluorescence sets new speed record"
A nanoscale view of the new superfast fluorescent system using a transmission electron microscope. The silver cube is just 75-nanometers wide. The quantum dots (red) are sandwiched between the silver cube and a thin gold foil. (Image: Maiken Mikkelsen, Duke University) The study was published online on July 27 in ("Ultrafast Spontaneous Emission Source Using Plasmonic Nanoantennas"). "This is something that the scientific community has wanted to do for a long time," said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke. "We can now start to think about making fast-switching devices based on this research, so there's a lot of excitement about this demonstration." The new speed record was set using plasmonics. When a laser shines on the surface of a silver cube just 75 nanometers wide, the free electrons on its surface begin to oscillate together in a wave. These oscillations create their own light, which reacts again with the free electrons. Energy trapped on the surface of the nanocube in this fashion is called a plasmon. The plasmon creates an intense electromagnetic field between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away. This field interacts with quantum dots -- spheres of semiconducting material just six nanometers wide -- that are sandwiched in between the nanocube and the gold. The quantum dots, in turn, produce a directional, efficient emission of photons that can be turned on and off at more than 90 gigahertz. "There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons," said Gleb Akselrod, a postdoctoral research in Mikkelsen's laboratory. "Now we have made an important step towards solving these problems." "The eventual goal is to integrate our technology into a device that can be excited either optically or electrically," said Thang Hoang, also a postdoctoral researcher in Mikkelsen's laboratory. "That's something that I think everyone, including funding agencies, is pushing pretty hard for." The group is now working to use the plasmonic structure to create a single photon source -- a necessity for extremely secure quantum communications -- by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil. They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible. Aside from its potential technological impacts, the research demonstrates that well-known materials need not be limited by their intrinsic properties. "By tailoring the environment around a material, like we've done here with semiconductors, we can create new designer materials with almost any optical properties we desire," said Mikkelsen. "And that's an emerging area that's fascinating to think about."
The future with nanoionics
Living systems such as the brain conduct electric signals via ions – charged atoms or molecules – rather than electrons. Like the living ones, artificial systems that run on ions could in some ways be more efficient than today’s electronic devices. Nanoionics is a new area of research in which ionic currents are conducted on the scale of nanometers; and it may one day lead to innovative technologies. Weizmann Institute scientists have now made an important step toward the construction of artificial ionic circuits. As reported recently in ("Single-layer ionic conduction on carboxyl-terminated silane monolayers patterned by constructive lithography"), the Weizmann researchers have developed a method for building ion-conducting channels with planned shapes and dimensions on the surface of a solid material. This method answers two technical challenges: The motion of ions through solids is rather sluggish at room temperature, and no previous method has been available for confining such motion to well-defined, predetermined paths. 
Future devices may run on ions flowing in nano-sized channels. An ion-conducting nanochannel viewed under an atomic force microscope (left) and scanning electron microscope (right). To create ion-conducting channels on a solid surface, the Institute team – Dr. Jonathan Berson (then a Ph.D. student), Dr. Doron Burshtain and Dr. Assaf Zeira, Alexander Yoffe, Dr. Rivka Maoz and Prof. Jacob Sagiv, all of the Materials and Interfaces Department – started out with a self-assembled organic monolayer – that is, a one-molecule-thick layer of highly ordered organic material laid down on a silicon wafer by a process known as surface self-assembly. The scientists then “wrote” the desired ion-conducting channels on the top surface of the monolayer using an atomic force microscope. This microscope “feels out” a material’s surface with a fine-tip needle, but it can also chemically modify that surface by means of a small electric current flowing through the tip. Thus the researchers could modify selected sites on the monolayer surface, literally writing custom surface patterns of ion-conducting channels, with widths down to ten nanometers and lengths up to a hundred micrometers. Similarly, using special electric stamps instead of a sharp needle, the scientists could “print” larger, macroscopic ion-conducting channels covering surface areas of up to several square centimeters. The researchers examined the resulting ion-conducting patterns using a combination of advanced tools: atomic force microscopy, surface-adapted infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, along with sensitive electrical measurements. Driven by a small electric potential, ions of silver, titanium and other metals were found to effectively move along these channels. The new method may one day be used in the manufacture of artificial nanoionic systems that could mimic biological ones, even if they do not necessarily employ the same mechanisms of action. And this, in turn, may in the future help create nanoionic devices for use in science and in day-to-day life that would offer significant advantages over some of today’s electronic technologies – just as, for example, the brain, a natural ionic system, still outperforms electronic computers at many tasks. Though ionic devices are not expected to be faster than electronic ones, they may require less energy for writing and erasing information. Most prominent among their potential applications are ion-based computer memory devices that promise to be much longer-lasting than the electronic ones existing today, a feature that is essential for safe long-term storage of information. In the more immediate future, the Weizmann Institute methodology offers new possibilities for exploring previously unknown properties of matter, as the properties of materials differ on the nanoscale compared with the macroscale. For example, the Weizmann scientists observed an unusually fast conduction of ions in their nanochannels – a finding that opens up a new direction of research.
