Institute of Materials Science

Agía Paraskeví, Greece

Institute of Materials Science

Agía Paraskeví, Greece

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

"Stabilizing nanocrystals at elevated temperatures is a common challenge," says Peiman Shahbeigi-Roodposhti, a postdoctoral research scholar with UConn's Institute of Materials Science and the study's lead author. "In certain alloys, we found that high levels of oxygen can lead to a significant reduction in their efficiency." Using a special milling process in an enclosed glove box filled with argon gas, UConn scientists, working in collaboration with researchers from North Carolina State University, were able to synthesize nano-sized crystals of Iron-Chromium and Iron-Chromium-Hafnium with oxygen levels as low as 0.01 percent. These nearly oxygen-free alloy powders appeared to be much more stable than their commercial counterparts with higher oxygen content at elevated temperatures and under high levels of stress. "In this study, for the first time, optimum oxygen-free nanomaterials were developed," says Sina Shahbazmohamadi, an assistant professor of biomedical engineering at UConn and a co-author on the paper. "Various characterization techniques, including advanced aberration corrected transmission electron microscopy, revealed a significant improvement in grain size stability at elevated temperatures." Grain size stability is important for scientists seeking to develop the next generation of advanced materials. Like fine links in an intricately woven mesh, grains are the small solids from which metals are made. Studies have shown that smaller grains are better when it comes to making stronger and tougher metals that are less prone to cracking, better conductors of electricity, and more durable at high temperatures and under extreme stress. Recent advances in technology have allowed materials scientists to develop grains at the scale of just 10 nanometers, which is tens of thousands of times smaller than the thickness of a sheet of paper or the width of a human hair. Such nanocrystals can only be viewed under extremely powerful microscopes. But the process isn't perfect. When some nanograins are created in bulk for applications such as semiconductors, the stability of their size can fluctuate under higher temperatures and stress. It was during the investigation of this instability that Shahbeigi-Roodposhti and the UConn research team learned the role oxygen played in weakening the nanocrystals' stability at high temperatures. "This is only a first step, but this line of investigation could ultimately lead to developing faster jet engines, more capacity in semiconductors, and more sensitivity in biosensors," Shahbeigi-Roodposhti says. Moving forward, the UConn researchers intend to test their theory on other alloys to see whether the presence or absence of oxygen impacts their performance at elevated temperatures. The study, "Effect of oxygen content on thermal stability of grain size for nanocrystalline Fe10Cr and Fe14Cr4Hf alloy powders," which was supported by funding from the U.S. Department of Energy, currently appears online in the Journal of Alloys and Compounds. Explore further: New model should expedite development of temperature-stable nano-alloys More information: Peiman Shahbeigi Roodposhti et al, Effect of oxygen content on thermal stability of grain size for nanocrystalline Fe10Cr and Fe14Cr4Hf alloy powders, Journal of Alloys and Compounds (2017). DOI: 10.1016/j.jallcom.2017.05.261


