University of Cincinnati researchers create all-electric spintronics

Multidisciplinary team of UC researchers first to find an innovative and novel way to control an electron’s spin orientation using purely electrical means.

A multidisciplinary team of UC researchers is the first to find an innovative and novel way to control an electron's spin orientation using purely electrical means.

Their findings were recently published in the prestigious, high-profile journal "Nature Nanotechnology," in an article titled "All-Electric Quantum Point Contact Spin-Polarizer."

For decades, the transistors inside radios, televisions and other everyday electronic items have transmitted data by controlling the movement of the charge of an electron.

Caption: Professors Philippe Debray (left) and Marc Cahay discuss their spintronics reseach with graduate students Partha Pratim Das (on stepladder) and Krishna Chetry (far right). Credit: Lisa Ventre, UC Photo Services. Usage Restrictions: None. Caption: (Left) Scanning electron micrograph of the quantum point contact schematically illustrates unpolarized (spin up and spin down) electrons incident on the left coming out of the device spin-polarized with spin up. (Right) Spatial distribution of spin polarization in the quantum point contact constriction. Credit: Illustration by Professor Philippe Debray, University of Cincinnati, Usage Restrictions: None.

Scientists have since discovered that transistors that function by controlling an electron's spin instead of its charge would use less energy, generate less heat and operate at higher speeds. This has resulted in a new field of research — spin electronics or spintronics — that offers one of the most promising paradigms for the development of novel devices for use in the post-CMOS (complementary metal–oxide–semiconductor) era. Until now, scientists have attempted to develop spin transistors by incorporating local ferromagnets into device architectures. This results in significant design complexities, especially in view of the rising demand for smaller and smaller transistors," says Philippe Debray, research professor in the Department of Physics in the McMicken College of Arts & Sciences. "A far better and practical way to manipulate the orientation of an electron's spin would be by using purely electrical means, like the switching on and off of an electrical voltage. This will be spintronics without ferromagnetism or all-electric spintronics, the holy grail of semiconductor spintronics." The team of researchers led by Debray and Professor Marc Cahay (Department of Electrical and Computer Engineering) is the first to find an innovative and novel way to control an electron's spin orientation using purely electrical means. "We used a quantum point contact — a short quantum wire — made from the semiconductor indium arsenide to generate strongly spin-polarized current by tuning the potential confinement of the wire by bias voltages of the gates that create it," Debray says.

In the diagram at left, (Left) Scanning electron micrograph of the quantum point contact schematically illustrates unpolarized (spin up and spin down) electrons incident on the left coming out of the device spin-polarized with spin up. (Right) Spatial distribution of spin polarization in the quantum point contact constriction.

Debray continues, "The key condition for the success of the experiment is that the potential confinement of the wire must be asymmetric — the transverse opposite edges of the quantum point contact must be asymmetrical. This was achieved by tuning the gate voltages. This asymmetry allows the electrons — thanks to relativistic effects — to interact with their surroundings via spin-orbit coupling and be polarized. The coupling triggers the spin polarization and the Coulomb electron–electron interaction enhances it."

Controlling spin electronically has major implications for the future development of spin devices. The work by Debray's team is the first step. The next experimental step would be to achieve the same results at a higher temperature using a different material such as gallium arsenide. ###

This work was supported by National Science Foundation awards ECCS 0725404 and DMR 0710581.

Contact: Wendy Beckman wendy.beckman@uc.edu 513-556-1826 University of Cincinnati

Study shows how carbon nanotubes can affect lining of the lungs

Carbon nanotubes are being considered for use in everything from sports equipment to medical applications, but a great deal remains unknown about whether these materials cause respiratory or other health problems. Now a collaborative study from North Carolina State University, The Hamner Institutes for Health Sciences, and the National Institute of Environmental Health Sciences shows that inhaling these nanotubes can affect the outer lining of the lung, though the effects of long-term exposure remain unclear.

Using mice in an animal model study, the researchers set out to determine what happens when multi-walled carbon nanotubes are inhaled.

Inhaled carbon nanotubes accumulate within cells at the pleural lining of the lung as visualized by light microscopy.

