In motor learning, it’s actions, not intentions, that count

In motor learning, it’s actions, not intentions, that count

Posted by Biomechanism 

Research from Harvard’s Neuromotor Control Lab contradicts a common assumption about how the body learns to make accurate movements

(“Biomechanism.com“) Albert Einstein defined insanity as “doing the same thing over and over again and expecting different results.” Practicing the same task repetitively, though, tends to be the default procedure when trying to learn a new motor skill.

Caption: Lead author Nicolas Gonzalez Castro demonstrates the use of the robotic arm to measure motor learning. Photo courtesy of the Harvard SEAS Neuromotor Control Lab. Credit: Photo courtesy of the laboratory of Maurice Smith, Harvard School of Engineering and Applied Sciences.

A study led by Maurice Smith and colleagues at the Harvard School of Engineering and Applied Sciences (SEAS) suggests that simple task repetition may not be the most efficient way for the brain to learn a new move.

Their results, published in PLoS Computational Biology, demonstrate “motion-referenced learning.” In essence, when people make an imperfect movement during practice, their brains learn less about what they plan to do than about what they actually do.

With that in mind, the researchers propose a new approach to neurological rehabilitation: one that continually adjusts the goals of practice movements so that systematic differences (errors) between these movements and the intended motion can be reduced.

In order to perform any movement accurately—whether that means reaching for a glass of juice without knocking it over, or swimming across a pool without sinking—the brain has to learn exactly which muscles to activate, and in what manner.

The muscle activation required for a given movement depends on the environment. For example, producing a swimming motion on the pool deck is not the same as doing it in the water, and picking up a glass of juice requires a different motion when your arm is weighed down by a heavy bag.

“Individuals learn to accommodate varying physical dynamics, making errors when encountering new situations, but quickly improving with practice. The brain builds internal models of these dynamics, producing patterns of muscle activation that account for external conditions,” explains Smith, an Assistant Professor of Biomedical Engineering at SEAS.

Yet, for people who have suffered neurological damage, such as victims of stroke, the simplest of actions can be difficult to relearn.

The researchers recreated the learning process in healthy subjects by simulating a new physical environment, with unfamiliar forces.

The test subjects were asked to make reaching movements while holding the handle of a robotic arm so that a dot on an LCD screen moved toward a target. At the same time, the robot pushed the handle off course with a varying but predictable amount of force that depended on the hand’s velocity.

One experiment provided evidence of motion-referenced learning, and a second investigated whether an understanding of this mechanism could be used to improve the rate of learning.

Caption: The brain learns associations most efficiently when it is repeating the same actual movement, not the same plan. In the Harvard experiments, the robot pushed the subject's hand to the right. The most efficient way for a subject to learn to hit a target that was directly ahead, then, was to begin with a target that was off to the left (Left-Shifted Training, LST), and in subsequent trials move that target slightly to the right. Credit: Image courtesy of Maurice Smith, Harvard School of Engineering and Applied Sciences.

“We designed a training procedure specifically tailored to this aspect of motor learning,” says lead author Nicolas Gonzalez Castro, a graduate student in Smith’s Neuromotor Control Lab.

“For a simple reaching task, we found that when we adjusted the target position from one trial to the next, so that adaptations could build up around the intended movement, our subjects learned 50 percent faster than when they just practiced the intended movement.”

The findings reveal that the brain is “wired” to maximize stability, a concept that has been essential in the development of algorithms for machine learning, but perhaps underplayed in scientists’ previous understanding of human learning.

“Motion-referenced learning is required for stability when we learn about the physical dynamics of an environment, because the forces imposed by physical dynamics always depend on the actual motion experienced,” explains Smith.

As algorithms for machine learning already incorporate motion-referenced learning (for example, when training a mechanical arm to grasp an object), Smith’s research suggests that scientists may have been better at teaching robots than at teaching humans.

“If you’re intending to teach a person something, there’s an implicit assumption that in the process of practicing what you’re trying to teach them, they’re learning what you want them to learn,” says Smith.

In reality, he says, rehabilitation might be more efficient if the assigned tasks were modified to take advantage of the mechanisms by which the nervous system actually forms associations.

As learning a complex task like walking is much more complicated than moving a handle to a target, adapting the rehabilitation plan for such a task will not be easy.

The findings, however, advance scientists’ understanding of human motor learning at a fundamental level.

“There’s a lot of talk about biologically inspired engineering and how understanding biology helps you engineer things better, but of course there are also a huge number of things that aren’t well understood in biology,” says Smith.

