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News Article | April 17, 2017
Site: phys.org

Graphene is an extremely thin layer of carbon that is promising for optoelectronics, and researchers are trying to develop graphene-based photodetectors, devices that are critical for many technologies. However, typical photodetectors made of graphene have only a small area that is sensitive to light, limiting their performance. Now, researchers have solved the problem by combining graphene with a comparatively much larger silicon carbide substrate, creating graphene field-effect transistors, or GFETs, which can be activated by light, said Yong Chen, a Purdue University professor of physics and astronomy and electrical and computer engineering, and director of the Purdue Quantum Center. High-performance photodetectors might be useful for applications including high-speed communications and ultra-sensitive cameras for astrophysics, as well as sensing applications and wearable electronics. Arrays of the graphene-based transistors might bring high-resolution imaging and displays. "In most cameras you need lots of pixels," said Igor Jovanovic, a professor of nuclear engineering and radiological sciences at the University of Michigan. "However, our approach could make possible a very sensitive camera where you have relatively few pixels but still have high resolution." New findings are detailed in a research paper appearing this week in the journal Nature Nanotechnology. The work was performed by researchers at Purdue, the University of Michigan and Pennsylvania State University. "In typical graphene-based photodetectors demonstrated so far, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size," Jovanovic said. "However, for many optoelectronic device applications, it is desirable to obtain photoresponse and positional sensitivity over a much larger area." New findings show the device is responsive to light even when the silicon carbide is illuminated at distances far from the graphene. The performance can be increased by as much as 10 times depending on which part of the material is illuminated. The new phototransistor also is "position-sensitive," meaning it can determine the location from which the light is coming, which is important for imaging applications and for detectors. "This is the first time anyone has demonstrated the use of a small piece of graphene on a large wafer of silicon carbide to achieve non-local photodetection, so the light doesn't have to hit the graphene itself," Chen said. "Here, the light can be incident on a much larger area, almost a millimeter, which has not been done before." A voltage is applied between the back side of the silicon carbide and the graphene, setting up an electric field in the silicon carbide. Incoming light generates "photo carriers" in the silicon carbide. "The semiconductor provides the media that interact with light," Jovanovic said. "When light comes in, part of the device becomes conducting and that changes the electric field acting on graphene." This change in the electric field also changes the conductivity of graphene itself, which is detected. The approach is called field-effect photo detection. The silicon carbide is "un-doped," unlike conventional semiconductors in silicon-based transistors. Being un-doped makes the material an insulator unless it is exposed to light, which temporarily causes it to become partially conductive, changing the electric field on the graphene. "This is a novelty of this work," Chen said. The research is related to work to develop new graphene-based sensors designed to detect radiation and was funded with a joint grant from the National Science Foundation and the U.S. Department of Homeland Security and another grant from the Defense Threat Reduction Agency. "This particular paper is about a sensor to detect photons, but the principles are the same for other types of radiation," Chen said. "We are using the sensitive graphene transistor to detect the changed electric field caused by photons, light in this case, interacting with a silicon carbide substrate." Light detectors can be used in devices called scintillators, which are used to detect radiation. Ionizing radiation creates brief flashes of light, which in scintillators are detected by devices called photo multiplier tubes, a roughly century-old technology. "So there is a lot of interest in developing advanced semiconductor-based devices that can achieve the same function," Jovanovic said. The paper was authored by former Purdue postdoctoral research associate Biddut K. Sarker; former Penn State graduate student Edward Cazalas; Purdue graduate student Ting-Fung Chung; former Purdue graduate student Isaac Childres; Jovanovic; and Chen. The researchers also explained their findings with a computational model. The transistors were fabricated at the Birck Nanotechnology Center in Purdue's Discovery Park. Future research will include work to explore applications such as scintillators, imaging technologies for astrophysics and sensors for high-energy radiation.


