'Spincasting' Holds Promise for Creation of Nanoparticle Thin Films

Spincasting, which utilizes centrifugal force to distribute a liquid onto a solid substrate, already has a variety of uses. For example, it is used in the electronics industry to deposit organic thin films on silicon wafers to create transistors.

For this study, the researchers first dispersed magnetic nanoparticles coated with ligands into a solution. The ligands, small organic molecules that bond directly to metals, facilitate the even distribution of the nanoparticles in the solution -- and, later, on the substrate itself.

A drop of the solution was then placed on a silicon chip that had been coated with a layer of silicon nitride. The chip was then rotated at high speed, which spread the nanoparticle solution over the surface of the chip. As the solution dried, a thin layer of nanoparticles was left on the surface of the substrate.

Using this technique, the researchers were able to create an ordered layer of nanoparticles on the substrate, over an area covering a few square microns."The results are promising, and this approach definitely merits further investigation," says Dr. Joe Tracy, an assistant professor of materials science and engineering at NC State and co-author of a paper describing the study.

Tracy explains that one benefit of spincasting is that it is a relatively quick way to deposit a layer of nanoparticles."It also has commercial potential as a cost-effective way of creating nanoparticle thin films," Tracy says.

However, the approach still faces several hurdles. Tracy notes that modifications to the technique are needed, so that it can be used to coat a larger surface area with nanoparticles. Additional research is also needed to learn how, or whether, the technique can be modified to achieve a more even distribution of nanoparticles over that surface area.

Analysis of the nanoparticle films created using spincasting led to another development as well. The researchers adapted analytical tools to evaluate transmission electron microscopy images of the films they created. One benefit of using these graphical tools is their ability to identify and highlight defects in the crystalline structure of the layer."These methods for image analysis allow us to gain a detailed understanding of how the nanoparticle size and shape distributions affect packing into monolayers," Tracy says.

The paper,"Formation and Grain Analysis of Spin Cast Magnetic Nanoparticle Monolayers," was published online March 24 by the journalLangmuir. The paper was co-authored by Tracy; NC State Ph.D. student Aaron Johnston-Peck; and former NC State post-doctoral research associate Dr. Junwei Wang. The research was funded by the National Science Foundation, the U.S. Department of Education, and Protochips, Inc.

NC State's Department of Materials Science and Engineering is part of the university's College of Engineering.

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First Practical Nanogenerator Produces Electricity With Pinch of the Fingers

"This development represents a milestone toward producing portable electronics that can be powered by body movements without the use of batteries or electrical outlets," said lead scientist Zhong Lin Wang, Ph.D."Our nanogenerators are poised to change lives in the future. Their potential is only limited by one's imagination."

The latest improvements have resulted in a nanogenerator powerful enough to drive commercial liquid-crystal displays, light-emitting diodes and laser diodes. By storing the generated charges using a capacitor, the output power is capable to periodically drive a sensor and transmit the signal wirelessly.

"If we can sustain the rate of improvement, the nanogenerator may find a broad range of other applications that require more power," he added. Wang cited, for example, personal electronic devices powered by footsteps activating nanogenerators inside the sole of a shoe; implanted insulin pumps powered by a heart beat; and environmental sensors powered by nanogenerators flapping in the breeze.

Wang and colleagues demonstrated commercial feasibility of the latest nanogenerator by using it to power an LED light and a liquid crystal display like those widely used in many electronic devices, such as calculators and computers. The power came from squeezing the nanogenerator between two fingers.

The key to the technology is zinc oxide (ZnO) nanowires. ZnO nanowires are piezoelectric -- they can generate an electric current when strained or flexed. That movement can be virtually any body movement, such as walking, a heartbeat, or blood flowing through the body. The nanowires can also generate electricity in response to wind, rolling tires, or many other kinds of movement.

The diameter of a ZnO nanowire is so small that 500 of the wires can fit inside the width of a single human hair. Wang's group found a way to capture and combine the electrical charges from millions of the nanoscale zinc oxide wires. They also developed an efficient way to deposit the nanowires onto flexible polymer chips, each about a quarter the size of a postage stamp. Five nanogenerators stacked together produce about 1 micro Ampere output current at 3 volts -- about the same voltage generated by two regular AA batteries (about 1.5 volts each).

"While a few volts may not seem like much, it has grown by leaps and bounds over previous versions of the nanogenerator," said Wang, a scientist at Georgia Institute of Technology."Additional nanowires and more nanogenerators, stacked together, could produce enough energy for powering larger electronics, such as an iPod or charging a cell phone."

Wang said the next step is to further improve the output power of the nanogenerator and find a company to produce the nanogenerator. It could hit the market in three to five years, he estimated. The device's first application is likely to be as a power source for tiny environmental sensors and sensors for infrastructure monitoring.

The scientists acknowledge funding from the Defense Advanced Research Projects Agency (of the U.S. Department of Defense), the Department of Energy, the National Institutes of Health and the National Science Foundation, and the U.S. Air Force.