read more "The future with nanoionics"
Future devices may run on ions flowing in nano-sized channels. An ion-conducting nanochannel viewed under an atomic force microscope (left) and scanning electron microscope (right). To create ion-conducting channels on a solid surface, the Institute team – Dr. Jonathan Berson (then a Ph.D. student), Dr. Doron Burshtain and Dr. Assaf Zeira, Alexander Yoffe, Dr. Rivka Maoz and Prof. Jacob Sagiv, all of the Materials and Interfaces Department – started out with a self-assembled organic monolayer – that is, a one-molecule-thick layer of highly ordered organic material laid down on a silicon wafer by a process known as surface self-assembly. The scientists then “wrote” the desired ion-conducting channels on the top surface of the monolayer using an atomic force microscope. This microscope “feels out” a material’s surface with a fine-tip needle, but it can also chemically modify that surface by means of a small electric current flowing through the tip. Thus the researchers could modify selected sites on the monolayer surface, literally writing custom surface patterns of ion-conducting channels, with widths down to ten nanometers and lengths up to a hundred micrometers. Similarly, using special electric stamps instead of a sharp needle, the scientists could “print” larger, macroscopic ion-conducting channels covering surface areas of up to several square centimeters. The researchers examined the resulting ion-conducting patterns using a combination of advanced tools: atomic force microscopy, surface-adapted infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, along with sensitive electrical measurements. Driven by a small electric potential, ions of silver, titanium and other metals were found to effectively move along these channels. The new method may one day be used in the manufacture of artificial nanoionic systems that could mimic biological ones, even if they do not necessarily employ the same mechanisms of action. And this, in turn, may in the future help create nanoionic devices for use in science and in day-to-day life that would offer significant advantages over some of today’s electronic technologies – just as, for example, the brain, a natural ionic system, still outperforms electronic computers at many tasks. Though ionic devices are not expected to be faster than electronic ones, they may require less energy for writing and erasing information. Most prominent among their potential applications are ion-based computer memory devices that promise to be much longer-lasting than the electronic ones existing today, a feature that is essential for safe long-term storage of information. In the more immediate future, the Weizmann Institute methodology offers new possibilities for exploring previously unknown properties of matter, as the properties of materials differ on the nanoscale compared with the macroscale. For example, the Weizmann scientists observed an unusually fast conduction of ions in their nanochannels – a finding that opens up a new direction of research.
Graphene - from science fundamentals to low-cost production
Keynote presentations on the third day of Graphene Week 2015 offered an eclectic mix of fundamental science and practical chemical engineering. Here we report briefly on each of the talks, beginning with an introduction to optoelectronics in 2d semiconductors and heterostructures, and concluding with an outline of a highly promising ‘kitchen sink’ approach to graphene production. Xiaodong Xu of the University of Washington in Seattle set the ball rolling with a look at the optoelectronics and valleytronics of 2d semiconductors and heterostructures, such as those based on the sulphides and selenides of molybdenum and tungsten. Xu began with a recap of the electronic properties of MoS2, including the change in band gap with number of layers. Of particular interest to Xu are valley-specific interlayer excitons in monolayer WSe2-MoSe2 vertical heterostructures. Optical pumping leads to coupled spin-valley polarisation of interlayer excitons, with measured lifetimes of more than 30 nanoseconds. Such long-lived polarisation allows for the visualisation of both spin and valley diffusion over length scales of several microns. In practical terms, the effect is important in laser technologies with a tuneable carrier density. Marco Polini of the Scuola Normale Superiore in Pisa discussed plasmon damping mechanisms, with a focus on graphene sheets encapsulated in boron nitride. With 2d materials, plasmon lifetimes are two orders of magnitude longer than with bulk materials such as silicon and silver. With graphene and boron nitride, lifetimes as long as a picosecond have been observed. Damping mechanisms outlined by Polini included electron-electron, electron-impurity, and electron-phonon collisions. He went on to discuss experimental and theoretical work on hybrid plasmon-phonon polaritons. In the second part of his talk, Polini looked at direct current transport in graphene, with hydrodynamic flow and current whirlpools observed at length scales of half a micron. Amir Yacoby from Harvard University in Cambridge, Massachusetts discussed observations of edge currents using Josephson interferometry. The idea here is to use superconductivity to study the intrinsic physical properties of graphene. As for the origin of the observed edge currents, Yacoby suggested that certain edge shapes may guide the current. An alternative explanation is guided electron fibre-optic states at the Dirac point. This guided mode theory can explain edge current observations in bilayer as well as in single layer graphene.
Relevant applications include imaging of topological currents at domain boundaries in bilayer graphene, and induced superconductivity in the quantum spin Hall regime.
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Relevant applications include imaging of topological currents at domain boundaries in bilayer graphene, and induced superconductivity in the quantum spin Hall regime.
Graphene is a two-dimensional polymer, noted Klaus Müllen of the Max-Planck Institute in Mainz, and this makes it something of a challenge for materials synthesis. Müllen looked at both bottom-up and top-down production protocols, including the flattening of 3d, propeller-like molecules. The most promising approach to graphene synthesis is electrochemical exfoliation.
Applications of electrochemically exfoliated graphene identified by Müllen include organic photodetectors and transparent conductive electrodes, with the ability to produce ultrathin and flexible devices. Energy storage is another possibility, using exfoliated graphene and colloidal nanoparticles. Such nanoparticles, wrapped in graphene, offer high reversible charge capacity, retention and Coulomb efficiency.
Müllen concluded his talk with some 3d simulations of carbon networks, and noted, with the illustration of a beehive, that nature sometimes makes mistakes.
Manish Chhowalla of Rutgers University in New Jersey began his talk with an overview of molybdenum and tungsten disulphides. These layered semiconductor materials have a number of interesting properties, but the key problem in using them for electronics applications has been high contact resistance with metals deposited on the semiconducting 2H phase.
