Taxel addressable matrix of vertical-nanowire piezotronic transistors for active/adaptive tactile imaging,

Using bundles of vertical zinc oxide nanowires, researchers have fabricated arrays of piezotronic transistors capable of converting mechanical motion directly into electronic controlling signals. The arrays could help give robots a more adaptive sense of touch, provide better security in handwritten signatures and offer new ways for humans to interact with electronic devices.

The arrays include more than 8,000 functioning piezotronic transistors, each of which can independently produce an electronic controlling signal when placed under mechanical strain. These touch-sensitive transistors – dubbed "taxels" – could provide significant improvements in resolution, sensitivity and active/adaptive operations compared to existing techniques for tactile sensing. Their sensitivity is comparable to that of a human fingertip.

The vertically-aligned taxels operate with two-terminal transistors. Instead of a third gate terminal used by conventional transistors to control the flow of current passing through them, taxels control the current with a technique called "strain-gating." Strain-gating based on the piezotronic effect uses the electrical charges generated at the Schottky contact interface by the piezoelectric effect when the nanowires are placed under strain by the application of mechanical force.

The research will be reported on April 25 in the journal Science online, at the Science Express website, and will be published in a later version of the print journal Science. The research has been sponsored by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the U.S. Air Force (USAF), the U.S. Department of Energy (DOE) and the Knowledge Innovation Program of the Chinese Academy of Sciences.

Caption: Georgia Tech researcher Wenzhuo Wu holds an array of piezotronic transistors capable of converting mechanical motion directly into electronic controlling signals. The arrays are fabricated on flexible substrates. Credit: Georgia Tech Photo: Gary Meek. Usage Restrictions: None.

"Any mechanical motion, such as the movement of arms or the fingers of a robot, could be translated to control signals," explained Zhong Lin Wang, a Regents' professor and Hightower Chair in the School of Materials Science and Engineering at the Georgia Institute of Technology. "This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface." Mimicking the sense of touch electronically has been challenging, and is now done by measuring changes in resistance prompted by mechanical touch. The devices developed by the Georgia Tech researchers rely on a different physical phenomenon – tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or placed under strain. In the piezotronic transistors, the piezoelectric charges control the flow of current through the wires just as gate voltages do in conventional three-terminal transistors. The technique only works in materials that have both piezoelectric and semiconducting properties. These properties are seen in nanowires and thin films created from the wurtzite and zinc blend families of materials, which includes zinc oxide, gallium nitride and cadmium sulfide.

In their laboratory, Wang and his co-authors – postdoctoral fellow Wenzhuo Wu and graduate research assistant Xiaonan Wen – fabricated arrays of 92 by 92 transistors. The researchers used a chemical growth technique at approximately 85 to 90 degrees Celsius, which allowed them to fabricate arrays of strain-gated vertical piezotronic transistors on substrates that are suitable for microelectronics applications. The transistors are made up of bundles of approximately 1,500 individual nanowires, each nanowire between 500 and 600 nanometers in diameter.

In the array devices, the active strain-gated vertical piezotronic transistors are sandwiched between top and bottom electrodes made of indium tin oxide aligned in orthogonal cross-bar configurations. A thin layer of gold is deposited between the top and bottom surfaces of the zinc oxide nanowires and the top and bottom electrodes, forming Schottky contacts. A thin layer of the polymer Parylene is then coated onto the device as a moisture and corrosion barrier.

The array density is 234 pixels per inch, the resolution is better than 100 microns, and the sensors are capable of detecting pressure changes as low as 10 kilopascals – resolution comparable to that of the human skin, Wang said. The Georgia Tech researchers fabricated several hundred of the arrays during a research project that lasted nearly three years.

The arrays are transparent, which could allow them to be used on touch-pads or other devices for fingerprinting. They are also flexible and foldable, expanding the range of potential uses.