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Rapid Etching X-Rayed: Physicists Unveil Processes During Fast Chemical Dissolution

The results appear in theJournal of the American Chemical Society.

For their research the scientists used the intense X-ray radiation of the experimental station ID32, one of the ESRF's instruments. The X-ray beam was directed onto a gold surface while it dissolved in diluted hydrochloric acid. Because the reflected X-rays are sensitive to tiny changes in the atomic arrangement at the material's surface, the metal removal during the reaction can be precisely measured.

"Such studies were only possible during very slow changes of the material so far," Olaf Magnussen explains. To gain insight into the fast reactions going on in industrially employed processes the speed of the measurements had to be increased more than a hundredfold. Even during very fast etching the removal of the metal proceeded very uniformly."The material dissolves quasi atomic layer by atomic layer, without formation of deeper holes," Magnussen remarks. In a similar way, the team could follow the attachment of atoms during the chemical coating of materials.

Among the diverse industrial applications of chemical etching and coating are high-tech manufacturing processes, for example in the production of electronic devices. These require precisely controlled reactions. In order to optimize such etching and coating processes they are intensely studied worldwide. Until now it was only possible to analyse the finished product. With the method developed by the scientists, changes within a few thousandth seconds may be detected so that the reactions at the material's surface can be tracked on the atomic scale under realistic conditions.

Christian-Albrechts-Universität zu Kiel is a North German research university with proven international expertise in the field of nanoscience, including research using synchrotron radiation. In a number of research networks, funded by the German Federal Ministry of Education and Research, Kiel scientists develop new methods and instruments. In addition, the CAU competes for a Cluster of Excellence in the area of nanoscience and surface science within the ongoing round of the German Excellence Initiative.

The ESRF is a European research institution, funded by 19 nations, providing and utilizing brilliant synchrotron X-rays for advanced scientific research.

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Templated Growth Technique Produces Graphene Nanoribbons With Metallic Properties

"We can now make very narrow, conductive nanoribbons that have quantum ballistic properties," said Walt de Heer, a professor in the School of Physics at the Georgia Institute of Technology."These narrow ribbons become almost like a perfect metal. Electrons can move through them without scattering, just like they do in carbon nanotubes."

De Heer was scheduled to discuss recent results of this graphene growth process March 21st at the American Physical Society's March 2011 Meeting in Dallas. The research was sponsored by the National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC).

First reported Oct. 3 in the advance online edition of the journal Nature Nanotechnology, the new fabrication technique allows production of epitaxial graphene structures with smooth edges. Earlier fabrication techniques that used electron beams to cut graphene sheets produced nanoribbon structures with rough edges that scattered electrons, causing interference. The resulting nanoribbons had properties more like insulators than conductors.

"In our templated growth approach, we have essentially eliminated the edges that take away from the desirable properties of graphene," de Heer explained."The edges of the epitaxial graphene merge into the silicon carbide, producing properties that are really quite interesting."

The"templated growth" technique begins with etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons and other structures of specific widths and shapes without the use of cutting techniques that produce the rough edges.

In creating these graphene nanostructures, de Heer and his research team first use conventional microelectronics techniques to etch tiny"steps" -- or contours -- into a silicon carbide wafer whose surface has been made extremely flat. They then heat the contoured wafer to approximately 1,500 degrees Celsius, which initiates melting that polishes any rough edges left by the etching process.

Established techniques are then used for growing graphene from silicon carbide by driving off the silicon atoms from the surface. Instead of producing a consistent layer of graphene across the entire surface of the wafer, however, the researchers limit the heating time so that graphene grows only on portions of the contours.

The width of the resulting nanoribbons is proportional to the depth of the contours, providing a mechanism for precisely controlling the nanoribbon structures. To form complex structures, multiple etching steps can be carried out to create complex templates.

"This technique allows us to avoid the complicated e-beam lithography steps that people have been using to create structures in epitaxial graphene," de Heer noted."We are seeing very good properties that show these structures can be used for real electronic applications."

Since publication of the Nature Nanotechnology paper, de Heer's team has been refining its technique."We have taken this to an extreme -- the cleanest and narrowest ribbons we can make," he said."We expect to be able to do everything we need with the size ribbons that we are able to make right now, though we probably could reduce the width to 10 nanometers or less."

While the Georgia Tech team is continuing to develop high-frequency transistors -- perhaps even at the terahertz range -- its primary effort now focuses on developing quantum devices, de Heer said. Such devices were envisioned in the patents Georgia Tech holds on various epitaxial graphene processes.

"This means that the way we will be doing graphene electronics will be different," he explained."We will not be following the model of using standard field-effect transistors (FETs), but will pursue devices that use ballistic conductors and quantum interference. We are headed straight into using the electron wave effects in graphene."

Taking advantage of the wave properties will allow electrons to be manipulated with techniques similar to those used by optical engineers. For instance, switching may be carried out using interference effects -- separating beams of electrons and then recombining them in opposite phases to extinguish the signals.

