Introducing plug-and-play nanoelectromechanical systems (NEMS)

Engineerblogger
March 8, 2012

Silicon nitride beam flanked by two gold electrodes.  Schematic illustration of the 55-µm long silicon nitride beam (green) flanked by two gold electrodes (yellow). Artwork by Christoph Hohmann, Nanosystems Initiative Munich (NIM).

The measurement of very low concentrations of various agents plays an important role in medicine, pharmacology and food technology. So-called “nanomechanical resonators” – vibrating nanostrings – represent promising candidates for suitable detectors, because their oscillating motion is extremely sensitive to the binding of substances of interest. In recent years scientists have refined these techniques to the point where single atoms can now be detected. These analyses, however, have their shortcomings. They tend to be time-consuming, require expensive instrumentation and frequently operate only at temperatures near absolute zero. Recently, a group of physicists at the LMU developed a compact sensor architecture on the nanometer scale, which is easy to handle and works at room temperature.

The group is led by Dr. Eva Weig, who is also a member of the Nanosystems Initiative Munich (NIM). The new work builds on their initial demonstration of an efficient electrical interface for nanomechanical resonators which was published in Nature in 2009. They now describe a fully integrated nanomechanical sensor platform that permits robust and sensitive detection of tiny displacements.

The most important part of the nanosensor is a thin beam of highly stressed silicon nitride, about 50 micrometers in length and 200 nanometers wide, suspended between two silica supports. The large pre-stress on this “nano guitar string” allows one to drive its resonant motion with low excitation energy and gives rise to a high mechanical quality factor. The beam is flanked on each side by slightly elevated, parallel gold electrodes. An electric voltage is applied to the two gold electrodes, which act as a capacitor. The resulting electric field couples to the resonator. In the preceding 2009 Nature publication, this effect was employed to control and drive the vibration of the beam. In the new work, it is utilized to sense its motion. The measurement scheme is based on a simple effect: when the nanobeam oscillates up and down within the electric field, the capacitance between the two electrodes varies slightly. In order to pick up this tiny signal, the scientists devised an elegant extension of the existing setup. They incorporated a so-called microwave cavity into the design, which allows them to detect even the thermal motion of the suspended nanobeam.
The microwave cavity can be described as an electrical circuit formed by an inductor and a capacitor, which is connected to the gold electrodes. It is powered by a microwave signal and transmits the combined response of nanobeam and microwave cavity. This effectively allows one to employ the microwave cavity as an amplifier to enhance the signal generated by the moving nanoresonator. The measurement scheme combines two major advantages. Besides considerably enhancing the sensitivity, the microwave cavity can be easily connected to a whole set of nanobeams, which dramatically simplifies operation.

“This will enable the development of highly integrated sensors in the future,” says Thomas Faust, who is first author of the publication. In addition, the scientists have also demonstrated a back-action of the microwave cavity field on the oscillation of the nanomechanical resonator. In this way it is possible to directly drive the resonator motion into self-oscillation and to narrow the width of the peak down to only a few Hz. This offers a means of further enhancing the sensitivity of any future sensor. Furthermore, this latest version of the device is much easier to utilize than other existing solutions. “You only need to connect two cables and, in principle, you can obtain the read-out from thousands of resonators at the touch of a button.” explains Eva Weig. Because the system is simple to operate and is not susceptible to external influences, the new method should be suitable for use even under the non-ideal conditions found outside physics labs. (bige, NIM)

The work has been funded by the German Research Foundation (DFG) and the FET-Open project QNEMS of the European Commission.

Source:  Nanosystems Initiative Munich

Additional Information:

• Microwave cavity-enhanced transduction for plug and play nanomechanics at room temperature. T. Faust, P. Krenn, S. Manus, J.P. Kotthaus, and E.M. Weig. Nature Communications

NIST Measurements May Help Optimize Organic Solar Cells

Engineerblogger
March 8, 2012

Light that strikes this organic solar cell causes electrons to flow between its layers, creating an electric current. Measurements made by the NIST/NRL research team determined the best thickness for the layers, a finding that could help optimize the cells performance.  Credit: NIST

Organic solar cells may be a step closer to market because of measurements taken at the National Institute of Standards and Technology (NIST) and the U.S. Naval Research Laboratory (NRL), where a team of scientists has developed a better fundamental understanding of how to optimize the cells’ performance.

Prototype solar cells made of organic materials currently lag far behind conventional silicon-based photovoltaic cells in terms of electricity output. But if even reasonably efficient organic cells can be developed, they would have distinct advantages of their own: They would cost far less to produce than conventional cells, could cover larger areas, and conceivably could be recycled far more easily.

The cells the team studied are made by stacking up hundreds of thin layers that alternate between two different organic materials—zinc pthalocyanine and C60, the soccer-ball shaped carbon molecules sometimes called buckminsterfullerenes, or “buckyballs.” Light that strikes this multilayered film excites all its layers from top to bottom, causing them to give up electrons that flow between the buckyball and pthalocyanine layers, creating an electric current.

Each layer is only a few nanometers thick, and varying their thickness has a dramatic effect on how much electrical current the overall cell puts out. According to NIST chemist Ted Heilweil, determining the ideal thickness of the layers is crucial to making the best-performing cells.

“In essence, if the layers are too thin, they don’t generate enough electrons for a substantial current to flow, but if they are too thick, many of the electrons get trapped in the individual layers,” says Heilweil. “We wanted to find the sweet spot.”

Finding that “sweet spot” involved exploring the relationship between layer thickness and two different aspects of the material. When light strikes the film, the layers generate an initial “spike” in current that then decays fairly quickly; the ideal cell would generate electrons as steadily as possible. Changing the layer thickness affects the initial decay rate, but it also affects the overall capacity of the material to carry electrons, so the team wanted to find the optimum combination of these two factors.

Paul Lane of NRL grew a number of films that had layers of different thickness, and the team made measurements at both labs that took the two factors into account, finding that layers of roughly two nanometers thick give the best performance. Heilweil says the results encourage him to think prototype cells based on this geometry can be optimized, though one engineering hurdle remains: finding the best way to get the electricity out.

“It’s still unclear how to best incorporate such thin nanolayers in devices,” he says. “We hope to challenge engineers who can help us with that part.”

Source: NIST

Additional Information:

• P.A. Lane, P.D. Cunningham, J.S. Melinger, G.P. Kushto, O. Esenturk and E.J. Heilweil. Photoexcitation dynamics in films of C60 and Zn-Phthalocyanine with a layered nanostructure. Physical Review Letters, DOI: 10.1103/PhysRevLett.108.077402. Published 15 Feb 2012.

Exotic Material Shows Promise as Flexible, Transparent Electrode

Engineerblogger
March 8, 2012

An array of microcircuits made of a 10-nanometer-thick film of bismuth sulfide, an exotic material called a topological insulator, on an insulating mica substrate can be flexed without damaging its electrical properties.

Photo by Hailin Peng, Peking University.

An international team of scientists with roots at SLAC and Stanford has shown that ultra-thin sheets of an exotic material remain transparent and highly conductive even after being deeply flexed 1,000 times and folded and creased like a piece of paper.

The result could open this class of unusual materials, called topological insulators, to its first practical applications: flexible, transparent electrodes for solar cells, sensors and optical communications devices.

“It’s rare for a good conductor to be both transparent and durable as well,” said Zhi-Xun Shen of SLAC and Stanford’s Institute for Materials and Energy Sciences (SIMES).

Researchers led by Shen, Zhongfan Liu and Hailin Peng of Peking University in China, and Yulin Chen of Oxford University in England published their results last week in Nature Chemistry. Until recently, Peng and Chen were graduate students and postdoctoral researchers at Stanford and SIMES. They have continued to collaborate with Shen’s research team after being named professors at their current universities.

The researchers made and tested samples of a compound in which sheets of bismuth and selenium, each just one atom thick, alternate to form five-layer units. The bonds between the units are weak, allowing the overall material to flex while retaining its durability. And as a topological insulator – a new state of quantum matter – the material conducts electricity only on its surface while its interior remains insulating, an unexpected property with unknown potential for fundamental research and practical applications.