News Article | April 28, 2017
Site: www.cemag.us

A “chemical imaging” system that uses a special type of laser beam to penetrate deep into tissue might lead to technologies that eliminate the need to draw blood for analyses including drug testing and early detection of diseases such as cancer and diabetes. The system, called stimulated Raman projection microscopy and tomography, makes possible “volumetric imaging” without using fluorescent dyes that might affect biological functions and hinder accuracy, said Ji-Xin Cheng, a professor in Purdue University’s Weldon School of Biomedical Engineering, Department of Chemistry and Birck Nanotechnology Center. “Volumetric chemical imaging allows a better understanding of the chemical composition of three-dimensional complex biological systems such as cells,” he said. The technology uses a type of laser beam called a Bessel beam, which maintains focus for a longer distance than a traditional “Gaussian beam” used in other imaging technologies, making it possible to penetrate deep into tissue. Stimulated Raman spectroscopy eliminates the need for fluorescent dyes. The technology yields more accurate data than other methods because it allows imaging of the entire cell by “adding up” signals produced from the scanning beam, Cheng said. Because the Bessel beam makes possible deep-tissue imaging, it could lead to systems that eliminate the need to draw blood for analyses such as drug testing and detection of biomarkers for non-invasive early diagnosis of diseases, Cheng said. “This is a long-term goal,” he said. “In the meantime, much more research is needed to improve the system.” The researchers proved the concept by imaging fat storage in living cells. Findings are detailed in a research paper appearing on April 24 in the journal Nature Communications. The reported technology yields information about chemical composition, collecting a series of images while rotating the sample and reconstructing the 3-D structure through image reconstruction algorithms. The Bessel beam is produced using a pair of cone-shaped “axicon” lenses and is combined with a microscope objective. Its use for volumetric fluorescence imaging was previously demonstrated by physicist Eric Betzig, who won the Nobel Prize in chemistry in 2014 for his pioneering contribution to super-resolution fluorescence microscopy. Super-resolution technology allows researchers to resolve structural features far smaller than the wavelength of visible light, sidestepping the “diffraction limit” that normally prevents imaging of features smaller than about 250 nanometers, which is large compared to certain biological molecules and structures in cells. However, fluorescence microscopy usually requires the use of fluorescent tags, which may interfere with biological processes and hinder accuracy for determining chemical structure. Future research will include work to increase the detection sensitivity of the system and improve the imaging quality and speed. “There is plenty of room for improvement,” Cheng said. “The system is based on a bulky and relatively expensive femtosecond laser, which limits its potential for broad use and clinical translation. Nevertheless, we anticipate that this limitation can be circumvented through engineering innovations to reduce the cost and size of our technology. We also note that the Bessel beam can be produced using fibers, which could simplify the system and enable endoscopic applications.” The paper was authored by Xueli Chen, a visiting scholar from Xidian University in China; Purdue postdoctoral research associate Chi Zhang; Purdue doctoral students Peng Lin and Kai-Chih Huang; Xidian University researchers Jimin Liang and Jie Tian; and Cheng. The research was supported by funds from the Keck Foundation and National Institutes of Health.


