Posted: Mar 06, 2018

A treasure trove for nanotechnology experts (Nanowerk News) 2D materials, which consist of a few layers of atoms, may well be the future of nanotechnology. They offer potential new applications and could be used in small, higher-performance and more energy-efficient devices. 2D materials were first discovered almost 15 years ago, but only a few dozen of them have been synthesized so far.

Now, thanks to an approach developed by researchers from EPFL’s Theory and Simulation of Materials Laboratory (THEOS) and from NCCR-MARVEL for Computational Design and Discovey of Novel Materials, many more promising 2D materials may now be identified. Their work was recently published in the journal Nature Nanotechnology („Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds”), and even got a mention on the cover page.

EPFL and NCCR-MARVEL scientists identified more than 1,000 2-D materials, focusing on the feasibility of exfoliation. (Image: G.Pizzi, EPFL)

The first 2D material to be isolated was graphene, in 2004, earning the researchers who discovered it a Nobel Prize in 2010. This marked the start of a whole new era in electronics, as graphene is light, transparent and resilient and, above all, a good conductor of electricity. It paved the way to new applications in numerous fields such as photovoltaics and optoelectronics.

„To find other materials with similar properties, we focused on the feasibility of exfoliation,” explains Nicolas Mounet, a researcher in the THEOS lab and lead author of the study. „But instead of placing adhesive strips on graphite to see if the layers peeled off, like the Nobel Prize winners did, we used a digital method.”

More than 100,000 materials analyzed

The researchers developed an algorithm to review and carefully analyze the structure of more than 100,000 3D materials recorded in external databases. From this, they created a database of around 5,600 potential 2D materials, including more than 1,000 with particularly promising properties. In other words, they’ve created a treasure trove for nanotechnology experts.

To build their database, the researchers used a step-by-step process of elimination. First, they identified all of the materials that are made up of separate layers. „We then studied the chemistry of these materials in greater detail and calculated the energy that would be needed to separate the layers, focusing primarily on materials where interactions between atoms of different layers are weak, something known as Van der Waals bonding,” says Marco Gibertini, a researcher at THEOS and the second author of the study.

A plethora of 2D candidates

Of the 5,600 materials initially identified, the researchers singled out 1,800 structures that could potentially be exfoliated, including 1,036 that looked especially easy to exfoliate. This represents a considerable increase in the number of possible 2D materials known today. They then selected the 258 most promising materials, categorizing them according to their magnetic, electronic, mechanical, thermal and topological properties.

„Our study demonstrates that digital techniques can really boost discoveries of new materials,” says Nicola Marzari, the director of NCCR-MARVEL and a professor at THEOS. „In the past, chemists had to start from scratch and just keep trying different things, which required hours of lab work and a certain amount of luck. With our approach, we can avoid this long, frustrating process because we have a tool that can single out the materials that are worth studying further, allowing us to conduct more focused research.”

It is also possible to reproduce the researchers’ calculations thanks to their software AiiDA, which describes the calculation process for each material discovered in the form of workflows and stores the full provenance of each stage of the calculation. „Without AiiDA, it would have been very difficult to combine and process different types of data,” explains Giovanni Pizzi, a senior researcher at THEOS and co-author of the study. „Our workflows are available to the public, so anyone in the world can reproduce our calculations and apply them to any material to find out if it can be exfoliated.”

Source: Ecole Polytechnique Fédérale de Lausanne

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Posted: Mar 05, 2018

Capturing brain signals with soft electronics (Nanowerk News) Klas Tybrandt, principal investigator at the Laboratory of Organic Electronics at Linköping University, has developed new technology for long-term stable neural recording. It is based on a novel elastic material composite, which is biocompatible and retains high electrical conductivity even when stretched to double its original length.

The result has been achieved in collaboration with colleagues in Zürich and New York. The breakthrough, which is crucial for many applications in biomedical engineering, is described in an article published in the prestigious scientific journal Advanced Materials („High-Density Stretchable Electrode Grids for Chronic Neural Recording”).

Soft electrodes. (Image: Klas Tybrandt)

The coupling between electronic components and nerve cells is crucial not only to collect information about cell signalling, but also to diagnose and treat neurological disorders and diseases, such as epilepsy.

It is very challenging to achieve long-term stable connections that do not damage neurons or tissue, since the two systems, the soft and elastic tissue of the body and the hard and rigid electronic components, have completely different mechanical properties.

“As human tissue is elastic and mobile, damage and inflammation arise at the interface with rigid electronic components. It not only causes damage to tissue; it also attenuates neural signals,” says Klas Tybrandt, leader of the Soft Electronics group at the Laboratory of Organic Electronics, Linköping University, Campus Norrköping.

Klas Tybrandt has developed a new conductive material that is as soft as human tissue and can be stretched to twice its length. The material consists of gold coated titanium dioxide nanowires, embedded into silicone rubber. The material is biocompatible – which means it can be in contact with the body without adverse effects – and its conductivity remains stable over time.