“Sometimes fundamental principles like the need for stability, which is of the utmost importance to us as engineers, and the properties of the learning rules engineers have designed to ensure stability, are things that we take into our study of biological systems.”

Researchers discover source for generating ‘green’ electricity

Researchers discover source for generating ‘green’ electricity

Posted by Biomechanism 

(“Biomechanism.com“) University of Minnesota engineering researchers in the College of Science and Engineering have recently discovered a new alloy material that converts heat directly into electricity. This revolutionary energy conversion method is in the early stages of development, but it could have wide-sweeping impact on creating environmentally friendly electricity from waste heat sources.

In this image, at the University of Minnesota lab; researchers perform a small-scale demonstration of the new alloy material that becomes magnetic when heated.

Researchers say the material could potentially be used to capture waste heat from a car’s exhaust that would heat the material and produce electricity for charging the battery in a hybrid car. Other possible future uses include capturing rejected heat from industrial and power plants or temperature differences in the ocean to create electricity. The research team is looking into possible commercialization of the technology.

“This research is very promising because it presents an entirely new method for energy conversion that’s never been done before,” said University of Minnesota aerospace engineering and mechanics professor Richard James, who led the research team.”It’s also the ultimate ‘green’ way to create electricity because it uses waste heat to create electricity with no carbon dioxide.”

To create the material, the research team combined elements at the atomic level to create a new multiferroic alloy, Ni45Co5Mn40Sn10. Multiferroic materials combine unusual elastic, magnetic and electric properties. The alloy Ni45Co5Mn40Sn10 achieves multiferroism by undergoing a highly reversible phase transformation where one solid turns into another solid. During this phase transformation the alloy undergoes changes in its magnetic properties that are exploited in the energy conversion device.

During a small-scale demonstration in a University of Minnesota lab, the new material created by the researchers begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil. Some of this heat energy is lost in a process called hysteresis. A critical discovery of the team is a systematic way to minimize hysteresis in phase transformations. The team’s research was recently published in the first issue of the new scientific journal Advanced Energy Materials.

In addition to Professor James, other members of the research team include University of Minnesota aerospace engineering and mechanics post-doctoral researchers Vijay Srivastava and Kanwal Bhatti, and Ph.D. student Yintao Song. The team is also working with University of Minnesota chemical engineering and materials science professor Christopher Leighton to create a thin film of the material that could be used, for example, to convert some of the waste heat from computers into electricity.

“This research crosses all boundaries of science and engineering,” James said. “It includes engineering, physics, materials, chemistry, mathematics and more. It has required all of us within the university’s College of Science and Engineering to work together to think in new ways.”

Fossilized pollen reveals climate history of northern Antarctica

Researchers ascertained the exact species of plants that existed on the Antarctic Peninsula over the past 36 million years during a three-year examination of thousands of grains of fossilized pollen, including this grain from the tree Nothofagus fusca. Credit: S. Warny/LSU

A painstaking examination of the first direct and detailed climate record from the continental shelves surrounding Antarctica reveals that the last remnant of Antarctic vegetation existed in a tundra landscape on the continent's northern peninsula about 12 million years ago. The research, which was led by researchers at Rice University and Louisiana State University, appears online this week and will be featured on the cover of the July 12 issue of the Proceedings of the National Academy of Sciences.

The new study contains the most detailed reconstruction to date of the climatic history of the Antarctic Peninsula, which has warmed significantly in recent decades. The rapid decline of glaciers along the peninsula has led to widespread speculation about how the rest of the continent's ice sheets will react to risingglobal temperatures.

"The best way to predict future changes in the behavior of Antarctic ice sheetsand their influence on climate is to understand their past," said Rice University marine geologist John Anderson, the study's lead author. The study paints the most detailed picture to date of how the Antarctic Peninsula first succumbed to ice during a prolonged period of global cooling.

In the warmest period in Earth's past 55 million years, Antarctica was ice-free and forested. The continent's vast ice sheets, which today contain more than two-thirds of Earth's freshwater, began forming about 38 million years ago. The Antarctic Peninsula, which juts farther north than the rest of the continent, was the last part of Antarctica to succumb to ice. It's also the part that has experienced the most dramatic warming in recent decades; its mean annual temperatures rose as much as six times faster than mean annual temperatures worldwide.