News Article | April 27, 2017
Site: phys.org

The system, called stimulated Raman projection microscopy and tomography, makes possible "volumetric imaging" without using fluorescent dyes that might affect biological functions and hinder accuracy, said Ji-Xin Cheng, a professor in Purdue University's Weldon School of Biomedical Engineering, Department of Chemistry and Birck Nanotechnology Center. "Volumetric chemical imaging allows a better understanding of the chemical composition of three-dimensional complex biological systems such as cells," he said. The technology uses a type of laser beam called a Bessel beam, which maintains focus for a longer distance than a traditional "Gaussian beam" used in other imaging technologies, making it possible to penetrate deep into tissue. Stimulated Raman spectroscopy eliminates the need for fluorescent dyes. The technology yields more accurate data than other methods because it allows imaging of the entire cell by "adding up" signals produced from the scanning beam, Cheng said. Because the Bessel beam makes possible deep-tissue imaging, it could lead to systems that eliminate the need to draw blood for analyses such as drug testing and detection of biomarkers for non-invasive early diagnosis of diseases, Cheng said. "This is a long-term goal," he said. "In the meantime, much more research is needed to improve the system." The researchers proved the concept by imaging fat storage in living cells. Findings are detailed in a research paper appearing on April 24 in the journal Nature Communications. The reported technology yields information about chemical composition, collecting a series of images while rotating the sample and reconstructing the 3-D structure through image reconstruction algorithms. The Bessel beam is produced using a pair of cone-shaped "axicon" lenses and is combined with a microscope objective. Its use for volumetric fluorescence imaging was previously demonstrated by physicist Eric Betzig, who won the Nobel Prize in chemistry in 2014 for his pioneering contribution to super-resolution fluorescence microscopy. Super-resolution technology allows researchers to resolve structural features far smaller than the wavelength of visible light, sidestepping the "diffraction limit" that normally prevents imaging of features smaller than about 250 nanometers, which is large compared to certain biological molecules and structures in cells. However, fluorescence microscopy usually requires the use of fluorescent tags, which may interfere with biological processes and hinder accuracy for determining chemical structure. Future research will include work to increase the detection sensitivity of the system and improve the imaging quality and speed. "There is plenty of room for improvement," Cheng said. "The system is based on a bulky and relatively expensive femtosecond laser, which limits its potential for broad use and clinical translation. Nevertheless, we anticipate that this limitation can be circumvented through engineering innovations to reduce the cost and size of our technology. We also note that the Bessel beam can be produced using fibers, which could simplify the system and enable endoscopic applications." The paper was authored by Xueli Chen, a visiting scholar from Xidian University in China; Purdue postdoctoral research associate Chi Zhang; Purdue doctoral students Peng Lin and Kai-Chih Huang; Xidian University researchers Jimin Liang and Jie Tian; and Cheng. Explore further: Imaging uses 'photothermal effect' to peer into living cells More information: Xueli Chen et al. Volumetric chemical imaging by stimulated Raman projection microscopy and tomography, Nature Communications (2017). DOI: 10.1038/ncomms15117