Since surface atoms dominate the structure of bismuth selenide, it is an exceptionally good electrical conductor – as good as gold. Unlike gold, however, bismuth selenide is transparent to infrared light, which we know as heat. While about half the solar energy that hits the Earth comes in the form of infrared light, few of today’s solar cells are able to collect it. The transparent electrodes on the surfaces of most cells are either too fragile or not transparent or conducting enough. The new material could get around that problem and allow cells to harvest more of the sun’s spectrum of wavelengths.

The researchers’ experiments also showed that bismuth selenide does not degrade significantly in humid environments or when exposed to oxygen treatments that are common in manufacturing.

“In addition to being a scientific success,” Chen said, “this demonstration should alert engineers and companies that topological insulators can also be important commercially.”

Peng added, “Infrared light pulses carry phone calls and data through optical fiber networks, so bismuth selenide may be useful in communications devices. This material could also improve infrared sensors common in scientific equipment and aerospace systems.”

Peng and colleagues made the bismuth selenide samples and conducted the flexing, conductivity and transparency tests in China. The researchers confirmed that the samples were topological insulators at the Stanford Synchrotron Radiation Lightsource’s Beam Line 5-4 at SLAC.

Theorists first proposed topological insulators in 2004, and experimentalists made the first examples, using mercury telluride at very low temperatures, two years later. Guided by theory, Chen, Shen and colleagues proved in 2009 that cheaper, more abundant and easier-to-handle bismuth telluride and similar compounds containing antimony and selenium are topological insulators at room temperature. Also in 2009, Peng, Shen and colleagues discovered important electrical conduction behavior in bismuth selenide nanoribbons.





Source:  SLAC National Accelerator Laboratory

Graphene Battery Turns Ambient Heat Into Electric Current

Engineerblogger
March 7, 2012

Credit: Technology Review

Here's an interesting idea for a battery. The thermal velocity of ions in aqueous solution is huge--hundreds of metres per second at room temperature. And yet few people have studied this process or its potential to generate current.

Step forward Zihan Xu at The Hong Kong Polytechnic University and a few buddies who have not only studied this process but seemingly mastered it too.

These guys have created a circuit consisting of an LED connected to a strip of graphene by some wire. They simply placed the graphene in a solution of copper chloride and watched. Sure enough, the LED lights up. (Actually, they needed six of these graphene circuits in series to generate the 2V needed to make the LED light up but you get the picture.)

Here's what's going on, according to Zihan and co. The copper ions, which have a double positive charge, move through the solution at a rate of about 300 metres per second thanks to the thermal energy of the solution at room temperature.

When an ion smashes into the graphene strip, the collision generates enough energy to kick a delocalised electron out of the graphene.

The electron then has two options: it can either leave the graphene strip and combine with the copper ion or it can travel through the graphene strip and into the circuit.

It turns out that the mobility of electrons is much higher in graphene than it is through the solution, so the electron naturally chooses the route through the circuit. It is this that lights up the LED.

"The released electrons prefer to travel across the graphene surface...instead of going into the electrolyte solution. That is how the voltage was produced by our device," say Zihan and co.

So the energy generated by this device comes from ambient heat. These guys say there were able to increase the current by heating the solution and also by accelerating the copper ions with ultrasound. They even claim to have kept their graphene battery running for 20 days on nothing but ambient heat.

But there's an important question mark. One alternative hypothesis is that some kind of chemical reaction is generating the current, just as in an ordinary battery.

However, Zihan and co say they ruled this out with a couple of control experiments. However, these are described in some supplementary material that they do not appear to have put on the arXiv. They'll need to make this available before others will take the claim seriously, of course.

Taken at face value, however, this looks to be a hugely important result. Others have generated current in graphene simply by passing moving water over it, so it's not really a surprise that moving ions can do the job as well.

It raises the prospect of clean, green batteries powered by nothing but ambient heat. As Zihan and co modestly put it: "it represents a huge breakthrough for the research of self-powered technology".

Let's hope they're right. But for the moment at least, the jury must remain undecided.

Source: Technology Review

Additional Information:

•  To read the abstract"Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment" in the Cornell University Library.

Nanotrees harvest the sun's energy to turn water into hydrogen fuel

Engineerblogger
March 7, 2012

Schematic shows the light trapping effect in nanowire arrays. Photons on are bounced between single nanowires and eventually absorbed by them (R). By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts (L) where light is simply reflected off the surface. Image Credit: Wang Research Group, UC San Diego Jacobs School of Engineering.

University of California, San Diego electrical engineers are building a forest of tiny nanowire trees in order to cleanly capture solar energy without using fossil fuels and harvest it for hydrogen fuel generation. Reporting in the journal Nanoscale, the team said nanowires, which are made from abundant natural materials like silicon and zinc oxide, also offer a cheap way to deliver hydrogen fuel on a mass scale.

“This is a clean way to generate clean fuel,” said Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.

The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, according to Wang. That’s because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it, Wang said, adding that it is also similar to retinal photoreceptor cells in the human eye. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.

Wang’s team has mimicked this structure in their “3D branched nanowire array” which uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to be used as fuel. This process uses clean energy with no green-house gas byproduct. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels

“Hydrogen is considered to be clean fuel compared to fossil fuel because there is no carbon emission, but the hydrogen currently used is not generated cleanly,” said Ke Sun, a PhD student in electrical engineering who led the project.

By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts. Wang is also affiliated with the California Institute of Telecommunications and Information Technology and the Material Science and Engineering Program at UC San Diego.

The vertical branch structure also maximizes hydrogen gas output, said Sun. For example, on the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, very small gas bubbles of hydrogen can be extracted much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” said Sun.

In the long run, what Wang’s team is aiming for is even bigger: artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.

In this experiment, nanotree electrodes are submersed in water and illuminated by simulated sun light to measure electricity output of the device. Photo Credit: Joshua Knoff, UC San Diego Jacobs School of Engineering.

“We are trying to mimic what the plant does to convert sunlight to energy,” said Sun. “We are hoping in the near future our ‘nanotree’ structure can eventually be part of an efficient device that functions like a real tree for photosynthesis."

The team is also studying alternatives to zinc oxide, which absorbs the sun’s ultraviolet light, but has stability issues that affect the lifetime usage of the nanotree structure.

Source: University of California, San Diego

The World's First Sterilizable Flexible Organic Transistor

Engineerblogger
March 7, 2012

Figure 1: A highly thermostable organic transistor manufactured on a thin plastic film. The team succeeded in building a low drive-voltage and a high thermostable organic circuit on a plastic film by using SAM molecule for the gate insulator, and high heat resistant semiconductors for semiconductor layer.

An international research team has succeeded in manufacturing on a polymeric film the world’s first flexible organic transistor that is robust enough under high temperature medical sterilization process. The study published online in Nature Communications on March 6, 2012.

In a serious aging society with a declining birthrate, electronics are increasing their importance in the health and medical area as more IT devices are being introduced. Upon this background, an expectation is getting higher on an organic transistor, which is a soft electronic switch. A flexible organic transistor can easily be manufactured on a biocompatible polymeric film, and this is the reason why it is expected to adopt it to a wearable health monitor without a stress, and/or implantable devices such as a soft pace maker. For practical implementation, it is crucial (1) to make the best use of its softness and biocompatibility, simultaneously (2) to decrease driving voltage down to a few V, and (3) to decrease the risk of infections by sterilization, for a security reason. Up until now, however, the existing organic transistors had huge obstacles towards the practical usage in the health and medical field. For example, typical driving voltage for displays is high (i.e. 20 to 80 V) and/or and it is not durable under high temperature sterilization.

The team has succeeded in manufacturing on a polymeric film an organic transistor that has high thermal stability and driving voltage of 2V at the same time. The new type organic transistor can be sterilized in a standard sterilization process (150 °C heat treatment) without being deteriorated in its electrical performances. The key to realize heat resistant organic transistor is in the forming technique of an ultrathin insulator film: The team develops a technique to form extraordinarily densely packed self-assembled monolayer (SAM) films, whose thickness is as small as 2 nanometers, on a polymeric film. This allows them to elevate substrate temperature up to 150 °C without creating pinholes through SAM films during the high temperature treatment. It is believed that ultrathin monolayer film like SAM degrades easily by thermal processes; however, it is unexpectedly demonstrated that densely packed SAM is stable at 150 °C or higher. This result is also proved by systematic characterization of crystallographic structures of SAM using a synchrotron radiation beam. Furthermore, by adopting a novel encapsulation layer comprising organic/metal composite materials and extremely thermally stable and high mobility organic semiconductors, the thermal stability of organic transistors is now improved up to 150 °C.