Specifically, researchers wanted to determine whether the nanotubes would be able to reach the pleura, which is the tissue that lines the outside of the lungs and is affected by exposure to certain types of asbestos fibers which cause the cancer mesothelioma. The researchers used inhalation exposure and found that inhaled nanotubes do reach the pleura and cause health effects. Short-term studies described in the paper do not allow conclusions about long-term responses such as cancer.

However, the inhaled nanotubes "clearly reach the target tissue for mesothelioma and cause a unique pathologic reaction on the surface of the pleura, and caused fibrosis," says Dr. James Bonner, associate professor of environmental and molecular toxicology at NC State and senior author of the study. The "unique reaction" began within one day of inhalation of the nanotubes, when clusters of immune cells (lymphocytes and monocytes) began collecting on the surface of the pleura. Localized fibrosis, or scarring on parts of the pleural surface that is also found with asbestos exposure, began two weeks after inhalation.

The study showed the immune response and fibrosis disappeared within three months of exposure. However, this study used only a single exposure to the nanotubes. "It remains unclear whether the pleura could recover from chronic, or repeated, exposures," Bonner says. "More work needs to be done in that area and it is completely unknown at this point whether inhaled carbon nanotubes will prove to be carcinogenic in the lungs or in the pleural lining."

The mice received a single inhalation exposure of six hours as part of the study, and the effects on the pleura were only evident at the highest dose used by the researchers – 30 milligrams per cubic meter (mg/m3). The researchers found no health effects in the mice exposed to the lower dose of one mg/m3. ###

The study, "Inhaled Carbon Nanotubes Reach the Sub-Pleural Tissue in Mice," was co-authored by Bonner, Dr. Jessica Ryman-Rasmussen, Dr. Arnold Brody, and Dr. Jeanette Shipley-Phillips of NC State, Dr. Jeffrey Everitt who is an adjunct faculty at NC State, Dr. Mark Cesta of the National Institute of Environmental Health Sciences (NIEHS), Earl Tewksbury, Dr. Owen Moss, Dr. Brian Wong, Dr. Darol Dodd and Dr. Melvin Andersen of The Hamner Institutes for Health Sciences. The study is published in the Oct. 25 issue of Nature Nanotechnology and was funded by The Hamner Institutes for Health Sciences, NIEHS and NC State's College of Agriculture and Life Sciences.

Note to Editors: The presentation abstract follows.

“Inhaled Carbon Nanotubes Reach the Sub-Pleural Tissue in Mice”

Authors: Jessica Ryman-Rasmussen, Arnold Brody, Jeanette Shipley-Phillips, James Bonner, Jeffrey Everitt, North Carolina State University; Mark Cesta, National Institute of Environmental Health Sciences; Earl Tewksbury, Owen Moss, Brian Wong, Darol Dodd, Melvin Andersen, The Hamner Institutes for Health Sciences.

Published: Oct. 25, 2009, Nature Nanotechnology.

Abstract: Carbon nanotubes are shaped like fibres and can stimulate inflammation at the surface of the peritoneum when injected into the abdominal cavity of mice, raising concerns that inhaled nanotubes may cause pleural fibrosis and/or mesothelioma. Here, we show that multiwalled carbon nanotubes reach the subpleura in mice after a single inhalation exposure of 30 mg m-3 for 6 h. Nanotubes were embedded in the subpleural wall and within subpleural macrophages. Mononuclear cell aggregates on the pleural surface increased in number and size after 1 day and nanotube-containing macrophages were observed within these foci. Subpleural fibrosis unique to this form of nanotubes increased after 2 and 6 weeks following inhalation. None of these effects was seen in mice that inhaled carbon black nanoparticles or a lower dose of nanotubes (1 mg m-3). This work suggests that minimizing inhalation of nanotubes during handling is prudent until further long-term assessments are conducted.

Contact: Matt Shipman matt_shipman@ncsu.edu 919-515-6386 North Carolina State University

Penn study: Transforming nanowires into nano-tools using cation exchange reactions

PHILADELPHIA –- A team of engineers from the University of Pennsylvania has transformed simple nanowires into reconfigurable materials and circuits, demonstrating a novel, self-assembling method for chemically creating nanoscale structures that are not possible to grow or obtain otherwise.

