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  • richardmitnick 1:23 pm on September 23, 2018 Permalink | Reply
    Tags: , , , , New battery gobbles up carbon dioxide, Scanning Electron microscopy   

    From MIT News: “New battery gobbles up carbon dioxide” 

    MIT News
    MIT Widget

    From MIT News

    September 21, 2018
    David L. Chandler

    1
    This scanning electron microscope image shows the carbon cathode of a carbon-dioxide-based battery made by MIT researchers, after the battery was discharged. It shows the buildup of carbon compounds on the surface, composed of carbonate material that could be derived from power plant emissions, compared to the original pristine surface (inset). Courtesy of the researchers

    Scanning transmission electron microscope Wikipedia

    Lithium-based battery could make use of greenhouse gas before it ever gets into the atmosphere.

    A new type of battery developed by researchers at MIT could be made partly from carbon dioxide captured from power plants. Rather than attempting to convert carbon dioxide to specialized chemicals using metal catalysts, which is currently highly challenging, this battery could continuously convert carbon dioxide into a solid mineral carbonate as it discharges.

    While still based on early-stage research and far from commercial deployment, the new battery formulation could open up new avenues for tailoring electrochemical carbon dioxide conversion reactions, which may ultimately help reduce the emission of the greenhouse gas to the atmosphere.

    The battery is made from lithium metal, carbon, and an electrolyte that the researchers designed. The findings are described today in the journal Joule, in a paper by assistant professor of mechanical engineering Betar Gallant, doctoral student Aliza Khurram, and postdoc Mingfu He.

    Currently, power plants equipped with carbon capture systems generally use up to 30 percent of the electricity they generate just to power the capture, release, and storage of carbon dioxide. Anything that can reduce the cost of that capture process, or that can result in an end product that has value, could significantly change the economics of such systems, the researchers say.

    However, “carbon dioxide is not very reactive,” Gallant explains, so “trying to find new reaction pathways is important.” Generally, the only way to get carbon dioxide to exhibit significant activity under electrochemical conditions is with large energy inputs in the form of high voltages, which can be an expensive and inefficient process. Ideally, the gas would undergo reactions that produce something worthwhile, such as a useful chemical or a fuel. However, efforts at electrochemical conversion, usually conducted in water, remain hindered by high energy inputs and poor selectivity of the chemicals produced.

    Gallant and her co-workers, whose expertise has to do with nonaqueous (not water-based) electrochemical reactions such as those that underlie lithium-based batteries, looked into whether carbon-dioxide-capture chemistry could be put to use to make carbon-dioxide-loaded electrolytes — one of the three essential parts of a battery — where the captured gas could then be used during the discharge of the battery to provide a power output.

    This approach is different from releasing the carbon dioxide back to the gas phase for long-term storage, as is now used in carbon capture and sequestration, or CCS. That field generally looks at ways of capturing carbon dioxide from a power plant through a chemical absorption process and then either storing it in underground formations or chemically altering it into a fuel or a chemical feedstock.

    Instead, this team developed a new approach that could potentially be used right in the power plant waste stream to make material for one of the main components of a battery.

    While interest has grown recently in the development of lithium-carbon-dioxide batteries, which use the gas as a reactant during discharge, the low reactivity of carbon dioxide has typically required the use of metal catalysts. Not only are these expensive, but their function remains poorly understood, and reactions are difficult to control.

    By incorporating the gas in a liquid state, however, Gallant and her co-workers found a way to achieve electrochemical carbon dioxide conversion using only a carbon electrode. The key is to preactivate the carbon dioxide by incorporating it into an amine solution.

    “What we’ve shown for the first time is that this technique activates the carbon dioxide for more facile electrochemistry,” Gallant says. “These two chemistries — aqueous amines and nonaqueous battery electrolytes — are not normally used together, but we found that their combination imparts new and interesting behaviors that can increase the discharge voltage and allow for sustained conversion of carbon dioxide.”

    They showed through a series of experiments that this approach does work, and can produce a lithium-carbon dioxide battery with voltage and capacity that are competitive with that of state-of-the-art lithium-gas batteries. Moreover, the amine acts as a molecular promoter that is not consumed in the reaction.