It should be benefited more from applying this heat-resistant organic transistor to long term implantable devices, or to some medical devices such as a smart catheter. With these applications, it is expected to broaden the usage of the transistor to medical apparatus such as thin film sensor that will detect tumors, inflammations, and or cancers.

The international team is led by Dr. Takao Someya, who is a professor of the University of Tokyo (President: Jyunichi Hamada, Ph.D.), a research director of ERATO (Exploratory Research for Advanced Technology) “Someya Bio-Harmonized Electronics Project” of Japan Science and Technology Agency (JST, President: Michiharu Nnakamura, D.Sc.), and a global scholar of Princeton University (President: Shirley M. Tilghman, Ph.D.), in collaborations with Associate Professor Tsuyoshi Sekitani of the University of Tokyo and Professor Yueh-Lin (Lynn) Loo of Princeton University. This joint research project was also carried out with the following institutions: Max Planck Institute for Solid State Research, Germany, National Institute of Standards and Technology, NIST, U.S., Hiroshima University, and Nippon Kayaku Co., Japan.

Background

In consequence of a serious declining birthrate and a growing proportion of elderly, information technology (IT) devices are rapidly introduced in the health and medical area. One of the good examples is the internet connection of a healthcare device between a patient’s home and a hospital. The internet allowed a doctor to monitor patience’s heart rates and weights away from his/her home. The miniaturization of medical apparatuses such as endoscopes succeeded in minimizing patients’ burdens and/or invasiveness. In this way, in the medical and the healthcare field, electronics are increasing their importance. Indeed, in the health and medical market, electronics are expected to grow 120% every year successively until 2015.

In this background, an organic transistor, which is a flexible electronic switch, attracts much attention because it is easily manufactured on a biocompatible polymeric film. A biocompatible organic transistor would be suitable for applications to a stress free wearable health monitoring system and implantable devices such as a soft pacemaker. For practical implementation, it is crucial (1) to make the best use of its softness and biocompatibility, simultaneously (2) to decrease driving voltage down to a few V, and (3) to decrease the risk of infections by sterilization, for a security reason. Up until now, however, the existing organic transistors had huge obstacles towards the practical usage in the health and medical field. For example, typical driving voltage for displays is high (i.e. 20 to 80 V) and/or and it is not durable under high temperature sterilization.

Results in details

The team has succeeded in manufacturing on a polymeric film an organic transistor that has world’s first 150 °C thermostability and simultaneously its driving voltage of 2V. The keys to realize the heat resistant organic transistor are (1) self-assembled monolayer (SAM) and (2) a sealing film, which are to be discussed later. The highly thermal stability that we had realized exploded the typical theory that an ultrathin monolayer film of nanometers in size was easily affected by heat. This result was also proved by the systematic analysis of precise crystallographic characterizations using a synchrotron radiation beam, which will be described in (3) in detail. Furthermore, the organic transistor has successfully been sterilized under a standard sterilization process (150 °C heat treatment) without being electrically deteriorated. This will be discussed in (4).

(1) Highly thermostable self-assembled monolayer (SAM) gate insulator

A key technology towards the development of sterilizable organic transistor is the 2-nm-thick ultrathin self-assembled monolayer (SAM) film. To reduce a thickness of a gate insulator film is known as the effective way to reduce the driving voltage of an organic transistor. From the security reasons, it is necessary to thin down a gate insulator film to a few nanometers thickness in order to reduce the driving voltage down to 2V. The team employed SAM film for a gate insulator in the past. They attempted to optimize manufacturing process of SAM from heat resistance point of view. As a result, by substantially improving crystalline ordering of densely packed SAM films on a polymeric film, they succeed in forming an insulator film that does not create pinholes, the cause of a leakage current, even under a high heat treatment. This becomes possible by optimizing plasma condition during the shaping process of aluminum-oxide thin films on top of the polymeric film, resulting in a way to avoid the film from being damaged during a plasma process.

(2) An encapsulation layer comprising organic and metal composite films

An improvement of thermal stability of a SAM gate insulator is not enough to accomplish the high thermal stability of an organic transistor. Normally, organic semiconductors that compose the channel layer in organic transistor are known to be easily degraded by heat. Thereby, an organic semiconductor, which is carefully chosen among heat resistant materials, is dinaphtho-thieno-thiophene (DNTT) in the experiment. Furthermore, after manufacturing an organic transistor, the transistor is completely covered by a flexible, heat-resistant encapsulation layer comprising organic and metal composite films (Figure 2). The encapsulation layer restrains DNTT from subliming with heat, and it prevents elements from substantial deterioration. Moreover, it is demonstrated that electronic characteristic of organic transistor remains practically unchanged even after dipped in the boiling water.

Figure 2: A schematic device structure (a) and a picture (b) of a thermally stable organic transistor. An organic transistor is covered with a flexible encapsulation layer that has both sealing characteristic as well as thermal stability.

(3) Structural characterization of nanometer-thick films by synchrotron radiation beams

The crystallographic structures of SAM films are examined. To be accurate, the gate insulator film used in the experiment consists of two layers, namely, 4-nm-thick aluminum-oxide and 2-nm-thick self-assembled monolayer. The thermal resistance of aluminum-oxide has been long known; however, there has been no report published on a structural analysis on SAM film, nor a report to prove structural stability of SAM film embedded in the devices at high temperature. This is because of the difficulty in analyzing the structure of such a thin SAM film with single molecular layer thickness using x-ray analysis.

The team attempted to precisely characterize crystallographic structures of a SAM film in order to evaluate the heat resistance of an organic transistor. Note that the thickness of a SAM film is as small as 2 nanometers. By using a synchrotron radiation beam, it is proven, for the very first time, to the best of our knowledge, that crystallographic structure of a SAM film exhibits any deterioration in molecular ordering even at 150 °C or higher temperature. This outcome unexpectedly overthrew what it had been believed that an ultrathin monolayer film of a few nanometers thinness must degrade easily by heat.

The analysis was carried out together with Professor Yueh-Lin (Lynn) Loo from Princeton University and a group at NIST, and a synchrotron radiation beam at Brookhaven National Laboratory is used.

(4) The creation of medical flexible electronics

The high thermostable organic transistors are capable of being sterilized without electrically deteriorated. The team evaluated elements’ heat resistance for three different standard heating sterilization processes that are widely used to sterilize medical apparatuses: they are (1) a heat treatment at a temperature of 150 °C for 20 seconds at atmospheric pressure, (2) a heat treatment at 2 atmospheric pressures, 121 °C for 20 seconds, and (3) a sterilization by boiling.

First, the thermal stability of the manufactured organic transistor is improved by annealing process at 160 °C, which is slightly high than the typical annealing temperature for sterilization. Second, bacteria are cultured on the above mentioned transistor. Finally, the number of bacteria and the electric characteristics are measured before and after the medical sterilization process. As a result, almost all the bacteria died off after the sterilization; however, electrical characteristics of the transistor are practically unchanged (a negligible level).

The team’s development in the past

Unlike the conventional inorganic materials, organic transistors are capable of making lightweight and mechanically flexible electronic devices, since they can be built on polymeric film by a low temperature processing. Organic transistors can be manufactured through printing process as well: This allowed a drastic cost reduction when making large area transistors, compared with those made with silicon. One of the major driving applications for organic transistors is e-paper. Up until now, Someya and his coworkers have intensively investigated the application of organic transistors to large-area sensors or large-area actuators. The team has shown the feasiblity of implementing organic transistors to large area electronics. A series of their achievements include a robot e-skin (2003), a sheet type scanner (2004), an ultrathin braille sheet display (2005), a wireless power transmission sheet (2006), a communication sheet (2007), an ultrasonic sheet (2008), a flash memory (2009).

Recently, organic transistors are longed to be implemented to medical and healthcare devices because of their biocompatibility. However, it is indispensable that those devices are sterilized. Therefore, it has been required that those organic circuits built on plastic films to be stable through heat treatment, and that they are driven with low voltage.