News Article | July 17, 2017
Site: www.eurekalert.org

Researchers at the University of Connecticut have found that reducing oxygen in some nanocrystalline materials may improve their strength and durability at elevated temperatures, a promising enhancement that could lead to better biosensors, faster jet engines, and greater capacity semiconductors. "Stabilizing nanocrystals at elevated temperatures is a common challenge," says Peiman Shahbeigi-Roodposhti, a postdoctoral research scholar with UConn's Institute of Materials Science and the study's lead author. "In certain alloys, we found that high levels of oxygen can lead to a significant reduction in their efficiency." Using a special milling process in an enclosed glove box filled with argon gas, UConn scientists, working in collaboration with researchers from North Carolina State University, were able to synthesize nano-sized crystals of Iron-Chromium and Iron-Chromium-Hafnium with oxygen levels as low as 0.01 percent. These nearly oxygen-free alloy powders appeared to be much more stable than their commercial counterparts with higher oxygen content at elevated temperatures and under high levels of stress. "In this study, for the first time, optimum oxygen-free nanomaterials were developed," says Sina Shahbazmohamadi, an assistant professor of biomedical engineering at UConn and a co-author on the paper. "Various characterization techniques, including advanced aberration corrected transmission electron microscopy, revealed a significant improvement in grain size stability at elevated temperatures." Grain size stability is important for scientists seeking to develop the next generation of advanced materials. Like fine links in an intricately woven mesh, grains are the small solids from which metals are made. Studies have shown that smaller grains are better when it comes to making stronger and tougher metals that are less prone to cracking, better conductors of electricity, and more durable at high temperatures and under extreme stress. Recent advances in technology have allowed materials scientists to develop grains at the scale of just 10 nanometers, which is tens of thousands of times smaller than the thickness of a sheet of paper or the width of a human hair. Such nanocrystals can only be viewed under extremely powerful microscopes. But the process isn't perfect. When some nanograins are created in bulk for applications such as semiconductors, the stability of their size can fluctuate under higher temperatures and stress. It was during the investigation of this instability that Shahbeigi-Roodposhti and the UConn research team learned the role oxygen played in weakening the nanocrystals' stability at high temperatures. "This is only a first step, but this line of investigation could ultimately lead to developing faster jet engines, more capacity in semiconductors, and more sensitivity in biosensors," Shahbeigi-Roodposhti says. Moving forward, the UConn researchers intend to test their theory on other alloys to see whether the presence or absence of oxygen impacts their performance at elevated temperatures. The study, "Effect of oxygen content on thermal stability of grain size for nanocrystalline Fe10Cr and Fe14Cr4Hf alloy powders," which was supported by funding from the U.S. Department of Energy, currently appears online in the Journal of Alloys and Compounds. Also serving as co-authors on the paper were Mostafa Saber, an assistant professor at Portland State University; and Professors Ronald Scattergood and Carl Koch from North Carolina State University. DOE funding supporting the research was acquired by Scattergood's lab.


News Article | July 6, 2017
Site: www.cemag.us

One afternoon, Carnegie Mellon University Materials Science and Engineering (MSE)'s Mohammad Islam walked into colleague Paul Salvador's office and asked what the biggest problem was in photocatalysis that he'd like to be able to solve. Salvador's answer: He'd like to determine how the oxidation and reduction reactions in photocatalysis could be separated into distinct channels in order to increase performance. A photocatalyst, which uses energy from light to accelerate a reaction, typically facilitates two reactions: an oxidation reaction and a reduction reaction. They are used in generating hydrogen, in remediating environmental biofouling, and potentially for killing drug-resistant bacteria. "We're making open carbon nanotubes," responded Islam, research professor of MSE, "so how about we put the photocatalyst on the outside and the co-catalyst on the inside of each nanotube?" Salvador, professor of MSE, said he thought that was an elegant solution—but was it possible? Thus was formed a team including Islam, Salvador, and MSE Professor and Department Head Greg Rohrer, with Ph.D. student Hang-Ah Park, master's student Siyuan Liu, and former postdoc Youngseok Oh (currently a senior scientist at the Korea Institute of Materials Science). Recently, the team published a paper on their new approach to optimizing photocatalysts. Like many Carnegie Mellon research projects, the project started with a problem that could only be solved through collaboration. The challenge: photocatalysts need to be cheap, efficient, and environmentally friendly. Though current photocatalysts may be inexpensive, they either have high toxicity or don't perform well. In a photocatalyst, both the oxidation reaction and the reduction reaction need to be optimized, as does the space between these reactions. Typically, a photocatalyst that is good at performing one type of reaction (such as oxidation) has a co-catalyst added to it that is good at performing the opposite reaction (reduction). Though this helps with optimization, the reactions are not entirely separated, and therefore, products such as hydrogen and oxygen are generated in the same space. "Imagine that you have a micrometer-sized sphere known to be good at oxidation, and you add onto it small co-catalyst hemispheres known to be good at reduction (typically 10 nanometers)," says Rohrer. "Even though the reactions are technically separated, they are still occurring in close proximity, which decreases the photocatalyst's performance. So, we put them in completely different channels." What makes their work novel is not the complete separation of the channels, which is well known in standard photoelectrochemical cells (PECs), but that they brought a PEC down to the nanoscale, developed massively parallel arrays of those nanoscale PECs, and maintained complete separation. "It's a very simple idea," says Salvador. "Many of us have done lab experiments in high school or college using traditional PECs, which separate products into two large beakers. We have taken that huge PEC from chemistry lab and brought it down to the nanoscale, and then we fabricated thousands of them that operate in parallel. In that process, we found some interesting new fundamental materials behavior, including high activity in visible light, and saw a phenomenal performance that has many applications." A big application of photocatalysts is in remediating environmental biofouling, or removing organisms like barnacles and algae from surfaces such as pipes. Another application is in killing drug-resistant bacteria. Many hospitals, for example, use paints loaded with titania and irradiated with UV light to disinfect walls or other surfaces. But with the new photocatalytic method, they can use visible light, which is much safer. Finally, during hydrogen generation their photocatalysts suppress the mixing of product gases, an important advancement. "The question now is, why is it doing a lot better?" says Islam. "Why did it become photoactive in the visible light when I am doing this with carbon nanotubes and titanium? What are the parameters that we can tweak to make it better? That's the direction we're going."