    The key was developing the right electrolyte system, Khurram explains. In this initial proof-of-concept study, they decided to use a nonaqueous electrolyte because it would limit the available reaction pathways and therefore make it easier to characterize the reaction and determine its viability. The amine material they chose is currently used for CCS applications, but had not previously been applied to batteries.

    This early system has not yet been optimized and will require further development, the researchers say. For one thing, the cycle life of the battery is limited to 10 charge-discharge cycles, so more research is needed to improve rechargeability and prevent degradation of the cell components. “Lithium-carbon dioxide batteries are years away” as a viable product, Gallant says, as this research covers just one of several needed advances to make them practical.

    But the concept offers great potential, according to Gallant. Carbon capture is widely considered essential to meeting worldwide goals for reducing greenhouse gas emissions, but there are not yet proven, long-term ways of disposing of or using all the resulting carbon dioxide. Underground geological disposal is still the leading contender, but this approach remains somewhat unproven and may be limited in how much it can accommodate. It also requires extra energy for drilling and pumping.

    The researchers are also investigating the possibility of developing a continuous-operation version of the process, which would use a steady stream of carbon dioxide under pressure with the amine material, rather than a preloaded supply the material, thus allowing it to deliver a steady power output as long as the battery is supplied with carbon dioxide. Ultimately, they hope to make this into an integrated system that will carry out both the capture of carbon dioxide from a power plant’s emissions stream, and its conversion into an electrochemical material that could then be used in batteries. “It’s one way to sequester it as a useful product,” Gallant says.

    “It was interesting that Gallant and co-workers cleverly combined the prior knowledge from two different areas, metal-gas battery electrochemistry and carbon-dioxide capture chemistry, and succeeded in increasing both the energy density of the battery and the efficiency of the carbon-dioxide capture,” says Kisuk Kang, a professor at Seoul National University in South Korea, who was not associated with this research.

    “Even though more precise understanding of the product formation from carbon dioxide may be needed in the future, this kind of interdisciplinary approach is very exciting and often offers unexpected results, as the authors elegantly demonstrated here,” Kang adds.

    See the full article here .


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  • richardmitnick 5:00 pm on January 3, 2018 Permalink | Reply
    Tags: , Bottom-up nanofabrication or molecular self-assembly, Center for Functional Nanomaterials(CFN), Directed self-assembly - combining the two approaches, Gregory Doerk at CFN, Lithography is generally faster–and more reliably produces the designed structures–but requires expensive complex tools, , , , Scanning Electron microscopy, Self-assembly is often slower and less predictive but it is inexpensive and can be easier, Top-down nanofabrication, X-ray scattering experiments,   

    From BNL- “CFN Scientist Spotlight: Gregory Doerk Guides the Self-Assembly of Materials to Make Diverse Nanoscale Patterns” 

    Brookhaven Lab

    December 7, 2017 [They just now put this up n social media.]
    Interview with a CFN scientist

    1
    Materials scientist Gregory Doerk in the materials processing lab at CFN.

    Some materials have the unique ability to self-assemble into organized molecular patterns and structures. Materials scientist Gregory Doerk of the Electronic Nanomaterials Group at the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory—takes advantage of this ability in materials called block copolymers. Using these self-assembling materials, which have chains of two or more distinct molecules linked together by chemical bonds, Doerk directs the formation of such patterns and structures at the nanoscale. The ultimate goal is to leverage these nanoscale architectures to control the properties of materials for applications including solar energy conversion and storage, catalysis, and optics.

    Self-assembly has gained a lot of attention recently as a nanofabrication approach. How does it differ from the approach that has been traditionally used?

    Broadly speaking, there are two platforms for nanofabrication. One platform is top-down nanofabrication, which is the traditional approach used to make computer chips and other microelectronics. Light is used to create patterns that are then carved into silicon wafers. The patterning technique using light is called optical lithography. The other approach is bottom-up nanofabrication, or molecular self-assembly. The properties of the materials are encoded in their weak interactions, which drive certain materials to come together and form specific configurations—kind of like Lego bricks but the bricks all build up on their own to form some structure.