News Article | April 20, 2017
Site: www.cemag.us

Researchers have shown how to create a rechargeable “spin battery” made out of materials called topological insulators, a step toward building new spintronic devices and quantum computers. Unlike ordinary materials that are either insulators or conductors, topological insulators are both at the same time — they are insulators inside but conduct electricity on the surface. The materials might be used for spintronic devices and quantum computers more powerful than today's technologies. Electrons can be thought of as having two spin states: up or down, and a phenomenon known as superposition allows electrons to be in both states at the same time. Such a property could be harnessed to perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks. The conducting electrons on the surface of topological insulators have a key property known as “spin momentum locking,” in which the direction of the motion of electrons determines the direction of its spin. This spin could be used to encode or carry information by using the down or up directions to represent 0 or 1 for spin-based information processing and computing, or spintronics. “Because of the spin-momentum locking, you can make the spin of electrons line up or ‘locked’ in one direction if you pass a current through the topological insulator material, and this is a very interesting effect,” says Yong P. Chen, a Purdue University professor of physics and astronomy and electrical and computer engineering and director of the Purdue Quantum Center. Applying an electric current to the material induces an electron “spin polarization” that might be used for spintronics. Ordinarily, the current must remain turned on to maintain this polarization. However, in new findings, Purdue researchers are the first to induce a long-lived electron spin polarization lasting two days even when the current is turned off. The electron spin polarization is detected by a magnetic voltage probe, which acts as a spin-sensitive voltmeter in a technique known as “spin potentiometry”. The new findings are detailed in a research paper in the journal Science Advances. The experiment was led by postdoctoral research associate Jifa Tian. “Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information systems,” Tian says. This “writing current” could be likened to recording the ones and zeroes in a computer’s memory. “However, a better analog is that of a battery,” Chen says. “The writing current is like a charging current. It’s slow, just like charging your iPhone for an hour or two, and then it can output power for several days. That’s the similar idea. We charge up this spin battery using this writing current in half an hour or one hour and then the spins stay polarized for two days, like a rechargeable battery.” “This was not predicted nor something we were looking for when we started the experiment,” he says. “It was an accidental discovery, thanks to Jifa’s patience and persistence, running and repeating the measurements many times, and effectively charging up the spin battery to output a measurable persistent spin polarization signal.” The researchers are unsure what causes the effect. However, one theory is that the spin- polarized electrons might be transferring their polarization to the atomic nuclei in the material. This hypothesis as a possible explanation to the experiment was proposed by Supriyo Datta, Purdue’s Thomas Duncan Distinguished Professor of Electrical and Computer Engineering and the leader of the recently launched Purdue “spintronics preeminent team initiative.” “In one meeting, Professor Datta made the critical suggestion that the persistent spin signal Jifa observed looked like a battery,” Chen says. “There were some analogous experiments done earlier on a nuclear spin powered battery, although they typically required much more challenging conditions such as high magnetic fields. Our observation so far is consistent with the effect also arising from the nuclear spins, even though we don’t have direct evidence.” Nuclear spin has implications for development of quantum memory and quantum computing. “And now we have an electrical way to achieve this, meaning it is potentially useful for quantum circuits because you can just pass current and you polarize nuclear spin,” Chen says. “Traditionally that has been very difficult to achieve. Our spin battery based on topological insulators works even at zero magnetic field, and moderately low temperatures such as tens of kelvins, which is very unusual.” Seokmin Hong, a former Purdue doctoral student working with Datta who is now a software engineer at Intel Corp., said, “While an ordinary charged battery outputs a voltage that can be used to drive a charge current, a ‘spin battery’ outputs a ‘spin voltage,’ or more precisely a chemical potential difference between the spin up and spin down electrons, that can be used to drive a non-equilibrium spin current.” The researchers used small flakes of a material called bismuth tellurium selenide. It is in the same class of materials as bismuth telluride, which is behind solid-state cooling technologies such as commercial thermoelectric refrigerators. However, unlike the commercial grade material that is a “doped” bulk semiconductor, the material used in the experiment was carefully produced to have ultra-high-purity and little doping in the bulk so the conduction is dominated by the spin-polarized electrons on the surface. It was synthesized by research scientist Ireneusz Miotkowski in the semiconductor bulk crystal lab managed by Chen in Purdue’s Department of Physics and Astronomy. The devices were fabricated by Tian in the Birck Nanotechnology Center in Purdue’s Discovery Park. The paper was authored by Tian; Hong; and Miotkowski, Datta, and Chen. Future research will include work to probe what causes the effect by directly probing the nuclear spin, and also to explore how this spin battery can be used in potential practical applications. The research was funded by the U.S. Defense Advanced Research Projects Agency, the Birck Nanotechnology Center, and the Midwestern Institute for Nanoelectronics Discovery of Nanoelectronics Research Initiative, and National Science Foundation.