News Article | July 7, 2017
Site: phys.org

A photocatalyst, which uses energy from light to accelerate a reaction, typically facilitates two reactions: an oxidation reaction and a reduction reaction. They are used in generating hydrogen, in remediating environmental biofouling, and potentially for killing drug-resistant bacteria. "We're making open carbon nanotubes," responded Islam, research professor of MSE, "so how about we put the photocatalyst on the outside and the co-catalyst on the inside of each nanotube?" Salvador, professor of MSE, said he thought that was an elegant solution—but was it possible? Thus was formed a team including Islam, Salvador, and MSE Professor and Department Head Greg Rohrer, with Ph.D. student Hang-Ah Park, master's student Siyuan Liu, and former postdoc Youngseok Oh (currently a senior scientist at the Korea Institute of Materials Science). Recently, the team published a paper on their new approach to optimizing photocatalysts. Like many Carnegie Mellon research projects, the project started with a problem that could only be solved through collaboration. The challenge: photocatalysts need to be cheap, efficient, and environmentally friendly. Though current photocatalysts may be inexpensive, they either have high toxicity or don't perform well. In a photocatalyst, both the oxidation reaction and the reduction reaction need to be optimized, as does the space between these reactions. Typically, a photocatalyst that is good at performing one type of reaction (such as oxidation) has a co-catalyst added to it that is good at performing the opposite reaction (reduction). Though this helps with optimization, the reactions are not entirely separated, and therefore, products such as hydrogen and oxygen are generated in the same space. "Imagine that you have a micrometer-sized sphere known to be good at oxidation, and you add onto it small co-catalyst hemispheres known to be good at reduction (typically 10 nanometers)," says Rohrer. "Even though the reactions are technically separated, they are still occurring in close proximity, which decreases the photocatalyst's performance. So, we put them in completely different channels." What makes their work novel is not the complete separation of the channels, which is well known in standard photoelectrochemical cells (PECs), but that they brought a PEC down to the nanoscale, developed massively parallel arrays of those nanoscale PECs, and maintained complete separation. "It's a very simple idea," says Salvador. "Many of us have done lab experiments in high school or college using traditional PECs, which separate products into two large beakers. We have taken that huge PEC from chemistry lab and brought it down to the nanoscale, and then we fabricated thousands of them that operate in parallel. In that process, we found some interesting new fundamental materials behavior, including high activity in visible light, and saw a phenomenal performance that has many applications." A big application of photocatalysts is in remediating environmental biofouling, or removing organisms like barnacles and algae from surfaces such as pipes. Another application is in killing drug-resistant bacteria. Many hospitals, for example, use paints loaded with titania and irradiated with UV light to disinfect walls or other surfaces. But with the new photocatalytic method, they can use visible light, which is much safer. Finally, during hydrogen generation their photocatalysts suppress the mixing of product gases, an important advancement. "The question now is, why is it doing a lot better?" says Islam. "Why did it become photoactive in the visible light when I am doing this with carbon nanotubes and titanium? What are the parameters that we can tweak to make it better? That's the direction we're going." Explore further: Nanomaterials with potential for environmentally friendly hydrogen production More information: Hang-Ah Park et al, Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels, ACS Nano (2017). DOI: 10.1021/acsnano.6b08387