“The microfabrication of soft electrically conductive composites involves several challenges. We have developed a process to manufacture small electrodes that also preserves the biocompatibility of the materials. The process uses very little material, and this means that we can work with a relatively expensive material such as gold, without the cost becoming prohibitive,” says Klas Tybrandt.

The electrodes are 50 µm in size and are located at a distance of 200 µm from each other. The fabrication procedure allows 32 electrodes to be placed onto a very small surface. The final probe, shown in the photograph, has a width of 3.2 mm and a thickness of 80 µm.

The soft microelectrodes have been developed at Linköping University and ETH Zürich, and researchers at New York University and Columbia University have subsequently implanted them in the brain of rats. The researchers were able to collect high-quality neural signals from the freely moving rats for 3 months. The experiments have been subject to ethical review, and have followed the strict regulations that govern animal experiments.

“When the neurons in the brain transmit signals, a voltage is formed that the electrodes detect and transmit onwards through a tiny amplifier. We can also see which electrodes the signals came from, which means that we can estimate the location in the brain where the signals originated. This type of spatiotemporal information is important for future applications. We hope to be able to see, for example, where the signal that causes an epileptic seizure starts, a prerequisite for treating it. Another area of application is brain-machine interfaces, by which future technology and prostheses can be controlled with the aid of neural signals. There are also many interesting applications involving the peripheral nervous system in the body and the way it regulates various organs,” says Klas Tybrandt.

The breakthrough is the foundation of the reseach area Soft Electronics, currently being established at Linköping University, with Klas Tybrandt as principal investigator.

 

Source: Linköping University

Posted: Mar 05, 2018

Insulator or superconductor? Physicists find graphene is both (Nanowerk News) It’s hard to believe that a single material can be described by as many superlatives as graphene can. Since its discovery in 2004, scientists have found that the lacy, honeycomb-like sheet of carbon atoms — essentially the most microscopic shaving of pencil lead you can imagine — is not just the thinnest material known in the world, but also incredibly light and flexible, hundreds of times stronger than steel, and more electrically conductive than copper.

Now physicists at MIT and Harvard University have found the wonder material can exhibit even more curious electronic properties. In a paper published in Nature („Unconventional superconductivity in magic-angle graphene superlattices”), the team reports it can tune graphene to behave at two electrical extremes: as an insulator, in which electrons are completely blocked from flowing; and as a superconductor, in which electrical current can stream through without resistance.

Physicists at MIT and Harvard University have found that graphene, a lacy, honeycomb-like sheet of carbon atoms, can behave at two electrical extremes: as an insulator, in which electrons are completely blocked from flowing; and as a superconductor, in which electrical current can stream through without resistance. (Image: MIT)

Researchers in the past, including this team, have been able to synthesize graphene superconductors by placing the material in contact with other superconducting metals — an arrangement that allows graphene to inherit some superconducting behaviors. This time around, the team found a way to make graphene superconduct on its own, demonstrating that superconductivity can be an intrinsic quality in the purely carbon-based material.

The physicists accomplished this by creating a “superlattice” of two graphene sheets stacked together — not precisely on top of each other, but rotated ever so slightly, at a “magic angle” of 1.1 degrees. As a result, the overlaying, hexagonal honeycomb pattern is offset slightly, creating a precise moiré configuration that is predicted to induce strange, “strongly correlated interactions” between the electrons in the graphene sheets. In any other stacked configuration, graphene prefers to remain distinct, interacting very little, electronically or otherwise, with its neighboring layers.

The team, led by Pablo Jarillo-Herrero, an associate professor of physics at MIT, found that when rotated at the magic angle, the two sheets of graphene exhibit nonconducting behavior, similar to an exotic class of materials known as Mott insulators. When the researchers then applied voltage, adding small amounts of electrons to the graphene superlattice, they found that, at a certain level, the electrons broke out of the initial insulating state and flowed without resistance, as if through a superconductor.

“We can now use graphene as a new platform for investigating unconventional superconductivity,” Jarillo-Herrero says. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices.”

A large-scale interpretation of the moiré patterns formed when one graphene lattice is slightly rotated at a “magic angle,” with respect to a second graphene lattice. (Image: MIT)

A 30-year gap

A material’s ability to conduct electricity is normally represented in terms of energy bands. A single band represents a range of energies that a material’s electrons can have. There is an energy gap between bands, and when one band is filled, an electron must embody extra energy to overcome this gap, in order to occupy the next empty band.

A material is considered an insulator if the last occupied energy band is completely filled with electrons. Electrical conductors such as metals, on the other hand, exhibit partially filled energy bands, with empty energy states which the electrons can fill to freely move.

Mott insulators, however, are a class of materials that appear from their band structure to conduct electricity, but when measured, they behave as insulators. Specifically, their energy bands are half-filled, but because of strong electrostatic interactions between electrons (such as charges of equal sign repelling each other), the material does not conduct electricity. The half-filled band essentially splits into two miniature, almost-flat bands, with electrons completely occupying one band and leaving the other empty, and hence behaving as an insulator.