    Lithography is generally faster–and more reliably produces the designed structures–but requires expensive, complex tools. Self-assembly is often slower and less predictive but it is inexpensive and can be easier. There are ways of combining the two approaches, and that combination is known as directed self-assembly. Self-assembling materials such as thin films of block copolymers are ordered using templates patterned by standard lithography. Directed self-assembly enhances current manufacturing processes, expanding the spectrum of pattern geometries that are possible, and lowers the cost of nanofabrication.

    Your self-assembly research is focused on block copolymers. What makes these materials so special?

    3
    The length of the polymer blocks (red and blue squiggly lines) and the strength of their interaction determine the shape of the resulting patterns in block copolymer self-assembly: (from left to right) spheres, cylinders, and lamellae (sheets).

    Since the 1950s, people have been using triblock copolymers (three polymers joined together) in materials like synthetic rubber. Most polymers will not mix. Trying to mix polymers is kind of like trying to mix oil and water.

    But in block copolymers, the polymer chains are chemically bound to each other. For example, diblock (two) copolymer chains are joined through a covalent bond. This bonding frustrates their drive to “demix.” Instead, if block copolymers are given energy to mobilize their chains—for example by annealing (heating) the polymer film on a hot plate to above its glass transition temperature (when a polymer transitions from a hard, glassy material to a soft, rubbery one)—the chains will reconfigure and assemble into nanoscale phase-separated domains. These domains are ordered into nanoscale patterns based on inherent traits of the block copolymer. For example, let us say we have two polymers, A and B, joined together (diblock copolymer). The length of block A relative to the length of both blocks, the overall length of the polymer chains, and the properties of the polymers and the strength of their interaction (how much the polymers want to pull apart) govern the size and morphology, or shape, of the resulting structures. If the chains are too short or their repulsive interactions are too weak, the polymers will mix, causing the ordered state to become a disordered state and thus no pattern will form.

    At CFN, a major part of our work is to transfer the patterns and structures made through self-assembly, electron-beam lithography, or optical lithography at our Nanofabrication Facility into other materials to control material properties. A great example of this is CFN Director Chuck Black’s work using block copolymers to create a self-assembled pattern that serves as a template for etching nanosized cones onto silicon surfaces. With these nanotextures, the silicon surfaces transformed from reflective mirrors to entirely black. That is one way we can use block copolymers.

    But we want to expand the range of things we can do with self-assembly—and thus the range of applications. Diversifying the patterns possible with block copolymer self-assembly is a big part of my research.

    Have you been able to expand this range?

    One way I am interested in expanding what we can do through self-assembly is by creating inorganic replicas of the self-assembled structures. We can accomplish this replication through a process called infiltration synthesis.

    4
    Plan view (top) and cross-sectional (bottom) scanning electron microscope images of an inorganic nanomesh created through iterative self-assembly and infiltration synthesis.

    For example, say you have a block copolymer that forms lamellae (stripes) perpendicular to a substrate. Using an atomic-layer-deposition tool, it is possible to infiltrate those lamellae with a metal oxide and then remove the polymer, leaving behind metal oxide lines in a pattern determined by the polymer self-assembly. It is even possible to perform self-assembly and replica formation on top of previous replicas in an iterative way. What is really interesting is that the topography from the initial layer actually acts as a template dictating how polymer domains in the next layer line up. In the case of a second layer of lamellae perpendicular with the substrate, a self-assembled nanomesh is naturally created.

    What other ways can you expand the range of self-assembled patterns?

    One area in which I have collaborated with other members of the Electronic Nanomaterials Group is to study the blending of block copolymers that form lamellae with block copolymers that form cylinders. The really cool thing we learned is that you can precisely control which of these two patterns emerge in different areas of a substrate through an approach we call selective directed self-assembly. In a diblock copolymer, one of the blocks may be relatively more hydrophobic (water repelling) and the other may be relatively more hydrophilic (water attracting). So if a chemical pattern was made up of alternating hydrophobic and hydrophilic lines, the block copolymer molecules would self-assemble accordingly. By changing the spacing and width of the chemical line gratings (patterns) on the substrate, we were able to direct the self-assembling blocks into specific arrangements—either forming striped patterns (lamellae) or hexagonal dot arrays (cylinders). We can locally adjust the spacing and linewidth of the underlying chemical template to precisely control exactly where these line or dot patterns form on the same substrate, too.