Someya and his coworkers have succeeded in making an organic transistor which stays undeteriorated after heating up to 150 °C in 2004. Though, a thick organic polymer that was used as an insulator film caused the driving voltage to be very high, and it was the reason why it did not suit for bio/medical usage. The team had attempted to build a few nm organic/inorganic materials on a plastic film using a molecular self-assembly, and they have finally proved the feasibility of heat resistance of SAM film for the first time.

In the last year, they invented a new medical electronics called “an intelligent catheter” using flexible organic transistor technique: the new narrow catheter is covered with a pressure sensor network (published in Nature Materials, UK in 2010). It was inevitable to develop a thermostable organic transistor so that the new catheter to be used practically at the hospitals. They finally overcame the barrier.

Outlook for the future

Organic transistors are mechanically flexible and expectedly biocompatible since they are made of soft organic electronic materials such as organic semiconductors. Attractive applications that are expected to be realized by flexible biocompatible organic transistors include “a wearable electronics” which reads out bio-information from outside of a skin, or “an implantable electronics” that directly extracts bio-information by implanting the electronics in a body. Indeed, Someya and his coworker also came up with applying the ultraflexible organic electronics to cover a narrow catheter. This opens a new path to the development of a thin film sensor that detects tumors, inflammations, early cancers. The invention will surely broaden the usage of the organic transistors as medical devices. Since a flexibility, a large coverage, and an electric stability are indispensable for implementation of these medical devices, the present invention will serve as the core technology when developing the future medical devices.

Up to this point, displays and solar cells have been considered as main driving applications of organic devices. Organic EL displays and organic flexible solar cells are implemented rapidly. However, they are only a glimpse of vast potentials that organic devices possess. Indeed, world’s researchers are competing in developing health and medical applications utilizing softness of organic devices. The team has led the field of flexible devices by achieving the world’s smallest minimum bending radius (100 µm). With the feasibility shown with these sterilizable, flexible organic transistors, the contribution will accelerate the researches on the medical applications.

Source: University of Tokyo via Brookhaven National Laboratory

Additional Information:

• The paper " Organic transistors with high thermal stability for medical applications" published online in Nature Communications on March 6, 2012

Researchers Discover New Method of Making Nanoparticles

Engineerblogger
March 6, 2012

Keith Roper, University of Arkansas

An engineering researcher at the University of Arkansas and his colleagues at the University of Utah have discovered a new method of making nanoparticles and nanofilms to be used in developing better electronic devices, biosensors and certain types of high-powered and highly specific microscopes used for scientific research.

The never-ending quest to build faster, more efficient and more reliable electronic devices starts deep down below the molecular level, where nanoparticles – far too small for the human eye to detect – make up the building blocks of the latest processing hardware. In pursuit of this goal, scientists and engineers are constantly investigating new materials and better methods of developing or assembling these materials.

The researchers’ nanoparticles, made of gold and deposited onto silicon substrates by a unique chemical process, are nontoxic and inexpensive to make and have superior dimensions, densities and distribution when compared to other nanoparticles and conventional methods of producing nanoparticles. The unique deposition technique has the further advantage of being able to rapidly coat fragile, three-dimensional and internal surfaces at the temperature and pressure of its surroundings without requiring conductive substrates or expensive, sophisticated equipment.

“Using successive thermal treatments, we characterized optical and structural features of an inexpensive, molecule-to-molecule, bottoms-up approach to create thermally stable, gold-nanoparticle ensembles on silica,” said Keith Roper, associate professor of chemical engineering at the University of Arkansas. “Images and analysis from scanning electron microscopy and atomic force microscopy revealed that particle densities are the highest reported to date. Our method also allows faster preparation than self-assembly or lithography and allows directed assembly of nanoparticle ensembles on 3D surfaces.”

The researchers’ unique approach improves upon a method that involves depositing atoms from a solution onto a substrate with a tin-sensitized surface. The researchers use a novel continuous-deposition process and then heat these deposited atoms to transform “islands” of nanoparticle material into desired forms. The resulting spherical nanoparticles can have diameters between 5 and about 300 nanometers. A nanometer is a billionth of a meter. A human hair typically has a diameter of 70,000 nanometers.

Roper said that microscopic images and spectroscopic data suggest that ultrathin films prepared by their new approach are smoother than conventional “sputtered” or evaporated gold films and may exhibit better optical features, such as reduced surface-roughness scattering. These features are desirable in devices such as photovoltaic cells in which narrow metal layers significantly affect local electromagnetic fields. Smoother thin films also could improve the limits of detection, sensitivity and photocurrent, respectively, in such applications.

The researchers’ recent studies in this area have been published in Langmuir and Journal of Physical Chemistry C, journals of the American Chemical Society. The researchers were awarded U.S. Patent No. 8,097,295 on Jan. 17 for the development.

Roper is holder of the Charles W. Oxford Professorship of Emerging Technologies. He is also assistant director of the graduate program in microelectronics/photonics.

Source:  University of Arkansas

Student Innovation at Rensselaer Polytechnic Institute Could Enable Better, Cheaper Detection of Hazardous Gases

Engineerblogger
March 6, 2012

Credit: RPI

Fazel Yavari has developed a new sensor to detect extremely small quantities of hazardous gases. Made from a 3-D foam of the world’s thinnest material—graphene—this sensor is durable, inexpensive to make, and opens the door to a new generation of gas detectors for use by bomb squads, defense and law enforcement officials, as well as applications in industrial settings.

Yavari, a doctoral student in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer Polytechnic Institute, is one of three finalists for the 2012 $30,000 Lemelson-MIT Rensselaer Student Prize. A public ceremony announcing this year’s winner will be held at 6:45 p.m. on Wednesday, March 7, in the auditorium of the Rensselaer Center for Biotechnology and Interdisciplinary Studies. For more information on the ceremony visit: http://www.eng.rpi.edu/lemelson

Fazel Yavari: Credit: RPI

Yavari’s project is titled “High Sensitivity Detection of Hazardous Gases Using a Graphene Foam Network,” and his faculty adviser is Nikhil Koratkar, professor of mechanical, aerospace, and nuclear engineering at Rensselaer.

Detecting trace amounts of hazardous gases present within air is a critical safety and health consideration in many different situations, from industrial manufacturing and chemical processing to bomb detection and environmental monitoring. Conventional gas sensors are either too bulky and expensive, which limits their use in many applications, or they are not sensitive enough to detect trace amounts of gases. Also, many commercial sensors require very high temperatures in order to adequately detect gases, and in turn require large amounts of power.

Researchers have long sought to leverage the power of nanomaterials for gas detection. Individual nanostructures like graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire fence, are extremely sensitive to chemical changes. However, creating a device based on a single nanostructure is costly, highly complex, and the resulting devices are extremely fragile, prone to failure, and offer inconsistent readings.

Yavari has overcome these hurdles and created a device that combines the high sensitivity of a nanostructured material with the durability, low price, and ease of use of a macroscopic device. His new graphene foam sensor, about the size of a postage stamp and as thick as felt, works at room temperature, is considerably less expensive to make, and still very sensitive to tiny amounts of gases. The sensor works by reading the changes in the graphene foam’s electrical conductivity as it encounters gas particles and they stick to the foam’s surface. Another benefit of the Yavari’s device is its ability to quickly and easily remove these stuck chemicals by applying a small electric current.

The new graphene foam sensor has been engineered to detect the gases ammonia and nitrogen dioxide, but can be configured to work with other gases as well. Ammonia detection is important as the gas is commonly used in industrial processes, and ammonia is a byproduct of several explosives. Nitrogen dioxide is also a byproduct of several explosives, as well as a closely monitored pollutant found in combustion exhaust and auto emissions. Yavari’s sensor can detect both gases in quantities as small as 0.5 parts-per-million at room temperature.

When he’s not studying or working in the lab, Yavari likes to keep active by playing tennis, cycling, or skiing. He also enjoys making time to travel around the United States and overseas. At home in Isfahan, Iran, Yavari’s parents are both high school teachers. They encouraged him as a child to study math and science, and today they are very proud of his accomplishments and cheering for him to win the $30,000 Lemelson-MIT Rensselaer Student Prize.

Yavari received his bachelor’s degree in mechanical engineering from Shahrekord University in Iran, and his master’s degrees in mechanical engineering from the University of Tehran.