“This means all the electrons are blocked, so it’s an insulator because of this strong repulsion between the electrons, so nothing can flow,” Jarillo-Herrero explains. “Why are Mott insulators important? It turns out the parent compound of most high-temperature superconductors is a Mott insulator.”

In other words, scientists have found ways to manipulate the electronic properties of Mott insulators to turn them into superconductors, at relatively high temperatures of about 100 Kelvin. To do this, they chemically “dope” the material with oxygen, the atoms of which attract electrons out of the Mott insulator, leaving more room for remaining electrons to flow. When enough oxygen is added, the insulator morphs into a superconductor. How exactly this transition occurs, Jarillo-Herrero says, has been a 30-year mystery.

“This is a problem that is 30 years and counting, unsolved,” Jarillo-Herrero says. “These high-temperature superconductors have been studied to death, and they have many interesting behaviors. But we don’t know how to explain them.”

A precise rotation

Jarillo-Herrero and his colleagues looked for a simpler platform to study such unconventional physics. In studying the electronic properties in graphene, the team began to play around with simple stacks of graphene sheets. The researchers created two-sheet superlattices by first exfoliating a single flake of graphene from graphite, then carefully picking up half the flake with a glass slide coated with a sticky polymer and an insulating material of boron nitride.

They then rotated the glass slide very slightly and picked up the second half of the graphene flake, adhering it to the first half. In this way, they created a superlattice with an offset pattern that is distinct from graphene’s original honeycomb lattice.

The team repeated this experiment, creating several “devices,” or graphene superlattices, with various angles of rotation, between 0 and 3 degrees. They attached electrodes to each device and measured an electrical current passing through, then plotted the device’s resistance, given the amount of the original current that passed through.

“If you are off in your rotation angle by 0.2 degrees, all the physics is gone,” Jarillo-Herrero says. “No superconductivity or Mott insulator appears. So you have to be very precise with the alignment angle.”

At 1.1 degrees — a rotation that has been predicted to be a “magic angle” — the researchers found the graphene superlattice electronically resembled a flat band structure, similar to a Mott insulator, in which all electrons carry the same energy regardless of their momentum.

“Imagine the momentum for a car is mass times velocity,” Jarillo-Herrero says. “If you’re driving at 30 miles per hour, you have a certain amount of kinetic energy. If you drive at 60 miles per hour, you have much higher energy, and if you crash, you could deform a much bigger object. This thing is saying, no matter if you go 30 or 60 or 100 miles per hour, they would all have the same energy.”

“Current for free”

For electrons, this means that, even if they are occupying a half-filled energy band, one electron does not have any more energy than any other electron, to enable it to move around in that band. Therefore, even though such a half-filled band structure should act like a conductor, it instead behaves as an insulator — and more precisely, a Mott insulator.

This gave the team an idea: What if they could add electrons to these Mott-like superlattices, similar to how scientists doped Mott insulators with oxygen to turn them into superconductors? Would graphene assume superconducting qualities in turn?

To find out, they applied a small gate voltage to the “magic-angle graphene superlattice,” adding small amounts of electrons to the structure. As a result, individual electrons bound together with other electrons in graphene, allowing them to flow where before they could not. Throughout, the researchers continued to measure the electrical resistance of the material, and found that when they added a certain, small amount of electrons, the electrical current flowed without dissipating energy — just like a superconductor.

“You can flow current for free, no energy wasted, and this is showing graphene can be a superconductor,” Jarillo-Herrero says.

Perhaps more importantly, he says the researchers are able to tune graphene to behave as an insulator or a superconductor, and any phase in between, exhibiting all these diverse properties in one single device. This is in contrast to other methods, in which scientists have had to grow and manipulate hundreds of individual crystals, each of which can be made to behave in just one electronic phase.

“Usually, you have to grow different classes of materials to explore each phase,” Jarillo-Herrero says. “We’re doing this in-situ, in one shot, in a purely carbon device. We can explore all those physics in one device electrically, rather than having to make hundreds of devices. It couldn’t get any simpler.” Source: By Jennifer Chu, MIT

Posted: Mar 05, 2018

Scientists create multifunctional protein-polymer films (Nanowerk News) A team from Lomonosov Moscow State University (MSU) together with foreign and Russian colleagues found that when mixing dendrimers (tree-like polymers) and proteins, multilayer films are spontaneously formed. They are easily formed and retain the activity and function of protein enzymes, which determines their potential as a material for creating biosensors and medical products. The results of the study are published in the journal Polymer(„Spontaneous formation of nanofilms under interaction of 4th generation pyrydylphenylene dendrimer with proteins”).