Contact resistance in MoS2 can be reduced by inducing a metallic (1T) phase on 2H phase nanosheets. Hybrid field-effect transistors with 2H monolayer MoS2 as the channel, and 1T source and drain contacts, display high electron mobilities, low subthreshold swing values, high on/off ratios and drive currents, and excellent current saturation. Deposition of different metals has a limited influence on transistor performance, suggesting that the 1T-2H interface controls carrier injection into the channel. In practical terms, the MoS2 channel must be locally patterned in order to make such structures. This can be done with a PMMA mask to partially cover certain areas. The result is a contact resistance of 0.2 kiloohms per micrometre. In comparison, 2H phase MoS2 has a contact resistance of 1.12 kiloohms per micron. Jonathan Coleman from Trinity College Dublin spoke of his research group's much-lauded graphene production process known as liquid phase exfoliation, aka kitchen-blender graphene. And not only graphene, as the technique can be used to produce nanoscale flakes of a range of 2d materials. Coleman discussed the fundamentals and practicalities of liquid-phase exfoliation, focusing on such matters as control of flake size. The bulk of Coleman's presentation was given to applications, and here he identified a number of areas. These include the mechanical improvement of composite materials, strain and other motion sensors based on electrical conductivity changes, electrical energy storage and printed electronics. The next challenge for liquid exfoliation is to achieve industrial-scale production of graphene and related 2d materials. To this end, Coleman highlighted a collaboration between his research group and chemical manufacturer Thomas Swan.Changing the color of light
Researchers at the University of Delaware have received a $1 million grant from the W.M. Keck Foundation to explore a new idea that could improve solar cells, medical imaging and even cancer treatments. Simply put, they want to change the color of light. They won’t be tinkering with what you see out your window: no purple days or chartreuse nights, no edits to rainbows and blazing sunsets. Their goal is to turn low-energy colors of light, such as red, into higher-energy colors, like blue or green. Changing the color of light would give solar technology a considerable boost. A traditional solar cell can only absorb light with energy above a certain threshold. Infrared light passes right through, its energy untapped. However, if that low-energy light could be transformed into higher-energy light, a solar cell could absorb much more of the sun’s clean, free, abundant energy. The team predicts that their novel approach could increase the efficiency of commercial solar cells by 25 to 30 percent. 
The UD research team aims to develop new nanostructures that act like a ratchet to combine the energy of two red photons of light into a single blue photon, which has higher energy. Such an advance could improve solar cell efficiency to chemotherapy treatments. The research team, based in UD’s College of Engineering, is led by Matthew Doty, associate professor of materials science and engineering and associate director of UD’s Nanofabrication Facility. Doty’s co-investigators include Joshua Zide, Diane Sellers and Chris Kloxin, all in the Department of Materials Science and Engineering; and Emily Day and John Slater, both in the Department of Biomedical Engineering. “This prestigious $1 million grant from the Keck Foundation underscores the excellence and innovation of our University of Delaware faculty,” says Nancy Targett, acting president of the University. “Clearly, the University of Delaware is pursuing big ideas in renewable energy and biomedicine with the potential to benefit the world.” “The University’s Delaware Will Shine strategic plan challenges us to think boldly as we seek solutions to problems facing society,” Domenico Grasso, UD’s provost, adds. “We congratulate the research team in the College of Engineering for this major award, and we look forward to their findings.” Changing the color of light “A ray of light contains millions and millions of individual units of light called photons,” says project leader Matthew Doty. “The energy of each photon is directly related to the color of the light — a photon of red light has less energy than a photon of blue light. You can’t simply turn a red photon into a blue one, but you can combine the energy from two or more red photons to make one blue photon.” This process, called “photon upconversion,” isn’t new, Doty says. However, the UD team’s approach to it is. They want to design a new kind of semiconductor nanostructure that will act like a ratchet. It will absorb two red photons, one after the other, to push an electron into an excited state when it can emit a single high-energy (blue) photon. These nanostructures will be so teeny they can only be viewed when magnified a million times under a high-powered electron microscope. “Think of the electrons in this structure as if they were at a water park,” Doty says. “The first red photon has only enough energy to push an electron half-way up the ladder of the water slide. The second red photon pushes it the rest of the way up. Then the electron goes down the slide, releasing all of that energy in a single process, with the emission of the blue photon. The trick is to make sure the electron doesn’t slip down the ladder before the second photon arrives. The semiconductor ratchet structure is how we trap the electron in the middle of the ladder until the second photon arrives to push it the rest of the way up.” The UD team will develop new semiconductor structures containing multiple layers of different materials, such as aluminum arsenide and gallium bismuth arsenide, each only a few nanometers thick. This “tailored landscape” will control the flow of electrons into states with varying potential energy, turning once-wasted photons into useful energy. The UD team has shown theoretically that their semiconductors could reach an upconversion efficiency of 86 percent, which would be a vast improvement over the 36 percent efficiency demonstrated by today’s best materials. What’s more, Doty says, the amount of light absorbed and energy emitted by the structures could be customized for a variety of applications, from lightbulbs to laser-guided surgery. How do you even begin to make structures so tiny they can only be seen with an electron microscope? In one technique the UD team will use, called molecular beam epitaxy, nanostructures will be built by depositing layers of atoms one at a time. Each structure will be tested to see how well it absorbs and emits light, and the results will be used to tailor the structure to improve performance. The researchers also will develop a milk-like solution filled with millions of identical individual nanoparticles, each one containing multiple layers of different materials. The multiple layers of this structure, like multiple candy shells in an M&M, will implement the photon ratchet idea. Through such work, the team envisions a future upconversion “paint” that could be easily applied to solar cells, windows and other commercial products. Improving medical tests and treatments While the initial focus of the three-year project will be on improving solar energy harvesting, the team also will explore biomedical applications. A number of diagnostic tests and medical treatments, ranging from CT and PET scans to chemotherapy, rely on the release of fluorescent dyes and pharmaceutical drugs. Ideally, such payloads are delivered both at specific disease sites and at specific times, but this is hard to control in practice. The UD team aims to develop an upconversion nanoparticle that can be triggered by light to release its payload. The goal is to achieve the controlled release of drug therapies even deep within diseased human tissue while reducing the peripheral damage to normal tissue by minimizing the laser power required. “This is high-risk, high-reward research,” Doty says. “High-risk because we don’t yet have proof-of-concept data. High-reward because it has such a huge potential impact in renewable energy to medicine. It’s amazing to think that this same technology could be used to harvest more solar energy and to treat cancer. We’re excited to get started!