Among the potential applications:

Multidimensional signature recording, in which not only the graphics of the signature would be included, but also the pressure exerted at each location during the creation of the signature, and the speed at which the signature is created.
Shape-adaptive sensing in which a change in the shape of the device is measured. This would be useful in applications such as artificial/prosthetic skin, smart biomedical treatments and intelligent robotics in which the arrays would sense what was in contact with them.
Active tactile sensing in which the physiological operations of mechanoreceptors of biological entities such as hair follicles or the hairs in the cochlea are emulated.
Because the arrays would be used in real-world applications, the researchers evaluated their durability. The devices still operated after 24 hours immersed in both saline and distilled water.

Future work will include producing the taxel arrays from single nanowires instead of bundles, and integrating the arrays onto CMOS silicon devices. Using single wires could improve the sensitivity of the arrays by at least three orders of magnitude, Wang said.

"This is a fundamentally new technology that allows us to control electronic devices directly using mechanical agitation," Wang added. "This could be used in a broad range of areas, including robotics, MEMS, human-computer interfaces and other areas that involve mechanical deformation."

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This research was supported by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF) under grant CMMI-0946418, the U.S. Air Force (USAF) under grant FA2386-10-1-4070, the U.S. Department of Energy (DOE) Office of Basic Energy Sciences under award DE-FG02-07ER46394 and the Knowledge Innovation Program of the Chinese Academy of Sciences under grant KJCX2-YW-M13. The content is solely the responsibility of the authors and does not necessarily represent the official views of DARPA, the NSF, the USAF or the DOE.

Contact: John Toon jtoon@gatech.edu 404-894-6986 Georgia Institute of Technology

Research News Georgia Institute of Technology 177 North Avenue Atlanta, Georgia 30332-0181 Media Relations Contact: John Toon (404-894-6986)(jtoon@gatech.edu) Writer: John Toon

Diamond-integrated optomechanical circuits

A Revolution in Optomechanical Materials Enables Manufacturing of Monolithic Components / Easy Industrial Processing of Robust Polycrystalline Diamonds / Precision Sensors for Oscillation Measurement.

The application of light for information processing opens up a multitude of possibilities. However, to be able to adequately use photons in circuits and sensors, materials need to have particular optical and mechanical properties. Researchers at the Karlsruhe Institute of Technology (KIT) have now for the first time used polycrystalline diamond to manufacture optical circuits and have published their results online in Nature Communications (DOI: 10.1038/ncomms2710).

“Diamond has several properties that allow us to manufacture all components of a ready-to-use optomechanical circuit monolithically, so to speak,” says KIT research group leader Wolfram Pernice. “The elements thus manufactured that is, the resonators, circuits, and the wafer, are attractive because of their high quality.”

Diamond is optically transparent to light waves of a wide range of wavelengths including the visible spectrum between 400 and 750 nm. It is due to this fact that it can be used specifically in optomechanical circuits for applications in sensor technology and fluorescence imaging, or for novel optical biological measuring methods. Whereas the high refractive index of diamond and the absence of absorption allow an efficient photon transport, its high modulus of elasticity makes it a robust material which adapts excellently to rough surfaces and releases heat rapidly.

Two parallel free-standing waveguides made of polycrystalline diamond serve as mechanical resonators. Optical fields (red/blue) are observed to propagate inside of them. (Graphic: KIT/CFN/Pernice)

So far, optical circuits have been manufactured using monocrystalline diamond substrates i.e., highly pure crystals with typically no more than one impurity atom to every one billion diamond atoms. Such circuits are bound to be small and their application to optical systems has required sophisticated fabrication methods. Now, for the first time, the research group headed by Pernice used polycrystalline diamond for the fabrication of wafer-based optomechanical circuits. Even though its crystal structures are more irregular, polycrystalline diamond is robust and thus can be more easily processed. It is due to these specific properties that polycrystalline diamond can be used on much larger areas than monocrystalline material. Polycrystalline diamond conducts photons almost as efficiently as the monocrystalline substrate and is suitable for industrial use. As a matter of fact, monolithic optomechanical components could not have been manufactured without this new material. Optomechanics combines integrated optics with mechanical elements e.g., with nanomechanical resonators in the case of the optomechanical circuit developed by Pernice and his group. These oscillatory systems react to a certain frequency. When that frequency occurs, the resonator is excited into vibration. “Nanomechanical resonators are among today’s most sensitive sensors and are used in various precision measurements. It is extremely difficult, however, to address such smallest components through conventional measuring methods,” explains Patrik Rath, first author of the study.