Rice University scientist John Anderson discusses what researchers have learned from studying the first direct and detailed climate record from the continental shelves surrounding Antarctica, and he describes the years of effort that went into obtaining it. Credit: B. Martin/Rice University

"There's a longstanding debate about how rapidly glaciation progressed in Antarctica," said Sophie Warny, a Louisiana State University geologist who specializes in palynology (the study of fossilized pollen and spores) and led the palynological reconstruction. "We found that the fossil record was unambiguous; glacial expansion in the Antarctic Peninsula was a long, gradual process that was influenced by atmospheric, tectonic and oceanographic changes." 

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Warny, her students and colleague Rosemary Askin were able to ascertain the exact species of plants that existed on the peninsula over the past 36 million years after a painstaking, three-year examination of thousands of individual grains of pollen that were preserved in muddy sediments beneath the sea floor just off the coast.

"The pollen record in the sedimentary layers was beautiful, both in its richness and depth," Warny said. "It allowed us to construct a detailed picture of the rapid decline of the forests during the late Eocene -- about 35 million years ago -- and the widespread glaciation that took place in the middle Miocene -- about 13 million years ago."

Obtaining the sedimentary samples wasn't easy. The muddy treasure trove was locked away beneath almost 100 feet of dense sedimentary rock. It was also off the coast of the peninsula in shallow waters that are covered by ice most of the year and beset by icebergs the rest. Anderson, a veteran of more than 25 research expeditions to Antarctica, and colleagues spent more than a decade building a case for the funding to outfit an icebreaker with the right kind of drilling equipment to bore through the rock.

In 2002, the National Science Foundation (NSF) funded the project, which was dubbed SHALDRIL. Three years later, the NSF research vessel Nathaniel B. Palmer left on the first of two drilling cruises.

"It was the worst ice year that any of us could remember," Anderson said. "We'd spend most of a day lowering drill string to the ocean floor only to pull it back up to get out of the way of approaching icebergs."

The next year was little better, but the SHALDRIL team managed to obtain enough core samples to cover the past 36 million years, thanks to the logistical planning of marine geologist Julia Wellner and to the skill of the drilling crew. By end of the second season, Anderson said, the crew could drill as much as a meter every five minutes.

Reconstructing a detailed climate record from the sample was another Herculean task. In addition to the three-year palynological analysis at LSU, University of Southampton palaeoceanographer Steven Bohaty led an effort to nail down the precise age of the various sediments in each core sample. Wellner, now at the University of Houston, examined the characteristics of the sediments to determine whether they formed below an ice sheet, in open marine conditions or in a combined glacial-marine setting. Other members of the team had to count, categorize and even examine the surface texture of thousands of sand grains that were preserved in the sediments. Gradually, the team was able to piece together a history of how much of the peninsula was covered by glaciers throughout the past 36 million years.

"SHALDRIL gave us the first reliable age constraints on the timing of ice sheet advance across the northern peninsula," Anderson said. "The rich mosaic of organic and geologic material that we found in the sedimentary record has given us a much clearer picture of the climatic history of the Antarctic Peninsula. This type of record is invaluable as we struggle to place in context the rapid changes that we see taking place in the peninsula today."

Provided by Rice University (news : web)

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Scientists discover dielectron charging of water nano-droplet

This image shows the surface attachment mode of two excess electrons to a water cluster (average diameter of close to 2nm) comprised of 105 molecules. The shown configuration was obtained from first-principles quantum simulations. The two wave functions each occupied by one excess electron, depicted in blue and green, are localized at opposite sides of the cluster and they are shown superimposed on the water molecules. The oxygens and hydrogens of the water molecules are represented by red and gray spheres, respectively. Credit: Uzi Landman/Georgia Tech

Scientists have discovered fundamental steps of charging of nano-sized water droplets and unveiled the long-sought-after mechanism of hydrogen emission from irradiated water. Working together at the Georgia Institute of Technology and Tel Aviv University, scientists have discovered when the number of water molecules in a cluster exceeds 83, two excess electrons may attach to it — forming dielectrons — making it a doubly negatively charged nano droplet. Furthermore, the scientists found experimental and theoretical evidence that in droplets comprised of 105 molecules or more, the excess dielectrons participate in a water-splitting process resulting in the liberation of molecular hydrogen and formation of two solvated hydroxide anions. The results appear in the June 30 issue of the Journal of Physical Chemistry A.

It has been known since the early 1980s that while single electrons may attach to small water clusters containing as few as two molecules, only much larger clusters may attach more than single electrons. Size-selected, multiple-electron, negatively-charged water clusters have not been observed — until now.

