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Northwestern University researchers have developed a first-of-its-kind technique for creating entirely new classes of optical materials and devices that could lead to light bending and cloaking devices — news to make the ears of Star Trek’s Spock perk up.

Using DNA as a key tool, the interdisciplinary team took gold nanoparticles of different sizes and shapes and arranged them in two and three dimensions to form optically active superlattices. Structures with specific configurations could be programmed through choice of particle type and both DNA-pattern and sequence to exhibit almost any color across the visible spectrum, the scientists report.

“Architecture is everything when designing new materials, and we now have a new way to precisely control particle architectures over large areas,” said Chad A. Mirkin, the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences at Northwestern. “Chemists and physicists will be able to build an almost infinite number of new structures with all sorts of interesting properties. These structures cannot be made by any known technique.”

The technique combines an old fabrication method — top-down lithography, the same method used to make computer chips — with a new one — programmable self-assembly driven by DNA. The Northwestern team is the first to combine the two to achieve individual particle control in three dimensions.

The study was published online by the journal Science today (Jan. 18). Mirkin and Vinayak P. Dravid and Koray Aydin, both professors in Northwestern’s McCormick School of Engineering, are co-corresponding authors.

Scientists will be able to use the powerful and flexible technique to build metamaterials — materials not found in nature — for a range of applications including sensors for medical and environmental uses.

The researchers used a combination of numerical simulations and optical spectroscopy techniques to identify particular nanoparticle superlattices that absorb specific wavelengths of visible light. The DNA-modified nanoparticles — gold in this case — are positioned on a pre-patterned template made of complementary DNA. Stacks of structures can be made by introducing a second and then a third DNA-modified particle with DNA that is complementary to the subsequent layers.

In addition to being unusual architectures, these materials are stimuli-responsive: the DNA strands that hold them together change in length when exposed to new environments, such as solutions of ethanol that vary in concentration. The change in DNA length, the researchers found, resulted in a change of color from black to red to green, providing extreme tunability of optical properties.

“Tuning the optical properties of metamaterials is a significant challenge, and our study achieves one of the highest tunability ranges achieved to date in optical metamaterials,” said Aydin, assistant professor of electrical engineering and computer science at McCormick.

“Our novel metamaterial platform — enabled by precise and extreme control of gold nanoparticle shape, size and spacing — holds significant promise for next-generation optical metamaterials and metasurfaces,” Aydin said.

The study describes a new way to organize nanoparticles in two and three dimensions. The researchers used lithography methods to drill tiny holes — only one nanoparticle wide — in a polymer resist, creating “landing pads” for nanoparticle components modified with strands of DNA. The landing pads are essential, Mirkin said, since they keep the structures that are grown vertical.

The nanoscopic landing pads are modified with one sequence of DNA, and the gold nanoparticles are modified with complementary DNA. By alternating nanoparticles with complementary DNA, the researchers built nanoparticle stacks with tremendous positional control and over a large area. The particles can be different sizes and shapes (spheres, cubes and disks, for example).

“This approach can be used to build periodic lattices from optically active particles, such as gold, silver and any other material that can be modified with DNA, with extraordinary nanoscale precision,” said Mirkin, director of Northwestern’s International Institute for Nanotechnology.

Mirkin also is a professor of medicine at Northwestern University Feinberg School of Medicine and professor of chemical and biological engineering, biomedical engineering and materials science and engineering in the McCormick School.

The success of the reported DNA programmable assembly required expertise with hybrid (soft-hard) materials and exquisite nanopatterning and lithographic capabilities to achieve the requisite spatial resolution, definition and fidelity across large substrate areas. The project team turned to Dravid, a longtime collaborator of Mirkin’s who specializes in nanopatterning, advanced microscopy and characterization of soft, hard and hybrid nanostructures.

Dravid contributed his expertise and assisted in designing the nanopatterning and lithography strategy and the associated characterization of the new exotic structures. He is the Abraham Harris Professor of Materials Science and Engineering in McCormick and the founding director of the NUANCE center, which houses the advanced patterning, lithography and characterization used in the DNA-programmed structures.

Scientists used spiraling X-rays at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) to observe, for the first time, a property that gives handedness to swirling electric patterns – dubbed polar vortices – in a synthetically layered material.

This property, also known as chirality, potentially opens up a new way to store data by controlling the left- or right-handedness in the material’s array in much the same way magnetic materials are manipulated to store data as ones or zeros in a computer’s memory.

Researchers said the behavior also could be explored for coupling to magnetic or optical (light-based) devices, which could allow better control via electrical switching.