After earning his doctoral degree later this year, Yavari plans to continue conducting research either in academia or the private sector.

About the $30,000 Lemelson-MIT Rensselaer Student Prize
The $30,000 Lemelson-MIT Rensselaer Student Prize is funded through a partnership with the Lemelson-MIT Program, which has awarded the $30,000 Lemelson-MIT Student Prize to outstanding student inventors at MIT since 1995.

ABOUT THE LEMELSON-MIT PROGRAMCelebrating innovation, inspiring youth

The Lemelson-MIT Program celebrates outstanding innovators and inspires young people to pursue creative lives and careers through invention.

Jerome H. Lemelson, one of U.S. history’s most prolific inventors, and his wife, Dorothy, founded the Lemelson-MIT Program at the Massachusetts Institute of Technology in 1994. It is funded by The Lemelson Foundation and administered by the School of Engineering. The Foundation sparks, sustains, and celebrates innovation and the inventive spirit. It supports projects in the U.S. and developing countries that nurture innovators and unleash invention to advance economic, social, and environmentally sustainable development. To date The Lemelson Foundation has donated or committed more than U.S. $150 million in support of its mission.

Source: Rensselaer Polytechnic Institute (RPI)


Additional Information:

• The report "High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network" in the Nature Scientific Report.

Engineer discovers spider silk conducts heat as well as metals

Engineeerblogger
March 6, 2012

Xinwei Wang, Guoqing Liu and Xiaopeng Huang, left to right, show the instruments they used to study the thermal conductivity of spider silk. Photo by Bob Elbert.

Xinwei Wang had a hunch that spider webs were worth a much closer look.

So he ordered eight spiders - Nephila clavipes, golden silk orbweavers - and put them to work eating crickets and spinning webs in the cages he set up in an Iowa State University greenhouse.

Wang, an associate professor of mechanical engineering at Iowa State, studies thermal conductivity, the ability of materials to conduct heat. He's been looking for organic materials that can effectively transfer heat. It's something diamonds, copper and aluminum are very good at; most materials from living things aren't very good at all.

But spider silk has some interesting properties: it's very strong, very stretchy, only 4 microns thick (human hair is about 60 microns) and, according to some speculation, could be a good conductor of heat. But nobody had actually tested spider silk for its thermal conductivity.

And so Wang, with partial support from the Army Research Office and the National Science Foundation, decided to try some lab experiments. Xiaopeng Huang, a post-doctoral research associate in mechanical engineering; and Guoqing Liu, a doctoral student in mechanical engineering, helped with the project.

"I think we tried the right material," Wang said of the results.

What Wang and his research team found was that spider silks - particularly the draglines that anchor webs in place - conduct heat better than most materials, including very good conductors such as silicon, aluminum and pure iron. Spider silk also conducts heat 1,000 times better than woven silkworm silk and 800 times better than other organic tissues.

A paper about the discovery - "New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and its Abnormal Change under Stretching" - has just been published online by the journal Advanced Materials.

"Our discoveries will revolutionize the conventional thought on the low thermal conductivity of biological materials," Wang wrote in the paper.

The paper reports that using laboratory techniques developed by Wang - "this takes time and patience" - spider silk conducts heat at the rate of 416 watts per meter Kelvin. Copper measures 401. And skin tissues measure .6.

"This is very surprising because spider silk is organic material," Wang said. "For organic material, this is the highest ever. There are only a few materials higher - silver and diamond."

Even more surprising, he said, is when spider silk is stretched, thermal conductivity also goes up. Wang said stretching spider silk to its 20 percent limit also increases conductivity by 20 percent. Most materials lose thermal conductivity when they're stretched.

That discovery "opens a door for soft materials to be another option for thermal conductivity tuning," Wang wrote in the paper.

And that could lead to spider silk helping to create flexible, heat-dissipating parts for electronics, better clothes for hot weather, bandages that don't trap heat and many other everyday applications.

What is it about spider silk that gives it these unusual heat-carrying properties?

Wang said it's all about the defect-free molecular structure of spider silk, including proteins that contain nanocrystals and the spring-shaped structures connecting the proteins. He said more research needs to be done to fully understand spider silk's heat-conducting abilities.

Wang is also wondering if spider silk can be modified in ways that enhance its thermal conductivity. He said the researchers' preliminary results are very promising.

And then Wang marveled at what he's learning about spider webs, everything from spider care to web unraveling techniques to the different silks within a single web. All that has one colleague calling him Iowa State's Spiderman.

"I've been doing thermal transport for many years," Wang said. "This is the most exciting thing, what I'm doing right now."

Source: Iowa State University

Additional Information:

• The paper "New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and its Abnormal Change under Stretching" has just been published online by the journal Advanced Materials.

Related Articles:

• A Spider Web's Strength Lies in More Than its Silk 
• Artificial Spider Silk Could Be Used for Armor, More 
• Cow Used in Man-Made Spider Web

Energy Squeeze: Squeezing polymers produces chemical energy

Engineerblogger
March 5, 2012

Bartosz A. Grzybowski

A polymer is a mesh of chains, which slowly break over time due to the pressure from ordinary wear and tear. When a polymer is squeezed, the pressure breaks chemical bonds and produces free radicals: ions with unpaired electrons, full of untapped energy. These molecules are responsible for aging, DNA damage and cancer in the human body.

In a new study, Northwestern University scientists turned to squeezed polymers and free radicals in a search for new energy sources. They found incredible promise but also some real problems. Their report is published by the journal Angewandte Chemie.

The researchers demonstrated that radicals from compressed polymers generate significant amounts of energy that can be used to power chemical reactions in water. This energy has typically been unused but now can be harnessed when polymers are under stress in ordinary circumstances -- as in shoe soles, car tires or when compacting plastic bags.

They also discovered during the study that a silicone polymer commonly used in implants for cosmetic procedures releases a large quantity of harmful free radicals when the polymer is under only a moderate amount of pressure. These findings suggest the safety of certain polymer-based medical implants should be looked at more closely.

“We have established that polymers under stress create free radicals with overall efficiencies of up to 30 percent and shoot the radicals out into the surrounding medium where they can drive chemical reactions,” said Bartosz A. Grzybowski, an author of the paper and the Kenneth Burgess Professor of Physical Chemistry and Chemical Systems Engineering. “These radicals can be useful or they can be harmful, depending on the situation.”

Grzybowski and his team are the first to use this energy to drive chemical reactions by simply surrounding the compressed polymer with water containing desired reagents.

The radicals created in the polymer migrate toward the polymer/water interface where they produce hydrogen peroxide, which then can drive chemical processes.

“You can get a surprisingly large amount of chemical energy from a polymer under compression,” Grzybowski said. “This energy is, in a sense, free for the taking. Under normal circumstances, the energy is virtually never retrieved from deformed polymers, which then age unproductively. But you could recharge a battery from the energy produced by walking or driving a car. And you could capture even more energy when compacting millions of plastic bags.”

Grzybowski is also director of Northwestern’s Non-Equilibrium Energy Research Center, which is funded by the U.S. Department of Energy.

“We are interested in new sources of chemical energy, and this energy from the simple breaking of polymers’ bonds is not being used,” he said. “By surrounding the polymer with a medium, such as water, we can produce environmentally friendly chemical energy. One direction we are pursuing is to use this energy to sanitize water in developing countries. This is possible because hydrogen peroxide produced by squeezed polymers kills bacteria.”

The researchers confirmed that mechanical deformation -- moderate squeezing -- created free radicals in the polymers. They also determined the number of radicals produced in a polymer under pressure is approximately 1016 (10 to the 16th) radicals per cubic centimeter of polymer -- a substantial amount.

They next filled polymer tubes with water, squeezed the tubes and measured the total number of radicals that migrated into the surrounding solution. They found that nearly 80 percent of the radicals made the trip.

Grzybowski and his team demonstrated they can squeeze a polymer, such as what might be found in a shoe, tire or plastic bag, and get a mechanical-to-chemical energy conversion of up to 30 percent -- approaching the energy efficiency of a car engine.

The hydrogen peroxide produced when a polymer surrounded by water is squeezed can power a variety of chemical reactions, including fluorescence, nanoparticle synthesis and dye bleaching, the researchers showed.