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The UD research team aims to develop new nanostructures that act like a ratchet to combine the energy of two red photons of light into a single blue photon, which has higher energy. Such an advance could improve solar cell efficiency to chemotherapy treatments. The research team, based in UD’s College of Engineering, is led by Matthew Doty, associate professor of materials science and engineering and associate director of UD’s Nanofabrication Facility. Doty’s co-investigators include Joshua Zide, Diane Sellers and Chris Kloxin, all in the Department of Materials Science and Engineering; and Emily Day and John Slater, both in the Department of Biomedical Engineering. “This prestigious $1 million grant from the Keck Foundation underscores the excellence and innovation of our University of Delaware faculty,” says Nancy Targett, acting president of the University. “Clearly, the University of Delaware is pursuing big ideas in renewable energy and biomedicine with the potential to benefit the world.” “The University’s Delaware Will Shine strategic plan challenges us to think boldly as we seek solutions to problems facing society,” Domenico Grasso, UD’s provost, adds. “We congratulate the research team in the College of Engineering for this major award, and we look forward to their findings.” Changing the color of light “A ray of light contains millions and millions of individual units of light called photons,” says project leader Matthew Doty. “The energy of each photon is directly related to the color of the light — a photon of red light has less energy than a photon of blue light. You can’t simply turn a red photon into a blue one, but you can combine the energy from two or more red photons to make one blue photon.” This process, called “photon upconversion,” isn’t new, Doty says. However, the UD team’s approach to it is. They want to design a new kind of semiconductor nanostructure that will act like a ratchet. It will absorb two red photons, one after the other, to push an electron into an excited state when it can emit a single high-energy (blue) photon. These nanostructures will be so teeny they can only be viewed when magnified a million times under a high-powered electron microscope. “Think of the electrons in this structure as if they were at a water park,” Doty says. “The first red photon has only enough energy to push an electron half-way up the ladder of the water slide. The second red photon pushes it the rest of the way up. Then the electron goes down the slide, releasing all of that energy in a single process, with the emission of the blue photon. The trick is to make sure the electron doesn’t slip down the ladder before the second photon arrives. The semiconductor ratchet structure is how we trap the electron in the middle of the ladder until the second photon arrives to push it the rest of the way up.” The UD team will develop new semiconductor structures containing multiple layers of different materials, such as aluminum arsenide and gallium bismuth arsenide, each only a few nanometers thick. This “tailored landscape” will control the flow of electrons into states with varying potential energy, turning once-wasted photons into useful energy. The UD team has shown theoretically that their semiconductors could reach an upconversion efficiency of 86 percent, which would be a vast improvement over the 36 percent efficiency demonstrated by today’s best materials. What’s more, Doty says, the amount of light absorbed and energy emitted by the structures could be customized for a variety of applications, from lightbulbs to laser-guided surgery. How do you even begin to make structures so tiny they can only be seen with an electron microscope? In one technique the UD team will use, called molecular beam epitaxy, nanostructures will be built by depositing layers of atoms one at a time. Each structure will be tested to see how well it absorbs and emits light, and the results will be used to tailor the structure to improve performance. The researchers also will develop a milk-like solution filled with millions of identical individual nanoparticles, each one containing multiple layers of different materials. The multiple layers of this structure, like multiple candy shells in an M&M, will implement the photon ratchet idea. Through such work, the team envisions a future upconversion “paint” that could be easily applied to solar cells, windows and other commercial products. Improving medical tests and treatments While the initial focus of the three-year project will be on improving solar energy harvesting, the team also will explore biomedical applications. A number of diagnostic tests and medical treatments, ranging from CT and PET scans to chemotherapy, rely on the release of fluorescent dyes and pharmaceutical drugs. Ideally, such payloads are delivered both at specific disease sites and at specific times, but this is hard to control in practice. The UD team aims to develop an upconversion nanoparticle that can be triggered by light to release its payload. The goal is to achieve the controlled release of drug therapies even deep within diseased human tissue while reducing the peripheral damage to normal tissue by minimizing the laser power required. “This is high-risk, high-reward research,” Doty says. “High-risk because we don’t yet have proof-of-concept data. High-reward because it has such a huge potential impact in renewable energy to medicine. It’s amazing to think that this same technology could be used to harvest more solar energy and to treat cancer. We’re excited to get started!
A 'nanomachine' for surgery with no incision
A surgical operation has long been considered one of the first options in cancer treatment; however, a number of issues have been recognized: a highly invasive procedure; a decline in the Quality of Life (QOL) after an operation; the possibility of a recurrence due to missed cancer cells; extended hospitalization, sometimes for as long as one month; and the economic costs. Against such a background, recently neutron capture therapy(1) has been drawing attention. By irradiating the affected area with a pinpoint light beam, ultrasonic waves, and thermal neutrons, which can be safely administered to living organisms, specific chemical compounds (neutron sensitizer elements) are activated and kill the cancer cells. This therapy has a lower burden on patients. However, the technological development to deliver the neutron sensitizer molecules to cancer cells has been a great challenge. A research team led by Professor Kazunori Kataoka, Department of Bioengineering, School of Engineering, The University of Tokyo (concurrently serving as the Director of the Innovation Center of NanoMedicine, Kawasaki Institute of Industry Promotion), and Professor Nobuhiro Nishiyama, Chemical Resources Laboratory, Tokyo Institute of Technology, has successfully developed a nano crystal aggregate (nanomachine) technology to deliver a gadolinium complex (Gd-DTPA or magnevist) — broadly used as an MRI contrast agent — to the affected area ("Hybrid Calcium Phosphate-Polymeric Micelles Incorporating Gadolinium Chelates for Imaging-Guided Gadolinium Neutron Capture Tumor Therapy"). More specifically, it is a drug delivery system (DDS) whereby a nano-level contrast agent (Gd)-DTPA is prepared, and introduced into the interior of calcium phosphate, a bone constituent, and is delivered to cancer tissues. The research team has clarified that selective accumulation of the developed nanomachine in a cancer tumor enables contrast imaging of a solid cancer. Moreover, when the Team applied the nanomachine to cancer neutron capture therapy, they confirmed a remarkable curative effect. This nanomachine therapy enables an imaging-guided thermal neutron irradiation treatment; thus it can be expected to lead to a reliable cancer treatment with no missed cancer cells. The realization of surgery with no incision (chemical surgery) by nanomachine allows us to anticipate outpatient treatment with no need of hospitalization.