Understanding the nature of excess electrons in water has captured the attention of scientists for more than half a century, and the hydrated electrons are known to appear as important reagents in charge-induced aqueous reactions and molecular biological processes. Moreover, since the discovery in the early 1960s that the exposure of water to ionizing radiation causes the emission of gaseous molecular hydrogen, scientists have been puzzled by the mechanism underlying this process. After all, the bonds in the water molecules that hold the hydrogen atoms to the oxygen atoms are very strong. The dielectron hydrogen-evolution (DEHE) reaction, which produces hydrogen gas and hydroxide anions, may play a role in radiation-induced reactions with oxidized DNA that have been shown to underlie mutagenesis, cancer and other diseases.

"The attachment of multiple electrons to water droplets is controlled by a fine balancing act between the forces that bind the electrons to the polar water molecules and the strong repulsion between the negatively charged electrons," said Uzi Landman, Regents' and Institute Professor of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech.

"Additionally, the binding of an electron to the cluster disturbs the equilibrium arrangements between the hydrogen-bonded water molecules and this too has to be counterbalanced by the attractive binding forces. To calculate the pattern and strength of single and two-electron charging of nano-size water droplets, we developed and employed first-principles quantum mechanical molecular dynamics simulations that go well beyond any ones that have been used in this field," he added.

Investigations on controlled size-selected clusters allow explorations of intrinsic properties of finite-sized material aggregates, as well as probing of the size-dependent evolution of materials properties from the molecular nano-scale to the condensed phase regime. 

In the 1980s Landman, together with senior research scientists in the CCMS Robert Barnett, the late Charles Cleveland and Joshua Jortner, professor of chemistry at Tel Aviv University, discovered that there are two ways that single excess electrons can attach to water clusters – one in which they bind to the surface of the water droplet, and the other where they localize in a cavity in the interior of the droplet, as in the case of bulk water. Subsequently, Landman, Barnett and graduate student Harri-Pekka Kaukonen reported in 1992 on theoretical investigations concerning the attachment of two excess electrons to water clusters. They predicted that such double charging would occur only for sufficiently large nano-droplets. They also commented on the possible hydrogen evolution reaction. No other work on dielectron charging of water droplets has followed since.

That is until recently, when Landman, now one of the world leaders in the area of cluster and nano science, and Barnett teamed up with Ori Chesnovsky, professor of chemistry, and research associate Rina Giniger at Tel Aviv University, in a joint project aimed at understanding the process of dielectron charging of water clusters and the mechanism of the ensuing reaction - which has not been observed previously in experiments on water droplets. Using large-scale, state-of-the-art first-principles dynamic simulations, developed at the CCMS, with all valence and excess electrons treated quantum mechanically and equipped with a newly constructed high-resolution time-of-flight mass spectrometer, the researchers unveiled the intricate physical processes that govern the fundamental dielectron charging processes of microscopic water droplets and the detailed mechanism of the water-splitting reaction induced by double charging.

Enlarge

This image shows the internal attachment mode of two electrons to a water cluster (average diameter of close to 2nm) comprised of 105 molecules. The shown configuration was obtained from first-principles quantum simulations. The wave function of the two excess electrons is depicted in pink and it is shown superimposed on the water molecules of the cluster. The compact dielectron distribution is localized in a hydration cavity in the interior of the cluster. The configuration shown here corresponds to the start of the dielectron hydrogen-evolution reaction. The protons of the two reacting neighboring water molecules, located approximately in the middle of the figure, are depicted by smaller blue spheres. Together with the dielectron these protons would form, in the course of the reaction, a hydrogen molecule. The oxygens and hydrogens of the water molecules are represented by red and gray spheres, respectively. Credit: Uzi Landman/Georgia Tech

The mass spectrometric measurements, performed at Tel Aviv, revealed that singly charged clusters were formed in the size range of six to more than a couple of hundred water molecules. However, for clusters containing more than a critical size of 83 molecules, doubly charged clusters with two attached excess electrons were detected for the first time. Most significantly, for clusters with 105 or more water molecules, the mass spectra provided direct evidence for the loss of a single hydrogen molecule from the doubly charged clusters.

The theoretical analysis demonstrated two dominant attachment modes of dielectrons to water clusters. The first is a surface mode (SS'), where the two repelling electrons reside in antipodal sites on the surface of the cluster (see the two wave functions, depicted in green and blue, in Figure 1). The second is another attachment mode with both electrons occupying a wave function localized in a hydration cavity in the interior of the cluster — the so-called II binding mode (see wave function depicted in pink in Figure 2). While both dielectron attachment modes may be found for clusters with 105 molecules and larger ones, only the SS' mode is stable for doubly charged smaller clusters.