To illustrate the process, they converted a Nike Air LeBron shoe into a “lightning shoe,” where the air pockets in the polymeric sole are filled with a solution of a compound that lights up in the presence of radicals. After a person walked in the shoe for 30 minutes or more, enough radicals were created to generate a blue glow visible to the naked eye.

The researchers studied seven different polymers, including a number of particular public interest. Poly(dimethylsiloxane), a silicon-based material commonly used in medical implants, was one of them. In the lab experiments, the medium surrounding the polymer and the amount of pressure exerted on the material were similar to what would be found in the human body, Grzybowski pointed out.

“Our findings are somewhat worrisome since every polymeric implant in the human body experiences mechanical stresses and, as we now know, can produce harmful free radicals and liberate them into surrounding tissues, which may contribute to diseases such as cancer, stroke, myocardial infarction, diabetes and other major disorders,” Grzybowski said. “With this knowledge, I am quite happy to have a metal implant in my knee, rather than a polymer implant.

“From a scientific perspective, our work proves yet again that a phenomenon can be useful or harmful depending on how we implement it,” he said. “The same polymer can be a useful source of energy when outside of a human body, yet a potential risk hazard when implanted into it.”

The U.S. Department of Energy funded the research.

Source: Northwestern University

Additional Information:

• The title of the paper is “Mechanoradicals Created in ‘Polymeric Sponges’ Drive Reactions in Aqueous Media.” In addition to Grzybowski, other authors of the paper are H. Tarik Baytekin and Bilge Baytekin.
  

An electrical switch for magnetic current

Engineerblogger
March 2, 2012

View of a ferroelectric tunnel junction: This image created by the atomic force microscope shows the extremely regular structure of the ferroelectric lead zirconate titanite layer. The yellow bumps are the ferromagnetic cobalt electrodes. Each tunnel junction can be targeted via the cobalt electrodes.

© Marin Alexe / MPI of Microstructure Physics

A new mechanism will make it possible to switch data storage in the future. Researchers at the Max Planck Institute of Microstructure Physics in Halle use a short electric pulse to change the magnetic transport properties of a material sandwich consisting of a ferroelectric layer between two ferromagnetic materials. It could be assumed that an electric pulse only influences the electric transport properties. With the help of the new switching mechanism, information can be placed in four instead of just two states of a storage point. This consequently increases storage density. This mechanism may also prove useful in spintronics. This type of electronics should be particularly efficient at processing data, as it does not just utilise the electrons’ charge, but also their spin, which could be regarded as their own spin momentum.

Such behaviour would have quite curious effects for a light switch: In the case of a lamp dimmer switch, the connection, which the physicists at the Max Planck Institute of Microstructure Physics in Halle have discovered, would not only cause the light to change brightness, but also to change colour – from green to red, for instance. Although both properties are characteristics of light, they cannot be manipulated with one switch simultaneously. However, the researchers in Halle have now succeeded in doing something similar with the tunnel current that flows between two ferromagnetic electrode layers of a multiferroic material sandwich.

In this case, multiferroic means that the parcel includes the two ferromagnetic substances, as well as a ferroelectric substance. In ferroelectric materials, voltage switches between the two directions of an electric polarisation – depending on its polarity – not unlike when a magnetic field permanently reverses the polarity of a ferromagnet. As ions shift within the material structure during this process, the polarisation remains intact, even after the voltage has been reduced. It is possible, however, to reverse the switch again with a similarly large voltage with reversed polarity.

Changing the direction of the polarisation with electric and magnetic effects

The Halle-based researchers prepared their multiferroic material sandwich by steaming an extremely accurately structured ferromagnetic lanthan strontium manganate (LSMO) layer on a base. The thickness of this layer is just under 30 nanometres − that is one millionth of a millimetre. On top of that, they deposited a layer of ferroelectric lead zirconate titanite (PZT) only three nanometres thick and with a very regular structure; a top layer of ferromagnetic cobalt finished the sandwich.

The physicists then placed the tip of an atomic force microscope over the cobalt cover of the material stack to create voltage at the multiferroic sandwich. Although the non-conductive PZT layer prevents current flow in the traditional sense between cobalt and LSMO, some electrons may overcome the barrier in the quantum-physical tunnel process. The researchers in Halle were interested in precisely these properties of tunnel current.

The strength of this tunnel current depends on the polarisation of the ferroelectric PZT. The polarisation impacts the height of the tunnel barrier, meaning that the tunnel resistance is different for both polarisation directions. The physicists also used electrical voltage to switch between the two polarisation directions. To do this, they applied a voltage pulse, which was considerably stronger than the voltage needed for the tunnel current but lasted less than a millisecond.

“Surprisingly, not only the component of the tunnel junction resistance that depends on the direction of the polarisation, but also the component that usually only depends on the direction of the magnetisation of the electrodes, the so-called tunnel magnetoresistance, changes when reversing the polarity of the ferroelectric fluid,” says Dietrich Hesse, who heads the research team together with Marin Alexe. This tunnel magnetoresistance (TMR) always appears when electrons are tunnelling between two different ferromagnets. It is usually smaller for two ferromagnetic electrodes that are magnetised in the same direction than for two electrodes that are magnetised in opposite directions – in this case, physicists talk of normal tunnel magnetoresistance.

The physicists in Halle actually observed the normal tunnel magnetoresistance in one of the two electric polarisation directions in the ferroelectric PZT layer. In the case of the other electric polarisation direction, however, the conditions surprisingly reversed themselves – now there was inverted TMR. The tunnel connection conducts current with little resistance when both ferromagnets are magnetised in the opposite direction.

Apart from the effect on the electric resistance, the TMR also acts as an electron spin filter. In simple terms, the spin is the direction in which the electrons are spinning; it provides every electron with its own magnetic momentum, which may go one way, then the other. In the simplest case of normal TMR, only electrons whose magnetic momentum is going in the same direction as the magnetisation of the two ferromagnetic electrodes manage to get through the PZT layer. “A change in the electric polarisation direction therefore influences the strength of the tunnel current and also results in electrons with a certain spin being filtered out,” explains Marin Alexe. “We can achieve the same effect by changing the magnetisation in both ferromagnetic layers with an external magnetic field, but this method uses a lot more energy.”

The influence of the electric polarisation on the filter effect of the tunnel junction for electron spins and the electric resistance are interesting for applications, because, all in all, the multiferroic tunnel connection can take four differently sized electric resistances: two for each electric polarisation direction, one for the same magnetisation and one for opposite magnetisation of both ferromagnets. “This enables us to deposit three times as much information in a multiferroic tunnel junction than in ordinary binary magnetic storage,” comments Marin Alexe. This means that the size of magnetic random access memories (MRAM) can be considerably reduced. MRAMs provide an alternative to conventional electrically operated RAMs. They would make it unnecessary to load data from the hard drive to a user memory when booting up a computer, and the device would be ready to use at the push of a button.

Diagram of a multiferroic material sandwich: The orange-red base carries a green layer of ferromagnetic lanthan strontium manganate (LSMO). Above that is the isolating ferroelectric lead zirconate titanite (PZT) layer. The nano condensers are closed by ferromagnetic cobalt electrodes. They get their shape from correspondingly structured shadow masks. In reality, the PZT layer is considerably thinner than the LSMO layer; it is barely three nanometres thick.

© Marin Alexe / MPI of Microstructure Physics

Better understanding to pave the way for spintronics

“As a filter that can be simply switched and sort electrons according to their spin direction, the multiferroic tunnel junction could also find use in spintronics,” says Dietrich Hesse. This could be a possible future development of electronics, which is why many physicists around the world are researching its basic principles. Spintronics uses both the charge and spin of the electrons to process data with a higher density than is possible in conventional electronics.

In order to advance the multiferroic material sandwich as a spin filter, physicists are first striving to understand precisely how a change of the PZT’s electric polarisation direction affects the magnetic tunnel resistance. Up to now, they only know the details of what happens in the case of the electric polarisation direction that goes hand in hand with the normal tunnel magnetoresistance. “We are unable as yet to explain exactly why the inverted tunnel magnetoresistance appears when the polarisation direction is reversed,” comments Dietrich Hesse. One reason could be the interaction between the ferromagnetic cobalt and the adjoining titan ions in the PZT. The latter change their position with the polarisation direction. When they get closer to the cobalt layer, they take on their own magnetic momentum on account of the intensive interaction. This magnetic momentum affects the spin direction of the tunnelling electrons.