News Article | August 1, 2017
Site: www.materialstoday.com

Researchers at the University of Connecticut have found that reducing oxygen in some nanocrystalline materials may improve their strength and durability at elevated temperatures. This is a promising enhancement, reported in a paper in the Journal of Alloys and Compounds, that could lead to better biosensors, faster jet engines and greater capacity semiconductors. "Stabilizing nanocrystals at elevated temperatures is a common challenge," says Peiman Shahbeigi-Roodposhti, a postdoctoral research scholar with UConn's Institute of Materials Science and the paper's lead author. "In certain alloys, we found that high levels of oxygen can lead to a significant reduction in their efficiency." Using a special milling process in an enclosed glove box filled with argon gas, UConn scientists, working in collaboration with researchers from North Carolina State University, were able to synthesize nano-sized crystals of iron chromium and iron chromium hafnium with oxygen levels as low as 0.01%. These nearly oxygen-free alloy powders appeared to be much more stable at elevated temperatures and under high levels of stress than their commercial counterparts with higher oxygen contents. "In this study, for the first time, optimum oxygen-free nanomaterials were developed," explains Sina Shahbazmohamadi, an assistant professor of biomedical engineering at UConn and a co-author on the paper. "Various characterization techniques, including advanced aberration corrected transmission electron microscopy, revealed a significant improvement in grain size stability at elevated temperatures." Grain size stability is important for scientists seeking to develop the next generation of advanced materials. Like fine links in an intricately woven mesh, grains are the small solids from which metals are made. Studies have shown that smaller grains are better when it comes to making stronger and tougher metals that are less prone to cracking, better at conducting electricity, and more durable at high temperatures and under extreme stress. Recent advances in technology have allowed materials scientists to develop grains at the scale of just 10nm. Such nanocrystals can only be viewed under extremely powerful microscopes. But the process isn't perfect. When some nanograins are created in bulk for applications such as semiconductors, the stability of their size can fluctuate under higher temperatures and stress. It was while investigating this instability that Shahbeigi-Roodposhti and the UConn research team learned the role oxygen played in weakening the nanocrystals' stability at high temperatures. "This is only a first step, but this line of investigation could ultimately lead to developing faster jet engines, more capacity in semiconductors and more sensitivity in biosensors," Shahbeigi-Roodposhti says. Moving forward, the UConn researchers intend to test their theory on other alloys to see whether the presence or absence of oxygen impacts their performance at elevated temperatures. This story is adapted from material from the University of Connecticut, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.