Dendrimer molecule is marked yellow. The surface of the protein molecule is color-coded in accordance with the chemical properties of amino acid residues (red – positive charge, blue – negative charge, green – polarized, and white – hydrophobic). (Image: Vladimir Muronetz)

Today a huge number of polymers is known, with different structure, composition, and properties. One of the most interesting examples are dendrimers – tree-like macromolecules. They are arranged as follows: three chemical groups are attached to the central core, each one branches into two same groups, and so on, and so on. Such a structure is achieved by a multistage synthesis.

The number of branching points determines generation number of the dendrimer: the first generation polymer has one branching point, the second one has two, the third has three, and so on. The further, the denser and more spherical becomes the structure, and its physical and chemical properties are preset by the functional groups on the outer layer. Dendrimers are able to form surface cavities into which molecules can be enveloped, and this property was the main prerequisite for the active study of these polymers. However, MSU scientists have found a completely different application for dendrimers.

„We found that upon mixing of proteins with fourth generation dendrimers the self-assembly of multilayer nanofilms 200-700 nm thick occurs. They can be adjacent to glass or plastic surfaces, as well as be formed at the liquid-air interface,” says Vladimir Muronetz, professor of the Faculty of Bioengineering and Bioinformatics and the Faculty of Biology, MSU, and Head of the Animal Cell Biochemistry Department in A.N. Belozersky Institute of Physico-Chemical Biology, MSU.

Scientists have conducted experiments on the incorporation of various enzymes into the composition of nanofilms: lysozyme, which destroys the bacterial cell walls, and several types of proteinases break down proteins. It was concluded that in the composition of nanofilms these proteins are able to preserve activity for about two weeks. This two-component system is resistant to the action of cleaning agents (detergents), to changes in the medium acidity, and is generally stable during storage. However, the main advantage of the produced films is the simplicity of their formation process due to self-assembly.

„We believe that protein-dendrimer films are promising materials for creating biosensors, and can also be used in medicine as bioactive dressing material,” concludes Vladimir Muronetz.

Source: Lomonosov Moscow State University

Posted: Mar 02, 2018

Graphene material strengthens nerve signaling in the brain (Nanowerk News) Less than 20 years after it was developed, a thin, resilient sheet of carbon atoms with remarkable properties known as graphene is transforming biomedical fields as far flung as tissue engineering, neuroprosthetics and drug discovery.

Because it readily conducts heat and electricity, graphene also may be a good biosensor. But it’s not neutral. When Vanderbilt University scientists tried to use graphene to measure electrical activity in the brain, they found that it actually enhanced nerve cell signaling.

It did so by enabling nerve cell membranes to pull in more cholesterol. The fatty substance was used to make more of the vesicles that package neurotransmitters, the chemical messengers that relay signals between nerve cells. More vesicles and more neurotransmitter meant stronger signals.

These findings, reported in the journal Nature Communications („Membrane cholesterol mediates the cellular effects of monolayer graphene substrates”), came as a complete surprise to the investigators. They raise the possibility that graphene may enable researchers to change how cells communicate with each other by manipulating the cholesterol content in the cell membrane.

Qi Zhang, PhD, Kristina Kitko and colleagues are studying the way graphene impacts nerve cell signaling in the brain.

Graphene “not only may be a very good vehicle to deliver drugs but also a way to potentiate the drug effect,” said the paper’s senior author, Qi Zhang, PhD, assistant professor of Pharmacology in the Vanderbilt University School of Medicine.

The researchers began their experiment by growing neurons — nerve cells — on a sheet of graphene.

The paper’s first author, Kristina Kitko, a graduate student in interdisciplinary material science, picks up the story from there in an invited column she wrote for a Nature Chemistry blog: “We observed an increase in neuronal firing frequency on graphene,” she wrote. “This was substantial and our curiosity was piqued. When we began to look at more targeted measurements of presynaptic activity, what consistently emerged was surprising: synaptic potentiation on graphene.”

Realizing that graphene was likely acting at the nerve cell membrane, which consists primarily of cholesterol, the next question was: “Might cholesterol somehow be involved?”

“This led us to the idea of manipulating membrane cholesterol levels,” Kitko continued. “Cholesterol plays important and fundamental roles to cells in a variety of ways, but with particular relevance to drug discovery we demonstrated the effect of graphene on G protein-coupled receptors, whose activity are mediated by cholesterol.”

Half of all drugs target these receptors.

Source: By Bill Snyder, Vanderbilt University

Posted: Mar 01, 2018

The shape of things to come for quantum materials? (Nanowerk News) For the first time, researchers isolated and characterized atomically thin 2-D crystals of pentagons bonded together in palladium diselenide (PdSe2). The research confirmed predictions that the puckered structure would be stable.