Quantum devices would be smaller than conventional transistors and operate at lower power. Because of its ability to transport electrons with virtually no resistance, epitaxial graphene may be the ideal material for such devices, de Heer said.

"Using the quantum properties of electrons rather than the standard charged-particle properties means opening up new ways of looking at electronics," he predicted."This is probably the way that electronics will evolve, and it appears that graphene is the ideal material for making this transition."

De Heer's research team hopes to demonstrate a rudimentary switch operating on the quantum interference principle within a year.

Epitaxial graphene may be the basis for a new generation of high-performance devices that will take advantage of the material's unique properties in applications where higher costs can be justified. Silicon, today's electronic material of choice, will continue to be used in applications where high-performance is not required, de Heer said.

"This is an important step in the process," he added."There are going to be a lot of surprises as we move into these quantum devices and find out how they work. We have good reason to believe that this can be the basis for a new generation of transistors based on quantum interference."

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Scientists Control Light Scattering in Graphene

The research team, led by Feng Wang of Berkeley Lab's Materials Sciences Division, made the first direct observation, in graphene, of so-called quantum interference in Raman scattering. Raman scattering is a form of"inelastic" light scattering. Unlike elastic scattering, in which the scattered light has the same color (the same energy) as the incident light, inelastically scattered light either loses energy or gains it.

Raman scattering occurs in graphene and other crystals when an incoming photon, a particle of light, excites an electron, which in turn generates a phonon together with a lower-energy photon. Phonons are vibrations of the crystal lattice, which are also treated as particles by quantum mechanics.

Quantum particles are as much waves as particles, so they can interfere with one another and even with themselves. The researchers showed that light emission can be controlled by controlling these interference pathways. They present their results in a forthcoming issue of the journalNature, now available in Advance Online Publication.

Manipulating quantum interference, in life and in the lab

"A familiar example of quantum interference in everyday life is antireflective coating on eyeglasses," says Wang, who is also an assistant professor of physics at UC Berkeley."A photon can follow two pathways, scattering from the coating or from the glass. Because of its quantum nature it actually follows both, and the coating is designed so that the two pathways interfere with each other and cancel light that would otherwise cause reflection."

Wang adds,"The hallmark of quantum mechanics is that if different paths are nondistinguishable, they must always interfere with each other. We can manipulate the interference among the quantum pathways that are responsible for Raman scattering in graphene because of graphene's peculiar electronic structure."

In Raman scattering, the quantum pathways are electronic excitations, which are optically stimulated by the incoming photons. These excitations can only happen when the initial electronic state is filled (by a charged particle such as an electron), and the final electronic state is empty.

Quantum mechanics describes electrons filling a material's available electronic states much as water fills the space in a glass: the"water surface" is called the Fermi level. All the electronic states below it are filled and all the states above it are empty. The filled states can be reduced by"doping" the material in order to shift the Fermi energy lower. As the Fermi energy is lowered, the electronic states just above it are removed, and the excitation pathways originating from these states are also removed.

"We were able to control the excitation pathways in graphene by electrostatically doping it -- applying voltage to drive down the Fermi energy and eliminate selected states," Wang says."An amazing thing about graphene is that its Fermi energy can be shifted by orders of magnitude larger than conventional materials. This is ultimately due to graphene's two-dimensionality and its unusual electronic bands."

The Fermi energy of undoped graphene is located at a single point, where its electronically filled bands, graphically represented as an upward-pointing cone, meet its electronically empty bands, represented as a downward-pointing cone. To move the Fermi energy appreciably requires a strong electric field.

Team member Rachel Segalman, an associate professor of chemical engineering at UC Berkeley and a faculty scientist in Berkeley Lab's Materials Sciences Division, provided the ion gel that was key to the experimental device. An ion gel confines a strongly conducting liquid in a polymer matrix. The gel was laid over a flake of graphene, grown on copper and transferred onto an insulating substrate. The charge in the graphene was adjusted by the gate voltage on the ion gel.

"So by cranking up the voltage we lowered the graphene's Fermi energy, sequentially getting rid of the higher energy electrons," says Wang. Eliminating electrons, from the highest energies on down, effectively eliminated the pathways that, when impinged upon by incoming photons, could absorb them and then emit Raman-scattered photons.

What comes of interference, constructive and destructive

"People have always known that quantum interference is important in Raman scattering, but it's been hard to see," says Wang."Here it's really easy to see the contribution of each state."

Removing quantum pathways one by one alters the ways they can interfere. The changes are visible in the Raman-scattering intensity emitted by the experimental device when it was illuminated by a beam of near-infrared laser light. Although the glow from scattering is much fainter than the near-infrared excitation, changes in its brightness can be measured precisely.

"In classical physics, you'd expect to see the scattered light get dimmer as you remove excitation pathways," says Wang, but the results of the experimenter came as a surprise to everyone."Instead the signal got stronger!"

The scattered light grew brighter as the excitation pathways were reduced -- what Wang calls"a canonical signature of destructive quantum interference."