"Moreover, starting from the II, internal cavity attachment mode in a cluster comprised of 105 water molecules, our quantum dynamical simulations showed that the concerted approach of two protons from two neighboring water molecules located on the first shell of the internal hydration cavity, leads, in association with the cavity-localized excess dielectron (see Figure 2), to the formation of a hydrogen molecule. The two remnant hydroxide anions diffuse away via a sequence of proton shuttle processes, ultimately solvating near the surface region of the cluster, while the hydrogen molecule evaporates," said Landman.

"What's more, in addition to uncovering the microscopic reaction pathway, the mechanism which we discovered requires initial proximity of the two reacting water molecules and the excess dielectron. This can happen only for the II internal cavity attachment mode. Consequently, the theory predicts, in agreement with the experiments, that the reaction would be impeded in clusters with less than 105 molecules where the II mode is energetically highly improbable. Now, that's a nice consistency check on the theory," he added.

As for future plans, Landman remarked, "While I believe that our work sets methodological and conceptual benchmarks for studies in this area, there is a lot left to be done. For example, while our calculated values for the excess single electron detachment energies are found to be in quantitative agreement with photoelectron measurements in a broad range of water cluster sizes — containing from 15 to 105 molecules — providing a consistent interpretation of these measurements, we would like to obtain experimental data on excess dielectron detachment energies to compare with our predicted values," he said.

"Additionally, we would like to know more about the effects of preparation conditions on the properties of multiply charged water clusters. We also need to understand the temperature dependence of the dielectron attachment modes, the influence of metal impurities, and possibly get data from time-resolved measurements. The understanding that we gained in this experiment about charge-induced water splitting may guide our research into artificial photosynthetic systems, as well as the mechanisms of certain bio-molecular processes and perhaps some atmospheric phenomena."

"You know," he added. "We started working on excess electrons in water clusters quite early, in the 1980s — close to 25 years ago. If we are to make future progress in this area, it will have to happen faster than that."

Provided by Georgia Institute of Technology (news : web)

Scientists pioneer nanoscale nuclear materials testing capability

Scientists at Lawrence Berkeley National Laboratory and the University of California at Berkeley conducted compression tests of copper specimens irradiated with high-energy protons, designed to model how damage from radiation affects the mechanical properties of copper. By using a specialized in situ mechanical testing device in a transmission electron microscope at the National Center for Electron Microscopy, the team could examine -- with nanoscale resolution -- the localized nature of this deformation. (Scales in nanometers, millionths of a meter.) Credit: Minor et al, Lawrence Berkeley National Laboratory

Nuclear power is a major component of our nation's long-term clean-energy future, but the technology has come under increased scrutiny in the wake of Japan's recent Fukushima disaster. Indeed, many nations have called for checks and "stress tests" to ensure nuclear plants are operating safely.

In the United States, about 20 percent of our electricity and almost 70 percent of the electricity from emission-free sources, including renewable technologies and hydroelectric power plants, is supplied by nuclear power. Along with power generation, many of the world's nuclear facilities are used for research, materials testing, or the production of radioisotopes for the medical industry. The service life of structural and functional material components in these facilities is therefore crucial for ensuring reliable operation and safety.

Now scientists at Berkeley Lab, the University of California at Berkeley, and Los Alamos National Laboratory have devised a nanoscale testing technique for irradiated materials that provides macroscale materials-strength properties. This technique could help accelerate the development of new materials for nuclear applications and reduce the amount of material required for testing of facilities already in service.

"Nanoscale mechanical tests always give you higher strengths than the macroscale, bulk values for a material. This is a problem if you actually want use a nanoscale test to tell you something about the bulk-material properties," said Andrew Minor, a faculty scientist in the National Center for Electron Microscopy(NCEM) and an associate professor in the materials science and engineeringdepartment at UC Berkeley. "We have shown you can actually get real properties from irradiated specimens as small as 400 nanometers in diameter, which really opens up the field of nuclear materials to take advantage of nanoscale testing."

In this study, Minor and his colleagues conducted compression tests of copper specimens irradiated with high-energy protons, designed to model how damage from radiation affects the mechanical properties of copper. By using a specialized in situ mechanical testing device in a transmission electron microscope at NCEM, the team could examine — with nanoscale resolution — the nature of the deformation and how it was localized to just a few atomic planes.

