Category Archives: Process Materials

The road to more versatile wearable technology is dotted with iron. Specifically, quantum dots of iron arranged on boron nitride nanotubes (BNNTs). The new material is the subject of a study to be published in Scientific Reports later this week, led by Yoke Khin Yap, a professor of physics at Michigan Technological University.

Yap says the iron-studded BNNTs are pushing the boundaries of electronics hardware. The transistors modulating electron flow need an upgrade.

“Look beyond semiconductors,” he says, explaining that materials like silicon semiconductors tend to overheat, can only get so small and leak electric current.

The key to revamping the fundamental base of transistors is creating a series of stepping-stones that use quantum tunneling.

The nanotubes are the mainframe of this new material. BNNTs are great insulators and terrible at conducting electricity. While at first that seems like an odd choice for electronics, the insulating effect of BNNTs is crucial to prevent current leakage and overheating. Additionally, electron flow will only occur across the metal dots on the BNNTs.

In past research, Yap and his team used gold for quantum dots, placed along a BNNT in a tidy line. With enough energy potential, the electrons are repelled by the insulating BNNT and hopscotch from gold dot to gold dot. This electron movement is called quantum tunneling.

“Imagine this as a river, and there’s no bridge; it’s too big to hop over,” Yap says. “Now, picture having stepping stones across the river–you can cross over, but only when you have enough energy to do so.”

Unlike with semiconductors, there is no classical resistance with quantum tunneling. No resistance means no heat. Plus, these materials are very small; the nanomaterials enable the transistors to shrink as well. An added bonus is that BNNTs are also quite flexible, a boon for wearable electronics.

Using bundled strands of DNA to build Tinkertoy-like tetrahedral cages, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have devised a way to trap and arrange nanoparticles in a way that mimics the crystalline structure of diamond. The achievement of this complex yet elegant arrangement, as described in a paper published February 5, 2016, in Science, may open a path to new materials that take advantage of the optical and mechanical properties of this crystalline structure for applications such as optical transistors, color-changing materials, and lightweight yet tough materials.

This is a schematic illustration of the experimental strategy: Double stranded DNA bundles (gray) form tetrahedral cages. Single stranded DNA strands on the edges (green) and vertices (red) match up with complementary strands on gold nanoparticles. This results in a single gold particle being trapped inside each tetrahedral cage, and the cages binding together by tethered gold nanoparticles at each vertex. The result is a crystalline nanoparticle lattice that mimics the long-range order of crystalline diamond. The images below the schematic are (left to right): a reconstructed cryo-EM density map of the tetrahedron, a caged particle shown in a negative-staining TEM image, and a diamond superlattice shown at high magnification with cryo-STEM. Credit: Brookhaven National Laboratory

This is a schematic illustration of the experimental strategy: Double stranded DNA bundles (gray) form tetrahedral cages. Single stranded DNA strands on the edges (green) and vertices (red) match up with complementary strands on gold nanoparticles. This results in a single gold particle being trapped inside each tetrahedral cage, and the cages binding together by tethered gold nanoparticles at each vertex. The result is a crystalline nanoparticle lattice that mimics the long-range order of crystalline diamond. The images below the schematic are (left to right): a reconstructed cryo-EM density map of the tetrahedron, a caged particle shown in a negative-staining TEM image, and a diamond superlattice shown at high magnification with cryo-STEM. Credit: Brookhaven National Laboratory

“We solved a 25-year challenge in building diamond lattices in a rational way via self-assembly,” said Oleg Gang, a physicist who led this research at the Center for Functional Nanomaterials (CFN) at Brookhaven Lab in collaboration with scientists from Stony Brook University, Wesleyan University, and Nagoya University in Japan.

The scientists employed a technique developed by Gang that uses fabricated DNA as a building material to organize nanoparticles into 3D spatial arrangements. They used ropelike bundles of double-helix DNA to create rigid, three-dimensional frames, and added dangling bits of single-stranded DNA to bind particles coated with complementary DNA strands.

“We’re using precisely shaped DNA constructs made as a scaffold and single-stranded DNA tethers as a programmable glue that matches up particles according to the pairing mechanism of the genetic code-A binds with T, G binds with C,” said Wenyan Liu of the CFN, the lead author on the paper. “These molecular constructs are building blocks for creating crystalline lattices made of nanoparticles.”