The research team, using only chemical reactants, transformed semiconducting nanowires into a variety of useful, nanoscale materials including nanoscale metal strips with periodic stripes and semiconducting patterns, purely metallic nanowires, radial heterostructures and hollow semiconducting nanotubes in addition to other morphologies and compositions.

Caption: Using only chemical reactants, engineers transformed semiconducting nanowires into a variety of useful, nanoscale materials. Credit: Ritesh Agarwal, the University of Pennsylvania. Usage Restrictions: None.

Researchers used ion exchange, one of the two most common techniques for solid phase transformation of nanostructures. Ion (cation/anion) exchange reactions exchange positive or negative ions and have been used to modify the chemical composition of inorganic nanocrystals, as well as create semiconductor superlattice structures. It is the chemical process, for example, that turns hard water soft in many American households.

Future applications of nanomaterials in electronics, catalysis, photonics and bionanotechnology are driving the exploration of synthetic approaches to control and manipulate the chemical composition, structure and morphology of these materials. To realize their full potential, it is desirable to develop techniques that can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase, to be compatible with nanowire growth schemes. The assembly, however, is an expensive and labor-intensive process that prohibits cost-effective production of these materials.

Recent research in the field has enabled the transformation of nanomaterials via solid-phase chemical reactions into nonequilibrium, or functional structures that cannot be obtained otherwise.

In this study, researchers transformed single-crystalline cadmium sulfide nanowires into composition-controlled nanowires, core−shell heterostructures, metal-semiconductor superlattices, single-crystalline nanotubes and metallic nanowires by utilizing size-dependent cation-exchange reactions along with temperature and gas-phase reactant delivery control. This versatile, synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits.

Researchers also found that the speed of the cation exchange process was determined by the size of the starting nanowire and that the process temperature affected the final product, adding new information to the conditions that affect reaction rates and assembly.

"This is almost like magic that a single-component semiconductor nanostructure gets converted into metal-semiconductor binary superlattice, a completely hollow but single crystalline nanotube and even a purely metallic material," said Ritesh Agarwal, assistant professor in the Department of Materials Science and Engineering at Penn. "The important thing here is that these transformations cannot take place in bulk materials where the reaction rates are incredibly slow or in very small nanocrystals where the rates are too fast to be precisely controlled. These unique transformations take place at 5-200 nanometer-length scales where the rates can be controlled very accurately to enable such intriguing products. Now we are working with theoreticians and designing new experiments to unravel this 'magic' at the nanoscale."

The fundamental revelation in this study is a further clarification of nanoscale chemical phenomena. The study also provides new data on how manufacturers can assemble these tiny circuits, electrically connecting nanoscale structures through chemical self-assembly.

It also opens up new possibilities for the transformation of nanoscale materials into the tools and circuits of the future, for example, self-assembling nanoscale electrical contacts to individual nanoscale components, smaller electronic and photonic devices such as a series of electrically connected quantum dots for LEDs or transistors, as well as improved storage capacities for batteries. ###

The study, published in the current issue of the journal Nano Letters, was conducted by Bin Zhang, Yeonwoong Jung, Lambert Van Vug and Agarwal of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science.

The work was supported by a National Science Foundation Career Award and a Penn Materials Research Science and Engineering Center grant.

Contact: Jordan Reese jreese@upenn.edu 215-573-6604 University of Pennsylvania

Tissue engineering could improve hand use for wounded soldiers

Animal studies at University of Michigan Health System also show potential to restore sense of touch.

Modern tissue engineering developed at the University of Michigan could improve the function of prosthetic hands and possibly restore the sense of touch for injured patients.

Researchers will present their updated findings Wednesday at the 95th annual Clinical Congress of the American College of Surgeons.

The research project, funded by the Department of Defense, arose from a need for better prosthetic devices for troops wounded in Afghanistan and Iraq. “Most of these individuals are typically using a prosthesis design that was developed decades ago,” says Paul S. Cederna, M.D., a plastic and reconstructive surgeon at U-M Health System and associate professor of surgery at the U-M Medical School. “This effort is to make a prosthesis that moves like a normal hand.”