Why"destructively?" Because phonons and scattered photons can be excited by many different, nondistinguishable pathways that interfere with one another, blocking one path can either decrease or increase the light from scattering, depending on whether that pathway was interfering constructively or destructively with the others. In graphene, the lower and higher-energy pathways interfered destructively. Removing one of them thus increased the brightness of the emission.

"What we've demonstrated is the quantum-interference nature of Raman scattering," Wang says."It was always there, but it was so hard to see that it was often overlooked."

In a second observation, the researchers found yet another unexpected example of inelastic light scattering. This one,"hot electron luminescence," didn't result from blocked quantum pathways, however.

When a strong voltage is applied and the graphene's Fermi energy is lowered, higher-energy electron states are emptied from the filled band. Electrons that are highly excited by incoming photons, enough to jump to the unfilled band, thus find additional chances to fall back to the now-vacant states in what was the filled band. But these"hot" electrons can only fall back if they emit a photon of the right frequency. The hot electron luminescence observed by the researchers has an integrated intensity a hundred times stronger than the Raman scattering.

The road taken

The poet Robert Frost wrote of coming upon two roads that diverged in a wood, and was sorry he could not travel both. Not only can quantum processes take both roads at once, they can interfere with themselves in doing so.

The research team, working at UC Berkeley and at Berkeley Lab's Advanced Light Source, has shown that inelastic light scattering can be controlled by controlling interference between the intermediate states between photon absorption and emission. Manipulating that interference has enabled new kinds of quantum control of chemical reactions, as well as of"spintronic" states, in which not charge but the quantum spins of electrons are affected. Strongly enhanced Raman scattering can be a boon to nanoscale materials research. Hot luminescence is potentially attractive for optoelectronics and biological research, in which near-infrared tags -- even weak ones -- could be very useful.

"Likewise the phenomenon of hot electron luminescence, because it immediately follows excitation by a probe laser, could become a valuable research tool," says Wang,"particularly for studying ultrafast electron dynamics, one of the chief unusual characteristics of graphene."

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3-D Printing Method Advances Electrically Small Antenna Design

"Recent attention has been directed toward producing antennas by screen-printing, inkjet printing, and liquid metal-filled microfluidics in simple motifs, such as dipoles and loops," explained Jennifer T. Bernhard, a professor of electrical and computer engineering at Illinois."However, these fabrication techniques are limited in both spatial resolution and dimensionality, yielding planar antennas that occupy a large area relative to the achieved performance."

"Omnidirectional printing of metallic nanoparticle inks offers an attractive alternative for meeting the demanding form factors of 3D electrically small antennas (ESAs)," stated Jennifer A. Lewis, the Hans Thurnauer Professor of Materials Science and Engineering and director of the Frederick Seitz Materials Research Laboratory at Illinois.

"To our knowledge, this is the first demonstration of 3D printed antennas on curvilinear surfaces," Lewis stated. The research findings and fabrication methods developed by Bernhard, Lewis, and their colleagues are featured in the cover article,"Illinois Calling" of the March 18 issue of Advanced Materials.

According to Bernhard, these antennas are electrically small relative to a wavelength (typically a twelfth of a wavelength or less) and exhibit performance metrics that are an order of magnitude better than those realized by monopole antenna designs.

"There has been a long-standing problem of minimizing the ratio of energy stored to energy radiated -- the Q -- of an ESA," Bernhard explained."By printing directly on the hemispherical substrate, we have a highly versatile single-mode antenna with a Q that very closely approaches the fundamental limit dictated by physics (known as the Chu limit).

Conformal printing allows the antenna's meander lines to be printed on the outside or inside of hemispherical substrates, adding to its flexibility.

"Unlike planar substrates, the surface normal is constantly changing on curvilinear surfaces, which presents added fabrication challenges," Lewis noted. To conformally print features on hemispherical substrates, the silver ink must strongly wet the surface to facilitate patterning even when the deposition nozzle (100μm diameter) is perpendicular to the printing surface.

To fabricate an antenna that can withstand mechanical handling, for example, the silver nanoparticle ink is printed on the interior surface of glass hemispheres. Other non-spherical ESAs can be designed and printed using a similar approach to enable integration of low Q antennas on, for example, the inside of a cell phone case or the wing of an unmanned aerial vehicle. The antenna's operating frequency is determined primarily by the printed conductor cross-section and the spacing (or pitch) between meander lines within each arm.

According to the researchers, their design can be rapidly adapted to new specifications, including other operating frequencies, device sizes, or encapsulated designs that offer enhanced mechanical robustness.

"This conformal printing technique can be extended other potential applications, including flexible, implantable, and wearable antennas, electronics, and sensors," Lewis said.

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Room-Temperature Spintronic Computers Coming Soon? Silicon Spin Transistors Heat Up and Spins Last Longer

"Electronic devices mostly use the charge of the electrons -- a negative charge that is moving," says Ashutosh Tiwari, an associate professor of materials science and engineering at the University of Utah."Spintronic devices will use both the charge and the spin of the electrons. With spintronics, we want smaller, faster and more power-efficient computers and other devices."