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Plasmonics study suggests how to maximize production of 'hot electrons'
New research from Rice University could make it easier for engineers to harness the power of light-capturing nanomaterials to boost the efficiency and reduce the costs of photovoltaic solar cells. Although the domestic solar-energy industry grew by 34 percent in 2014, fundamental technical breakthroughs are needed if the U.S. is to meet its national goal of reducing the cost of solar electricity to 6 cents per kilowatt-hour. In a study published July 13 in ("Distinguishing between plasmon-induced and photoexcited carriers in a device geometry"), scientists from Rice’s Laboratory for Nanophotonics (LANP) describe a new method that solar-panel designers could use to incorporate light-capturing nanomaterials into future designs. By applying an innovative theoretical analysis to observations from a first-of-its-kind experimental setup, LANP graduate student Bob Zheng and postdoctoral research associate Alejandro Manjavacas created a methodology that solar engineers can use to determine the electricity-producing potential for any arrangement of metallic nanoparticles. 
Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which did not filter electrons based on energy level, included a thin layer of titanium between the gold and the titanium dioxide. CREDIT: B. Zheng/Rice University LANP researchers study light-capturing nanomaterials, including metallic nanoparticles that convert light into plasmons, waves of electrons that flow like a fluid across the particles’ surface. For example, recent LANP plasmonic research has led to breakthroughs in color-display technology, solar-powered steam production and color sensors that mimic the eye. “One of the interesting phenomena that occurs when you shine light on a metallic nanoparticle or nanostructure is that you can excite some subset of electrons in the metal to a much higher energy level,” said Zheng, who works with LANP Director and study co-author Naomi Halas. “Scientists call these ‘hot carriers’ or ‘hot electrons.’” Halas, Rice’s Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, said hot electrons are particularly interesting for solar-energy applications because they can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces. Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium. Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides. In addition to being less expensive to make, the plasmonic nanostructures have optical properties that can be precisely controlled by modifying their shape. “We can tune plasmonic structures to capture light across the entire solar spectrum,” Halas said. “The efficiency of semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.” The plasmonic approach has been tried before but with little success. Zheng said, “Plasmonic-based photovoltaics have typically had low efficiencies, and it hasn’t been entirely clear whether those arose from fundamental physical limitations or from less-than-optimal designs.” He and Halas said Manjavacas, a theoretical physicist in the group of LANP researcher Peter Nordlander, conducted work in the new study that offers a fundamental insight into the underlying physics of hot-electron-production in plasmonic-based devices. Manjavacas said, “To make use of the photon’s energy, it must be absorbed rather than scattered back out. For this reason, much previous theoretical work had focused on understanding the total absorption of the plasmonic system.” He said a recent example of such work comes from a pioneering experiment by another Rice graduate student, Ali Sobhani, where the absorption was concentrated near a metal semiconductor interface. “From this perspective, one can determine the total number of electrons produced, but it provides no way of determining how many of those electrons are actually useful, high-energy, hot electrons,” Manjavacas said. He said Zheng’s data allowed a deeper analysis because his experimental setup selectively filtered high-energy hot electrons from their less-energetic counterparts. To accomplish this, Zheng created two types of plasmonic devices. Each consisted of a plasmonic gold nanowire atop a semiconducting layer of titanium dioxide. In the first setup, the gold sat directly on the semiconductor, and in the second, a thin layer of pure titanium was placed between the gold and the titanium dioxide. The first setup created a microelectronic structure called a Schottky barrier and allowed only hot electrons to pass from the gold to the semiconductor. The second setup allowed all electrons to pass. “The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system,” Manjavacas said. “In particular, we found that hot electrons were not correlated with total absorption. They were driven by a different, plasmonic mechanism known as field-intensity enhancement.” LANP researchers and others have spent years developing techniques to bolster the field-intensity enhancement of photonic structures for single-molecule sensing and other applications. Zheng and Manjavacas said they are conducting further tests to modify their system to optimize the output of hot electrons. Halas said, “This is an important step toward the realization of plasmonic technologies for solar photovoltaics. This research provides a route to increasing the efficiency of plasmonic hot-carrier devices and shows that they can be useful for converting sunlight into usable electricity.”
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Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which did not filter electrons based on energy level, included a thin layer of titanium between the gold and the titanium dioxide. CREDIT: B. Zheng/Rice University LANP researchers study light-capturing nanomaterials, including metallic nanoparticles that convert light into plasmons, waves of electrons that flow like a fluid across the particles’ surface. For example, recent LANP plasmonic research has led to breakthroughs in color-display technology, solar-powered steam production and color sensors that mimic the eye. “One of the interesting phenomena that occurs when you shine light on a metallic nanoparticle or nanostructure is that you can excite some subset of electrons in the metal to a much higher energy level,” said Zheng, who works with LANP Director and study co-author Naomi Halas. “Scientists call these ‘hot carriers’ or ‘hot electrons.’” Halas, Rice’s Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, said hot electrons are particularly interesting for solar-energy applications because they can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces. Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium. Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides. In addition to being less expensive to make, the plasmonic nanostructures have optical properties that can be precisely controlled by modifying their shape. “We can tune plasmonic structures to capture light across the entire solar spectrum,” Halas said. “The efficiency of semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.” The plasmonic approach has been tried before but with little success. Zheng said, “Plasmonic-based photovoltaics have typically had low efficiencies, and it hasn’t been entirely clear whether those arose from fundamental physical limitations or from less-than-optimal designs.” He and Halas said Manjavacas, a theoretical physicist in the group of LANP researcher Peter Nordlander, conducted work in the new study that offers a fundamental insight into the underlying physics of hot-electron-production in plasmonic-based devices. Manjavacas said, “To make use of the photon’s energy, it must be absorbed rather than scattered back out. For this reason, much previous theoretical work had focused on understanding the total absorption of the plasmonic system.” He said a recent example of such work comes from a pioneering experiment by another Rice graduate student, Ali Sobhani, where the absorption was concentrated near a metal semiconductor interface. “From this perspective, one can determine the total number of electrons produced, but it provides no way of determining how many of those electrons are actually useful, high-energy, hot electrons,” Manjavacas said. He said Zheng’s data allowed a deeper analysis because his experimental setup selectively filtered high-energy hot electrons from their less-energetic counterparts. To accomplish this, Zheng created two types of plasmonic devices. Each consisted of a plasmonic gold nanowire atop a semiconducting layer of titanium dioxide. In the first setup, the gold sat directly on the semiconductor, and in the second, a thin layer of pure titanium was placed between the gold and the titanium dioxide. The first setup created a microelectronic structure called a Schottky barrier and allowed only hot electrons to pass from the gold to the semiconductor. The second setup allowed all electrons to pass. “The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system,” Manjavacas said. “In particular, we found that hot electrons were not correlated with total absorption. They were driven by a different, plasmonic mechanism known as field-intensity enhancement.” LANP researchers and others have spent years developing techniques to bolster the field-intensity enhancement of photonic structures for single-molecule sensing and other applications. Zheng and Manjavacas said they are conducting further tests to modify their system to optimize the output of hot electrons. Halas said, “This is an important step toward the realization of plasmonic technologies for solar photovoltaics. This research provides a route to increasing the efficiency of plasmonic hot-carrier devices and shows that they can be useful for converting sunlight into usable electricity.”