    How big are the features of these self-assembled nanoscale patterns?

    Block copolymers typically self-assemble into ordered periodic structures with a tunable repeat spacing between around 25 to 50 nanometers. Actually, one of the projects I am currently working on is to increase the size range over which block copolymers form patterns. There is a lot of work in the scientific community to go to smaller and smaller sizes (approaching a few nanometers) for lithographic applications such as making computer chips. But for certain applications, you need larger sizes.

    For example, the wavelength range of visible light is about 400 (purple) to 700 (red) nanometers. Even 400 nanometers is still about 10 times larger than the approximately 40-nanometer length scale possible with most block copolymers. As a result, light does not “see” the individual features of block copolymers.

    However, patterns with features closer to 200 nanometers in size can influence light in new ways, and I am working to scale block copolymer assembly to these sizes. One exciting application is using this approach to make “structural colors.” Colors are typically made using dyes or pigments. However, structural colors emerge from the way the light interacts with the nanomaterial—and so potentially one could make new types of lower-power displays through structural color.

    Unfortunately, the process of forming patterns from block copolymers slows drastically, and even stops altogether, for these larger sizes. The development of self-assembled patterns is impeded by defective structures that form at the start of self-assembly. For these defects to “heal,” the block polymer must reconfigure, which involves pulling one chain through the domain of the other polymer, overcoming a large energy barrier to do so. As the size of the polymers increases, the energy barrier goes up exponentially—so exponentially slower healing!

    Have you come up with any solutions to overcome this challenge?

    My colleagues and I found that blending small-molecular-weight homopolymers (polymers made up of the same type of molecules) with the block copolymers makes it easier for the block copolymer chains to move around. Adding homopolymers promotes dramatic increases in the size of well-ordered pattern areas, or “grains.” However, this addition alone does not let us form patterns with larger-size features from block copolymers. We also need to anneal the materials in a solvent vapor. Solvent vapor annealing involves putting a volatile solvent in the region of the diblock copolymer, causing the polymer film to swell. As the polymer film swells, the solvent molecules intersperse between polymer chains. This process has a plasticizing effect, making it easier for the polymer to move. So both mixing the block copolymer with a homopolymer and swelling it with a solvent are needed to speed the formation of large-scale patterns.

    5
    Without blending, the size of grains (single-color regions) of a diblock copolymer with a molecular weight of 36 kilograms/mole barely changes with thermal annealing over time (top row). Blending the block copolymer with homopolymers increases the grain size and speeds up the ordering process (bottom row).

    After annealing, we image the resulting patterns with scanning electron microscopes at CFN. We also perform x-ray scattering experiments at the Complex Materials Scattering beamline at Brookhaven Lab’s National Synchrotron Light Source II [also a DOE Office of Science User Facility] to get a measure of the periodicity during the annealing process so we can determine how quickly the material self-assembles into an ordered pattern.

    BNL NSLS-II

    BNL NSLS-II

    How do all of the different self-assembled patterns you generate translate to possible applications?

    6
    A comparison of scanning electron microscope images after solvent vapor annealing of a large block copolymer (top left) and the same copolymer with added homopolymer (top right) shows that the homopolymer can significantly improve pattern quality. The bottom image is a cross-sectional image of the top right sample.

    The work we do at CFN establishes the basis for producing new materials. For example, consider the mesh pattern I described. My colleague Chang-Yong Nam and I are working on making a mesh of zinc oxide nanowires through infiltration synthesis of block copolymers. Zinc oxide is a versatile semiconducting material, and these nanowires have a lot of surface area, making them attractive for a number of applications—including photoelectrochemical water splitting (a way of converting sunlight into fuel by splitting water into hydrogen and oxygen). These semiconducting nanowires are also very responsive to environmental cues like light or chemicals. Given their large area uniformity, the meshes could be easily integrated into widely deployable gas sensors.