The difficulty of diamond

As Liu explained, “Building diamond superlattices from nano- and micro-scale particles by means of self-assembly has proven remarkably difficult. It challenges our ability to manipulate matter on small scales.”

The reasons for this difficulty include structural features such as a low packing fraction-meaning that in a diamond lattice, in contrast to many other crystalline structures, particles occupy only a small part of the lattice volume-and strong sensitivity to the way bonds between particles are oriented. “Everything must fit together in just such a way without any shift or rotation of the particles’ positions,” Gang said. “Since the diamond structure is very open, many things can go wrong, leading to disorder.”

“Even to build such structures one-by-one would be challenging,” Liu added, “and we needed to do so by self-assembly because there is no way to manipulate billions of nanoparticles one-by-one.”

Gang’s previous success using DNA to construct a wide range of nanoparticle arrays suggested that a DNA-based approach might work in this instance.

DNA guides assembly

The team first used the ropelike DNA bundles to build tetrahedral “cages”-a 3D object with four triangular faces. They added single-stranded DNA tethers pointing toward the interior of the cages using T,G,C,A sequences that matched up with complementary tethers attached to gold nanoparticles. When mixed in solution, the complementary tethers paired up to “trap” one gold nanoparticle inside each tetrahedron cage.

The arrangement of gold nanoparticles outside the cages was guided by a different set of DNA tethers attached at the vertices of the tetrahedrons. Each set of vertices bound with complementary DNA tethers attached to a second set of gold nanoparticles.

When mixed and annealed, the tetrahedral arrays formed superlattices with long-range order where the positions of the gold nanoparticles mimics the arrangement of carbon atoms in a lattice of diamond, but at a scale about 100 times larger.

“Although this assembly scenario might seem hopelessly unconstrained, we demonstrate experimentally that our approach leads to the desired diamond lattice, drastically streamlining the assembly of such a complex structure,” Gang said.

The proof is in the images. The scientists used cryogenic transmission electron microscopy (cryo-TEM) to verify the formation of tetrahedral frames by reconstructing their 3D shape from multiple images. Then they used in-situ small-angle x-ray scattering (SAXS) at the National Synchrotron Light Source (NSLS, https://www.bnl.gov/ps/), and cryo scanning transmission electron microscopy (cryo-STEM) at the CFN, to image the arrays of nanoparticles in the fully constructed lattice.

“Our approach relies on the self-organization of the triangularly shaped blunt vertices of the tetrahedra (so called ‘footprints’) on isotropic spherical particles. Those triangular footprints bind to spherical particles coated with complementary DNA, which allows the particles to coordinate their arrangement in space relative to one another. However, the footprints can arrange themselves in a variety of patterns on a sphere. It turns that one particular placement is more favorable, and it corresponds to the unique 3D placement of particles that locks the diamond lattice,” Gang said.

The team supported their interpretation of the experimental results using theoretical modeling that provided insight about the main factors driving the successful formation of diamond lattices.

Sparkling implications

“This work brings to the nanoscale the crystallographic complexity seen in atomic systems,” said Gang, who noted that the method can readily be expanded to organize particles of different material compositions. The group has demonstrated previously that DNA-assembly methods can be applied to optical, magnetic, and catalytic nanoparticles as well, and will likely yield the long-sought novel optical and mechanical materials scientists have envisioned.

“We’ve demonstrated a new paradigm for creating complex 3D-ordered structures via self-assembly. If you can build this challenging lattice, the thinking is you can build potentially a variety of desired lattices,” he said.

Researchers at the National University of Singapore (NUS) and Yale-NUS College have established the mechanisms for spin motion in molybdenum disulfide, an emerging two-dimensional (2D) material. Their discovery resolves a research question on the properties of electron spin in single layers of 2D materials, and paves the way for the next generation of spintronics and low-power devices. The work was published online in the journal Physical Review Letters on 29 January 2016.

Molybdenum disulfide (MoS2), a class of transition metal dichalcogenide compounds, has attracted great attention due to wide recognition of its potential for manipulating novel quantum degrees of freedom such as spin and valley. Due to its unique material properties, a single layer of MoS2 has the potential to be used for spin transistors, where both electric current and spin current can be switched on and off independently. Despite this potential for application, there have not been any experimental studies on the mechanism for spin dynamics in MoS2.