U-M researchers may help overcome some of the shortcomings of existing robotic prosthetics, which have limited motor control, provide no sensory feedback and can be uncomfortable and cumbersome to wear. “There is a huge need for a better nerve interface to control the upper extremity prostheses,” says Cederna.

When a hand is amputated, the nerve endings in the arm continue to sprout branches, growing a mass of nerve fibers that send flawed signals back to the brain.

The researchers created what they called an “artificial neuromuscular junction” composed of muscle cells and a nano-sized polymer placed on a biological scaffold. Neuromuscular junctions are the body's own nerve-muscle connections that enable the brain to control muscle movement.

That bioengineered scaffold was placed over the severed nerve endings like a sleeve. The muscle cells on the scaffold and in the body bonded and the body’s native nerve sprouts fed electrical impulses into the tissue, creating a stable nerve-muscle connection.

In laboratory rats, the bioengineered interface relayed both motor and sensory electrical impulses and created a target for the nerve endings to grow properly.

“The polymer has the ability to pick up signals coming out of the nerve, and the nerve does not grow an abnormal mass of nerve fibers,” explains Cederna.

The animal studies indicate the interface may not only improve fine motor control of prostheses, but can also relay sensory perceptions such as touch and temperature back to the brain.

Laboratory rats with the interface responded to tickling of feet with appropriate motor signals to move the limb, says Cederna.

The Department of Defense and the Army have already provided $4.5 million in grants to support the research. Meanwhile, the research team has submitted a proposal to the Defense Advance Research Project Agency to begin testing the bioengineered interface in humans in three years.

Additional U-M authors of the study include William M. Kuzon, Jr., M.D., Ph.D., professor of surgery and head of the Division of Plastic Surgery; David C. Martin, Ph.D., chair of materials science and engineering at the University of Delaware and former professor of materials science and engineering and macromolecular science at U-M; Daryl R. Kipke, Ph.D. professor of biomedical engineering; Melanie Urbancheck, Ph.D., assistant research professor; and Brent M. Egeland, M.D., surgical resident.

Resources: U-M Department of Surgery Division of Plastic Surgery //surgery.med.umich.edu/plastic/

American College of Surgeons 95^th Annual Clinical Congress //www.facs.org/clincon2009/

Contact: Jennifer Burke Labriola burkepr@gmail.com 203-405-1479 American College of Gastroenterology

Berkeley Researchers Find New Route to Nano Self-Assembly

By adding select small molecules to mixtures of nanoparticles and polymers, Berkeley researchers can direct the self-assembly of the nanoparticles into arrays of one, two and even three dimensions with no chemical modifications.

BERKELEY, CA - If the promise of nanotechnology is to be fulfilled, nanoparticles will have to be able to make something of themselves. An important advance towards this goal has been achieved by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) who have found a simple and yet powerfully robust way to induce nanoparticles to assemble themselves into complex arrays.

This electron micrograph shows a self-assembled composite in which nanoparticles of lead sulfide have arranged themselves in a hexagonal grid.

By adding specific types of small molecules to mixtures of nanoparticles and polymers, the researchers are able to direct the self-assembly of the nanoparticles into arrays of one, two and even three dimensions with no chemical modification of either the nanoparticles or the block copolymers. In addition, the application of external stimuli, such as light and/or heat, can be used to further direct the assemblies of nanoparticles for even finer and more complex structural details.

“We’ve demonstrated a simple yet versatile approach to precisely controlling the spatial distribution of readily available nanoparticles over multiple length scales, ranging from the nano to the macro,” says Ting Xu, a polymer scientist who led this project and who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California, Berkeley’s Departments of Materials Sciences and Engineering, and Chemistry. “Our technique can be used on a wide variety of nanoparticle and should open new routes to the fabrication of nanoparticle-based devices including highly efficient systems for the generation and storage of solar energy.”

Xu is the corresponding author on a paper describing this work that has been published by the journal Nature Materials. The paper is titled: “Small molecule-directed nanoparticle assembly towards stimuli-responsive nanocomposites.” Co-authoring this paper with her were Yue Zhao, Kari Thorkelsson, Alexander Mastroianni, Thomas Schilling, Joseph Luther, Benjamin Rancatore, Kazuyuki Matsunaga, Hiroshi Jinnai, Yue Wu, Daniel Poulsen, Jean Fréchet and Paul Alivisatos.