Theory predicted the existence of materials composed of pentagonal “tiles” (purple, teal, green, and pink). Now, researchers have proven these materials exist. Scientists made and characterized the first-ever sheets composed of pentagons of palladium diselenide. Rather than being flat, the material is puckered. The top image shows this puckered structure with blue palladium atoms and gold selenium atoms. The puckered structure leads to interesting electronic properties. These tiles are promising building blocks for air-stable electronics, photonics, and other technologies. (Image: Oak Ridge National Laboratory)

The unique structure of the material results in beneficial properties. It has fast-moving electrons. Also, it is stable in air. Other 2-D semiconductors are not stable in air. These properties may enable advanced uses in detectors, solar cells, and transistors. Further, this material could advance superconductivity for electrical flow without losses, piezoelectric sensors, and energy-efficient computing.

Most 2-D crystals studied to date are a lattice of hexagons—for example, graphene, transition metal dichalcogenides, and black phosphorus—and the lattice layers may be planar or puckered. Theory predicted a family of 2-D crystalline materials with lattices made of pentagonal “tiles.” Sheets of pentagons are unusual even in nature.

Now, a team led by Oak Ridge National Laboratory has proven experimentally the existence of one member of this family (Journal of the American Chemical Society, „PdSe2: Pentagonal 2D layers with high air stability for electronics”).

The researchers used bulk crystals made by a group from Nanyang Technological University; they exfoliated the crystals to obtain puckered layers of PdSe2. Using high-resolution scanning transmission electron microscopy, they characterized layers having different thicknesses. Micro-absorption optical spectroscopy, Raman spectroscopy, and first-principles calculations showed layer thickness changed the band gap.

The band gap is the energy range in a solid at which electronic states cannot exist. The thickness changed the band gap from 0 in the bulk (multiple layers) to 1.3 electron volts in single monolayers.

That finding paves the way for pentagonal 2-D materials with tunable band gaps, which may bring new capabilities to electronics and photonics. Most 2-D materials have highly symmetric lattices and, as a result, exhibit isotropic behavior—that is, a physical property has the same value when measured in different directions. In contrast, pentagonal 2-D PdSe2 is anisotropic—meaning property values differ when measured in different directions.

Pentagonal 2-D materials may allow a new degree of freedom for designing conceptually new optoelectronic and electronic devices not possible using other 2-D materials. Moreover, the material is stable in air, which is not true of many other 2-D semiconductors. For example, it is less susceptible to oxidation than black phosphorus, another promising 2-D material with a tunable band gap.

The discovery of additional puckered pentagonal building blocks may advance applications in low-energy optoelectronics, piezoelectrics, thermoelectrics, and spintronics.

Source: U.S. Department of Energy, Office of Science

Posted: Mar 01, 2018

Durable wood carbon sponge could be the future of wearable sensors, pollutant treatment (Nanowerk News) Engineers at the University of Maryland, College Park (UMD) have for the first time demonstrated that wood can be directly converted into a carbon sponge capable of enduring repeated compression and other extreme mechanical conditions. The UMD engineers’ wood carbon sponge overcomes several limiting factors of other lightweight, compressible carbon sponges because it is simpler, less expensive, and more sustainable to produce. The new sponge can be used in various applications such as energy storage (e.g., batteries), pollutant treatment, and electronic devices and sensors.

A peer-reviewed paper about the research was published March 1 in the journal Chem („Scalable and Sustainable Approach toward Highly Compressible, Anisotropic, Lamellar Carbon Sponge”).

Most lightweight, compressible carbon sponges are made from raw materials that are usually unrenewable fossil resources–such as graphene–and by a complicated fabrication process that involves multiple steps and environmentally unfriendly chemicals. In contrast, the UMD researchers use a simple chemical process to transform balsa wood, a choice biomass-based material that is both renewable and abundant.

„Our results reveal that rigid and incompressible balsa can be made highly compressible by a chemical treatment and carbonization process, yielding a wood carbon sponge with mechanical compressibility and fatigue resistance and electrical response sensitivity surpassing those of most reported compressible carbonaceous materials,” says corresponding author Liangbing Hu, associate professor of materials science and engineering at UMD’s A. James Clark School of Engineering.

Hu and colleagues achieved the bendable yet resilient architecture of the wood carbon sponge by using common chemicals to destroy the stiff hemicellulose and lignin fibers that maintain the normal cell-wall structure of balsa wood, then heating the treated wood to 1,000C in order to turn the organic material into carbon alone. The net effect of the process was to collapse the repeated, regular, rectangular pockets typical of the microstructure of balsa and other woods and replace them with a stack of wavy, interlocking, arch-like carbon sheets, likened by Hu to a cross between a coiled spring and a honeycomb.

Normal carbonized wood–obtained from only the heating step without any chemical modifications–is so fragile that any reasonable applied force pulverizes it irreversibly into ash and dust. However, the wood carbon sponge withstood and rebounded from substantial compression for up to 10,000 consecutive trials before deformation set in. Such a performance initially surprised the research team.

„Our process for creating the wood carbon sponge is unique because we preserve the structure of the wood. This makes the sponge highly compressible and resistant to stress. This means the performance of our wood carbon sponge is one of the best among all lightweight and compressible carbonaceous materials ever reported,” says lead author Chaoji Chen, postdoctoral researcher at UMD’s A. James Clark School of Engineering.