“In our study, we have made use of the fact that today, nanophotonic components can be manufactured in the same sizes as nanoscale mechanical resonators. When the resonator responds, corresponding optical signals are transferred directly to the circuit.” This novel development has allowed the combining of once separate fields of research and has enabled the realization of highly efficient optomechanical circuits.

Integrated optics works in a similar manner to integrated electrical circuits. Whereas optical circuits transmit information via photons, conventional electronic circuits transfer data via electrons. Integrated optics aims to combine all components required for optical communication in an integrated optical circuit to avoid a detour via electrical signals. In both cases, the respective circuits are applied to slices less than one mm in thickness i.e., to the so-called wafers.

The polycrystalline diamond was manufactured in cooperation with the Fraunhofer Institute for Applied Solid State Physics and the company Diamond Materials in Freiburg, Germany. The prototypes manufactured within the Integrated Quantum Photonics-project at the DFG Center for Functional Nanostructures (CFN) in Karlsruhe open up new ways for entirely optically controlled platforms that are increasingly needed in fundamental research and advanced sensor technologies. These technologies include accelerometers that are integrated in various electronic devices such as airbag sensors or smartphone waterlevels.

The DFG Center of Functional Nanostructures (CFN) devotes its attention to the important area of nanotechnology and functional nanostructures. Its objective is to carry out excellent interdisciplinary and international research in order to produce nanostructures with new technical functions and take the first step from fundamental research to application. At the present time, more than 250 scientists and engineers cooperate in more than 80 subprojects networked through the CFN in Karlsruhe, focusing on the areas of nanophotonics, nanoelectronics, molecular nanostructures, nanobiology, and nanoenergy.
http://www.cfn.kit.edu

Karlsruhe Institute of Technology (KIT) is a public corporation according to the legislation of the state of Baden-Württemberg. It fulfills the mission of a university and the mission of a national research center of the Helmholtz Association. KIT focuses on a knowledge triangle that links the tasks of research, teaching, and innovation.

For further information, please contact: Tatjana Erkert DFG-Centrum für Funktionelle Nanostrukturen (CFN) Tel.: +49 721 608-43409 Fax: +49 721 608-48496 E-Mail: tatjana erkert∂kit edu

Microstructure and rheology of a flow-induced structured phase in wormlike micellar solutions

Sometimes cost saving comes in nanoscale packages. A new procedure that thickens and thins fluid at the micron level could save consumers and manufacturers money, particularly for soap products that depend on certain molecules to effectively deal with grease and dirt. Researchers at the University of Washington published their findings online April 9 in the Proceedings of the National Academy of Sciences.

Read the back of most shampoos and dishwashing detergents and you'll find the word "surfactant" in the list of active ingredients. Surfactant molecules are tiny, yet they are the reason dish soap can attack an oily spot and shampoo can rid the scalp of grease.

Surfactant molecules are made up of two main parts, a head and a tail. Heads are attracted to water, while the tails are oil-soluble. This unique structure helps them break down and penetrate grease and oil while immersed in water. It also makes the soaps, shampoos and detergents thicker, or more viscous.

Soap manufacturers add organic and synthetic surfactants – and often a slew of other ingredients – to their products to achieve a desired thickness and to help remove grease and dirt. These extra ingredients add volume to the soap products, which then cost more to manufacture, package and ship, ultimately shifting more costs to consumers, said Amy Shen, a UW associate professor of mechanical engineering and lead author of the paper.

Caption: A web-like, gel structure is formed after fluid passes through the flow device. The unit of measurement is 1 micron. Credit: Environmental Molecular Sciences Laboratory and University of Washington. Usage Restrictions: Photo credit required.