To explain this connection in detail, Dietrich Hesse and Marin Alexe asked theoretical physicists at the Institute and at the University of Halle for help. They will now calculate the magnetoelectric coupling due to the interaction between the titan ions of the PZT and the cobalt. However, the help of Dietrich Hesse’s and Marin Alexe’s team is still needed to draw a better comparison between the results of this calculation and the experiments. They are currently trying to steam the cobalt cover of their multiferroic material sandwich with a structure that is as regular as that of the other two layers. “Only if we understand exactly how the magnetoelectric coupling works in multiferroic tunnel junctions can we also use it for electronic applications.”

Source: Max-Planck-Gesellschaft

Graphene-Based Optical Modulators Poised to Break Speed Limits in Digital Communications

Engineerblogger
March 2, 2012

In yet another astounding application of the “wonder material” graphene, scientists at the University of California, Berkeley discovered that it makes an excellent active media for optical modulators. Graphene-based modulators are expected to significantly enhance ultrafast optical communication and computing. The team will report on their findings at the Optical Fiber Communication Conference and Exhibition/National Fiber Optic Engineers Conference (OFC/NFOEC) taking place next week in Los Angeles.

Modulators play a vital role in communications due to their switching ability, because this is what controls the speed that data packets can travel through networks. As the speed of data pulses sent out increases, it means that greater volumes of information can be transmitted.

“We demonstrated a graphene-based optical modulator with a broad optical bandwidth (1.35-1.6 µm), a small device footprint (25 µm2), and high operational speed (1.2 GHz at 3dB) under ambient conditions—all of which are essential for optical interconnects for future integrated optoelectronic systems,” says Ming Liu, a post-doctoral researcher working at UC Berkeley’s NSF Nanoscale Science and Engineering Center. “The modulation efficiency of a single layer of a hexagonal carbon atom is already comparable to, if not better than, traditional semiconductor materials, which are orders of magnitude larger in active volume.”

Looking into future applications, graphene-based modulators could be very compact and potentially perform at speeds up to 10 times faster than today’s technology allows. They may someday enable consumers to stream full-length, high-definition, 3-D movies onto their smartphones within mere seconds.

Liu’s talk, “Graphene-based optical modulators,” takes place Tuesday, March 6 at 3:30 p.m. in the Los Angeles Convention Center.

About OFC/NFOEC

For more than 35 years, the Optical Fiber Communication Conference and Exposition/ National Fiber Optic Engineers Conference (OFC/NFOEC) has been the premier destination for converging breakthrough research and innovation in telecommunications, optical networking and, recently, datacom and computing. Uniting service providers, systems companies, enterprise customers, IT businesses and component manufacturers, along with researchers, engineers and development teams, OFC/NFOEC combines dynamic business programming, an exposition of more than 500 companies and cutting-edge peer-reviewed research into one event that showcases the trends and pulse of the entire optical communications industry. OFC/NFOEC is managed by the Optical Society (OSA) and co-sponsored by OSA, the Institute of Electrical and Electronics Engineers/Communications Society (IEEE/ComSoc) and the IEEE Photonics Society. Acting as a non-financial technical co-sponsor is Telcordia Technologies

Source:  Optical Society of America (OSA)

Solved: The Mystery of the Nanoscale Crop Circles

Lawrence Berkeley National Laboratory
March 1, 2012




Almost three years ago a team of scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) was performing an experiment in which layers of gold mere nanometers (billionths of a meter) thick were being heated on a flat silicon surface and then allowed to cool. They watched in surprise as peculiar features expanded and changed on the screen of their electron microscope, finally settling into circles surrounded by irregular blisters.

The circles varied in diameter up to a few millionths of a meter, and in the center of each was a perfect square. The mysterious patterns were reminiscent of nothing so much as so‑called “alien” crop circles.

Until recently the cause of these strange formations remained a mystery. Now theoretical insights have explained what’s happening, and the results have been published online by Physical Review Letters.

Eagerly melting alloys

When two solids are combined in just the right proportions, changes in chemical bonding may produce an alloy that melts at a temperature far lower than either can melt by itself. Such an alloy is called eutectic, Greek for “good melting.” The eutectic alloy of gold and silicon – 81 percent gold and 19 percent silicon – is especially useful in processing nanoscale semiconductors such as nanowires, as well as for device interconnections in integrated circuits; it liquefies at a modest 363˚ Celsius, far lower than the melting point of either pure gold, 1064°C, or pure silicon, 1414°C.

“Gold-silicon eutectic liquid can safely solder chip layers together or form microscopic conducting wires, by flowing into channels in the substrate without burning up the surroundings,” says Berkeley Lab’s Junqiao Wu. “It’s particularly interesting for processing nanoscale materials and devices.” Wu cites the example of silicon nanowires, which can be grown from beads of eutectic liquid that form from droplets of gold. The beads catalyze the deposition of silicon from a chemical vapor and ride atop continually lengthening nanowire whiskers.

Understanding just how and why this happens has been a challenge. Although eutectic alloys are well studied as solids, the liquid state presents more obstacles, which are particularly formidable at the nanoscale because of greatly increased surface tension – the same surface forces that make it difficult to form ultra-thin films of water, for example, because they pull the water into droplets. At smaller scales the ratio of surface area to bulk increases markedly, and nanoscale structures have been described as virtually “all surface.”

These are the conditions that the team led by Wu, who is a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor in the Department of Materials Science and Engineering at the University of California at Berkeley, set out to examine, by creating the thinnest possible films of gold-silicon eutectic alloys. The researchers did so by starting with a substrate of pure silicon, on whose flat surface an extremely thin barrier layer (two nanometers thick) of silicon dioxide had formed. On this surface they laid layers of pure gold, varying the thickness from one trial to the next between just a few nanometers to a hefty 300 nanometers. The silicon dioxide barrier prevented the pure silicon from mixing with the gold.

The next step was to heat the layered sample to 600 °C for several minutes – not hot enough to melt the gold or silicon but hot enough to cause naturally existing pinholes in the thin silicon dioxide layer to enlarge into small weak spots, through which pure silicon could come in contact with the overlying gold. At the high temperature, silicon atoms quickly diffused out of the substrate and into the gold, forming a layer of eutectic gold-silicon alloy nearly the same thickness as the original gold and spreading in a virtually perfect circle from the central pinhole.

When the circular disk of eutectic alloy got large enough it suddenly broke up, disrupted by the high surface energy of the gold-silicon eutectic liquid. The debris was literally pulled to the edges of the disk, piling up around it to leave a central denuded zone of bare silicon dioxide.

In the center of the denuded zone, a perfect square of gold and silicon remained.
To read more click here...

Tiny 3D chips: Researchers develop a new approach to producing microchips

MIT News
Feb 29, 2012

A new approach helps researchers make tiny three-dimensional structures. Pictured are two packaged microchips, each with tiny bridges fabricated on their surfaces.

Microelectromechanical systems, or MEMS, are small devices with huge potential. Typically made of components less than 100 microns in size — the diameter of a human hair — they have been used as tiny biological sensors, accelerometers, gyroscopes and actuators.

For the most part, existing MEMS devices are two-dimensional, with functional elements engineered on the surface of a chip. It was thought that operating in three dimensions — to detect acceleration, for example — would require complex manufacturing and costly merging of multiple devices in precise orientations.

Now researchers at MIT have come up with a new approach to MEMS design that enables engineers to design 3-D configurations, using existing fabrication processes; with this approach, the researchers built a MEMS device that enables 3-D sensing on a single chip. The silicon device, not much larger than Abraham Lincoln’s ear on a U.S. penny, contains microscopic elements about the width of a red blood cell that can be engineered to reach heights of hundreds of microns above the chip’s surface.

Fabio Fachin, a postdoc in the Department of Aeronautics and Astronautics, says the device may be outfitted with sensors, placed atop and underneath the chip’s minuscule bridges, to detect three-dimensional phenomena such as acceleration. Such a compact accelerometer may be useful in several applications, including autonomous space navigation, where extremely accurate resolution of three-dimensional acceleration fields is key.