To address this gap, scientists from the Centre for Advanced 2D Materials at NUS used highly precise measurements of the classical and quantum motion of electrons to extract information on how long spins live in this new material.

The team of scientists led by Assistant Professor Goki Eda, co-leader of this study who is from the NUS Department of Physics and Department of Chemistry, thinned down a crystal of molybdenite, a mineral of MoS2, to less than one nanometer. Here, the electrons live in a purely 2D plane that is just one atom thick. The researchers then successfully injected a high density of electrons in this ultra-thin material to enable measurements in the quantum mechanical regime. Quantum transport measurements at low temperatures of 2 Kelvin ( 271 degrees Celsius) revealed a surprising transition, where quantum mechanical wave interference switched from constructive to destructive with increasing magnetic field.

Mr Indra Yudhistira, a Research Associate with the NUS Department of Physics who is under the supervision of Assistant Professor Shaffique Adam, co-leader of the NUS study who is from Yale-NUS College and NUS Department of Physics, demonstrated that this crossover was caused by spin dynamics.

By comparing the theoretical and experimental results, the two research groups were able to extract spin lifetimes and also determine that the relaxation was driven by the Dyakonov-Perel type where electron spins live longer in dirtier samples.

“Aside from investigating the fundamental properties of low field magnetotransport in molybdenum disulfide, our team was able to establish the mechanism for spin scattering to reveal the properties of the electron spin,” said Dr Hennrik Schmidt, who was a Research Fellow working under the supervision of Asst Prof Eda when the study was conducted.

Commenting on the significance of the discovery, Asst Prof Adam noted that spin-based devices would generally lead to lower energy consumption as compared to conventional electronics. He explained, “The combination of MoS2 being a semiconductor and the long spin lifetimes open up opportunities in spintronics, where the electron spin and not the electron charge is used to transport information. Such unconventional devices could allow for next generation low-power devices.”

Professor Yoshihiro Iwasa, Director of the Center for Quantum-Phase Electronics at the University of Tokyo, and a world expert on quantum devices who first reported superconductivity in this class of materials remarked, “2D materials have been anticipated as a promising platform for spintronics. I feel that this very comprehensive study of the analysis of the electron spin life time will provide crucial information for further pushing the research toward the realization of a new generation of spintronic devices.”

The end of Moore’s Law — the prediction that transistor density would double every two years — was one of the hottest topics in electronics-related discussions in 2015. Silicon-based technologies have nearly reached the physical limits of the number and size of transistors that can be crammed into one chip, but alternative technologies are still far from mass implementation. The amount of heat generated during operation and the sizes of atoms and molecules in materials used in transistor manufacturing are some of problems that need to be solved for Moore’s Law to make a comeback.

Atomic and molecular sizes cannot be changed, but the heat problem is not unsolvable. Recent research has shown that in two-dimensional systems, including semiconductors, electrical resistance decreases and can reach almost zero when they are subjected to magnetic and microwave influence. Electrical resistance produces a loss of energy in the form of heat; therefore, a decrease in resistance reduces heat generation. There are several different models and explanations for the zero-resistance phenomenon in these systems. however, the scientific community has not reached an agreement on this matter because semiconductors used in electronics are complex and processes in them are difficult to model mathematically.

Research conducted by the Quantum Dynamics Unit at Okinawa Institute of Science and Technology graduate University (OIST) could represent an important step in understanding two-dimensional semiconductors. The Unit’s latest paper, published in Physical Review Letters, describes anomalies in the behaviour of electrons in electrons on liquid helium two-dimensional system.

This is a cell (container) where the electrons on liquid helium experiments are conducted. Credit: OIST

This is a cell (container) where the electrons on liquid helium experiments are conducted. Credit: OIST

The system is maintained at a temperature close to absolute zero (-272.75ºC or 0.4K) to keep the helium liquefied. Extraneous electrons are bound to the helium surface because their presence causes slight changes in the orbits of helium electrons, inducing a subtle positive charge at the helium surface. At the same time, free electrons lack the energy required to penetrate the surface to enter the liquid. The resulting system is ideal for studying various electron properties, as it has virtually zero impurities, which avoids artefacts caused by defects of surface and structure, or due to the presence of other chemical elements. Prof. Denis Konstantinov, head of the Quantum Dynamics Unit, and his team study conditions under which electrons can violate selection rules regulating transitions from one state to another.