    How did you come to join CFN?

    After graduate school, I completed a postdoctoral appointment working at IBM in California. It was there that I learned about block copolymers, using them to make patterns for microelectronics. I did that for three years, and then in 2013 I joined a research group at HGST, a subsidiary of Western Digital that sells hard disk and solid-state drives. At first, I worked on a project to create patterned nanoscale magnetic media onto which data is written to and read from, in order to enhance data density and stability.

    Following that project, I moved into another area that was not very research oriented. At about this time, the current CFN Director, Chuck Black, gave a talk at HGST. Chuck’s talk piqued my interest in CFN, and soon thereafter a position that very well fit my skills opened up. I applied, and joined CFN in 2015.

    What was it like coming from industry to a national lab setting?

    It definitely takes some getting used to. There is a lot less pressure in some ways but more pressure in other ways. At CFN, you are expected to develop a lot more on your own as far as what direction you need to pursue on the basis of what is valuable to the lab. In industry, the goals of the project are clear, and there are deliverables to keep you on the narrow path to achieving those goals. This is not to say that I did not do exploratory research for a portion of my time in industry.

    The other difference at CFN is that I am more involved with the academic and industrial community at large. I have to manage my time so that I can perform my own research and help users with their research. I find it really cool that researchers from all over the world come to CFN. The talent and expertise of the staff are what make the CFN so great. The staff know backwards and forwards what they are doing in the areas they specialize in, and they are dedicated to helping others.

    What are some of the ways you have helped users?

    I regularly show users how to do block copolymer self-assembly and the etching process to transfer patterns into a substrate. Some users have employed these patterns as superhydrophobic textures to manipulate the flow of liquids in microfluidic devices, for example. In many cases, I help users develop a process when they need self-assembled patterns of varying sizes or shapes, to be applied to different substrates.

    I also help with the lithographic patterning of unusual materials. For example, one user I am working with is trying to pattern protein hydrogels, looking at how they mechanically respond to swelling and de-swelling to understand how they might operate if injected into the body. Such protein hydrogels could have applications in biosensing, drug delivery, and wound healing.

    Some users are interested in solvent vapor annealing, which is a very tricky process to control. So I am building a system with feedback control that can be set to maintain a solvent fraction of any given solvent. Otherwise, if the amount of solvent varies over time, the self-assembly might not work at all (too little solvent) or result in a disordered state (too much solvent).

    How did you become interested in research?

    As an undergraduate at Case Western Reserve University, I majored in chemical engineering. One of my summer internships involved doing fuel cell work at a research lab. This internship led to another one at a fuel cell start-up, where I worked on sensors and solar cells. These internships gave me the opportunity to come up with ideas of my own and try different things. This exploration spurred me to pursue a doctoral degree at the University of California, Berkeley, where I continued my studies in chemical engineering.

    I also think my undergraduate studies in philosophy, which I minored in, gave me a new understanding of the way science works. I really enjoyed reading about the philosophy of science, including books like Kuhn’s The Structure of Scientific Revolutions. I learned how the views we often have are naïve or not accurate. There are lots of things people get wrong for a long time. But that does not mean they were not learning. Science is not perfect—but that is okay.

    I was not the kid who grew up knowing he wanted to be a scientist. I liked philosophy and delving into critical thought. But I began to realize there is a lot of commonality between philosophy and science—in both fields, the aim is to gain knowledge about the world around us. But science is better because you can actually test things!

    See the full article here .

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    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. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 1:39 pm on December 15, 2017 Permalink | Reply
    Tags: Accelerating the Self-Assembly of Nanoscale Patterns for Next-Generation Materials, , , Homopolymers, , Scanning Electron microscopy, Scientists blended a block copolymer containing two chemically distinct "blocks" with significantly smaller homopolymers from each of these blocks, The polymer blocks do not readily mix with each other so they must overcome an extremely large energy barrier to reconfigure   

    From Brookhaven Lab: “Accelerating the Self-Assembly of Nanoscale Patterns for Next-Generation Materials” 

    Brookhaven Lab

    December 13, 2017
    Ariana Tantillo
    atantillo@bnl.gov

    Using polymer blends, scientists rapidly generated highly ordered patterns that could be used in the fabrication of microelectronics, antireflective surfaces, magnetic data storage systems, and fluid-flow devices.