News Article | September 27, 2016
Site: www.cemag.us

A team of scientists studying solar cells made from cadmium telluride, a promising alternative to silicon, has discovered that microscopic "fault lines" within and between crystals of the material act as conductive pathways that ease the flow of electric current. This research—conducted at the University of Connecticut and the U.S. Department of Energy's Brookhaven National Laboratory, and described in the journal Nature Energy—may help explain how a common processing technique turns cadmium telluride into an excellent material for transforming sunlight into electricity, and suggests a strategy for engineering more efficient solar devices that surpass the performance of silicon. "If you look at semiconductors like silicon, defects in the crystals are usually bad," said co-author Eric Stach, a physicist at Brookhaven Lab's Center for Functional Nanomaterials (CFN). As Stach explained, misplaced atoms or slight shifts in their alignment often act as traps for the particles that carry electric current—negatively charged electrons or the positively charged "holes" left behind when electrons are knocked loose by photons of sunlight, making them more mobile. The idea behind solar cells is to separate the positive and negative charges and run them through a circuit so the current can be used to power houses, satellites, or even cities. Defects interrupt this flow of charges and keep the solar cell from being as efficient as it could be. But in the case of cadmium telluride, the scientists found that boundaries between individual crystals and "planar defects"—fault-like misalignments in the arrangement of atoms—create pathways for conductivity, not traps. Members of Bryan Huey's group at the Institute of Materials Science at the University of Connecticut were the first to notice the surprising connection. In an effort to understand the effects of a chloride solution treatment that greatly enhances cadmium telluride's conductive properties, Justin Luria and Yasemin Kutes studied solar cells before and after treatment. But they did so in a unique way. Several groups around the world had looked at the surfaces of such solar cells before, often with a tool known as a conducting atomic force microscope. The microscope has a fine probe many times sharper than the head of a pin that scans across the material's surface to track the topographic features—the hills and valleys of the surface structure—while simultaneously measuring location-specific conductivity. Scientists use this technique to explore how the surface features relate to solar cell performance at the nanoscale. But no one had devised a way to make measurements beneath the surface, the most important part of the solar cell. This is where the UConn team made an important breakthrough. They used an approach developed and perfected by Kutes and Luria over the last two years to acquire hundreds of sequential images, each time intentionally removing a nanoscale layer of the material, so they could scan through the entire thickness of the sample. They then used these layer-by-layer images to build up a three-dimensional, high-resolution 'tomographic' map of the solar cell—somewhat like a computed tomography (CT) brain scan. "Everyone using these microscopes basically takes pictures of the 'ground,' and interprets what is beneath," Huey said. "It may look like there's a cave, or a rock shelf, or a building foundation down there. But we can only really know once we carefully dig, like archeologists, keeping track of exactly what we find every step of the way—though, of course, at a much, much smaller scale." The resulting CT-AFM maps uniquely revealed current flowing most freely along the crystal boundaries and fault-like defects in the cadmium telluride solar cells. The samples that had been treated with the chloride solution had more defects overall, a higher density of these defects, and what appeared to be a high degree of connectivity among them, while the untreated samples had few defects, no evidence of connectivity, and much lower conductivity. Huey's team suspected that the defects were so-called planar defects, usually caused by shifts in atomic alignments or stacking arrangements within the crystals. But the CTAFM system is not designed to reveal such atomic-scale structural details. To get that information, the UConn team turned to Stach, head of the electron microscopy group at the CFN, a DOE Office of Science User Facility. "Having previously shared ideas with Eric, it was a natural extension of our discovery to work with his group," Huey said. Said Stach, "This is the exact type of problem the CFN is set up to handle, providing expertise and equipment that university researchers may not have to help drive science from hypothesis to discovery." CFN staff physicist Lihua Zhang used a transmission electron microscope (TEM) and UConn's results as a guide to meticulously study how atomic scale features of chloride-treated cadmium telluride related to the conductivity maps. The TEM images revealed the atomic structure of the defects, confirming that they were due to specific changes in the stacking sequence of atoms in the material. The images also showed clearly that these planar defects connected different grains in the crystal, leading to high-conductivity pathways for the movement of electrons and holes. "When we looked at the regions with good conductivity, the planar defects linked from one crystal grain to another, forming continuous pathways of conductance through the entire thickness of the material," said Zhang. "So the regions that had the best conductivity were the ones that had a high degree of connectivity among these defects." The authors say it's possible that the chloride treatment helps to create the connectivity, not just more defects, but that more research is needed to definitively determine the most significant effects of the chloride solution treatment. In any case, Stach says that combining the CTAFM technique and electron microscopy, yields a "clear winner" in the search for more efficient, cost-competitive alternatives to silicon solar cells, which have nearly reached their limit for efficiency. "There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon. But all of these alternatives, because of their crystal structure, have a higher tendency to form defects," he said. "This work gives us a systematic method we can use to understand if the defects are good or bad in terms of conductivity. It can also be used to explore the effects of different processing methods or chemicals to control how defects form. In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance." This research was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE)—including its SunShot Initiative—and the DOE Office of Science. The cadmium telluride samples were provided by Andrew Moore of Colorado State University.