Make mine a decaf: breakthrough in knowledge of how nanoparticles grow
A team of researchers from the University of Leicester and France’s G2ELab-CNRS in Grenoble have for the first time observed the growth of free nanoparticles in helium gas in a process similar to the decaffeination of coffee, providing new insights into the structure of nanoparticles. Nanoparticles have a very large surface area compared with their volume and are often able to react very quickly. This makes them useful as catalysts in chemical reactions and they are often used in sports equipment, clothing and sunscreens. In a paper published by the ("Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression") and funded by the Royal Society, The Leverhulme Trust, the British Council and CONACYT, the teams from the University of Leicester’s Department of Physics and Astronomy and the CNRS in Grenoble measured how helium ions cluster with neutral helium atoms and grow into nanoparticles. 
Researchers observe how nanoparticles grow when exposed to helium. (© American Chemical Society) During the study they examined how helium ions drift through a cell filled with helium atoms. When the pressure of helium was increased the researchers observed a decrease in the mobility of the ions. Dr Klaus von Haeften from the University of Leicester’s Department of Physics and Astronomy, who has received a Visiting Professorship from the University Joseph Fourier, said: “We concluded that the increased pressure forced more and more helium atoms to bind to the ions gradually, until the clusters grew to nanometre-sized particles. This process continued until the nanoparticles reached the maximum size possible which also depended on the temperature. “Further increase of the pressure was found to reduce the size, which we interpreted as compression. These size changes could then be followed in great detail. For low and moderate pressures the size changed rather rapidly whereas in the high pressure region the changes were slow.” By analysing how quickly the particle volume changed with pressure the researchers were able to investigate the structure of the nanoparticles. Nelly Bonifaci from the G2ELab-CNRS said: "At low and moderate pressure the nanoparticles were much softer than solid helium and we concluded that they must be liquid. At high pressures they became progressively harder and eventually solid." Dr von Haeften added: “By choosing helium we were able to study a system of greatest possible purity and our results are therefore very precise. Similar processes occur in the decaffeination of coffee in high pressure carbon dioxide, in dry cleaning and in chemical manufacturing. In all these processes nanoparticles grow. By knowing their size we can much better understand these processes and improve them." This is the first time that researchers have been able to observe the growth of free nanoparticles in a large range of pressure in gaseous helium. Frédéric Aitken from the G2ELab-CNRS added: "Our work is an important benchmark for the research on the formation and size of nanoparticles.”
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Researchers observe how nanoparticles grow when exposed to helium. (© American Chemical Society) During the study they examined how helium ions drift through a cell filled with helium atoms. When the pressure of helium was increased the researchers observed a decrease in the mobility of the ions. Dr Klaus von Haeften from the University of Leicester’s Department of Physics and Astronomy, who has received a Visiting Professorship from the University Joseph Fourier, said: “We concluded that the increased pressure forced more and more helium atoms to bind to the ions gradually, until the clusters grew to nanometre-sized particles. This process continued until the nanoparticles reached the maximum size possible which also depended on the temperature. “Further increase of the pressure was found to reduce the size, which we interpreted as compression. These size changes could then be followed in great detail. For low and moderate pressures the size changed rather rapidly whereas in the high pressure region the changes were slow.” By analysing how quickly the particle volume changed with pressure the researchers were able to investigate the structure of the nanoparticles. Nelly Bonifaci from the G2ELab-CNRS said: "At low and moderate pressure the nanoparticles were much softer than solid helium and we concluded that they must be liquid. At high pressures they became progressively harder and eventually solid." Dr von Haeften added: “By choosing helium we were able to study a system of greatest possible purity and our results are therefore very precise. Similar processes occur in the decaffeination of coffee in high pressure carbon dioxide, in dry cleaning and in chemical manufacturing. In all these processes nanoparticles grow. By knowing their size we can much better understand these processes and improve them." This is the first time that researchers have been able to observe the growth of free nanoparticles in a large range of pressure in gaseous helium. Frédéric Aitken from the G2ELab-CNRS added: "Our work is an important benchmark for the research on the formation and size of nanoparticles.”
Artificial moth eyes enhance the performance of silicon solar cells
Mimicking the texture found on the highly antireflective surfaces of the compound eyes of moths, researchers at Brookhaven National Laboratory use block copolymer self assembly to produce precise and tunable nanotextured designs in the range of ~20 nm across macroscopic silicon solar cells ("Sub-50-nm Self-Assembled Nanotextures for Enhanced Broadband Antireflection in Silicon Solar Cells"). 
Moth eyes are highly antireflective due to their surface nanostructure. This nanoscale texturing imparts broadband antireflection properties and significantly enhances performance compared with typical antireflection coatings. Proper design of an antireflection coating involves managing the refractive index mismatch at an abrupt optical interface. The most straightforward approach introduces a single layer of an intermediate optical index atop of a surface to create a system that engenders destructive interference in reflected light. This usually provides full antireflection at only a single wavelength. 