Three-dimensional defects within the copper created by radiation can block the motion of one-dimensional defects in the crystal structure, called dislocations. This interaction causes irradiated materials to become brittle, and alters the amount of force a material can withstand before it eventually breaks. By translating nanoscale strength values into bulk properties, this technique could help reactor designers find suitable materials for engineering components innuclear plants.

"This small-scale testing technique could help extend the lifetime of a nuclear reactor," said co-author Peter Hosemann, an assistant professor in the nuclear engineering department at UC Berkeley. "By using a smaller specimen, we limit any safety issues related to the handling of the test material and could potentially measure the exact properties of a material already being used in a 40-year-old nuclear facility to make sure this structure lasts well into the future."

Minor adds, "Understanding how materials fail is a fundamental mechanistic question. This proof of principle study gives us a model system from which we can now start to explore real, practical materials applicable to nuclear energy. By understanding the role of defects on the mechanical properties of nuclear reactor materials, we can design materials that are more resistant to radiation damage, leading to more advanced and safer nuclear technologies."

More information: "In situ nanocompression testing of irradiated copper," Nature Materials (2011)

Provided by Lawrence Berkeley National Laboratory (news : web)

STM of individual grains in CVD-grown graphene

Users from Purdue University, working collaboratively with staff in the CNM Electronic & Magnetic Materials & Devices Group, studied CVD-grown graphene on polycrystalline copper foil for the first time at the atomic-scale. The ultrahigh vacuum scanning tunneling microscopy (UHV-STM) findings performed at CNM will help to guide the optimization of synthesis towards defect-free graphene.

The focus of this study was to investigate the quality of the films and relative orientations of different graphene domains using the UHV-STM facility at CNM. The recent paper also addresses the resulting implications for domain boundary effects on transport properties.

The work follows previous studies done at CNM that investigated graphene on single-crystal Cu(111). Similar to the single-crystal work, it was shown that domain boundaries dramatically affect the carrier mobility of the graphene sheets. The ability to synthesize high-quality graphene for large-scale integration is one of the key challenges for this material system. Fundamental STM experiments performed at the atomic-scale have enabled the study of defects in the as-grown films.

Researchers from the University of Houston, Texas State University, Carl Zeiss SMT, and the Center for Functional Nanomaterials also participated in the study, which is featured as the cover image of Nature Materials.

More information: Y. Qinkai et al. “Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition,” Nature Materials, 10, 443 (2011) doi:10.1038/nmat3010

Abstract 
The strong interest in graphene has motivated the scalable production of high-quality graphene and graphene devices. As the large-scale graphene films synthesized so far are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene’s electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman ‘D’ peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.

Provided by Argonne National Laboratory (news : web)

Breakthrough reported in transconductance in ink-jet printing

he inaugural Rapid Communications article details an important breakthrough by a team of Japanese and German researchers.

The researchers report that they have successfully achieved a transconductance of 0.76 S/m for organic thin-film transistors with 4 V-operation. The team writes: “This is the highest transconductance reported for organic TFTs fabricated using printing, to the best of our knowledge.”

The transconductance report is the first in the new journal’s Rapid Communications section (which is the concise presentation of a study with broad interest showing novel results).



The first published paper shows that, thanks to ultra-low volume (subfemtoliter) inkjet nozzles, small transistors (channel length ~ 1 μm) were fabricated using electrodes printed from nanoparticle metal inks.

The small dimensions allowed the authors to demonstrate low-power and high-speed operation (theoretically up to a few MHz) of organic transistors, a requirement for useful circuits. 



The researchers go on to show that organic and printed electronics are not limited to large and slow devices, but can be extended to fast and miniaturized circuits while remaining compatible with low-cost fabrication on cheap flexible substrates.

In general, these capabilities widen the spectrum of potential applications of this technology.

 

More information: Low-voltage organic transistor with subfemtoliter inkjet source–drain contacts,

Abstract 
We have successfully achieved a transconductance of 0.76 S/m for organic thin-film transistors with 4 V operation, which is the largest value reported for organic transistors fabricated using printing methods. Using a subfemtoliter inkjet, silver electrodes with a line width of 1 µm and a channel length of 1 µm were printed directly onto an air-stable, high-mobility organic semiconductor that was deposited on a single-molecule self-assembled monolayer-based gate dielectric. On reducing the droplet volume (0.5 fl) ejected from the inkjet nozzle, which reduces sintering temperatures down to 90 °C, the inkjet printing of silver electrodes was accomplished without damage to the organic semiconductor.

Provided by Cambridge University