In a macro-world we perceive transitions from one state to another as happening gradually. For example, a person travelling from town A to town B can make an infinite number of stops. In micro-world that is not always the case. Properties, such as energy, position, speed, and colour, can be quantised, i.e. they can occur only in discrete quantities. In other words, the traveller can be either in town A, or town B, but not somewhere in-between.

Since electron energy is quantised, electrons can occupy only specific energy levels. Quantum theory predicts that in a two-dimensional electron system, where moving electrons are confined to one plane, under a strong magnetic field electrons also will be restricted to climbing only one step of the energy level ladder at a time. however, the experiments show that electrons can jump to higher energy levels, skipping levels between. Prof. Konstantinov and his team are very excited about this discovery: “It is not everyday that we get a chance to observe the violation of quantum theory predictions!”

In order to study abnormalities in electron state changes, the scientists applied a strong vertical magnetic field and then bombarded the system with microwave photons. Under these conditions selection rules seem to stop working. Prof. Konstantinov says that his group had theorised that such a phenomenon is possible and now they have proven it.

Selection rules describe a theoretical, absolutely pure, and homogenous system with no disorders. Real-life systems are more complex. In the case of electrons on helium, the system is pure and homogenous, but the surface of liquid helium is nonetheless disturbed by capillary waves — ripples associated with the surface tension and similar to small, circular ripples in a pond when a pebble is tossed into the water. The height of these ripples is only the diameter of a hydrogen atom, but in combination with microwave radiation they create enough deviation from an ideal system for selection rules to change.

Conditions modelled in the Quantum Dynamics Unit’s experiment are similar to those that led to observations of zero resistance in semiconductors. however, the electrons on helium system is relatively simple and can be described mathematically with great precision. Studying this system will further the development of quantum physics and will contribute to our understanding of electrons and various electrical phenomena. moreover, with some adjustments models, based on electrons on helium systems can be adapted to more complex systems, such as two-dimensional semiconductors.

Sweden-based SiC CVD developer and manufacturer Epiluvac AB has entered into a collaboration with SAMCO, a semiconductor process equipment developer and manufacturer based in Japan, in which Epiluvac will introduce new clients to SAMCO in Sweden, Norway, Finland and Denmark.

SAMCO offers systems and services that revolve around three major technologies: 1) thin film deposition with PECVD, MOCVD and ALD systems, 2) microfabrication with ICP etching, RIE and DRIE systems, and 3) surface treatment with plasma cleaning and UV ozone cleaning systems.

“With this collaboration, Epiluvac and SAMCO are both acting as a one-stop solution,” says Bo Hammarlund, managing director of Epiluvac AB. “We offer our expertise to help customers decide upon the best combinations in terms of processing equipment for WBG materials, including both SiC and GaN materials.”

Since its establishment in 1979, SAMCO has earned a reputation both at home and overseas for dependable systems and reliable service. In order to continue serving the needs of its customers in Europe, SAMCO acquired samco-ucp ltd. in Liechtenstein in 2014, which now offers services and support for the company’s European customer base.

In addition to its main European office in Liechtenstein, SAMCO also operates from several locations in North America and across Asia. The company works with research institutions and manufacturers around the world and offers customizable systems designed to meet the unique needs and goals of its clients.

“Both companies, Epiluvac and SAMCO, have long-standing relations with major players in the quickly-growing market for power electronics,” says Hammarlund. “Many of the processes have to be more efficient in terms of improved yield.”

He highlights the additional need to handle larger wafers during the coming years, pointing to the fact that 8-inch protoypes of SiC wafers are already on the market.

“This rapid development results in a strong request for new, improved equipment and processes which Epiluvac and SAMCO together can offer their customers,” he adds.

New equipment and processes made by Epiluvac and SAMCO in order to meet customer demand, Hammarlund says, include “not just standard solutions, but also custom-designed equipment.”

A research team at Umeå University in Sweden has showed, for the first time, that a very efficient vertical charge transport in semiconducting polymers is possible by controlled chain and crystallite orientation. These pioneering results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices and were recently published in the journal Advanced Materials.

Conjugated semiconducting polymers (plastic) possess exceptional optical and electronic properties, which make them highly attractive in the production of organic opto-electronic devices, such as for instance photovoltaic solar cells (OPV), light emitting diodes (OLED) and lasers.