Unlike ordinary materials that are either insulators or conductors, topological insulators are both at the same time - they are insulators inside but conduct electricity on the surface. The materials might be used for spintronic devices and quantum computers more powerful than today's technologies. Electrons can be thought of as having two spin states: up or down, and a phenomenon known as superposition allows electrons to be in both states at the same time. Such a property could be harnessed to perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks. The conducting electrons on the surface of topological insulators have a key property known as "spin momentum locking," in which the direction of the motion of electrons determines the direction of its spin. This spin could be used to encode or carry information by using the down or up directions to represent 0 or 1 for spin-based information processing and computing, or spintronics. "Because of the spin-momentum locking, you can make the spin of electrons line up or 'locked' in one direction if you pass a current through the topological insulator material, and this is a very interesting effect," said Yong P. Chen, a Purdue University professor of physics and astronomy and electrical and computer engineering and director of the Purdue Quantum Center. Applying an electric current to the material induces an electron "spin polarization" that might be used for spintronics. Ordinarily, the current must remain turned on to maintain this polarization. However, in new findings, Purdue researchers are the first to induce a long-lived electron spin polarization lasting two days even when the current is turned off. The electron spin polarization is detected by a magnetic voltage probe, which acts as a spin-sensitive voltmeter in a technique known as "spin potentiometry". The new findings are detailed in a research paper appearing on April 14 in the journal Science Advances. The experiment was led by postdoctoral research associate Jifa Tian. "Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information systems," Tian said. This "writing current" could be likened to recording the ones and zeroes in a computer's memory. "However, a better analog is that of a battery," Chen said. "The writing current is like a charging current. It's slow, just like charging your iPhone for an hour or two, and then it can output power for several days. That's the similar idea. We charge up this spin battery using this writing current in half an hour or one hour and then the spins stay polarized for two days, like a rechargeable battery." "This was not predicted nor something we were looking for when we started the experiment," he said. "It was an accidental discovery, thanks to Jifa's patience and persistence, running and repeating the measurements many times, and effectively charging up the spin battery to output a measurable persistent spin polarization signal." The researchers are unsure what causes the effect. However, one theory is that the spin- polarized electrons might be transferring their polarization to the atomic nuclei in the material. This hypothesis as a possible explanation to the experiment was proposed by Supriyo Datta, Purdue's Thomas Duncan Distinguished Professor of Electrical and Computer Engineering and the leader of the recently launched Purdue "spintronics preeminent team initiative." "In one meeting, Professor Datta made the critical suggestion that the persistent spin signal Jifa observed looked like a battery," Chen said. "There were some analogous experiments done earlier on a nuclear spin powered battery, although they typically required much more challenging conditions such as high magnetic fields. Our observation so far is consistent with the effect also arising from the nuclear spins, even though we don't have direct evidence." Nuclear spin has implications for development of quantum memory and quantum computing. "And now we have an electrical way to achieve this, meaning it is potentially useful for quantum circuits because you can just pass current and you polarize nuclear spin," Chen said. "Traditionally that has been very difficult to achieve. Our spin battery based on topological insulators works even at zero magnetic field, and moderately low temperatures such as tens of kelvins, which is very unusual." Seokmin Hong, a former Purdue doctoral student working with Datta who is now a software engineer at Intel Corp., said, "While an ordinary charged battery outputs a voltage that can be used to drive a charge current, a 'spin battery' outputs a 'spin voltage,' or more precisely a chemical potential difference between the spin up and spin down electrons, that can be used to drive a non-equilibrium spin current." The researchers used small flakes of a material called bismuth tellurium selenide. It is in the same class of materials as bismuth telluride, which is behind solid-state cooling technologies such as commercial thermoelectric refrigerators. However, unlike the commercial grade material that is a "doped" bulk semiconductor, the material used in the experiment was carefully produced to have ultra-high-purity and little doping in the bulk so the conduction is dominated by the spin-polarized electrons on the surface. It was synthesized by research scientist Ireneusz Miotkowski in the semiconductor bulk crystal lab managed by Chen in Purdue's Department of Physics and Astronomy. The devices were fabricated by Tian in the Birck Nanotechnology Center in Purdue's Discovery Park. The paper was authored by Tian; Hong; and Miotkowski, Datta, and Chen. Future research will include work to probe what causes the effect by directly probing the nuclear spin, and also to explore how this spin battery can be used in potential practical applications. Explore further: Long-distance transport of electron spins for spin-based logic devices More information: Jifa Tian et al. Observation of current-induced, long-lived persistent spin polarization in a topological insulator: A rechargeable spin battery, Science Advances (2017). DOI: 10.1126/sciadv.1602531