Berkeley researchers including (from left) Kari Thorkelsson, Alexander Mastroianni, Benjamin Rancatore and Ting Xu devised a simple but powerful technique to induce nanoparticles to assemble themselves into complex arrays. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

Nano-sized particles - bits of matter a few billionths of a meter in size, or more than a hundred times smaller than the stuff of today’s microtechnologies - display highly coveted properties not found in macroscopic materials, including optical, electronic, magnetic, etc. The promise of nanotechnololgy is that exploiting these unique properties on a commercial scale could yield such “game-changers” as sustainable, clean and cheap energy, and the creation on demand of new materials with properties tailored to meet specific needs. Realizing this promise starts with nanoparticles being able to organize themselves into complex structures and hierarchical patterns, similar to what nature routinely accomplishes with proteins.

“Precise control of the spatial organization of nanoparticles and other nanoscopic building blocks over multiple length scales has been a bottleneck in the bottom-up generation of technologically important materials,” says Xu. “Most of the approaches that have been used so far have involved surface modifications.”

Small as they are, nanoparticles are essentially all surface so any process that modifies the surface of a nanoparticle can profoundly change the properties of that particle. Precisely arranging these nanoparticles is critical to tailoring the macroscopic properties during nanoparticle assembly. Although DNA has been used to induce self-assembly of nanoparticles with a high degree of precision, this approach only works well for organized arrays that are limited in size; it is impractical for large-scale fabrication. Xu believes a better approach is to use block copolymers - long sequences or “blocks” of one type of monomer molecule bound to blocks of another type of monomer molecule.

“Block copolymers readily self-assemble into well-defined arrays of nanostructures over macroscopic distances,” she says. “They would be an ideal platform for directing the assembly of nanoparticles except that block copolymers and nanoparticles are not particularly compatible with one another from a chemistry standpoint. A mediator is required to bring them together.”

Xu and her group found such a “mediator” in the form of small molecules that will join with nanoparticles and then able attach themselves and their nanoparticle partners to the surface of a block copolymer. For this study, Xu and her group used two different types of small molecules, surfactants (wetting agents) dubbed “PDP” and “OPAP.” These small molecules can be stimulated by light (PDP) or heat (OPAP) to sever their connection to the surface of a block copolymer and be repositioned to another location along the polymeric chain. In this manner, the spatial distribution of the small molecule mediators and their nanoparticle partners can be precisely directed with no need to modify either the nanoparticles or the polymers.

“The beauty of this technique is that it involves no sophisticated chemistry,” says Xu. “It really is a plug and play technique, in which you simply mix the nanoparticles with the block copolymers and then add whatever small molecules you need.”

For this study, Xu and her colleagues added PDP or OPAP small molecules to various blends of nanoparticles, such as cadmium selenide and lead sulfide, mixed in with a commercial block copolymer - polystyrene-block-poly (4-vinyl pyridine). While she and her group worked with light and heat, she says other stimuli, such as pH, could also be used to reposition small molecules and their nanoparticle partners along block copolymer formations. Strategic substitutions of different types of stimulus-responsive small molecules could serve as a mechanism for structural fine-tuning or for incorporating specific functional properties into nanocomposites. Xu and her colleagues are now in the process of adding functionality to their self-assembly technique.

“Bring together the right basic components - nanoparticles, polymers and small molecules - stimulate the mix with a combination of heat, light or some other factors, and these components will assemble into sophisticated structures or patterns,” says Xu. “It is not dissimilar from how nature does it.”

This research was supported in part by the U.S. Department of Energy’s Office of Science and in part by the Army Research Office and National Science Foundation. The nanoparticles were synthesized at Berkley Lab’s Molecular Foundry and characterizations of the nanoparticle assemblies were performed at Beamline 7.3.3 of Berkeley Lab’s Advanced Light Source. Both the Molecular Foundry and the Advanced Light Source are DOE Office of Science national user facilities.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our Website at www.lbl.gov/

Additional Information

For more information on the research of Ting Xu, visit her Website at //www.mse.berkeley.edu/groups/

Contact: Lynn Yarris, (510) 486-5375, lcyarris@lbl.gov