The research team's design could create the same thickening results without having to add extra ingredients. "Our flow procedure can potentially help companies and consumers save a lot of money," Shen said. "This way, companies don't have to add too many surfactants to their products." Researchers found that when they manipulated the flow of a liquid through microscopic channels, the resulting substance became thicker. Now, scientists add a lot of salt, or alter the temperature and level of acidity to induce this change, but these methods can be expensive and more toxic, Shen said. The team built a palm-sized tool called a microfluidics device that lets researchers pump water mixed with a little detergent and salt through a series of vertical posts. The distance between posts is about one-tenth the size of a single human hair. That micron-sized gap squeezes the liquid as it flows, causing it to quickly deform. The end result is a gel-like substance that's more viscous and elastic. When researchers looked at high-resolution images of the end product, they saw a series of wormlike rods attaching and intermingling with each other, creating an entangled web. This structure stayed intact after the procedure was complete, which suggests this process can create a permanent, scaffold-like network that could prove useful for biological applications, Shen said. She is collaborating with other UW researchers to try to create stable structures that could house enzymes and other biomarkers for detecting certain diseases.

Shen and her team also discovered that when they pumped a thicker, more elastic fluid through the device, the opposite effect happened – the gel became thinner and more porous. This could be useful in biomedical applications, Shen said, though it hasn't yet been tested. In theory, a semi-solid gel could be injected into veins, then transform into a thinner liquid, delivering drugs throughout the body.

Researchers hope one eventual outcome will be a scaled-up industrial design of their microfluidics device that could help manufacturers churn out soap products that aren't filled with an excess of added materials. Shen has presented her initial findings at Procter & Gamble Co.

"What we can provide are all of the important parameters for operating conditions so companies can have an industrial design to achieve their goals," Shen said.

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Research collaborators are Joshua Cardiel and Ya Zhao, UW doctoral students in mechanical engineering; Alice Dohnalkova, senior research scientist at Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory in Richland, Wash.; and Neville Dubash and Perry Cheung, former post-doctoral researchers in mechanical engineering.

The research was funded by the National Science Foundation.

For more information, contact Shen at amyshen@uw.edu or 206-708-3411.


The team built a palm-sized tool called a microfluidics device that lets researchers pump water mixed with a little detergent and salt through a series of vertical posts. The distance between posts is about one-tenth the size of a single human hair. That micron-sized gap squeezes the liquid as it flows, causing it to quickly deform. The end result is a gel-like substance that's more viscous and elastic.

When researchers looked at high-resolution images of the end product, they saw a series of wormlike rods attaching and intermingling with each other, creating an entangled web. This structure stayed intact after the procedure was complete, which suggests this process can create a permanent, scaffold-like network that could prove useful for biological applications, Shen said. She is collaborating with other UW researchers to try to create stable structures that could house enzymes and other biomarkers for detecting certain diseases.

Shen and her team also discovered that when they pumped a thicker, more elastic fluid through the device, the opposite effect happened – the gel became thinner and more porous. This could be useful in biomedical applications, Shen said, though it hasn't yet been tested. In theory, a semi-solid gel could be injected into veins, then transform into a thinner liquid, delivering drugs throughout the body.

Researchers hope one eventual outcome will be a scaled-up industrial design of their microfluidics device that could help manufacturers churn out soap products that aren't filled with an excess of added materials. Shen has presented her initial findings at Procter & Gamble Co.

"What we can provide are all of the important parameters for operating conditions so companies can have an industrial design to achieve their goals," Shen said.

###
Research collaborators are Joshua Cardiel and Ya Zhao, UW doctoral students in mechanical engineering; Alice Dohnalkova, senior research scientist at Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory in Richland, Wash.; and Neville Dubash and Perry Cheung, former post-doctoral researchers in mechanical engineering.

The research was funded by the National Science Foundation.

For more information, contact Shen at amyshen@uw.edu or 206-708-3411.