News Article | April 21, 2016
Site: www.nanotech-now.com

Abstract: Dynamic optoelectric trapping and deposition of multiwalled carbon nanotubes Avanish Mishra1, Katherine Clayton1, Vanessa Velasco2, Stuart J. Williams2 and Steven T. Wereley1 1Birck Nanotechnology Center, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA and 2Department of Mechanical Engineering, University of Louisville, KY 40292, USA. Correspondence: Steven T. Wereley In the path toward the realization of carbon nanotube (CNT)-driven electronics and sensors, the ability to precisely position CNTs at well-defined locations remains a significant roadblock. Highly complex CNT-based bottom–up structures can be synthesized if there is a method to accurately trap and place these nanotubes. In this study, we demonstrate that the rapid electrokinetic patterning (REP) technique can accomplish these tasks. By using laser-induced alternating current (AC) electrothermal flow and particle–electrode forces, REP can collect and maneuver a wide range of vertically aligned multiwalled CNTs (from a single nanotube to over 100 nanotubes) on an electrode surface. In addition, these trapped nanotubes can be electrophoretically deposited at any desired location onto the electrode surface. Apart from active control of the position of these deposited nanotubes, the number of CNTs in a REP trap can also be dynamically tuned by changing the AC frequency or by adjusting the concentration of the dispersed nanotubes. On the basis of a calculation of the stiffness of the REP trap, we found an upper limit of the manipulation speed, beyond which CNTs fall out of the REP trap. This peak manipulation speed is found to be dependent on the electrothermal flow velocity, which can be varied by changing the strength of the AC electric field. A system that uses a laser and electrical current to precisely position and align carbon nanotubes represents a potential new tool for creating electronic devices out of the tiny fibers. Because carbon nanotubes have unique thermal and electrical properties, they may have future applications in electronic cooling and as devices in microchips, sensors and circuits. Being able to orient the carbon nanotubes in the same direction and precisely position them could allow these nanostructures to be used in such applications. However, it is difficult to manipulate something so small that thousands of them would fit within the diameter of a single strand of hair, said Steven T. Wereley, a professor of mechanical engineering at Purdue University. "One of the things we can do with this technique is assemble carbon nanotubes, put them where we want and make them into complicated structures," he said. New findings from research led by Purdue doctoral student Avanish Mishra are detailed in a paper that has appeared online March 24 in the journal Microsystems and Nanoengineering, published by the Nature Publishing Group. The technique, called rapid electrokinetic patterning (REP), uses two parallel electrodes made of indium tin oxide, a transparent and electrically conductive material. The nanotubes are arranged randomly while suspended in deionized water. Applying an electric field causes them to orient vertically. Then an infrared laser heats the fluid, producing a doughnut-shaped vortex of circulating liquid between the two electrodes. This vortex enables the researchers to move the nanotubes and reposition them. "When we apply the electric field, they are immediately oriented vertically, and then when we apply the laser, it starts a vortex, that sweeps them into little nanotube forests," Wereley said. The research paper was authored by Mishra; Purdue graduate student Katherine Clayton; University of Louisville student Vanessa Velasco; Stuart J. Williams, an assistant professor of mechanical engineering at the University of Louisville and director of the Integrated Microfluidic Systems Laboratory; and Wereley. Williams is a former doctoral student at Purdue. The technique overcomes limitations of other methods for manipulating particles measured on the scale of nanometers, or billionths of a meter. In this study, the procedure was used for multiwalled carbon nanotubes, which are rolled-up ultrathin sheets of carbon called graphene. However, according to the researchers, using this technique other nanoparticles such as nanowires and nanorods can be similarly positioned and fixed in vertical orientation. The researchers have received a U.S. patent on the system. The experimental work was performed at the Birck Nanotechnology Center in Purdue's Discovery Park. Future research will explore using the technique to create devices. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


Wei D.,Birck Nanotechnology Center | Xu X.,Birck Nanotechnology Center
Applied Physics Letters | Year: 2012

We demonstrate laser direct growth of few layer graphene on a silicon substrate. In our study, a continuous wave laser beam was focused on a poly(methyl methacrylate) (PMMA)-coated silicon wafer to evaporate PMMA and melt the silicon wafer. Carbon atoms, decomposed from PMMA, were absorbed by the molten silicon surface, and then separated from silicon in the cooling process to form few-layer graphene. This Si-catalyzed method will provide a new approach and platform for applications of graphene. © 2012 American Institute of Physics.