Tiwari and Ph.D. student Nathan Gray report their creation of room-temperature, spintronic transistors on a silicon semiconductor this month in the journalApplied Physics Letters. The research -- in which electron"spin" aligned in a certain way was injected into silicon chips and maintained for a record 276 trillionths of a second -- was funded by the National Science Foundation.

"Almost every electronic device has silicon-based transistors in it," Gray says."The current thrust of industry has been to make those transistors smaller and to add more of them into the same device" to process more data. He says his and Tiwari's research takes a different approach.

"Instead of just making transistors smaller and adding more of them, we make the transistors do more work at the same size because they have two different ways {electron charge and spin} to manipulate and process data," says Gray.

A Quick Spin through Spintronics

Modern computers and other electronic devices work because negatively charged electrons flow as electrical current. Transistors are switches that reduce computerized data to a binary code of ones or zeros represented by the presence or absence of electrons in semiconductors, most commonly silicon.

In addition to electric charge, electrons have another property known as spin, which is like the electron's intrinsic angular momentum. An electron's spin often is described as a bar magnet that points up or down, which also can represent ones and zeroes for computing.

Most previous research on spintronic transistors involved using optical radiation -- in the form of polarized light from lasers -- to orient the electron spins in non-silicon materials such as gallium arsenide or organic semiconductors at supercold temperatures.

"Optical methods cannot do that with silicon, which is the workhorse of the semiconductor and electronics industry, and the industry doesn't want to retool for another material," Tiwari says.

"Spintronics will become useful only if we use silicon," he adds.

The Experiment

In the new study, Tiwari and Gray used electricity and magnetic fields to inject"spin polarized carriers" -- namely, electrons with their spins aligned either all up or all down -- into silicon at room temperature.

Their trick was to use magnesium oxide as a"tunnel barrier" to get the aligned electron spins to travel from one nickel-iron electrode through the silicon semiconductor to another nickel-iron electrode. Without the magnesium oxide, the spins would get randomized almost immediately, with half up and half down, Gray says.

"This thing works at room temperature," Tiwari says."Most of the devices in earlier studies have to be cooled to very low temperatures" -- colder than 200 below zero Fahrenheit -- to align the electrons' spins either all up or all down."Our new way of putting spin inside the silicon does not require any cooling."

The experiment used a flat piece of silicon about 1 inch long, about 0.3 inches wide and one-fiftieth of an inch thick. An ultra-thin layer of magnesium oxide was deposited on the silicon wafer. Then, one dozen tiny transistors were deposited on the silicon wafer so they could be used to inject electrons with aligned spins into the silicon and later detect them.

Each nickel-iron transistor had three contacts or electrodes: one through which electrons with aligned spins were injected into the silicon and detected, a negative electrode and a positive electrode used to measure voltage.

During the experiment, the researchers send direct current through the spin-injector electrode and negative electrode of each transistor. The current is kept steady, and the researchers measure variations in voltage while applying a magnetic field to the apparatus

"By looking at the change in the voltage when we apply a magnetic field, we can find how much spin has been injected and the spin lifetime," Tiwari says.

A 328 Nanometer, 276 Picosecond Step for Spintronics

For spintronic devices to be practical, electrons with aligned spins need to be able to move adequate distances and retain their spin alignments for an adequate time.

During the new study, the electrons retained their spins for 276 picoseconds, or 276 trillionths of a second. And based on that lifetime, the researchers calculate the spin-aligned electrons moved through the silicon 328 nanometers, which is 328 billionths of a meter or about 13 millionths of an inch.

"It's a tiny distance for us, but in transistor technology, it is huge," Gray says."Transistors are so small today that that's more than enough to get the electron where we need it to go."

"Those are very good numbers," Tiwari says."These numbers are almost 10 times bigger than what we need {for spintronic devices} and two times bigger than if you use aluminum oxide" instead of the magnesium oxide in his study.

He says Dutch researchers previously were able to inject aligned spins into silicon using aluminum oxide as the"tunneling medium," but the new study shows magnesium oxide works better.

The new study's use of electronic spin injection is much more practical than using optical methods such as lasers because lasers are too big for chips in consumer electronic devices, Tiwari says.

He adds that spintronic computer processors require little power compared with electronic devices, so a battery that may power an electronic computer for eight hours might last more than 24 hours on a spintronic computer.

Gray says spintronics is"the next big step to push the limits of semiconductor technology that we see in every aspect of our lives: computers, cell phones, GPS (navigation) devices, iPods, TVs."

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Toward Real Time Observation of Electron Dynamics in Atoms and Molecules

Made by a team of researchers from the Energy, Materials, and Telecommunications Center of INRS and the National Research Council Canada/University of Ottawa Joint Attosecond Science Laboratory, this scientific breakthrough opens new opportunities for investigating electron dynamics on the timescale of the attosecond (0.000,000,000,000,000,001 second).