Polythiophene polymers, such as poly(3-hexylthiophene), P3HT, have been among the most studied semiconducting polymers due to their strong optical absorbance and ease of processing into a thin film from solution. In both OPVs and OLEDs, charges must be transported in the out of plane (vertical) direction inside the polymer film.

However, until now the vertical charge carrier mobility of organic semiconductors, i.e. the ability of charges to move inside the material, has been too low to produce fast charge transport in electronic devices. Faster charge transport can occur along the polymer chain backbone. However, a method to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now.

In the present work, a team of chemists and materials scientists, led by Professor David R. Barbero at Umeå University, has found a new method to align chains vertically and to produce efficient transport of electric charges through the chain backbone. In this new study, moreover, high charge transport and high mobility were obtained without any chemical doping, which is often used to artificially enhance charge transport in polymers.

“The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film. The mobility measured was approximately one thousand times higher than previously reported in the same organic semiconductor,” says David Barbero.

In what way will these results affect the field of organic electronics?

“We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device. Organic-based devices have traditionally been slower and less efficient than inorganic ones (e.g. made of silicon), in part due to the low mobility of organic (plastic) semiconductors. Typically, plastic semiconductors, which are only semi-crystalline, have hole mobilities about 10,000 times lower than doped silicon, which is used in many electronic devices. Now we show it is possible to obtain much higher mobility, and much closer to that of silicon, by controlled vertical chain alignment, and without doping,” says David Barbero.

The charge transport was measured using nanoscopic electrical measurements, and gave a mobility averaging 3.1 cm2/V.s, which is the highest mobility ever measured in P3HT, and which comes close to a theoretical estimation of the maximum mobility in P3HT. Crystallinity and molecular packing characterisation of the polymer was performed by synchrotron X-ray diffraction at Stanford University’s National Accelerator (SLAC) and confirmed that the high mobilities measured were due to the re-orientation of the polymer chains and crystallites, leading to fast charge transport along the polymer backbones.

These results, published in Advanced Materials, may open up the route to produce more efficient organic electronic devices with vertical charge transport (e.g. OPV, OLED, lasers etc.), by a simple and inexpensive method, and without requiring chemical modification of the polymer.

The Center for Integrated Nanostructure Physics (CINAP) within IBS has reported results correlating the flake merging angle with grain boundary (GBs) properties, and proven that increasing the merging angle of GBs drastically improves the flow of electrons. This correlates to an increase in the carrier mobility from less than 1 cm2V-1s-1 for small angles, to 16cm2 V-1s-1 for angles greater than 20°. The paper, entitled, ‘Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries’ is published in the journal Nature Communications.

According to the paper, it is essential to understand the atomic structures of GBs in order to control and improve electrical transport properties in both bulk and low-dimensional materials. Grain boundaries are the direction that atoms are arranged in a material. For the experiments undertaken by scientists at CINAP, a monolayer molybdenum disulfide (MoS2) was grown by chemical vapor deposition (CVD) and subsequently transferred to a substrate of silicon dioxide (SiO2). The team’s reasoning for using MoS2 is twofold: firstly, it is a 2D semiconductor that features high electrical conductance and, crucially, has a natural bandgap, which enables it to be tuned on and off and; secondly, the grain boundaries are well-defined. This is paramount for successful experiments. Previous research from Northwestern University found that the GBs of MoS2 provided a unique way to modulate resistance; this was achieved by using a large electric field to spatially modulate the location of the grain boundaries.

The Northwestern results, published last year in Nature Nanotechnology, opened a pathway for future research, but the debate regarding the transport physics at the GB is still under dispute. This is due to a large device-to-device performance variation, poor single-domain carrier mobility, and, most importantly, a lack of correlation between transport properties and GB atomic structures in MoS2 research. The CINAP team, headed by the Center’s director Young Hee Lee, overcame these obstacles by directly correlating four-probe transport measurements across single GBs with both high-resolution transmission electron microscopy (TEM) imaging and first-principles calculations. TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An exact atomic-scale image is formed from the interaction of the electrons transmitted through the specimen.