Chirality is present in many forms and at many scales, from the spiral-staircase design of our own DNA to the spin and drift of spiral galaxies; it can even determine whether a molecule acts as a medicine or a poison in our bodies.

A molecular compound known as d-glucose, for example, which is an essential ingredient for human life as a form of sugar, exhibits right-handedness. Its left-handed counterpart, l-glucose, though, is not useful in human biology.

“Chirality hadn’t been seen before in this electric structure,” said Elke Arenholz, a senior staff scientist at Berkeley Lab’s Advanced Light Source (ALS), which is home to the X-rays that were key to the study. The study was published online this week in the journal Proceedings of the National Academy of Sciences.

The experiments can distinguish between left-handed chirality and right-handed chirality in the samples’ vortices. “This offers new opportunities for fundamentally new science, with the potential to open up applications,” she said.

“Imagine that one could convert a right-handed form of a molecule to its left-handed form by applying an electric field, or artificially engineer a material with a particular chirality,” said Ramamoorthy Ramesh, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and associate laboratory director of the Lab’s Energy Technologies Area, who co-led the latest study.

Ramesh, who is also a professor of materials science and physics at UC Berkeley, custom-made the novel materials at UC Berkeley.

Padraic Shafer, a research scientist at the ALS and the lead author of the study, worked with Arenholz to carry out the X-ray experiments that revealed the chirality of the material.

The samples included a layer of lead titanate (PbTiO3) and a layer of strontium titanate (SrTiO3) sandwiched together in an alternating pattern to form a material known as a superlattice. The materials have also been studied for their tunable electrical properties that make them candidates for components in precise sensors and for other uses.

Neither of the two compounds show any handedness by themselves, but when they were combined into the precisely layered superlattice, they developed the swirling vortex structures that exhibited chirality.

“Chirality may have additional functionality,” Shafer said, when compared to devices that use magnetic fields to rearrange the magnetic structure of the material.

The electronic patterns in the material that were studied at the ALS were first revealed using a powerful electron microscope at Berkeley Lab’s National Center for Electron Microscopy, a part of the Lab’s Molecular Foundry, though it took a specialized X-ray technique to identify their chirality.

“The X-ray measurements had to be performed in extreme geometries that can’t be done by most experimental equipment,” Shafer said, using a technique known as resonant soft X-ray diffraction that probes periodic nanometer-scale details in their electronic structure and properties.

Spiraling forms of X-rays, known as circularly polarized X-rays, allowed researchers to measure both left-handed and right-handed chirality in the samples.

Arenholz, who is also a faculty member of the UC Berkeley Department of Materials Science & Engineering, added, “It took a lot of time to understand the results, and a lot of modeling and discussions.” Theorists at the University of Cantabria in Spain and their network of computational experts performed calculations of the vortex structures that aided in the interpretation of the X-ray data.

The same science team is pursuing studies of other types and combinations of materials to test the effects on chirality and other properties.

“There is a wide class of materials that could be substituted,” Shafer said, “and there is the hope that the layers could be replaced with even higher functionality materials.”

Researchers also plan to test whether there are new ways to control the chirality in these layered materials, such as by combining materials that have electrically switchable properties with those that exhibit magnetically switchable properties.

“Since we know so much about magnetic structures,” Arenholz said, “we could think of using this well-known connection with magnetism to implement this newly discovered property into devices.”

A recent paper published in NANO showed the gas-solid reaction method provides a full coverage of the perovskite film and avoids damage from the organic solvent, which is beneficial for light capture and electrons transportation, resulting in a faster response time and stability for perovskite photodetectors.

A schematic illustration of hybrid perovskite photoconductivity visible region detector with high speed and high stability. The gas-solid reaction in replace of the traditional solution methods provides a non-solvent environment during the reaction process, constructs a high crystallization and a full coverage film to increase the light capture and transportation, as well as enhance a good stability in the humidity condition, leading to a high response performance for the photodetector. Credit: Dr. Guoqing Tong

A schematic illustration of hybrid perovskite photoconductivity visible region detector with high speed and high stability. The gas-solid reaction in replace of the traditional solution methods provides a non-solvent environment during the reaction process, constructs a high crystallization and a full coverage film to increase the light capture and transportation, as well as enhance a good stability in the humidity condition, leading to a high response performance for the photodetector. Credit: Dr. Guoqing Tong

Pervoskite materials have long been considered candidates in the semiconductor manufacturing due to their characteristics of high light absorption, carrier mobility and wider light spectrum. They are widely applied in solar cells, light-emitted devices and photodetectors. However, the organic solvent in the traditional solution method will damage the perovskite film and form unstable phases during the synthesis process, which makes the perovskite film decompose quickly in wet conditions, limiting the practical application of perovskite devices. Considering the significant influence of the solvent, a team of researchers from Dongchang college of Liaocheng University and Hefei University of Technology proposed a new gas-solid process to fabricate the perovskite film. This non-solvent approach provides high crystallization and full coverage film in lower vacuum and low temperature systems.