After conducting further mechanical and electrical tests on the sponge, the researchers were able to incorporate a slice of it into a strain sensor prototype suitable for attachment to a human finger, a quality desirable for use in wearable fitness or health-monitoring electronics.

The researchers believe that the wood carbon sponge could also be incorporated into water purification devices and energy storage and conversation technologies, such as supercapacitors and rechargeable batteries. „The abundant applications illustrate the value of a strategy that explores the hidden potentials of natural materials, such as trees, by drawing inspiration from other natural structures and sources,” Hu says.

Source: University of Maryland

Posted: Feb 27, 2018

Optical emission of two-dimensional arsenic sulfide prepared in plasma (Nanowerk News) Since the discovery of graphene in 2004, there has been a rapidly growing interest among scientists in the study of 2D materials „beyond graphene”. In the family of chalcogenide materials, 2D-layered transition-metal dichalcogenides demonstrate excellent electronic and optical properties, outstanding mechanical flexibility, and exceptional catalytic performance. At the same time, chalcogenides like As2S3, As2Se3, etc., have never been considered as materials capable of forming structures of this type.

It could be due to the limitations of the methods used for preparing those materials. A group of researchers from the laboratory of functional nanomaterials at the Lobachevsky State University of Nizhny Novgorod, the Alekseev Nizhny Novgorod State Technical University and the AGH University of Science and Technology (Krakow, Poland) show the possibility of forming honeycomb structural fragments in plasma using the example of the well-known arsenic sulfide chalcogenide system (Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, „Infrared and Raman spectroscopy study of As-S chalcogenide films prepared by plasma-enhanced chemical vapor deposition”).

For the first time, optical emission of two-dimensional „beyond graphene” arsenic sulfide prepared in plasma has been demonstrated. A strong structural photoluminescence exited by continuous wave operation lasers with a laser excitation wavelength of 473 nm and 632.8 nm has been observed.

The influence of excitation parameters, chemical composition, structure, and annealing conditions on the intensity of photoluminescence of the chalcogenide materials has been established. Mass-spectrometry and Raman spectroscopy were coupled with quantum-chemical calculations to reveal the fragments which are the building blocks of the 2D As-S materials.

Figure 1. SEM images of arsenic sulfide materials prepared by plasma discharge at different conditions: pole-like (a) and layered (b) structures. (Image: Lobachevsky University) (click on image to enlarge)

A plausible mechanism of formation and modification of the arsenic sulfide luminescent structural units has been proposed. The properties of 2D pole-structured and layered arsenic sulfide are suggested to be the key to advancing to 2D photosensitive devices.

Scanning Electron Microscopy (SEM) images typical of the arsenic sulfide with a composition As40S60 are illustrated in Figure 1 (a) and (b). The striking difference in the surface morphology and structure is due to very different conditions of plasma deposition.

Both pictures (1a and 1b) illustrate arsenic sulfide structures consisting of (As2S2)n-units (see below) formed in plasma by spherical structural fragments with a diameter of about 100 nm. Figure 1a shows a pole-structured material and Figure 1b depicts a 2D layered structure, the theoretical possibility of existence of which has been described recently.

The researchers have reported quantum-chemical estimations of these structures together with experimental results on the unusually broad transparency window of these materials (0.43 – 20 µm) in comparison with those of As2S3 (0.6 – 11 µm) prepared by traditional thermal methods.

Photoluminescence of the plasma prepared arsenic sulfide was measured by excitation at 473 nm and 632.8 nm employing continuous wave operation lasers at room temperature (RT, Figure 2).

Figure 2. This is a photoluminescence and mass spectra of the plasma prepared arsenic sulfide. (Image: Lobachevsky University) (click on image to enlarge)

The arsenic sulfide materials prepared in plasma were analyzed using the mass-spectrometry method to reveal and to gain insight into the main structural fragments affecting the luminescence intensity.

The mass-spectroscopy data presented clarify the main structural fragments affecting the PL intensity of the plasma prepared arsenic sulfide. According to the data obtained and taking into account the point of view formulated previously we may assume that the main reason for appearance and enhancing of the luminescence in arsenic sulfide materials prepared in plasma is the (As2S2)n cyclic structure unit playing the role of a „disk-like polarizability tensor”.

Due to its properties, two-dimensional pole-structured and layered „beyond graphene” arsenic sulfide is a promising material for developing 2D photosensitive devices. These properties are also useful when creating field-effect transistors, highly sensitive photodetectors and gas sensors.

Source: Lobachevsky University

Posted: Feb 27, 2018

Optical properties of 2D materials could advance infrared technology (Nanowerk News) University of Arkansas researchers have studied the optical characteristics of a special type of material made of a single layer of phosphorus atoms for the benefit of detecting and interacting with infrared light, which is invisible not only to the human eye but to many other materials proposed for use in optoelectronic systems.