Identifying Grain Boundaries

GBs in the MoS2 layers were identified and regions with no sign of wrinkling or multilayers were then selected to prevent misinterpretations. Four-probe transport measurements were then performed on the substrate with surprising results; when measuring flake misorientations of 8-20o, mobility increased from much less than 1 cm2V-1s-1 up to 16cm2 V-1s-1. Above 20o field effect mobility saturates at a 16cm2 V-1s-1 intra-domain cutoff. Thus, GBs between flakes having a misorientation angle of 20-60o show better transport properties.

The team has, as reported in their paper, “provided a more unified picture of the relationship between mobility, merging angle and atomistic structures of the GBs of monolayer MoS2.” The results provide practical expectations regarding transport properties in large-area films, which will be restricted largely by the poor mobility across GBs. The results obtained in this work are applicable to other similar 2D systems, and contribute to the fundamental understanding of transport in semiconductors.

Fujifilm Corporation recently announced that its semiconductor business subsidiary, FUJIFILM Electronic Materials Co.,Ltd. will build a new plant for manufacturing advanced semiconductor materials in the city of Tainan, to expand its production in Taiwan.

The new plant is expected to be operational in August 2016, and will begin with production of developing solutions. With the full-scale arrival of the age of the Internet of Things (IoT), the semiconductor market is projected to expand strongly in future. Taiwan has a high concentration of semiconductor factories with a large production share globally, making the region well-positioned to grow further as the hub of semiconductor production.

FFEM established FUJIFILM Electronic Materials Taiwan Co., Ltd. in Taiwan’s Hsinchu Country in 1996. The company began with the production of developing solutions, and has since expanded its array of production items to include photo resists.

The local production structure has constantly been enhanced, as seen in the launch of the second plant (Hsinchu) for producing cutting-edge NTI demand for advanced semiconductor materials.
FFEM will build its third plant in Taiwan for producing advanced semiconductor materials in Science Park in Tainan City, where the manufacturing facilities of many of its customers are based. Taking advantage of its close proximity to customers’ sites, the company strives to boost its customer-support capability and shorten its supply chain.

Moving beyond just catering to the expanding demand for advanced semiconductor materials in Taiwan, Fujifilm will have multiple production sites to distribute risk and ensure a stable supply of advanced semiconductor materials to customers – even in natural disasters. The new plant will begin producing developing solutions, and gradually expand its range of production items.
With this Tainan plant in Taiwan, joining the network of existing production sites in Asia, at Shizuoka (Japan), Hsinchu (Taiwan), Suzhou (China) and Cheonan (Korea), FFEM will continue to strengthen its stable supply of advanced semiconductor materials and improve quality control further in order to attain a higher level of customer satisfaction.

FFEM is a subsidiary at the core of Fujifilm’s semiconductor materials business, which forms part of the highly functional materials category on which Fujifilm has focused its commitment. The company produces and provides a worldwide supply of photoresists and image sensor materials process of semiconductors, as well as developers, cleaners, CMP slurries and other advanced semiconductor, CMP slurries and image sensor materials developers in 2014, in a bid to address the ever-expanding used in the manufacturing materials.

It will continue to supply products useful for semiconductor manufacturing, and expand the Customer support structure in its contributions to the advancement of the semiconductor industry.

Manufacture of longer, thinner, and uncontaminated carbon nanotubes, and successfully isolating them, have been ongoing challenges for researchers. A newly developed method has opened up new possibilities in carbon nanotube development.

As recently reported in an article published online at Scientific Reports, researchers at Kyushu University’s Department of Applied Chemistry have developed a method for obtaining high-quality single-walled carbon nanotubes. The relatively mild process uses an outer stimulus to yield undamaged carbon nanotubes that are purer and longer, and even gives researchers the ability to sort nanotubes according to their structure and length.

Hydrogen bonding allows a fluorene based polymer to grow on specific carbon nanotubes. This changes the solubility of the nanotube allowing it to be separated from other types of nanotubes. Credit: International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University

Hydrogen bonding allows a fluorene based polymer to grow on specific carbon nanotubes. This changes the solubility of the nanotube allowing it to be separated from other types of nanotubes. Credit: International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University

Previous approaches for isolating or sorting nanotubes have required use of more aggressive techniques. These can contaminate the nanotubes and are difficult to completely remove. They also involve processes that could damage the nanotubes and impair their functionality.

“Our approach involves introducing supramolecular hydrogen-bonding polymers, followed by simply shaking the mixture and changing the polarity of the solvent, rather than applying potentially destructive sonication or chemical modification,” says coauthor Naotoshi Nakashima. “In this way, we can obtain single-walled carbon nanotubes over two microns long that do a fine job maintaining structural integrity.”