The researchers investigated the morphology, light absorption and the crystal phases of the perovskite film at the different annealing temperature after gas-reaction to obtain the high-quality perovskite film. The devices exhibited high responsivity and detectivity of 5.87AW-1 and 1012 Jones. The response time of the device is estimated to be 248 μs/207 μs, which is faster than most previous reports via the solution method. Remarkably, the responsivity and detectivity are estimated to be 0.26 AW-1, 2.13×1010 Jones after lasting exposure in air (25oC, RH~40%) for up to two months. This improvement of the stability of the devices demonstrates that the well-controlled vapor deposition method allows a thorough removal of the residual solvents (i.e. DMF, DMSO et. al) and thus effectively promotes a high-quality crystallization of perovskite grains, reducing the metastable phases among the thin films.

This work was financed by Science and Technology Plan Project of Shandong Higher Education Institutions, NSFC and Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology of China.

A discovery by an international team of researchers from Princeton University, the Georgia Institute of Technology and Humboldt University in Berlin points the way to more widespread use of an advanced technology generally known as organic electronics.

The research, published in the journal Nature Materials, focused on organic semiconductors, a class of materials prized for their applications in emerging technologies such as flexible electronics, solar energy conversion, and high-quality color displays for smartphones and televisions. In the short term, the advancement could particularly help with organic light-emitting diodes that operate at high energy to emit colors such as green and blue.

Researchers used ultraviolet light to excite molecules in a semiconductor, triggering reactions that split up and activated a dopant. Credit: Princeton University / Jing Wang and Xin Lin

Researchers used ultraviolet light to excite molecules in a semiconductor, triggering reactions that split up and activated a dopant. Credit: Princeton University / Jing Wang and Xin Lin

“Organic semiconductors are ideal materials for the fabrication of mechanically flexible devices with energy-saving, low-temperature processes,” said Xin Lin, a doctoral student and a member of the Princeton research team. “One of their major disadvantages has been their relatively poor electrical conductivity. In some applications, this can lead to difficulties and inefficient devices. We are working to improve the electrical properties of organic semiconductors.”

Semiconductors, typically made of silicon, are the foundation of modern electronics because engineers can take advantage of their unique properties to control electrical currents. Among many applications, semiconductor devices are used for computing, signal amplification, and switching. They are used in energy-saving devices such as light-emitting diodes and devices that convert energy such as solar cells.

Essential to these functionalities is a process called doping, in which the semiconductor’s chemical makeup is modified by adding a small amount of chemicals or impurities. By carefully choosing the type and amount of dopant, researchers can alter semiconductors’ electronic structure and electrical behavior in a variety of ways.

In their Nature Materials paper, the researchers have described a new approach for greatly increasing the conductivity of organic semiconductors, formed of carbon-based molecules rather than silicon atoms. The dopant, a ruthenium-containing compound, was a reducing agent, which means it added electrons to the organic semiconductor as part of the doping process. The addition of the electrons was the key to increasing the semiconductor’s conductivity. The compound belongs to a newly-introduced class of dopants called dimeric organometallic dopants. Unlike many other powerful reducing agents, these dopants are stable when exposed to air but still work as strong electron donors both in solution and solid state.

Georgia Tech’s Seth Marder, a Regents Professor in the School of Chemistry and Biochemistry, and Stephen Barlow, a research scientist in the school, led the development of the new dopant. They called the ruthenium compound a “hyper-reducing dopant.”

They said it was unusual, not only in its combination of electron donation strength and air stability but also in its ability to work with a class of organic semiconductors that have previously been very difficult to dope. In studies conducted at Princeton, the researchers found that the new dopant increased the conductivity of these semiconductors by about a million times.

The ruthenium compound was a dimer, meaning it consisted of two identical molecules, or monomers, connected by a chemical bond.  As is, the compound proved relatively stable and, when added to these difficult-to-dope semiconductors, it did not react and remained in its equilibrium state. That posed a problem because to increase the conductivity of the organic semiconductor, the ruthenium dimer needed to split and release its two identical monomers.