An image of a silicon moth eye, fabricated by polymer self-assembly. Increasingly broadband coverage, for application in transparent window coatings, military camouflage, or solar cells, is possible using multilayered thin-film schemes. An alternative to thin-film coating strategies, nanoscale patterns applied to the surface of a material, can create an effective medium between the substrate and air. Such structures provide broadband antireflection over a wide range of incident light angles when nanoscale, sub-wavelength textures are sufficiently tall and closely spaced. In this work, the team enhances the broadband antireflection properties of a nanofabricated moth eye structure through simultaneous control of both the geometry and optical properties, using block copolymer self assembly to design nanotextures that are sufficiently small to take advantage of a beneficial material surface layer that is only a few nanometers thick. 
A polished, highly reflective silicon solar cell (right) turns completely black (left) after the application of surface nanotexture. Why does this matter? Self-assembly based approaches to produce texturing reduce reflections from silicon solar cell surfaces to less than 1% across entire visible and near infrared spectrum and across a wide range of incident light angles. Furhter, block-copolymer based approaches to material design are scalable for the manufacture of large-area photovoltaic devices, with potential for implementation in silicon, silicon nitride, and glass, among others.
read more "Artificial moth eyes enhance the performance of silicon solar cells"
Moth eyes are highly antireflective due to their surface nanostructure. This nanoscale texturing imparts broadband antireflection properties and significantly enhances performance compared with typical antireflection coatings. Proper design of an antireflection coating involves managing the refractive index mismatch at an abrupt optical interface. The most straightforward approach introduces a single layer of an intermediate optical index atop of a surface to create a system that engenders destructive interference in reflected light. This usually provides full antireflection at only a single wavelength. An image of a silicon moth eye, fabricated by polymer self-assembly. Increasingly broadband coverage, for application in transparent window coatings, military camouflage, or solar cells, is possible using multilayered thin-film schemes. An alternative to thin-film coating strategies, nanoscale patterns applied to the surface of a material, can create an effective medium between the substrate and air. Such structures provide broadband antireflection over a wide range of incident light angles when nanoscale, sub-wavelength textures are sufficiently tall and closely spaced. In this work, the team enhances the broadband antireflection properties of a nanofabricated moth eye structure through simultaneous control of both the geometry and optical properties, using block copolymer self assembly to design nanotextures that are sufficiently small to take advantage of a beneficial material surface layer that is only a few nanometers thick. A polished, highly reflective silicon solar cell (right) turns completely black (left) after the application of surface nanotexture. Why does this matter? Self-assembly based approaches to produce texturing reduce reflections from silicon solar cell surfaces to less than 1% across entire visible and near infrared spectrum and across a wide range of incident light angles. Furhter, block-copolymer based approaches to material design are scalable for the manufacture of large-area photovoltaic devices, with potential for implementation in silicon, silicon nitride, and glass, among others.
Researchers make scalable arrays of building blocks for ultrathin electronics
Semiconductors, metals and insulators must be integrated to make the transistors that are the electronic building blocks of your smartphone, computer and other microchip-enabled devices. Today's transistors are miniscule--a mere 10 nanometers wide--and formed from three-dimensional (3D) crystals. But a disruptive new technology looms that uses two-dimensional (2D) crystals, just 1 nanometer thick, to enable ultrathin electronics. Scientists worldwide are investigating 2D crystals made from common layered materials to constrain electron transport within just two dimensions. Researchers had previously found ways to lithographically pattern single layers of carbon atoms called graphene into ribbon-like "wires" complete with insulation provided by a similar layer of boron nitride. But until now they have lacked synthesis and processing methods to lithographically pattern junctions between two different semiconductors within a single nanometer-thick layer to form transistors, the building blocks of ultrathin electronic devices. Now for the first time, researchers at the Department of Energy's Oak Ridge National Laboratory have combined a novel synthesis process with commercial electron-beam lithography techniques to produce arrays of semiconductor junctions in arbitrary patterns within a single, nanometer-thick semiconductor crystal. The process relies upon transforming patterned regions of one existing, single-layer crystal into another. The researchers first grew single, nanometer-thick layers of molybdenum diselenide crystals on substrates and then deposited protective patterns of silicon oxide using standard lithography techniques. Then they bombarded the exposed regions of the crystals with a laser-generated beam of sulfur atoms. The sulfur atoms replaced the selenium atoms in the crystals to form molybdenum disulfide, which has a nearly identical crystal structure. The two semiconductor crystals formed sharp junctions, the desired building blocks of electronics. reports the accomplishment ("Patterned Arrays of Lateral Heterojunctions within Monolayer Two-Dimensional Semiconductors"). 