    1
    Materials scientist Gregory Doerk preparing a sample for electron microscopy at Brookhaven Lab’s Center for Functional Nanomaterials. The scanning electron microscope image on the computer screen shows a cross-sectional view of line patterns transferred into a layer of silicon dioxide.

    The ability to quickly generate ultra-small, well-ordered nanopatterns over large areas on material surfaces is critical to the fabrication of next-generation technologies in many industries, from electronics and computing to energy and medicine. For example, patterned media, in which data are stored in periodic arrays of magnetic pillars or bars, could significantly improve the storage density of hard disk drives.

    Scientists can coax thin films of self-assembling materials called block copolymers—chains of chemically distinct macromolecules (polymer “blocks”) linked together—into desired nanoscale patterns through heating (annealing) them on a substrate. However, defective structures that deviate from the regular pattern emerge early on during self-assembly.

    The presence of these defects inhibits the use of block copolymers in the nanopatterning of technologies that require a nearly perfect ordering—such as magnetic media, computer chips, antireflective surfaces, and medical diagnostic devices. With continued annealing, the block copolymer patterns can reconfigure to remove the imperfections, but this process is exceedingly slow. The polymer blocks do not readily mix with each other, so they must overcome an extremely large energy barrier to reconfigure.

    Adding small things with a big impact.

    2
    As shown in the illustration, a block copolymer consists of different molecule chains (red and blue) linked together; a homopolymer chain consists of identical molecules (red or blue). In this study, scientists blended a block copolymer containing two chemically distinct “blocks” with significantly smaller homopolymers from each of these blocks.

    Now, scientists from the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory—have come up with a way to massively speed up the ordering process.

    3
    BNL Center for Functional Nanomaterials (CFN) campus

    They blended a line-forming block copolymer with significantly smaller polymer chains made of only one type of molecule (homopolymers) from each of the two constituent blocks. The electron microscopy images they took after annealing the films for only a few minutes show that the addition of these two smaller homopolymers dramatically increases the size of well-ordered line-pattern areas, or “grains.”

    “Without the homopolymers, the same block copolymer cannot produce grains with these sizes,” said CFN materials scientist Gregory Doerk, who led the work, which was published online in an ACS Nano paper on December 1. “Blending in homopolymers that are less than one-tenth of the size of the block copolymer greatly accelerates the ordering process. In the resulting line patterns, there is a constant spacing between each of the lines, and the same directions of line-pattern orientations—for example, vertical or horizontal—persist over longer distances.”

    Doerk and coauthor Kevin Yager, leader of the Electronic Nanomaterials Group at CFN, used image analysis software to calculate the grain size and repeat spacing of the line patterns.

    4
    The scanning electron microscope images taken after thermal annealing at around 480 degrees Fahrenheit for five minutes show that the block copolymer/homopolymer blend generates a line pattern with a significantly higher degree of long-range order (b) than the unblended version (a), which shows a fingerprint-like pattern. Using image analysis software, the scientists generated colored maps to visualize the local line-pattern orientations in two block copolymers of different size (c). For both block copolymers, the size of well-ordered areas (indicated by the large individual colored regions, with the different line orientations designated by the corresponding color key) increases as more homopolymer is blended, up until a certain point, after which the pattern becomes disordered.

    While blending different concentrations of homopolymer to determine how much was needed to achieve the accelerated ordering, they discovered that the ordering sped up as more homopolymer was added. But too much homopolymer actually resulted in disordered patterns.

    “The homopolymers accelerate the self-assembly process because they are small enough to uniformly distribute throughout their respective polymer blocks,” said Doerk. “Their presence weakens the interface between the two blocks, lowering the energy barrier associated with the block copolymer reconfiguring to remove the defects. But if the interface is weakened too much through the addition of too much homopolymer, then the blocks will mix together, resulting in a completely disordered phase.”