Zwanenburg F.A.,MESA Institute for Nanotechnology | Zwanenburg F.A.,University of New South Wales | Dzurak A.S.,University of New South Wales | Morello A.,University of New South Wales | And 6 more authors.
Reviews of Modern Physics | Year: 2013

This review describes recent groundbreaking results in Si, Si/SiGe, and dopant-based quantum dots, and it highlights the remarkable advances in Si-based quantum physics that have occurred in the past few years. This progress has been possible thanks to materials development of Si quantum devices, and the physical understanding of quantum effects in silicon. Recent critical steps include the isolation of single electrons, the observation of spin blockade, and single-shot readout of individual electron spins in both dopants and gated quantum dots in Si. Each of these results has come with physics that was not anticipated from previous work in other material systems. These advances underline the significant progress toward the realization of spin quantum bits in a material with a long spin coherence time, crucial for quantum computation and spintronics. Published by the American Physical Society.


News Article | April 7, 2016
Site: www.cemag.us

Carbon fibers derived from a sustainable source, a type of wild mushroom, and modified with nanoparticles have been shown to outperform conventional graphite electrodes for lithium-ion batteries. Researchers at Purdue University have created electrodes from a species of wild fungus called Tyromyces fissilis. "Current state-of-the-art lithium-ion batteries must be improved in both energy density and power output in order to meet the future energy storage demand in electric vehicles and grid energy-storage technologies," says Vilas Pol, an associate professor in the School of Chemical Engineering and the School of Materials Engineering. "So there is a dire need to develop new anode materials with superior performance." Batteries have two electrodes, called an anode and a cathode. The anodes in most of today's lithium-ion batteries are made of graphite. Lithium ions are contained in a liquid called an electrolyte, and these ions are stored in the anode during recharging. Pol and doctoral student Jialiang Tang have found that carbon fibers derived from Tyromyces fissilis and modified by attaching cobalt oxide nanoparticles outperform conventional graphite in the anodes. The hybrid design has a synergistic result, Pol says. "Both the carbon fibers and cobalt oxide particles are electrochemically active, so your capacity number goes higher because they both participate," he says. The hybrid anodes have a stable capacity of 530 milliamp hours per gram, which is one and a half times greater than graphite's capacity. Findings are detailed in a paper appearing online in the American Chemical Society's Sustainable Chemistry & Engineering journal. One approach for improving battery performance is to modify carbon fibers by attaching certain metals, alloys or metal oxides that allow for increased storage of lithium during recharging. Tang got the idea of tapping fungi for raw materials while researching alternative sources for carbon fibers. "The methods now used to produce carbon fibers for batteries are often chemical heavy and expensive," Tang says. He noticed a mushroom growing on a rotting wood stump in his backyard and decided to study its potential as a source for carbon fibers. "I was curious about the structure so I cut it open and found that it has very interesting properties," he says. "It's very rubbery and yet very tough at the same time. Most interestingly, when I cut it open it has a very fibrous network structure." Comparisons with other fungi showed the Tyromyces fissilis was especially abundant in fibers. The fibers are processed under high temperatures in a chamber containing argon gas using a procedure called pyrolysis, yielding pure carbon in the original shape of the fungus fibers. The fibers have a disordered arrangement and intertwine like spaghetti noodles. The interconnected network brings faster electron transport, which could result in faster battery charging. Electron microscopy studies were performed at the Birck Nanotechnology Center in Purdue's Discovery Park. The work was supported by Purdue's School of Chemical Engineering. The electron microscopy studies at Birck were funded by a Kirk exploratory research grant and were conducted by former postdoctoral research associate Vinodkumar Etacheri.