Contact: Michelle Ma mcma@uw.edu 206-543-2580 University of Washington

Optimized broadband, low-loss optical metamaterial nanostructure

Theory and practice key to optimized broadband, low-loss optical metamaterials

The union of theory and practice makes broadband, low-loss optical devices practical, which is why two groups of Penn State engineers collaborated to design optical metamaterials that have custom applications that are easily manufactured.

Metamaterials are manufactured materials that derive their unusual properties from structure rather than only composition, and possess exotic properties not usually found in nature. Nanostructured metamaterials appear different for signals of different frequencies. They are dispersive, so that if researchers manipulate this material dispersion, they gain a comprehensive control of the device performance across a band of frequencies.

In the past, to control the optics of metamaterials, researchers used complicated structures including 3-dimensional rings and spirals that are difficult if not impossible to manufacture in large numbers and small sizes at optical wavelengths. From a practical perspective, simple and manufacturable nanostructures are necessary for creating high-performance devices.

Caption: This is a tilted view, field emission scanning electron microscopy image of the fabricated metamaterial nanostructure. Scale bar: 1000 nm. Credit: Penn State. Usage Restrictions: Non-commercial use only.

"We must design (nanostructures that can be fabricated," said Theresa S. Mayer, Distinguished Professor of Electrical Engineering and co-director of Penn State's nanofabrication laboratory. Designing materials that can allow a range of wavelengths to pass through while blocking other wavelengths is far more difficult than simply creating something that will transmit a single frequency. Minimizing the time domain distortion of the signal over a range of wavelengths is necessary, and the material also must be low loss. "We don't want the signal to change as it passes through the device," said Jeremy A. Bossard, postdoctoral fellow in electrical engineering. The majority of what goes in must come out with little absorption or distortions to the signal waveform due to the metamaterial dispersion. "What we do is use global optimization approaches to target, over wide bandwidths, the optical performance and nano fabrication constraints required by different design problems," said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering. "The design methodology coupled with the fabrication approach is critically important."

The design team looked at existing fishnet structured metamaterials and applied nature-inspired optimization techniques based on genetic algorithms. They optimized the dimensions of features such as the size of the fishnet and the thicknesses of the materials. One of the transformative innovations made by the researchers was the inclusion of nanonotches in the corners of the fishnet holes, creating a pattern that could be tuned to shape the dispersion over large bandwidths. They reported their approach in today's (Mar. 28) online issue of Scientific Reports.

"We introduced nanonotches in the corners of the air holes to give a lot more flexibility to independently control the properties of permittivity and permeability across a broad band," said Werner. "The conventional fishnet doesn't have much flexibility, but is easy to fabricate."

Permittivity measures the ease or difficulty of inducing an electric field in a material, while permeability measures the ease or difficulty of inducing a magnetic field. Theoretically, manipulating permittivity and permeability allows tuning of the metamaterial across a range of wavelengths and creates the desired index of refraction and impedance.

Theory may provide a solution, but can that solution become reality? The fabrication team placed constraints on the design to ensure that the material could be manufactured using electron-beam lithography and reactive ion etching. The initial material was a three-layer sandwich of gold, polyimide and gold on oxidized silicon. When the silicon dioxide mask and the electron beam resist are removed, the researchers were left with an optical metamaterial with the desired properties.

In this case they created a band pass filter, but the same principles can be applied to many optical devices used in optical communications systems, medicine, testing and characterization or even optical beam scanning if the metamaterial is shaped to form a prism.

Another use of this metamaterial could be in conjunction with natural materials that do not have the desired properties for a specific optical application.

"All materials have a natural dispersion," said Mayer. "We might want to coat a natural material in some regions to compensate for the dispersion."

According to Werner, currently the only way to compensate is to find another natural material that would do the job. Only rarely does such a material exist.

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Working on this project with Mayer, Werner and Bossard were Zhi Hao Jiang and Lan Lin, both graduate students in electrical engineering, and Seokho Yun, a former electrical engineering postdoctoral fellow.

The National Science Foundation's Materials Research Science and Engineering Center and National Nanotechnology Infrastructure Network supported this work.

Contact: A'ndrea Elyse Messer aem1@psu.edu 814-865-9481 Penn State