Researchers used a new laser source developed at ALLS by Professor François Légaré's team from the Energy, Materials, and Telecommunications Center in collaboration with colleagues from INRS University, NRC Canada, and the University of Ottawa. This laser source proves to be an ideal tool for HHG from atoms and molecules. The HHG spectra obtained through interaction of the laser source with xenon atoms provide information on electronic correlations by highlighting the giant resonance of xenon. In addition, results obtained at ALLS show that the laser source is ideal for developing a soft X-ray beamline delivering ultrafast x-ray laser pulses down to the nanometer wavelength.

Built on national scientific collaboration, this study was conducted at ALLS by researchers Bruno E. Schmidt, Jean-Claude Kieffer, and François Légaré of the Energy, Materials, and Telecommunications Center of INRS and by Andrew D. Shiner, Carlos Trallero-Herrero, Hans J. Wörner, Serguei Patchkovskii, Paul B. Corkum, and David M. Villeneuve of the NRC Canada/University of Ottawa Joint Attosecond Science Laboratory. The project was funded by the Natural Sciences and Engineering Research Council of Canada, Fonds québécois de recherche sur la nature et les technologies, the Canadian Institute for Photonic Innovations, and the Canada Foundation for Innovation.

Research results have just been published in the journalNature Physics. 

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Stretchable Balloon Electronics Get to the Heart of Cardiac Medicine

A team of materials scientists, mechanical and electrical engineers, and physicians has successfully integrated stretchable electronics technology with standard endocardial balloon catheters. Led by John A. Rogers, the Lee J. Flory-Founder Chair in Engineering at Illinois, the team published its work in the March 6 online edition ofNature Materials.

The team previously demonstrated a sensor-laden sheet that could laminate to the surface of the heart in 2010. Now they have expanded their technology to endocardial balloon catheters, one of the most common, least-invasive devices for cardiac procedures.

Catheters are long, flexible tubes that can be threaded through a vein or artery to reach the inside of the heart. Catheters with balloons at the end are commonly used for angioplasty, stent placement and other procedures as passive mechanical instruments. When in place, the balloon inflates and gently presses against the surrounding tissue to open blood vessels or valves.

Invasive cardiologists specializing in heart rhythm disorders use catheters with electrodes at the end for detecting and mapping arrhythmias and for ablation, or selectively killing small patches of cells that beat off-rhythm. Current invasive arrhythmia procedures involve two separate, rigid catheter devices: one that maps the heart point-by-point as a cardiologist maneuvers the tube in search of irregularities, and one with an electrode at the end that ablates spots identified as aberrant, one at a time.

The balloon device Rogers' team developed can perform both functions over large areas of the heart simultaneously, using integrated arrays of multifunctional sensors and ablation electrodes.

"It's all in one, so it maps and zaps," said Rogers, a professor of materials science and engineering who also is affiliated with the Beckman Institute for Advanced Science and Technology at Illinois."The idea here is instead of this single-point mapping and separate single-point zapping catheter, have a balloon that offers all that functionality, in a mode that can do spatial mapping in a single step. You just inflate it right into the cavity and softly push all of that electronics and functionality against the tissue."

The researchers created a meshwork of tiny sensor nodes that could mount directly onto a conventional catheter balloon. The device holds an array of sensors to measure electrical activity of the cardiac muscle, temperature, blood flow, and pressure as the balloon presses against the tissue, along with electrodes for ablation. The entire system is designed to operate reliably as the balloon inflates and deflates.

"It demands all the features and capabilities that we've developed in stretchable electronics over the years in a pretty aggressive way," Rogers says."It also really exercises the technology in an extreme, and useful, manner -- we put everything on the soft surface of a rubber balloon and blow it up without any of the devices failing."

The Illinois team collaborated with cardiologists at the University of Arizona and Massachusetts General Hospital (MGH) to determine what types of features would be most useful for patient care.

For example, the researchers added temperature sensors and mapped temperature distribution on actual tissue as areas were ablated. From this data they developed a model to predict temperature distribution so cardiologists know how deep into the tissue they are ablating.

"Adding a feature such as temperature detection and distribution gives us greater insight as to what we are actually doing to the tissue," said co-author Dr. Marvin J. Slepian, a practicing cardiologist and a professor of medicine at the Sarver Heart Center of the University of Arizona."This will enhance the safety and effectiveness of ablation catheters, providing a new level of precision that we have not had to date, while simultaneously shortening the length of procedure times, which is an overall 'win' for patients, physicians and hospitals."

Rogers' team also worked closely with mc10, a company he co-founded that is commercializing the underlying technology for both medical and non-medical applications. Several researchers at mc10 are co-authors of the paper. The company has tested the devices in live animal experiments with medical collaborators at Arizona and MGH.

The biggest challenge for the researchers was ensuring full functionality of the electronics at all levels of balloon inflation. Since the center of the balloon stretches more than the ends, they had to figure out the range of strain the sensors would encounter and how to accommodate it so that sensors at the most strained points would function the same as those at areas of lower strain.