The new technique is particularly useful because of the mildness and selectivity of the newly designed hydrogen-bonding polymers used. The presence of fluorene moieties within them enables the specific recognition of and binding to single-walled carbon nanotubes, and specific sorting of tubes with a small diameter. This is particularly beneficial because small-diameter nanotubes are exceedingly useful for optoelectronic devices, such as thin-film transistors and sensors.

“The nanotubes we can obtain using this method can be expected to have superior characteristics to those isolated by previous procedures,” says coauthor Fumiyuki Toshimitsu (Visiting Assistant Professor). “For example, by limiting contamination, their electrical and mechanical properties can be optimized. And by being able to sort nanotubes by length or chirality, we can more precisely customize those used for a particular application.”

Two MIT researchers have developed a thin-film material whose phase and electrical properties can be switched between metallic and semiconducting simply by applying a small voltage. The material then stays in its new configuration until switched back by another voltage. The discovery could pave the way for a new kind of “nonvolatile” computer memory chip that retains information when the power is switched off, and for energy conversion and catalytic applications.

The findings, reported in the journal Nano Letters in a paper by MIT materials science graduate student Qiyang Lu and associate professor Bilge Yildiz, involve a thin-film material called a strontium cobaltite, or SrCoOx.

Usually, Yildiz says, the structural phase of a material is controlled by its composition, temperature, and pressure.

“Here for the first time,” she says, “we demonstrate that electrical bias can induce a phase transition in the material. And in fact we achieved this by changing the oxygen content in SrCoOx.”

“It has two different structures that depend on how many oxygen atoms per unit cell it contains, and these two structures have quite different properties,” Lu explains.

One of these configurations of the molecular structure is called perovskite, and the other is called brownmillerite. When more oxygen is present, it forms the tightly-enclosed, cage-like crystal structure of perovskite, whereas a lower concentration of oxygen produces the more open structure of brownmillerite.

The two forms have very different chemical, electrical, magnetic, and physical properties, and Lu and Yildiz found that the material can be flipped between the two forms with the application of a very tiny amount of voltage — just 30 millivolts (0.03 volts). And, once changed, the new configuration remains stable until it is flipped back by a second application of voltage.

Strontium cobaltites are just one example of a class of materials known as transition metal oxides, which is considered promising for a variety of applications including electrodes in fuel cells, membranes that allow oxygen to pass through for gas separation, and electronic devices such as memristors — a form of nonvolatile, ultrafast, and energy-efficient memory device. The ability to trigger such a phase change through the use of just a tiny voltage could open up many uses for these materials, the researchers say.

Previous work with strontium cobaltites relied on changes in the oxygen concentration in the surrounding gas atmosphere to control which of the two forms the material would take, but that is inherently a much slower and more difficult process to control, Lu says. “So our idea was, don’t change the atmosphere, just apply a voltage.”

“Voltage modifies the effective oxygen pressure that the material faces,” Yildiz adds. To make that possible, the researchers deposited a very thin film of the material (the brownmillerite phase) onto a substrate, for which they used yttrium-stabilized zirconia.

In that setup, applying a voltage drives oxygen atoms into the material. Applying the opposite voltage has the reverse effect. To observe and demonstrate that the material did indeed go through this phase transition when the voltage was applied, the team used a technique called in-situ X-ray diffraction at MIT’s Center for Materials Science and Engineering.

The basic principle of switching this material between the two phases by altering the gas pressure and temperature in the environment was developed within the last year by scientists at Oak Ridge National Laboratory. “While interesting, this is not a practical means for controlling device properties in use,” says Yildiz. With their current work, the MIT researchers have enabled the control of the phase and electrical properties of this class of materials in a practical way, by applying an electrical charge.

In addition to memory devices, the material could ultimately find applications in fuel cells and electrodes for lithium ion batteries, Lu says.

“Our work has fundamental contributions by introducing electrical bias as a way to control the phase of an active material, and by laying the basic scientific groundwork for such novel energy and information processing devices,” Yildiz adds.

In ongoing research, the team is working to better understand the electronic properties of the material in its different structures, and to extend this approach to other oxides of interest for memory and energy applications, in collaboration with MIT professor Harry Tuller.