Princeton’s Lin, the study’s lead author, said the researchers looked for different ways to break up the ruthenium dimer and activate the doping. Eventually, he and Berthold Wegner, a visiting graduate student from the group of Norbert Koch at Humboldt University, took a hint from how photosynthetic systems work. They irradiated the system with ultraviolet light, which excited molecules in the semiconductor and initiated the reaction. Under exposure to the light, the dimers were able to dope the semiconductor, leading to a roughly 100,000 times increase in the conductivity.

After that, the researchers made an interesting observation.

“Once the light was turned off, one might naively expect the reverse reaction to occur and the increased conductivity to disappear,” said Georgia Tech’s Marder, who is also associate director of the Center for Organic Photonics and Electronics (COPE) at Georgia Tech. “However, this was not the case.”

The researchers found that the ruthenium monomers remained isolated in the semiconductor, increasing conductivity, even though thermodynamics should have returned the molecules to their original configuration as dimers. Antoine Kahn, a Princeton professor who led the research team, said the physical layout of the molecules inside the doped semiconductor provides a likely answer to this puzzle. The hypothesis is that the monomers are scattered in the semiconductor in such a way that it was very difficult for them to return to their original configuration and re-form the ruthenium dimer. To recombine, he said, the monomers would have to have faced in the correct orientation, but in the mixture, they remained askew. So, even though thermodynamics showed that dimers should reform, most never snapped back together.

“The question is why aren’t these things moving back together into equilibrium,” said Kahn, who is Stephen C. Macaleer ’63 Professor in Engineering and Applied Science. “The answer is they are kinetically trapped.”

In fact, the researchers observed the doped semiconductor for over a year and found very little decrease in the electrical conductivity. Also, by observing the material in light-emitting diodes fabricated by the group of Barry Rand, an assistant professor of electrical engineering at Princeton and the Andlinger Center for Energy and the Environment, the researchers discovered that doping was continuously re-activated by the light produced by the device.

“The light activates the system more, which leads to more light production and more activation until the system is fully activated, said Marder, who is Georgia Power Chair in Energy Efficiency. “This alone is a novel and surprising observation.”

The paper was co-authored by Kyung Min Lee, Michael A. Fusella, and Fengyu Zhang, of Princeton, and Karttikay Moudgil of Georgia Tech. Research was funded by the National Science Foundation (grants DMR-1506097, DMR-1305247), the Department of Energy’s Energy Efficiency & Renewable Energy Solid-State Lighting program (award DE-EE0006672) and the DoE’s Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (award DE-SC0012458), the Deutsche Forschungsgemeinschaft (project SFB 951) and the Helmholtz Energy-Alliance Hybrid Photovoltaics project.

Insulating oxides are oxygen containing compounds that do not conduct electricity, but can sometimes form conductive interfaces when they’re layered together precisely. The conducting electrons at the interface form a two-dimensional electron gas (2DEG) which boasts exotic quantum properties that make the system potentially useful in electronics and photonics applications.

Researchers at Yale University have now grown a 2DEG system on gallium arsenide, a semiconductor that’s efficient in absorbing and emitting light. This development is promising for new electronic devices that interact with light, such as new kinds of transistors, superconducting switches and gas sensors.

“I see this as a building block for oxide electronics,” said Lior Kornblum, now of the Technion – Israel Institute of Technology, who describes the new research appearing this week in the Journal of Applied Physics, from AIP publishing.

Oxide 2DEGs were discovered in 2004. Researchers were surprised to find that sandwiching together two layers of some insulating oxides can generate conducting electrons that behave like a gas or liquid near the interface between the oxides and can transport information.

Researchers have previously observed 2DEGs with semiconductors, but oxide 2DEGs have much higher electron densities, making them promising candidates for some electronic applications. Oxide 2DEGs have interesting quantum properties, drawing interest in their fundamental properties as well. For example, the systems seem to exhibit a combination of magnetic behaviors and superconductivity.

Generally, it’s difficult to mass-produce oxide 2DEGs because only small pieces of the necessary oxide crystals are obtainable, Kornblum said. If, however, researchers can grow the oxides on large, commercially available semiconductor wafers, they can then scale up oxide 2DEGs for real-world applications. Growing oxide 2DEGs on semiconductors also allows researchers to better integrate the structures with conventional electronics. According to Kornblum, enabling the oxide electrons to interact with the electrons in the semiconductor could lead to new functionality and more types of devices.

The Yale team previously grew oxide 2DEGs on silicon wafers. In the new work, they successfully grew oxide 2DEGs on another important semiconductor, gallium arsenide, which proved to be more challenging.

Most semiconductors react with oxygen in the air and form a disordered surface layer, which must be removed before growing these oxides on the semiconductor. For silicon, removal is relatively easy — researchers heat the semiconductor in vacuum. This approach, however, doesn’t work well with gallium arsenide.