Such systems strive to utilize light alongside electrons to speed up processing and to reduce heating and other energy-wasters in our ever-growing plethora of computational devices.

The group of researchers in the Department of Physics published their findings in a recent issue of Scientific Reports („Tuning Infrared Plasmon Resonance of Black Phosphorene Nanoribbon with a Dielectric Interface”), helping to advance the understanding of black phosphorus as an optically useful material. Physics doctoral student Desalegn Debu was the first author of this theoretical and computational work. Other authors include Stephen Bauman and David French, University of Arkansas graduate students; and Hugh Churchill and Joseph Herzog, assistant professors in the Department of Physics.

Similar to a pebble splash causing waves to move across the surface of a pond, light shining upon a plasmonic material causes electrons to move back and forth on the surface. This wave of electrons, known as a plasmon, can be tuned like a musical instrument to oscillate most strongly for certain frequencies (colors) of light, making them useful for applications ranging across much of the spectrum of visible and invisible signals. Tuning is key to the use of plasmonic materials for specific applications, just as tuning a musical instrument is critical to creating the desired note.

Two-dimensional materials such as graphene, a sheet of carbon with single-atom thickness, have experienced a lot of hype in the scientific world within the past decade. They offer the potential of reducing the size of various optoelectronic components down to single atoms thick as well as taking advantage of unique physical properties.

While graphene has been the ‘poster child’ of 2D materials and it may yet hold promise for improved electronics, material strength, or light-based technology, it is not perfect. Graphene lacks what is called a bandgap, a defining property of semiconductors. This limitation of graphene is being circumvented by looking into other materials that offer superior structural, electrical, thermal, or optical properties.

Black phosphorus, which has a corrugated structure of phosphorus atoms with two different crystalline directions, offers unique benefits over other previously studied options.

The study performed by Debu and his colleagues investigates the theoretical effects of changing material properties in the area surrounding a black phosphorus sheet. The results demonstrate that the wavelength of light absorbed by the material can be tuned by changing the surrounding material.

The study also advanced the understanding of the plasmonic nature of this material when it is patterned into nanoscale ribbons with different widths. Combining the tuning parameters of nanoribbon width and surrounding media, black phosphorus can become a very useful material for applications involving infrared light.

An additional benefit of black phosphorus is that its two different crystalline directions allow for light to interact differently with the electrons on the surface depending on the orientation of the light waves used in the application.

Source: University of Arkansas

Posted: Feb 26, 2018

Researchers find metal in ‘metal-free’ graphene catalysts

(Nanowerk News) Detective work by Rice University chemists has defined a deception in graphene catalysts that, until now, has defied description.

Graphene has been widely tested as a replacement for expensive platinum in applications like fuel cells, where the material catalyzes the oxygen reduction reaction (ORR) essential to turn chemical energy into electrical energy. Three transmission electron microscope images of nitrogen-doped graphene show the relative presence of manganese atoms

Three transmission electron microscope images of nitrogen-doped graphene show the relative presence of manganese atoms, contaminants from graphite precursors or reactants believed responsible for the material’s ability to catalyze oxygen-reduction reactions, according to Rice University scientists. The top image shows many manganese atoms (white) remain on graphene that has been washed once; few on twice-washed graphene in the center image; and none on graphene washed six times at bottom. Twice-washed graphene with a scattering of manganese atoms proved best for catalysis. (Image: Tour Group/Rice University)Because graphene, the atom-thick form of carbon, isn’t naturally metallic, researchers have been baffled by its catalytic activity when used as a cathode.

 

Wonder no more, said Rice chemist James Tour and his crew, who have discovered that trace quantities of manganese contamination from graphite precursors or reactants hide in the graphene lattice. Under the right conditions, those metal bits activate the ORR. Tour said they also provide insight into how ultrathin catalysts like graphene can be improved.

 

The research appears in the journal Carbon („Manganese deception on graphene and implications in catalysis”).

 

Because the contrast between carbon and manganese atoms is so slight, trace atoms of the contaminants can’t be seen with traditional characterization techniques like X-ray diffraction and X-ray photoelectron spectroscopy (XPS).

 

“Labs have reported ‘metal-free’ graphene catalysts, and the evidence they’ve gathered could easily be interpreted to show that,” Tour said. “In fact, the tools they were using simply weren’t sensitive enough to show the manganese atoms.”

 

A more sensitive tool, inductively coupled plasma mass spectrometry (ICP-MS), clearly saw the interlopers among samples made by the Rice lab.

 

Nitrogen-doped graphene test samples were reduced from graphene oxide and then acid-washed between one and six times. With each wash, the ICP-MS scan showed fewer manganese atoms and detected none in graphene samples washed six times. By the fifth wash, the catalytic activity totally changed and showed the former activity had been due to those residual metal atoms.

 

The lab reported no manganese atoms were observed in any of the same samples using conventional analytical tools, including XPS or transmission electron microscopy.