Through a collaboration with researchers at Northwestern University, led by Younggang Huang, the team solved this problem by mounting the sensors and electrodes on tiny rigid islands so they wouldn't be affected by the balloon stretching. They also used spring-like interconnects between the sensors to handle the 100 percent distance increase between the islands when the balloon inflates.

The fabrication techniques the engineers used in developing the balloon device could be exploited for integrating many classes of advanced semiconductor devices on a variety of surgical instruments. For example, the team also demonstrated surgical gloves with sensor arrays mounted on the fingertips to show that the electronics could be applied to other biomedical platforms.

Next, Rogers would like to further increase the density of sensors on the balloon, up to thousands of tiny, multiplexed devices on the surface. This design would enable the integration of sophisticated electronic systems with the capability for even greater resolution for mapping and the ability to ablate the minimal amount of tissue. He also plans to continue exploring medical device applications for stretchable, flexible electronic arrays in other surgical tools.

"Being able to embed these kinds of advanced semiconductor devices into tissue-like formats creates all kinds of new ways to do minimally invasive procedures," Rogers said."I'm hopeful that this will be the first of many devices that collectively can have a major impact on the way human health care is done."

This work was supported by the National Science Foundation and the Department of Energy. The authors will present related findings at the Heart Rhythm Society Meeting in San Francisco in May.

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Probing Atomic Chicken Wire: Mounting Graphene on Boron Nitride Dramatically Improves Electronic Properties

In addition to potential applications in integrated circuits, solar cells, miniaturized bio devices and gas molecule sensors, the material has attracted the attention of physicists for its unique properties in conducting electricity on an atomic level.

Graphene has very little resistance and allows electrons to behave as massless particles like photons, or light particles, while traveling through the hexagonal grid at very high speeds.

The study of the physical properties and potential applications of graphene, however, has suffered from a lack of suitable carrier materials that can support a flat graphene layer while not interfering with its electrical properties.

Researchers in the University of Arizona's physics department along with collaborators from the Massachusetts Institute of Technology and the National Materials Science Institute in Japan have now taken an important step forward toward overcoming those obstacles.

They found that by placing the graphene layer on a material almost identical in structure, instead of the commonly used silicon oxide found in microchips, they could significantly improve its electronic properties.

Substituting silicon wafers with boron nitride, a graphene-like structure consisting of boron and nitrogen atoms in place of the carbon atoms, the group was the first to measure the topography and electrical properties of the resulting smooth graphene layer with atomic resolution.

The results are published in the advance online publication ofNature Materials.

"Structurally, boron nitride is basically the same as graphene, but electronically, it's completely different," said Brian LeRoy, an assistant professor of physics and senior author of the study."Graphene is a conductor, boron nitride is an insulator."

"We want our graphene to sit on something insulating, because we are interested in studying the properties of the graphene alone. For example, if you want to measure its resistance, and you put it on metal, you're just going to measure the resistance of the metal because it's going to conduct better than the graphene."

Unlike silicon, which is traditionally used in electronics applications, graphene is a single sheet of atoms, making it a promising candidate in the quest for ever smaller electronic devices. Think going from a paperback to a credit card.

"It's as small as you can shrink it down," LeRoy said."It's a single layer, you'll never get half a layer or something like that. You could say graphene is the ultimate in making it small, yet it 's still a good conductor."

Stacked upon each other, 3 million sheets of graphene would amount to only 1 millimeter. The thinnest material on Earth, graphene brought the 2010 Nobel Prize to Andre Geim and Konstantin Novoselov, who were able to demonstrate its exceptional properties with relation to quantum physics.

"Using a scanning tunneling microscope, we can look at atoms and study them," he added."When we put graphene on silicon oxide and look at the atoms, we see bumps that are about a nanometer in height."

While a nanometer -- a billionth of a meter -- may not sound like much, to an electron whizzing along in a grid of atoms, it's quite a bump in the road.

"It's basically like a piece of paper that has little crinkles in it," LeRoy explains."But if you put the paper, in this case the graphene, on boron nitride, it's much flatter. It smooths out the bumps by an order of magnitude."

LeRoy admits the second effect achieved by his research team is a bit harder to explain.

"When you have graphene sitting on silicon oxide, there are trapped electric charges inside the silicon oxide in some places, and these induce some charge in the overlying graphene. You get quite a bit of variation in the density of electrons. If graphene sits on boron nitride, the variation is two orders of magnitude less."

In his lab, LeRoy demonstrates the first -- and surprisingly low-tech -- step in characterizing the graphene samples: He places a tiny flake of graphite -- the stuff that makes up pencil"lead" -- on sticky tape, folds it back on itself and peels it apart again, in a process reminiscent of a Rorschach Test.

"You fold this in half," he explained,"and again, and again, until it gets thin. Graphene wants to peel off into these layers, because the bonds between the atoms in the horizontal layer are strong, but weak between atoms belonging to different layers. When you put this under an optical microscope, there will be regions with one, two, three, four or more layers. Then you just search for single-layer ones using the microscope."