Carbon nanotube above a photonic crystal waveguide with electrodes. The structure converts electric signals into light. Credit: WWU Worldwide growing data volumes make conventional electronic processing reach its limits. Future information technology is therefore expected to use light as a medium for quick data transmission also within computer chips. Researchers under the direction of KIT have now demonstrated that carbon nanotubes are suited for use as on-chip light source for tomorrow's information technology, when nanostructured waveguides are applied to obtain the desired light properties. The scientists now present their results in Nature Photonics. On the large scale, data transmission by light has long become a matter of routine: Glass fiber cables as light waveguides transmit telephone and internet signals, for instance. For using the advantages of light, i.e. speed and energy efficiency, also on the small scale of computer chips, researchers of KIT have made an important step from fundamental research towards application. By the integration of smallest carbon nanotubes into a nanostructured waveguide, they have developed a compact miniaturized switching element that converts electric signals into clearly defined optical signals. "The nanostructures act like a photonic crystal and allow for customizing the properties of light from the carbon nanotube," Felix Pyatkov and Valentin Fütterling, the first authors of the study of KIT's Institute of Nanotechnology, explain. "In this way, we can generate narrow-band light in the desired color on the chip." Processing of the waveguide precisely defines the wavelength at which the light is transmitted. By engravings using electron beam lithography, the waveguides of several micrometers in length are provided with finest cavities of a few nanometers in size. They determine the waveguide's optical properties. The resulting photonic crystals reflect the light in certain colors, a phenomenon observed in nature on apparently colorful butterfly wings. As novel light sources, carbon nanotubes of about 1 micrometer in length and 1 nanometer in diameter are positioned on metal contacts in transverse direction to the waveguide. At KIT, a process was developed, by means of which the nanotubes can be integrated specifically into highly complex structures. The researchers applied the method of dielectrophoresis for deposition of carbon nanotubes from the solution and their arrangement vertically to the waveguide. This way of separating particles using inhomogeneous electric fields was originally used in biology and is highly suited to deposit nanoscaled objects on carrier materials. The carbon nanotubes integrated into the waveguide act as a small light source. When electric voltage is applied, they produce photons. The compact electricity/light signal converter presented now meets the requirements of the next generation of computers that combine electronic components with nanophotonic waveguides. The signal converter bundles the light about as strongly as a laser and responds to variable signals with high speed. Already now, the optoelectronic components developed by the researchers can be used to produce light signals in the gigahertz frequency range from electric signals. Among the leading researchers involved in the project were Ralph Krupke, who conducts research at the KIT Institute of Nanotechnology and at the Institute of Materials Science of TU Darmstadt, Wolfram H.P. Pernice, who moved from the KIT to the University of Münster one year ago, and Manfred M. Kappes, Institute of Physical Chemistry and Institute of Nanotechnology of KIT. The project was funded by the Science and Technology of Nanosystems (STN) programme of the Helmholtz Association. It is aimed at studying nanosystems of unique functionality and the potential of materials of a few nanometers in structural size. The Volkswagen Foundation financed a Ph.D. student position for the research project. In addition, the project was supported by the Karlsruhe Nano Micro Facility (KNMF) platform. Explore further: World-record micrometer-sized converter of electrical into optical signals More information: Felix Pyatkov et al. Cavity-enhanced light emission from electrically driven carbon nanotubes, Nature Photonics (2016). DOI: 10.1038/NPHOTON.2016.70