    Guiding the self-assembly of useful nanopatterns in minutes.

    5
    The unblended block copolymer aligns well close to the template guides (“sidewalls”), but this alignment degrades further in, as evident by the appearance of the fingerprint-like pattern in the center of the scanning electron microscope image in (a). Under the same annealing temperature and time (two minutes), the block copolymer/homopolymer blend retains the alignment across the entire area between the sidewalls (b).

    To demonstrate how the rapid ordering in the blended system could accelerate the self-assembly of well-aligned nanopatterns over large areas, Doerk and Yager used line-pattern templates they had previously prepared through photolithography. Used to build almost all of today’s digital devices, photolithography involves projecting light through a mask (a plate containing the desired pattern) that is positioned over a wafer (usually made of silicon) coated with a light-sensitive material. This template can then be used to direct the self-assembly of block copolymers, which fill in the spaces between the template guides. In this case, after only two minutes of annealing, the polymer blend self-assembles into lines that are aligned across these gaps. However, after the same annealing time, the unblended block copolymer self-assembles into a mostly unaligned pattern with many defects between the gaps.

    “The width of the gaps is more than 80 times the repeat spacing, so the fact that we got this degree of alignment with our polymer blend is really exciting because it means we can use templates with huge gaps, created with very low-resolution lithography,” said Doerk. “Typically, expensive high-resolution lithography equipment is needed to align block copolymer patterns over this large of an area.”

    6
    A scanning electron microscope image showing a cross-sectional view of the line patterns transferred into a silicon dioxide layer.

    For these patterns to be useful for many nanopatterning applications, they often need to be transferred to other more robust materials that can withstand harsh manufacturing processes—for example, etching, which removes layers from silicon wafer surfaces to create integrated circuits or make the surfaces antireflective. In this study, the scientists converted the nanopatterns into a metal-oxide replica. Through chemical etching, they then transferred the replica pattern into a silicon dioxide layer on a silicon wafer, achieving clearly defined line patterns.

    Doerk suspects that blending homopolymers with other block copolymers will similarly yield accelerated assembly, and he is interested in studying blended polymers that self-assemble into more complicated patterns. The x-ray scattering capabilities at the National Synchrotron Light Source II—another DOE Office of Science User Facility at Brookhaven—could provide the structural information needed to conduct such studies.


    BNL NSLS II

    “We have introduced a very simple and easily controlled way of immensely accelerating self-assembly,” concluded Doerk. “Our approach should substantially reduce the number of defects, helping to meet the demands of the semiconductor industry. At CFN, it opens up possibilities for us to use block copolymer self-assembly to make some of the new functional materials that we envision.”

    See the full article here .

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    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. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 3:39 pm on August 1, 2017 Permalink | Reply
    Tags: A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity, , , , , , Scanning Electron microscopy   

    From LBNL: “A Semiconductor That Can Beat the Heat” 

    Berkeley Logo

    Berkeley Lab

    July 31, 2017
    Jon Weiner
    jrweiner@lbl.gov
    (510) 486-4014

    A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

    A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

    These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.

    1
    Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). (Credit: Berkeley Lab/UC Berkeley)

    This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

    “Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciences journal. These are the first published results relating to the thermoelectric performance of this single crystal material.

    Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

    “We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

    Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

    “We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

    Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

    But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

    Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

    A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

    Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.

    3
    Scanning electron microscope images of suspended micro-island devices. Individual AIHP NW is suspended between two membranes. (Credit: Berkeley Lab/UC Berkeley).

    To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

    They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

    “A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

    Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

    The research team also included other scientists from Berkeley Lab’s Materials Sciences Division and the Molecular Foundry, the Kavli Energy NanoScience Institute at UC Berkeley and Berkeley Lab, and UC Berkeley’s Department of Chemistry.

    The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists from all over the world.

    This work was supported by the Department of Energy’s Office of Basic Energy Sciences.

    More information about Peidong Yang’s research group: http://nanowires.berkeley.edu/.

    See the full article here .

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