Instead, the research team coated a clean surface of a gallium arsenide wafer with a layer of arsenic. The arsenic protected the semiconductor’s surface from the air while they transferred the wafer into an instrument that grows oxides using a method called molecular beam epitaxy. This allows one material to grow on another while maintaining an ordered crystal structure across the interface.

Next, the researchers gently heated the wafer to evaporate the thin arsenic layer, exposing the pristine semiconductor surface beneath. They then grew an oxide called SrTiO3 on the gallium arsenide and, immediately after, another oxide layer of GdTiO3. This process formed a 2DEG between the oxides.

Gallium arsenide is but one of a whole class of materials called III-V semiconductors, and this work opens a path to integrate oxide 2DEGs with others.

“The ability to couple or to integrate these interesting oxide two-dimensional electron gases with gallium arsenide opens the way to devices that could benefit from the electrical and optical properties of the semiconductor,” Kornblum said. “This is a gateway material for other members of this family of semiconductors.”

A team of Russian, Czech and German researchers gained a new perspective on the properties of three materials of biological origin. Besides two reference materials with well-studied properties — serum albumin and cytochrome C — the researchers looked at the extracellular matrix of the Shewanella oneidensis MR-1 bacterium, which is used in biofuel cells. The team measured the materials’ dynamic conductivity and dielectric permittivity in a wide range of frequencies and temperatures. To interpret their findings, the researchers used theoretical approaches and concepts from condensed matter physics. The paper detailing the study was published in the journal Scientific Reports.

“So far, the formalism of condensed matter physics has only found limited use in classical biochemistry and biophysics. As a result, certain interesting effects evade our attention,” says Konstantin Motovilov, senior research scientist at the Laboratory of Terahertz Spectroscopy at Moscow Institute of Physics and Technology (MIPT). “When we do make use of this language, we acquire new ways of modeling observed phenomena and describing biological structures. In our paper, we characterize the behavior of proteins, considered as classical amorphous semiconductors, with the help of the formalism of condensed matter physics.”

Before discussing the study, here is a quick example of how solid-state physics explains the electrical properties of different materials.

There are in fact multiple mechanisms of electrical conductivity. For each, there is a corresponding theory that describes the properties of certain materials. For example, the conductivity in metals is adequately explained by the Drude theory. In the theory, there is no interaction between the conduction electrons, which are assumed to only occasionally collide with crystal lattice, impurities, and defects. Electrical conductivity is the inverse of electrical resistivity. Conductivity indicates how easy it is for an electric current to pass through a given material. Within the Drude model, this property does not depend strongly on frequency up to the frequency of the collisions between charge carriers and lattice or impurities. However, there is a large group of conductive materials that do not fit this description. Yet their behavior in an external electromagnetic field is quite interesting. Among them are glasses, ionic conductors, and amorphous semiconductors.

To qualitatively describe the electrical properties of such materials, another theory was proposed about 40 years ago by Andrzej Karol Jonscher, an English physicist. According to his theory, charge carriers — electrons, for example — can adequately be considered as free at room temperature, provided the alternating current frequency does not exceed several megahertz. Under these conditions, the Drude model is applicable and conductivity is nearly constant, i.e., it does not depend on the frequency of the external field. If, however, the frequency is higher, this description is no longer valid and there is an increase in conductivity proportional to a certain power — which is close to 0.8 — of frequency. The same effect is observed for materials that are gradually cooled, even if the frequency is kept constant.

Interestingly, different materials exhibit quite similar behavior in that regard. Moreover, if you restate the dependences — say, talk about the ratio between direct current (static) conductivity and alternating current conductivity, as opposed to conductivity as such — the relations for all materials turn out to be identical, revealing the so-called Universal Dielectric Response (UDR). This curious phenomenon was thoroughly investigated in a study that examined the conduction in glasses and other amorphous materials, offering new insights into their structure and properties.

The authors of the paper showed that Jonscher’s law for conductivity applies to three organic materials. Among them, two are well-known reference proteins: bovine serum albumin and bovine heart cytochrome C. Their structural, physical, and chemical properties have been investigated in detail, so the researchers used them as reference materials.

In addition, they examined the extracellular matrix and filaments (EMF) of the Shewanella oneidensis MR-1 bacterium, which can produce electricity in biological fuel cells. S. oneidensis has been used in many studies with a focus on alternative energy sources, so its electrical properties are of interest to both researchers and engineers. In 2010, a team of researchers based in the United States and Canada showed that the bacterium’s extracellular appendages behave a lot like p-type semiconductors. The electrical properties of S. oneidensis MR-1 have nevertheless not been studied in detail. The recently published paper is an attempt to remedy that.