"It's hard to find the sample because it's very, very small," said Jiamin Xue, a doctoral student in LeRoy's lab and the paper's leading author."Once we find it, we put it between two gold electrodes so we can measure the conductance."

To measure the topography of the graphene surface, the team uses a scanning tunneling microscope, which has an ultrafine tip that can be moved around.

"We move the tip very close to the graphene, until electrons start tunneling to it," Xue explained."That's how we can see the surface. If there is a bump, the tip moves up a bit."

For the spectroscopic measurement, Xue holds the tip at a fixed distance above the sample. He then changes the voltage and measures how much current flows as a function of that voltage and any given point across the sample. This allows him to map out different energy levels across the sample.

"You want as thin an insulator as possible," LeRoy added."The initial idea was to pick something flat but insulating. Because boron nitride essentially has the same structure as graphene, you can peel it into layers in the same way. Therefore, we use a metal as a base, put a thin layer of boron nitride on it and then graphene on top."

The UA portion of this research was funded by the U.S. Army Research Office and the National Science Foundation.

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Nanofabrication Tools May Make Silicon Optical Chips More Accessible

Silicon optical chips are critical to the Air Force because of their size, weight, power, rapid cycle time, program risk reduction and the improvements they can offer in data communications, lasers and detectors.

The Air Force Office of Scientific Research is funding this effort in silicon photonics called"Optoelectronic Systems Integration in Silicon" at UW's Nanophotonics Lab in Seattle. OpSIS is hosted by UW's Institute for Photonic Integration. Hochberg is in charge of the OpSIS research program, the Nanophotonics Lab and the Institute for Photonic Integration.

Hochberg emphasizes that the funding from the Air Force Research Laboratory and AFOSR is a critical component in getting the effort off the ground because it provides both a strong technical validation, and the resources to get started on the project.

Unlike most research groups that are designing, building and testing silicon photonic devices or optical chips in-house rather than by using commercial chip fabrication facilities, the UW researchers are using shared infrastructure at the foundry at BAE Systems in Manassas, Virginia. There they are working toward creating high-end, on-shore manufacturing capabilities that will be ultimately made available to the wider community. In the past few years, complex photonic circuitry has not been accessible to researchers because of the expense and a lack of standard processes.

The UW researchers are working on system design and validation so they can imitate what's been done in electronics by stabilizing and characterizing some processes so that the transition from photonics to systems can be smooth.

"The OpSIS program will help advance the field of silicon photonics by bringing prototyping capability within reach of startup companies and researchers," said Hochberg."They will provide design rules, device design support and design-flow development so that even non-experts will be able to design and integrate photonics and electronics."

Silicon photonics has developed over the last decade, and the transition from using devices to systems is something that has only recently occurred.

"The digital electronics revolution over the past 40 years has had a transformative effect on how the Air Force systems are built, and we're hoping to have a similar impact on photonic systems," he said.

The researchers' current goal is to work first on test runs for the new optical chips for commercial uses and on developing some software tools that will make the design process easier.

AFOSR program manager, Dr. Gernot S. Pomrenke, agrees with Prof. Hochberg."Integrating silicon photonics will impact Air Force, DoD and commercial avionics," he said."AFRL has been a leader in developing and supporting this technology over the last two decades and the OpSIS program will help in transitioning silicon photonics into new system capabilities."

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New Generation of Optical Integrated Devices for Future Quantum Computers

Quantum computers, holding the great promise of tremendous computational power for particular tasks, have been the goal of worldwide efforts by scientists for several years. Tremendous advances have been made but there is still a long way to go.

Building a quantum computer will require a large number of interconnected components -- gates -- which work in a similar way to the microprocessors in current personal computers. Currently, most quantum gates are large structures and the bulky nature of these devices prevents scalability to the large and complex circuits required for practical applications.

Recently, the researchers from the University of Bristol's Centre for Quantum Photonics showed, in several important breakthroughs, that quantum information can be manipulated with integrated photonic circuits. Such circuits are compact (enabling scalability) and stable (with low noise) and could lead in the near future to mass production of chips for quantum computers.

Now the team, in collaboration with Dr Terry Rudolph at Imperial College, London, shows a new class of integrated divides that promise further reduction in the number of components that will be used for building future quantum circuits.

These devices, based on optical multimode interference (and therefore often called MMIs) have been widely employed in classical optics as they are compact and very robust to fabrication tolerances."While building a complex quantum network requires a large number of basic components, MMIs can often enable the implementation with much fewer resources," said Alberto Peruzzo, PhD student working on the experiment.

Until now it was not clear how these devices would work in the quantum regime. Bristol researchers have demonstrated that MMIs can perform quantum interference at the high fidelity required.

Scientists will now be able to implement more compact photonics circuits for quantum computing. MMIs can generate large entangled states, at the heart of the exponential speedup promised by quantum computing.

"Applications will range from new circuits for quantum computation to ultra precise measurement and secure quantum communication," said Professor Jeremy O'Brien, director of the Centre for Quantum Photonics.

The team now plans to build new sophisticated circuits for quantum computation and quantum metrology using MMI devices.

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