 

The researchers characterized the samples’ ORR activity and found twice-washed nitrogen-graphene was most effective. These samples tended to incorporate single atoms of manganese into the graphene structure, which facilitated full reduction of oxygen through a four-electron process in which four electrons are transferred to oxygen atoms, usually from hydrogen.

 

“In a four-electron process, oxygen is reduced to water or hydroxide,” said Rice graduate student Ruquan Ye, the paper’s lead author. “However, peroxide is formed in a two-electron process, which results in a lower diffusion-limited current density and generates hazardous reactive oxygen species.” Ye said that without metal, the ORR in graphene is far less efficient.

 

Tour said the results should lead to investigation of the role of trace metals in other materials thought to be metal-free.

“Single-atom catalysts can hide among graphene, and their activity is profound,” he said. “So what has sometimes been attributed to the graphene was really the single metal buried into the graphene surface. Graphene is good in its own right, but in these cases, it was being made to look even better by these single metal-atom stowaways.”

Source: Rice University

Posted: Feb 26, 2018

First nanoscale look at how lithium ions navigate a molecular maze to reach battery electrode

(Nanowerk News) The lithium-ion batteries that power laptops, electric cars and so many other modern gadgets operate on a simple plan: Lithium ions shuttle back and forth between two electrodes, inserting themselves into one of the electrodes as the battery charges and moving across to the other as the battery drains. The speed and ease of their travel through the battery’s liquid electrolyte help determine how fast the battery can charge.

Now scientists have taken the first close look at what happens within a few nanometers of the electrode, where the normally freely moving electrolyte molecules organize themselves into layers that stand directly in the lithium ions’ paths.

They directly observed this layering for the first time in X-ray experiments at the Department of Energy’s SLAC National Accelerator Laboratory. The results suggest that changing the concentration of lithium ions in the electrolyte might change the arrangement of the molecular layers and make it easier for the ions to get in and out of the electrode.

An illustration of electrolyte molecules arranging themselves into layers within a few nanometers of a battery electrode

An illustration of electrolyte molecules arranging themselves into layers within a few nanometers of a battery electrode. Lithium ions (purple balls) must navigate through these layers on their way in and out of an electrode during battery charging and discharging. (Image: Greg Stewart/SLAC National Accelerator Laboratory)

“That process of the ions finding their way into the electrode is very important in terms of how fast you can charge the battery and how long the battery lasts,” said Michael Toney, a distinguished staff scientist at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and co-leader of the study. “Understanding the nanoscale details of how this works could suggest ways to increase charging speed and efficiency.”

The report has been accepted for publication in Energy & Environmental Science („The nanoscale structure of the electrolyte–metal oxide interface”), and an advance copy is postedon the journal’s website.

Probing a Commercial Electrolyte

In lithium-ion batteries, the electrolyte consists of lithium and other ions in a solvent, with the solvent molecules moving around as they would in any other liquid. But based on theory and previous computer simulations, scientists had a strong suspicion that something different happened in the tiny volume of the electrolyte that’s right up next to the electrode. Here, they thought, the presence of the electrode’s hard surface would induce the solvent molecules to line up and form orderly layers. However, confirming this through experiments proved difficult.

For these latest experiments, Toney’s team used a metal oxide material to represent the electrode, bathed in an electrolyte typically found in commercial lithium-ion batteries.

By focusing a high-brilliance X-ray beam from SSRL on the surface of the electrode and analyzing the X-rays that bounced back through the electrolyte, like light reflecting from a mirror, the researchers were able to determine the structures and positions of individual solvent molecules and lithium ions that were within a few billionths of a meter of the electrode surface, said Hans-Georg Steinrück, a postdoctoral researcher in Toney’s group and co-leader of the experiments. Molecular dynamics simulations complemented and agreed with the experimental results.

“We can see the positions of ions and solvent molecules near the electrode with angstrom resolution, and also see how they are oriented at the surface of the electrode,” Steinrück said. “They’re arranged in well-defined layers at the boundary, and the first layer lies flat, parallel to the surface of the electrode; then they become more disordered, more typical of a liquid, as you move out from the surface.” These ordered layers make it more difficult for the lithium ions to move quickly through the layers and into the electrode.

Shifting Ranks of Molecules

However, as the concentration of lithium ions in the electrolyte increased, the arrangement of the layers changed; it became a bit more orderly, and the layers were farther apart, Steinrück said. This led the researchers to a conclusion that seems almost the opposite of what you’d expect.

“Our hypothesis is that if you want to improve lithium ion transport, you want to decrease the amount of order in the layers, and that means decreasing the lithium ion concentration rather than increasing it,” he said.

Steinrück said the team will be exploring this avenue of research further, adding that the foundational knowledge obtained with this technique can also be applied to studies of other types of next-generation batteries and energy storage systems.

Source: By Glennda Chui, SLAC National Accelerator Laboratory