The authors measured the conductivity of the materials, as well as the energy losses in a frequency range from 1 hertz to 1.5 terahertz, or trillion hertz, for temperatures from -260 to 40 degrees Celsius. (Strictly speaking, the energy losses are given by the imaginary part of the complex dielectric permittivity.) Next, the researchers measured the direct current conductivity of EMF for temperatures from zero to 40 C, as well as the temperature dependence of their heat capacity. For each of the three materials, water content and ion concentration were also determined.

To do this, the researchers pressed the substances into pellets using a 1-centimeter mold. They then applied electrodes to the faces of the pellets to pass alternating current through them in order to measure the electrical conductivity and dielectric permittivity of the materials in the 1-300 million hertz range. For higher frequencies, this approach does not work, so for the 30-1,500 gigahertz, or billion hertz, range, the team obtained the spectra of complex dielectric permittivity using quasioptical terahertz spectroscopy. No measurements were made in the intermediate frequency range.

It turned out that at room temperature, EMF conductivity is nearly constant, and when the frequency is increased above several million hertz, or several megahertz, the conductivity is proportional to a certain power — which is close to 1 — of the frequency. Cytochrome C did not exhibit such behavior unless the frequency was low and the temperature high. In the case of albumin, it was not observed at all. This suggests that different conductivity mechanisms are at play in these materials. It is likely that EMF has nearly free charges at room temperature — just like in the Drude model — whereas albumin does not have them and cytochrome C is a mixed bag.

The dependence observed by the researchers can be explained in terms of the individual properties of the materials. Both cytochrome C and albumin are regular proteins. Although these materials do have some free charges, these are not nearly as many as it would be necessary to justify the Drude model. Comparing the conductivity in EMF to that in metals (conductors) is more realistic, as free charges are more easily generated in these molecules. However, a comparison even more valid would be that with a solution of table salt, which has a high concentration of free ions.

Naturally, a complete description is more complex and would require us to take the water content of materials and other factors into account. For instance, because EMF contains significant amounts of loosely bound water, its conductivity grows quadratically at temperatures of about -250 C and frequencies on the order of 100 billion hertz (sub-terahertz terahertz range). Temperatures that low cause the bulk water in the material to freeze, and high frequencies mean that the dielectric properties resulting from water dipole dynamics become non-negligible. The other materials, too, exhibit deviations from Jonscher’s predictions, but they are not as dramatic.

The authors have thus clearly shown the powerful methodology and instrumentation of condensed matter physics to be effective for fundamental research into the electrodynamics of biological objects. The next step could involve the application to biomaterials research of the wide range of other theories and models that have been effectively used by the physics community for many decades.

Scientists at Tokyo Institute of Technology (Tokyo Tech) and Tohoku University have developed high-quality GFO epitaxial films and systematically investigated their ferroelectric and ferromagnetic properties. They also demonstrated the room-temperature magnetocapacitance effects of these GFO thin films.

Spontaneous polarization appears to be parallel with the c-axis, while spontaneous magnetism appears to be parallel with the a-axis. Credit: None

Spontaneous polarization appears to be parallel with the c-axis, while spontaneous magnetism appears to be parallel with the a-axis. Credit: None

Multiferroic materials show magnetically driven ferroelectricity. They are attracting increasing attention because of their fascinating properties such as magnetic (electric) field-controlled ferroelectric (ferromagnetic) properties and because they can be used in novel technological applications such as fast-writing, power-saving, and nondestructive data storage. However, because multiferroicity is typically observed at low temperatures, it is highly desirable to develop multiferroic materials that can be observed at room temperature.

GaxFe2-xO3, or GFO for short, is a promising room-temperature multiferroic material because of its large magnetization. GFO thin films have already been successfully fabricated, and their polarization switching at room temperature has been demonstrated. However, their ferroelectric and ferromagnetic properties must be controlled to realize better magnetoelectric properties and applications of GFO films. In order to control these properties, it is essential to understand the relationship between the constituent composition at each cation site and the original character.

Therefore, the research team led by Mitsuru Ito at Tokyo Tech set out to systematically investigate multiferroicity as a function of the compositional ratio of Ga and Fe in GFO films. Specifically, they studied the ferroelectric properties of the GFO films using piezoresponse force microscopy, and found that GaxFe2-xO3 films with x = 1 and 0.6 show ferroelectricity at room temperature. The piezoresponse phase can be reversed by 180° when a voltage of more than 4.5 V is applied. This behavior is typical of ferroelectric materials and is a strong indicator of the presence of switchable polarization in the film at room temperature.