News Article | October 7, 2016
Site: www.nanotech-now.com

Home > Press > Exotic property confirmed in natural material could lead to fundamental studies Abstract: Auxetic Black Phosphorus: A 2D Material with Negative Poisson's Ratio Yuchen Du1,3, Jesse Maassen1,3,4,*, Wangran Wu1,3, Zhe Luo2,3, Xianfan Xu2,3,*, and Peide D. Ye1,3,* 1 School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 2 School of Mechanical Engineering, Purdue University 3 Birck Nanotechnology Center, Purdue University 4 Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada, * Address correspondence to: (P.D.Y.); (X.X.); (J.M.) The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus, and that our experimental observations are consistent with first principles simulations. Through applying uniaxial strain along armchair direction, we have succeeded in demonstrating a cross-plane interlayer negative Poisson's ratio on black phosphorus for the first time. Meanwhile, our results support the existence of a cross-plane intralayer negative Poisson's ratio in the constituent phosphorene layers under uniaxial deformation along the zigzag axis, which is in line with a previous theoretical prediction. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations. Researchers have confirmed the existence of a naturally occurring exotic property in which a material becomes thicker when stretched - the opposite of most materials - a discovery that could lead to new studies into the fundamental science of nano-materials behavior. The counterintuitive phenomenon, called auxetic behavior, has been extensively studied in engineered structures that have potential applications in medicine, tissue engineering, body armor and "fortified armor enhancement." However, until now the behavior has not been confirmed in natural materials, said Peide Ye, Purdue University's Richard J. and Mary Jo Schwartz Professor of Electrical and Computer Engineering. The auxetic behavior was discovered in a material called black phosphorous. The phenomenon is governed by a fundamental mechanical property of materials called the Poisson's ratio, which characterizes how a material behaves when stretched. Most materials when stretched become thinner and when compressed become thicker, and they are said to have a positive Poisson's ratio. "A negative Poisson's ratio is theoretically possible but until now has not, with few exceptions of man-made structures, been experimentally observed in any natural materials," Ye said. "Here, we show that the negative Poisson's ratio exists in the natural material black phosphorus." Findings are detailed in a research paper that appeared on Sept. 23 in the journal Nano Letters. "Until now, there has been a lack of experimental evidence since the measurement of internal deformation in auxetic materials, in particular at the atomic level, is extremely difficult," Ye said. Researchers used a technique called Raman spectroscopy to document the negative Poisson's ratio in extremely thin, individual layers of black phosphorous called phosphorene. The research was based at the Birck Nanotechnology Center in Purdue's Discovery Park. The Nano Letters paper was authored by doctoral student Yuchen Du; former postdoctoral research associate Jesse Maassen; graduate students Wangran Wu and Zhe Luo; Xianfan Xu, the James J. and Carol L. Shuttleworth Professor of Mechanical Engineering and professor of electrical and computer engineering; and Ye. Du carried out most of the experiments. Maassen performed the theoretical work critical to the research. He is now an assistant professor of physics at Dalhousie University in Nova Scotia, Canada. The researchers focused on the material's uniquely puckered crystal structure in which atoms are arranged in a wavy pattern. Like silicon, the material possesses a bandgap, a trait essential for a semiconductor's ability to switch on and off in electronic circuits. The material also has a relatively high "carrier mobility," meaning it is very conductive and could be useful for technological applications. Future research will include work to investigate whether the negative Poisson's ratio exists in other so-called "two-dimensional" materials, including extremely thin layers of graphite called graphene. The research was funded by the National Science Foundation, U.S. Air Force Office of Scientific Research, the U.S. Army Research Office, and the Natural Sciences and Engineering Research Council of Canada. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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