Home > Press > Nature Photonics: Light source for quicker computer chips: Waveguide with integrated carbon nanotubes for conversion of electric signals into light / quicker computer chips are feasible / publication in Nature Photonics Abstract: Worldwide growing data volumes make conventional electronic processing reach its limits. Future information technology is therefore expected to use light as a medium for quick data transmission also within computer chips. Researchers under the direction of KIT have now demonstrated that carbon nanotubes are suited for use as on-chip light source for tomorrow's information technology, when nanostructured waveguides are applied to obtain the desired light properties. The scientists now present their results in Nature Photonics. DOI: 10.1038/NPHOTON. 2016.70 On the large scale, data transmission by light has long become a matter of routine: Glass fiber cables as light waveguides transmit telephone and internet signals, for instance. For using the advantages of light, i.e. speed and energy efficiency, also on the small scale of computer chips, researchers of KIT have made an important step from fundamental research towards application. By the integration of smallest carbon nanotubes into a nanostructured waveguide, they have developed a compact miniaturized switching element that converts electric signals into clearly defined optical signals. "The nanostructures act like a photonic crystal and allow for customizing the properties of light from the carbon nanotube," Felix Pyatkov and Valentin Fütterling, the first authors of the study of KIT's Institute of Nanotechnology, explain. "In this way, we can generate narrow-band light in the desired color on the chip." Processing of the waveguide precisely defines the wavelength at which the light is transmitted. By engravings using electron beam lithography, the waveguides of several micrometers in length are provided with finest cavities of a few nanometers in size. They determine the waveguide's optical properties. The resulting photonic crystals reflect the light in certain colors, a phenomenon observed in nature on apparently colorful butterfly wings. As novel light sources, carbon nanotubes of about 1 micrometer in length and 1 nanometer in diameter are positioned on metal contacts in transverse direction to the waveguide. At KIT, a process was developed, by means of which the nanotubes can be integrated specifically into highly complex structures. The researchers applied the method of dielectrophoresis for deposition of carbon nanotubes from the solution and their arrangement vertically to the waveguide. This way of separating particles using inhomogeneous electric fields was originally used in biology and is highly suited to deposit nanoscaled objects on carrier materials. The carbon nanotubes integrated into the waveguide act as a small light source. When electric voltage is applied, they produce photons. The compact electricity/light signal converter presented now meets the requirements of the next generation of computers that combine electronic components with nanophotonic waveguides. The signal converter bundles the light about as strongly as a laser and responds to variable signals with high speed. Already now, the optoelectronic components developed by the researchers can be used to produce light signals in the gigahertz frequency range from electric signals. Among the leading researchers involved in the project were Ralph Krupke, who conducts research at the KIT Institute of Nanotechnology and at the Institute of Materials Science of TU Darmstadt, Wolfram H.P. Pernice, who moved from the KIT to the University of Münster one year ago, and Manfred M. Kappes, Institute of Physical Chemistry and Institute of Nanotechnology of KIT. The project was funded by the Science and Technology of Nanosystems (STN) programme of the Helmholtz Association. It is aimed at studying nanosystems of unique functionality and the potential of materials of a few nanometers in structural size. The Volkswagen Foundation financed a Ph.D. student position for the research project. In addition, the project was supported by the Karlsruhe Nano Micro Facility (KNMF) platform. About Karlsruhe Institute of Technology (KIT) Karlsruhe Institute of Technology (KIT) pools its three core tasks of research, higher education, and innovation in a mission. With about 9,300 employees and 25,000 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe. KIT - The Research University in the Helmholtz Association Since 2010, the KIT has been certified as a family-friendly university. 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.