The scientists also confirmed room-temperature ferrimagnetism of the films through magnetic characterization. Lastly, they were able to demonstrate the room-temperature magnetocapacitance effects of the GFO films. They reported that by changing x, the coercive electric field, coercive force, and saturated magnetism values could be controlled. They also showed that the ferroelectric and magnetic ranges of GFO-type iron oxides differ from those of the well-known room-temperature multiferroic BiFeO2 and may expand the variety of room-temperature multiferroic materials.

Carbon nanotubes bound for electronics need to be as clean as possible to maximize their utility in next-generation nanoscale devices, and scientists at Rice and Swansea universities have found a way to remove contaminants from the nanotubes.

Rice chemist Andrew Barron, also a professor at Swansea in the United Kingdom, and his team have figured out how to get nanotubes clean and in the process discovered why the electrical properties of nanotubes have historically been so difficult to measure.

Scientists at Rice and Swansea universities have demonstrated that heating carbon nanotubes at high temperatures eliminates contaminants that make nanotubes difficult to test for conductivity. They found when measurements are taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlap, which scrambles the results. The plot shows the deviation when probes test conductivity from minus 1 to 1 volt at distances greater or less than 4 microns. Credit: Barron Research Group/Rice University

Scientists at Rice and Swansea universities have demonstrated that heating carbon nanotubes at high temperatures eliminates contaminants that make nanotubes difficult to test for conductivity. They found when measurements are taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlap, which scrambles the results. The plot shows the deviation when probes test conductivity from minus 1 to 1 volt at distances greater or less than 4 microns. Credit: Barron Research Group/Rice University

Like any normal wire, semiconducting nanotubes are progressively more resistant to current along their length. But over the years, conductivity measurements of nanotubes have been anything but consistent. The Rice-Swansea team wanted to know why.

“We are interested in the creation of nanotube-based conductors, and while people have been able to make wires, their conduction has not met expectations,” Barron said. “We wanted to determine the basic science behind the variability observed by other researchers.”

They discovered that hard-to-remove contaminants — leftover iron catalyst, carbon and water — could easily skew the results of conductivity tests. Burning those contaminants away, Barron said, creates new possibilities for carbon nanotubes in nanoscale electronics.

The new study appears in the American Chemical Society journal Nano Letters.

The researchers first made multiwalled carbon nanotubes between 40 and 200 nanometers in diameter and up to 30 microns long. They then either heated the nanotubes in a vacuum or bombarded them with argon ions to clean their surfaces.

They tested individual nanotubes the same way one would test any electrical conductor: by touching them with two probes to see how much current passes through the material from one tip to the other. In this case, tungsten probes were attached to a scanning tunneling microscope.

In clean nanotubes, resistance got progressively stronger as the distance increased, as it should. But the results were skewed when the probes encountered surface contaminants, which increased the electric field strength at the tip. And when measurements were taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlapped, which further scrambled the results.

“We think this is why there’s such inconsistency in the literature,” Barron said. “If nanotubes are to be the next-generation lightweight conductor, then consistent results, batch-to-batch and sample-to-sample, are needed for devices such as motors and generators as well as power systems.”

Heating the nanotubes in a vacuum above 200 degrees Celsius (392 degrees Fahrenheit) reduced surface contamination, but not enough to eliminate inconsistent results, they found. Argon ion bombardment also cleaned the tubes but led to an increase in defects that degrade conductivity.

Ultimately the researchers discovered vacuum annealing nanotubes at 500 degrees Celsius (932 Fahrenheit) reduced contamination enough to measure resistance accurately.

Barron said engineers who use nanotube fibers or films in devices currently modify the material through doping or other means to get the conductive properties they require. But if the source nanotubes are sufficiently decontaminated, they should be able to get the desired conductivity by simply putting their contacts in the right spot.

“A key result of our work is that if contacts on a nanotube are less than 1 micron apart, the electronic properties of the nanotube change from conductor to semiconductor, due to the presence of overlapping depletion zones, which shrink but are still present even in clean nanotubes,” Barron said.

“This has a potential limiting factor on the size of nanotube-based electronic devices,” he said. “Carbon-nanotube devices would be limited in how small they could become, so Moore’s Law would only apply to a point.”

Researchers at the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS), have shown that defects in monolayer molybdenum disulfide (MoS2) exhibit electrical switching, providing new insights into the electrical properties of this material. As MoS2 is one of the most promising 2D semiconductors, it is expected that these results will contribute to its future use in opto-electronics.