“One of the main driving factors in the current MEMS industry is to try to make fully three-dimensional devices on a single chip, which would not only enable real 3-D sensing and actuation, but also yield significant cost benefits,” Fachin says. “A MEMS accelerometer could give you very accurate acceleration [measurements] with a very small footprint, which in space is critical.”

Fachin collaborated with Brian Wardle, an associate professor of aeronautics and astronautics at MIT, and Stefan Nikles, a design engineer at MEMSIC, an Andover, Mass., company that develops wireless-sensor technology. The team outlined the principles behind their 3-D approach in a paper accepted for publication in the Journal of Microelectromechanical Systems.
To read more click here...

New laser can point the way to new energy harvesting

Engineerblogger
Feb 29, 2012

Ismael Heisler next to a diffractive optic polarisation spectrometer. Credit: EPRSC

New ultrafast laser equipment, capable of generating intense pulses of light as short as a few femtoseconds from the UV to the Infra Red, will help scientists at the University of East Anglia (UEA) measure how energy is transferred from molecule to molecule and point the way to molecular structures for exploiting solar radiation.

Funded by a £466,000 grant from the Engineering and Physical Sciences Research Council, the new laser will be used for 2D electronic spectroscopy experiments that look at the very fastest reactions. By studying how energy transfers in natural and artificial systems such as proteins and molecular materials, researchers will in turn be able to help the design of new nanomachines and solar power collectors.

Steve Meech, Professor of Chemistry at UEA’s said:

"With this equipment we will be able to develop experiments which probe in exquisite detail the link between the efficiency of light driven processes in natural and synthetic systems and the underlying molecular architecture."

2D electronic spectroscopy is in many ways analogous to the much better known 2D Nuclear Magnetic Resonance method. It uses ultra fast visible light pulses to reveal coupling between electronic states whereas NMR uses radio frequency pulses to measure couplings between nuclear spins.

Twenty years ago most ultrafast experiments relied upon amplified dye lasers. These difficult to use and unstable devices severely limited the range of experiments possible. Starting with the discovery of the Titanium Sapphire laser, a whole new family of experiments became possible.

"It is because of the amazing stability and reliability of these modern devices that we can even consider 2D optical experiments, which may take days to run", added Meech.

Lesley Thompson, EPSRC’s Director of Research Base, said:

"The grant for equipment made by our strategic equipment panel will give UEA the tools they need, but EPSRC has also allocated a further £613,000 for staff and collaborations to drive this research forward."

The announcement coincides with the inaugural lecture by Professor Alf Adams at the Royal Society in London, to mark the 25th anniversary of his work on strained quantum well lasers, recently named as one of the Top Ten greatest UK scientific breakthroughs of all time.

The lecture, entitled Semiconductor Lasers TakeThe Strain, is the first in a series named in his honour.

Source: Engineering and Physical Sciences Research Council (EPSRC)

The Asian Research Network: a Collaboration to Boost Science

Engineerblogger
Feb 29, 2012

Prof. Haiwon Lee: “Giving is better than taking. So I thought to myself, what about giving something to the other people in Asia? I want to give something as long as I have something to give.” Credit : Asian Research Network

Hanyang University of Korea and RIKEN of Japan, along with other Asian research institutes, are launching the Asian Research Network (ARN). Recently ARN members succeeded in producing transparent touch sensors using carbon nanotubes and ink solutions that can print electronic circuits or change colour in exposure to heat or UV radiation.

“I say to people, ‘I’m a small, skinny guy and I have a dream, I want to do something for Asia,’” beams Prof. Haiwon Lee, Director of the Institute of Nanoscience and Technology at Hanyang University in South Korea.

Small as his stature may be, Lee’s wit, enthusiasm and intelligence make up for it in fair measure. Holding more professorships, directorships and editorial posts than there is space to mention here, it is immediately clear that here is a man who does not define himself by these titles, but by his actions. In particular, it is the Asian Research Network that he speaks of with a passion often rare in professors who are comfortably at the top of their game.

In 1989, on his own accord, Lee started yearly trips to Japan—a step made all the more significant by the historical tensions between the two countries. He sought to establish relationships with other researchers and institutes, integrating science in Asia for a better future. It was a slow process. Apart from exchanges on a company or government level it was, and perhaps still is, highly unusual for a South Korean individual to be promoting research, development and educational cooperation across borders.

Step-by-step Lee built a performance-based relationship with RIKEN. Nevertheless, it was not until 2003 that an alliance between RIKEN and Hanyang was formally established. The significance was profound. Never before had Japan opened up its doors for a private research university.

Next Lee sought to obtain funding for a cooperative research laboratory to give tangible structure to the Asian Research Network. In 2008, following grants from the Korean Ministry of Education, Science and Technology, Seoul’s mayor and Samsung electronics, the Hanyang-RIKEN Collaboration Centre was established. Here researchers from both institutions could work side by side to produce world-class research.

Many would be satisfied with these achievements. For Lee however, it is just the start. The alliance needs to go across Asia. “The idea is to exchange information and relationships at a high level,” he explains. ARN is starting with tangible goals, initially focusing on the areas of nanoscience and nanotechnology. Lee points to a poster advertising a recent joint Hanyang-RIKEN nanoscience conference. However, as they expand ARN is to encompass all science and technology and include other Asian partners such as China, India and Singapore.

“Our aim is to build a borderless research environment,” says Lee. He stresses that this is not just for Korea, but also for Asia and ultimately he aims to go global. The reason that Lee has made his dream a reality is due to his insistence on a pragmatic approach. He looks to innovate, change and truly engage rather than go through set patterns and motions.

“In the beginning, I was talking to government people who would always say, ‘Show me the MOU’ said Lee. A ‘memorandum of understanding’ or ‘MOU’ is a traditional document indicating a multilateral agreement between parties. MOU’s are popular across Asia, so Lee took me by surprise when he continued matter-of-factly: “MOU’s don’t mean anything – its just politics”.

He continued, “It took five years to get people onboard. They always wanted to wait and consider things endlessly, it was very difficult.” If there is one thing that is clear about Lee, it is that he is a man of deeds, not just words, who does not shy away from getting things done.

But why put so much effort into this? I asked. Of course there are huge benefits, but most academics are more concerned with climbing up the citation league table, (and it is clear that Lee has spent at least a hundred papers worth of time establishing ARN!). He looks at me with thoughtful eyes and stares into the distance. “I was born in 1954, right after the Korean war,” he says. “I was one of eight children, there was nothing left of Korea and it was miserable. Our parents sacrificed everything for our education. They did not spend even a single penny. I am not from a rich family, my mother only went to elementary school, but because of their efforts four of us are now professors. They knew how to save material, how to manage, how to change their country. This is the strength and spirit of our parents.”

And the spirit of cooperation is certainly helping the research productivity and output of ARN members. Take for example Choi Eunsuk and colleagues; they recently announced they had made a transparent touch sensor using carbon nanotube thin films (Journal of Nanoscience and Nanotechnology, vol. 11, 2011). These films are optically transparent and electrically conductive in thin layers. The applications are enormous: think of flexible electronic interfaces such as “e-paper”, or television screens that you can roll up.

Similarly, Jong-Man Kim and his team have managed to devise an ink solution that can repeatedly change colour upon exposure to heat or UV radiation. Their results in the Journal of Advanced Materials (Vol. 23, 2011) open the possibility of printing electronic circuits on paper. Being able to integrate such circuitry into lightweight, disposable materials such as paper using simple ‘inkjet’ technology is of great interest to manufacturers.

Prof. Lee meanwhile revels in this spirit of collaboration: “Giving is better than taking. So I thought to myself, what about giving something to the other people in Asia? I want to give something as long as I have something to give.”


Source: Asian Research Network

Additional Information:

• Choi Eunsuk, Kim Jinoh, et al. "Fabrication of a flexible and transparent touch sensor using single-walled carbon nanotube thin-films" Journal of Nanoscience and Nanotechnology Vol. 11 7 pp. 5845-5849 (2011) DOI: 10.1166/jnn.2011.4450

• B. Yoon, D.-Y. Ham, O. Yarimaga, H. An , C. W. Lee, and J.-M. Kim, "Inkjet Printing of Conjugated Polymer Precursors on Paper Substrates for Colorimetric Sensing and Flexible Electrothermochromic Display", Advanced Materials, 2011, 23, 5492-5497. DOI: 10.1002/adma.201103471