Complex, scalable arrays of semiconductor heterojunctions -- promising building blocks for future electronics -- were formed within a two-dimensional crystalline monolayer of molybdenum deselenide by converting lithographically exposed regions to molybdenum disulfide using pulsed laser deposition of sulfur atoms. Sulfur atoms (green) replaced selenium atoms (red) in lithographically exposed regions (top) as shown by Raman spectroscopic mapping (bottom). (Illustration: Oak Ridge National Laboratory, U.S. Dept. of Energy) "We can literally make any kind of pattern that we want," said Masoud Mahjouri-Samani, who co-led the study with David Geohegan. Geohegan, head of ORNL's Nanomaterials Synthesis and Functional Assembly Group at the Center for Nanophase Materials Sciences, is the principal investigator of a Department of Energy basic science project focusing on the growth mechanisms and controlled synthesis of nanomaterials. Millions of 2D building blocks with numerous patterns may be made concurrently, Mahjouri-Samani added. In the future, it might be possible to produce different patterns on the top and bottom of a sheet. Further complexity could be introduced by layering sheets with different patterns. Added Geohegan, "The development of a scalable, easily implemented process to lithographically pattern and easily form lateral semiconducting heterojunctions within two-dimensional crystals fulfills a critical need for 'building blocks' to enable next-generation ultrathin devices for applications ranging from flexible consumer electronics to solar energy." Tuning the bandgap "We chose pulsed laser deposition of sulfur because of the digital control it gives you over the flux of the material that comes to the surface," said Mahjouri-Samani. "You can basically make any kind of intermediate alloy. You can just replace, say, 20 percent of the selenium with sulfur, or 30 percent, or 50 percent." Added Geohegan, "Pulsed laser deposition also lets the kinetic energy of the sulfur atoms be tuned, allowing you to explore a wider range of processing conditions." It is important that by controlling the ratio of sulfur to selenium within the crystal, the researchers can tune the bandgap of the semiconductors, an attribute that determines electronic and optical properties. To make optoelectronic devices such as electroluminescent displays, microchip fabricators integrate semiconductors with different bandgaps. For example, molybdenum disulfide's bandgap is greater than molybdenum diselenide's. Applying voltage to a crystal containing both semiconductors causes electrons and "holes" (positive charges created when electrons vacate) to move from molybdenum disulfide into molybdenum diselenide and recombine to emit light at the bandgap of molybdenum diselenide. For that reason, engineering the bandgaps of monolayer systems can allow the generation of light with many different colors, as well as enable other applications such as transistors and sensors, Mahjouri-Samani said. Next the researchers will see if their pulsed laser vaporization and conversion method will work with atoms other than sulfur and selenium. "We're trying to make more complex systems in a 2D plane--integrate more ingredients, put in different building blocks--because at the end of the day, a complete working device needs different semiconductors and metals and insulators," Mahjouri-Samani said. To understand the process of converting one nanometer-thick crystal into another, the researchers used powerful electron microscopy capabilities available at ORNL, notably atomic-resolution Z-contrast scanning transmission electron microscopy, which was developed at the lab and is now available to scientists worldwide using the Center for Nanophase Materials Sciences. Employing this technique, electron microscopists Andrew Lupini and visiting scientist Leonardo Basile imaged hexagonal networks of individual columns of atoms in the nanometer-thick molybdenum diselenide and molybdenum disulfide crystals. "We could directly distinguish between sulfur and selenium atoms by their intensities in the image," Lupini said. "These images and electron energy loss spectroscopy allowed the team to characterize the semiconductor heterojunction with atomic precision."
read more "Researchers make scalable arrays of building blocks for ultrathin electronics"
Complex, scalable arrays of semiconductor heterojunctions -- promising building blocks for future electronics -- were formed within a two-dimensional crystalline monolayer of molybdenum deselenide by converting lithographically exposed regions to molybdenum disulfide using pulsed laser deposition of sulfur atoms. Sulfur atoms (green) replaced selenium atoms (red) in lithographically exposed regions (top) as shown by Raman spectroscopic mapping (bottom). (Illustration: Oak Ridge National Laboratory, U.S. Dept. of Energy) "We can literally make any kind of pattern that we want," said Masoud Mahjouri-Samani, who co-led the study with David Geohegan. Geohegan, head of ORNL's Nanomaterials Synthesis and Functional Assembly Group at the Center for Nanophase Materials Sciences, is the principal investigator of a Department of Energy basic science project focusing on the growth mechanisms and controlled synthesis of nanomaterials. Millions of 2D building blocks with numerous patterns may be made concurrently, Mahjouri-Samani added. In the future, it might be possible to produce different patterns on the top and bottom of a sheet. Further complexity could be introduced by layering sheets with different patterns. Added Geohegan, "The development of a scalable, easily implemented process to lithographically pattern and easily form lateral semiconducting heterojunctions within two-dimensional crystals fulfills a critical need for 'building blocks' to enable next-generation ultrathin devices for applications ranging from flexible consumer electronics to solar energy." Tuning the bandgap "We chose pulsed laser deposition of sulfur because of the digital control it gives you over the flux of the material that comes to the surface," said Mahjouri-Samani. "You can basically make any kind of intermediate alloy. You can just replace, say, 20 percent of the selenium with sulfur, or 30 percent, or 50 percent." Added Geohegan, "Pulsed laser deposition also lets the kinetic energy of the sulfur atoms be tuned, allowing you to explore a wider range of processing conditions." It is important that by controlling the ratio of sulfur to selenium within the crystal, the researchers can tune the bandgap of the semiconductors, an attribute that determines electronic and optical properties. To make optoelectronic devices such as electroluminescent displays, microchip fabricators integrate semiconductors with different bandgaps. For example, molybdenum disulfide's bandgap is greater than molybdenum diselenide's. Applying voltage to a crystal containing both semiconductors causes electrons and "holes" (positive charges created when electrons vacate) to move from molybdenum disulfide into molybdenum diselenide and recombine to emit light at the bandgap of molybdenum diselenide. For that reason, engineering the bandgaps of monolayer systems can allow the generation of light with many different colors, as well as enable other applications such as transistors and sensors, Mahjouri-Samani said. Next the researchers will see if their pulsed laser vaporization and conversion method will work with atoms other than sulfur and selenium. "We're trying to make more complex systems in a 2D plane--integrate more ingredients, put in different building blocks--because at the end of the day, a complete working device needs different semiconductors and metals and insulators," Mahjouri-Samani said. To understand the process of converting one nanometer-thick crystal into another, the researchers used powerful electron microscopy capabilities available at ORNL, notably atomic-resolution Z-contrast scanning transmission electron microscopy, which was developed at the lab and is now available to scientists worldwide using the Center for Nanophase Materials Sciences. Employing this technique, electron microscopists Andrew Lupini and visiting scientist Leonardo Basile imaged hexagonal networks of individual columns of atoms in the nanometer-thick molybdenum diselenide and molybdenum disulfide crystals. "We could directly distinguish between sulfur and selenium atoms by their intensities in the image," Lupini said. "These images and electron energy loss spectroscopy allowed the team to characterize the semiconductor heterojunction with atomic precision."
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