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

Defects can cause major changes in the properties of a material, leading to either desirable or unwanted effects. For example, petrochemical industry has long taken advantage of the catalytic activity of MoS2edges, characterized by the presence of a high concentration of defects, to produce petroleum products with reduced sulfur dioxide (SO2) emissions. On the other hand, having a pristine material is a must in electronics. Currently, silicon rules the industry, because it can be prepared in a virtually defect-free manner. In the case of MoS2, its suitability for electronic applications is currently limited by the presence of naturally occurring defects. So far, the precise link between these defects and the degraded properties of MoS2 has been an open question.

In IBS, a team of physicists, material scientists, and electrical engineers worked closely together to explore the electronic properties of sulfur vacancies in MoS2 monolayers, using a combination of atomic force microscopy (AFM) and noise analysis. The scientists used a metallic AFM tip to measure the noise signal, i.e., the variation of electrical current passing through a single layer of MoS2 placed on a metal substrate.

The most common defects in MoS2 are instances of missing single sulfur atoms, also known as sulfur monovacancies. In a perfect sample, each sulfur atom has two valence electrons that bind to two molybdenum electrons. However, where a sulfur atom is missing, these two molybdenum electrons are left unsaturated, defining the neutral state (0 state) of the defect. However, the team observed rapid switching events in their noise measurements, indicating the state of the vacancy switched between neutral (0 state) and charged (-1 state).

“The switching between 0 and -1 is happening continuously. While an electron resides at the vacancy for a while, it is missing from the current, such that we observe a current drop,” explains Michael Neumann, one of the co-first authors of the study. “This goes a long way towards understanding the known anomalies of MoS2, and it is very interesting that sulfur vacancies alone are enough to explain these anomalies, without requiring more complex defects.” According to the experiments and earlier calculations, two electrons can be also trapped at the vacancy (-2 state), but this does not seem to be energetically favored.

The new observation that sulfur vacancies can be charged (-1 and -2 states) sheds light on several MoS2 anomalies, including its reduced electron mobility observed in MoS2 monolayer samples: electrons move following the direction of an applied voltage, but get scattered by charged defects. “The -1 state is occupied around 50% of the time, which would lead to scattering of electrons, and thus explain why MoS2 has such poor mobility,” clarifies Neumann. Other MoS2 characteristics which can be explained by this study are the n-type doping of MoS2, and the unexpectedly large resistance at the MoS2-metal junction.

“This research opens up the possibility of developing a new noise nanospectroscopy device capable of mapping one or more defects on a nanoscale scale over a wide area of a 2D material,” concludes the corresponding author Young Hee Lee.

The full study is available on Nature Communications.

Researchers from North Carolina State University have found that the transfer of triplet excitons from nanomaterials to molecules also creates a feedback mechanism that returns some energy to the nanocrystal, causing it to photoluminesce on long time scales. The mechanism can be adjusted to control the amount of energy transfer, which could be useful in optoelectronic applications.

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Felix N. Castellano, Goodnight Innovation Distinguished Chair of Chemistry at NC State, had previously shown that semiconductor nanocrystals could transfer energy to molecules, thereby extending their excited state lifetimes long enough for them to be useful in photochemical reactions.

In a new contribution, Castellano and Cédric Mongin, a former postdoctoral researcher currently an assistant professor at École normale supérieure Paris-Saclay in France, have shown that not only does the transfer of triplet excitons extend excited state lifetimes, but also that some of the energy gets returned to the original nanomaterial in the process.

“When we looked at triplet exciton transfers from nanomaterials to molecules, we noticed that after the initial transfer the nanomaterial would still luminesce in a delayed fashion, which was unexpected,” says Castellano. “So we decided to find out what exactly was happening at the molecular level.”

Castellano and Mongin utilized cadmium selenide (CdSe) quantum dots as the nanomaterial and pyrenecarboxylic acid (PCA) as the acceptor molecule. At room temperature, they found that the close proximity of the relevant energy levels created a feedback mechanism that thermally repopulated the CdSe excited state, causing it to photoluminesce.

Taking the experiment one step further, the researchers then systematically varied the CdSe-PCA energy gap by changing the size of the nanocrystals. This resulted in predictable changes to the resultant excited state lifetimes. They also examined this process at different temperatures, yielding results consistent with a thermally activated energy transfer mechanism.

“Depending on relative energy separation, the system can be tuned to behave more like PCA or more like the CdSe nanoparticle,” says Castellano. “It’s a control dial for the system. We can make materials with unique photoluminescent properties simply by controlling the size of the nanoparticle and the temperature of the system.”