Home > Press > Crystalline Fault Lines Provide Pathway for Solar Cell Current: New tomographic AFM imaging technique reveals that microstructural defects, generally thought to be detrimental, actually improve conductivity in cadmium telluride solar cells Abstract: A team of scientists studying solar cells made from cadmium telluride, a promising alternative to silicon, has discovered that microscopic "fault lines" within and between crystals of the material act as conductive pathways that ease the flow of electric current. This research-conducted at the University of Connecticut and the U.S. Department of Energy's Brookhaven National Laboratory, and described in the journal Nature Energy-may help explain how a common processing technique turns cadmium telluride into an excellent material for transforming sunlight into electricity, and suggests a strategy for engineering more efficient solar devices that surpass the performance of silicon. "If you look at semiconductors like silicon, defects in the crystals are usually bad," said co-author Eric Stach, a physicist at Brookhaven Lab's Center for Functional Nanomaterials (CFN). As Stach explained, misplaced atoms or slight shifts in their alignment often act as traps for the particles that carry electric current-negatively charged electrons or the positively charged "holes" left behind when electrons are knocked loose by photons of sunlight, making them more mobile. The idea behind solar cells is to separate the positive and negative charges and run them through a circuit so the current can be used to power houses, satellites, or even cities. Defects interrupt this flow of charges and keep the solar cell from being as efficient as it could be. But in the case of cadmium telluride, the scientists found that boundaries between individual crystals and "planar defects"-fault-like misalignments in the arrangement of atoms-create pathways for conductivity, not traps. Members of Bryan Huey's group at the Institute of Materials Science at the University of Connecticut were the first to notice the surprising connection. In an effort to understand the effects of a chloride solution treatment that greatly enhances cadmium telluride's conductive properties, Justin Luria and Yasemin Kutes studied solar cells before and after treatment. But they did so in a unique way. Several groups around the world had looked at the surfaces of such solar cells before, often with a tool known as a conducting atomic force microscope. The microscope has a fine probe many times sharper than the head of a pin that scans across the material's surface to track the topographic features-the hills and valleys of the surface structure-while simultaneously measuring location-specific conductivity. Scientists use this technique to explore how the surface features relate to solar cell performance at the nanoscale. But no one had devised a way to make measurements beneath the surface, the most important part of the solar cell. This is where the UConn team made an important breakthrough. They used an approach developed and perfected by Kutes and Luria over the last two years to acquire hundreds of sequential images, each time intentionally removing a nanoscale layer of the material, so they could scan through the entire thickness of the sample. They then used these layer-by-layer images to build up a three-dimensional, high-resolution 'tomographic' map of the solar cell-somewhat like a computed tomography (CT) brain scan. "Everyone using these microscopes basically takes pictures of the 'ground,' and interprets what is beneath," Huey said. "It may look like there's a cave, or a rock shelf, or a building foundation down there. But we can only really know once we carefully dig, like archeologists, keeping track of exactly what we find every step of the way-though, of course, at a much, much smaller scale." The resulting CT-AFM maps uniquely revealed current flowing most freely along the crystal boundaries and fault-like defects in the cadmium telluride solar cells. The samples that had been treated with the chloride solution had more defects overall, a higher density of these defects, and what appeared to be a high degree of connectivity among them, while the untreated samples had few defects, no evidence of connectivity, and much lower conductivity. Huey's team suspected that the defects were so-called planar defects, usually caused by shifts in atomic alignments or stacking arrangements within the crystals. But the CTAFM system is not designed to reveal such atomic-scale structural details. To get that information, the UConn team turned to Stach, head of the electron microscopy group at the CFN, a DOE Office of Science User Facility. "Having previously shared ideas with Eric, it was a natural extension of our discovery to work with his group," Huey said. Said Stach, "This is the exact type of problem the CFN is set up to handle, providing expertise and equipment that university researchers may not have to help drive science from hypothesis to discovery." CFN staff physicist Lihua Zhang used a transmission electron microscope (TEM) and UConn's results as a guide to meticulously study how atomic scale features of chloride-treated cadmium telluride related to the conductivity maps. The TEM images revealed the atomic structure of the defects, confirming that they were due to specific changes in the stacking sequence of atoms in the material. The images also showed clearly that these planar defects connected different grains in the crystal, leading to high-conductivity pathways for the movement of electrons and holes. "When we looked at the regions with good conductivity, the planar defects linked from one crystal grain to another, forming continuous pathways of conductance through the entire thickness of the material," said Zhang. "So the regions that had the best conductivity were the ones that had a high degree of connectivity among these defects." The authors say it's possible that the chloride treatment helps to create the connectivity, not just more defects, but that more research is needed to definitively determine the most significant effects of the chloride solution treatment. In any case, Stach says that combining the CTAFM technique and electron microscopy, yields a "clear winner" in the search for more efficient, cost-competitive alternatives to silicon solar cells, which have nearly reached their limit for efficiency. "There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon. But all of these alternatives, because of their crystal structure, have a higher tendency to form defects," he said. "This work gives us a systematic method we can use to understand if the defects are good or bad in terms of conductivity. It can also be used to explore the effects of different processing methods or chemicals to control how defects form. In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance." This research was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE)-including its Sunshot Program-and the DOE Office of Science. The cadmium telluride samples were provided by Andrew Moore of Colorado State University. The University of Connecticut's Institute of Materials Science serves as the heart of materials science research at the University of Connecticut, with a mission of supporting materials research and industry throughout Connecticut and the Northeast. It houses the research labs of more than 30 core faculty, with an overall membership of 120 UConn faculty whose work benefits from the available facilities and expertise. About Brookhaven National Laboratory Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov. One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization. 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.


Pradhan S.K.,Institute of Materials Science
Journal of Materials Science: Materials in Electronics | Year: 2013

Monophasic rhombohedral structure of BiFeO3 electroceramic is successfully synthesized by conventional solid state reaction route followed by slow step sintering schedule. Effect of sintering temperature is found to greatly influence its structural, dielectric, ferroelectric, capacitance and leakage behavior of bulk ceramic. From XRD analysis it is seen that at lower sintering temperature (750 C) bulk BiFeO3 sample showed rhombohedral structure (R3c) along with few impurity phases, which become suppressed at higher sintering temperature and facilitates the compactness of grains and formation of dense microstructure. The leakage current and capacitive characteristic of the sample was improved significantly with increase in sintering temperature of BiFeO3 (850 C). At higher sintering temperature, ferroelectric behavior of the sample is found to change its shape from semi elliptical lossy P-E features to a typical ferroelectric loop with improvement of its remnant as well as saturation polarization value. Raman spectra over the frequency range of 100-700 cm-1 have been systematically investigated. Besides the changes of the peak position and the line width of all modes, the prominent frequency shift, the line broadening and variation of the intensity were observed with increase in sintering temperature. © 2013 Springer Science+Business Media New York.

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