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  • richardmitnick 11:00 am on August 1, 2019 Permalink | Reply
    Tags: "Oddball edge wins nanotube faceoff", A zigzag nanotube’s end looks like a saw tooth while an armchair is like a row of seats with armrests., , Carbon nanotubes, , , The two-faced “Janus” configuration   

    From Rice University: “Oddball edge wins nanotube faceoff” 

    Rice U bloc

    From Rice University

    July 29, 2019
    Mike Williams

    Rice theory shows peculiar ‘Janus’ interface a common mechanism in carbon nanotube growth.

    When is a circle less stable than a jagged loop? Apparently when you’re talking about carbon nanotubes.

    Rice University theoretical researchers have discovered that nanotubes with segregated sections of “zigzag” and “armchair” facets growing from a solid catalyst are far more energetically stable than a circular arrangement would be.

    2
    Rice University researchers have determined that an odd, two-faced “Janus” edge is more common than previously thought for carbon nanotubes growing on a rigid catalyst. The conventional nanotube at left has facets that form a circle, allowing the nanotube to grow straight up from the catalyst. But they discovered the nanotube at right, with a tilted Janus edge that has segregated sections of zigzag and armchair configurations, is far more energetically favored when growing carbon nanotubes via chemical vapor deposition. Illustration by Evgeni Penev.

    Under the right circumstances, they reported, the interface between a growing nanotube and its catalyst can reach its lowest-known energy state via the two-faced “Janus” configuration, with a half-circle of zigzags opposite six armchairs.

    The terms refer to the shape of the nanotube’s edge: A zigzag nanotube’s end looks like a saw tooth, while an armchair is like a row of seats with armrests. They are the basic edge configurations of the two-dimensional honeycomb of carbon atoms known as graphene (as well as other 2D materials) and determine many of the materials’ properties, especially electrical conductivity.

    The Brown School of Engineering team of materials theorist Boris Yakobson, researcher and lead author Ksenia Bets and assistant research professor Evgeni Penev reported their results in the American Chemical Society journal ACS Nano.

    The theory is a continuation of the team’s discovery last year that Janus interfaces are likely to form on a catalyst of tungsten and cobalt, leading to a single chirality, called (12,6), that other labs had reported growing in 2014.

    The Rice team now shows such structures aren’t unique to a specific catalyst, but are a general characteristic of a number of rigid catalysts. That’s because the atoms attaching themselves to the nanotube edge always seek their lowest energy states, and happen to find it in the Janus configuration they named A|Z.

    “People have assumed in studies that the geometry of the edge is a circle,” Penev said. “That’s intuitive — it’s normal to assume that the shortest edge is the best. But we found for chiral tubes the slightly elongated Janus edge allows it to be in much better contact with solid catalysts. The energy for this edge can be quite low.”

    In the circle configuration, the flat armchair bottoms rest on the substrate, providing the maximum number of contacts between the catalyst and the nanotube, which grows straight up. (Janus edges force them to grow at an angle.)

    Carbon nanotubes — long, rolled-up tubes of graphene — are difficult enough to see with an electron microscope. As yet there’s no way to observe the base of a nanotube as it grows from the bottom up in a chemical vapor deposition furnace. But theoretical calculations of the atom-level energy that passes between the catalyst and the nanotube at the interface can tell researchers a lot about how they grow.

    That’s a path the Rice lab has pursued for more than a decade, pulling at the thread that reveals how minute adjustments in nanotube growth can change the kinetics, and ultimately how nanotubes can be used in applications.

    “Generally, the insertion of new atoms at the nanotube edge requires breaking the interface between the nanotube and the substrate,” Bets said. “If the interface is tight, it would cost too much energy. That is why the screw dislocation growth theory proposed by Professor Yakobson in 2009 was able to connect the growth rate with the presence of kinks, the sites on the nanotube edge that disrupt the tight carbon nanotube-substrate contact.

    “Curiously, even though Janus edge configuration allows very tight contact with the substrate it still preserves a single kink that would allow continuous nanotube growth, as we demonstrated last year for the cobalt tungsten catalyst,” Bets said.

    Bets ran extensive computer simulations to model nanotubes growing on three rigid catalysts that showed evidence of Janus growth and one more “fluid” catalyst, tungsten carbide, that did not. “The surface of that catalyst is very mobile, so the atoms can move a lot,” Penev said. “For that one, we did not observe a clear segregation.”

    Yakobson compared Janus nanotubes to the Wulff shape of crystals. “It’s somewhat surprising that our analysis suggests a restructured, faceted edge is energetically favored for chiral tubes,” he said. “Assuming that the lowest energy edge must be a minimal-length circle is like assuming that a crystal shape must be a minimal-surface sphere but we know well that 3D shapes have facets and 2D shapes are polygons, as epitomized by the Wulff construction.

    “Graphene has by necessity several ‘sides,’ but a nanotube cylinder has one rim, making the energy analysis different,” he said. “This raises fundamentally interesting and practically important questions about the relevant structure of the nanotube edges.”

    The Rice researchers hope their discovery will advance them along the path toward those answers. “The immediate implication of this finding is a paradigm shift in our understanding of growth mechanisms,” Yakobson said. “That may become important in how one practically designs the catalyst for efficient growth, especially of controlled nanotube symmetry type, for electronic and optical utility.”

    Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and of Chemistry. The National Science Foundation (NSF) and the Air Force Office of Scientific Research supported the research.

    Computing resources were provided by the Department of Defense Supercomputing Resource Center; the National Energy Research Scientific Computing Center, supported by the Department of Energy Office of Science; the NSF-supported XSEDE supercomputer; and the NSF-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in partnership with Rice’s Ken Kennedy Institute for Information Technology.

    See the full article here .


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    Stem Education Coalition

    Rice U campus

    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
  • richardmitnick 8:13 am on October 23, 2017 Permalink | Reply
    Tags: Antennas, , “Specific radiation efficiency”, Carbon nanotubes, , Nanotube fiber antennas as capable as copper, ,   

    From Rice: “Nanotube fiber antennas as capable as copper” 

    Rice U bloc

    Rice University

    October 23, 2017
    Mike Williams

    1
    Rice University graduate student Amram Bengio sets up a nanotube fiber antenna for testing. Scientists at Rice and the National Institute of Standards and Technology have determined that nanotube fibers made at Rice can be as good as copper antennas but 20 times lighter. Photo by Jeff Fitlow

    Rice researchers show their flexible fibers work well but weigh much less

    Fibers made of carbon nanotubes configured as wireless antennas can be as good as copper antennas but 20 times lighter, according to Rice University researchers. The antennas may offer practical advantages for aerospace applications and wearable electronics where weight and flexibility are factors.

    The research appears in Applied Physics Letters.

    The discovery offers more potential applications for the strong, lightweight nanotube fibers developed by the Rice lab of chemist and chemical engineer Matteo Pasquali. The lab introduced the first practical method for making high-conductivity carbon nanotube fibers in 2013 and has since tested them for use as brain implants and in heart surgeries, among other applications.

    The research could help engineers who seek to streamline materials for airplanes and spacecraft where weight equals cost. Increased interest in wearables like wrist-worn health monitors and clothing with embedded electronics could benefit from strong, flexible and conductive fiber antennas that send and receive signals, Pasquali said.

    The Rice team and colleagues at the National Institute of Standards and Technology (NIST) developed a metric they called “specific radiation efficiency” to judge how well nanotube fibers radiated signals at the common wireless communication frequencies of 1 and 2.4 gigahertz and compared their results with standard copper antennas. They made thread comprising from eight to 128 fibers that are about as thin as a human hair and cut to the same length to test on a custom rig that made straightforward comparisons with copper practical.

    “Antennas typically have a specific shape, and you have to design them very carefully,” said Rice graduate student Amram Bengio, the paper’s lead author. “Once they’re in that shape, you want them to stay that way. So one of the first experimental challenges was getting our flexible material to stay put.”

    2
    Bengio prepares a sample nanotube fiber antenna for evaluation. The fibers had to be isolated in Styrofoam mounts to assure accurate comparisons with each other and with copper. Photo by Jeff Fitlow

    Contrary to earlier results by other labs (which used different carbon nanotube fiber sources), the Rice researchers found the fiber antennas matched copper for radiation efficiency at the same frequencies and diameters. Their results support theories that predicted the performance of nanotube antennas would scale with the density and conductivity of the fiber.

    “Not only did we find that we got the same performance as copper for the same diameter and cross-sectional area, but once we took the weight into account, we found we’re basically doing this for 1/20th the weight of copper wire,” Bengio said.

    “Applications for this material are a big selling point, but from a scientific perspective, at these frequencies carbon nanotube macro-materials behave like a typical conductor,” he said. Even fibers considered “moderately conductive” showed superior performance, he said.

    Although manufacturers could simply use thinner copper wires instead of the 30-gauge wires they currently use, those wires would be very fragile and difficult to handle, Pasquali said.

    “Amram showed that if you do three things right — make the right fibers, fabricate the antenna correctly and design the antenna according to telecommunication protocols — then you get antennas that work fine,” he said. “As you go to very thin antennas at high frequencies, you get less of a disadvantage compared with copper because copper becomes difficult to handle at thin gauges, whereas nanotubes, with their textile-like behavior, hold up pretty well.”

    Co-authors of the paper are, from Rice, graduate students Lauren Taylor and Peiyu Chen, alumnus Dmitri Tsentalovich and Aydin Babakhani, an associate professor of electrical and computer engineering, and, from NIST in Boulder, Colo., postdoctoral researcher Damir Senic, research engineer Christopher Holloway, physicist Christian Long, research scientists David Novotny and James Booth and physicist Nathan Orloff. Pasquali is a professor of chemical and biomolecular engineering, of materials science and nanoengineering and of chemistry.

    The U.S. Air Force supported the research.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Rice U campus

    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
  • richardmitnick 7:08 pm on July 31, 2016 Permalink | Reply
    Tags: , Carbon nanotubes,   

    From Technion: “Watch Out, Silicon Chips. Molecular Electronics Are Coming” 

    Technion bloc

    Technion

    July 11, 2016 [Just now in social media by American Technion Society]
    Kevin Hattori

    Technion breakthrough could replace silicon chips in the world of electronics

    Technion researchers have developed a method for growing carbon nanotubes that could lead to the day when molecular electronics replace the ubiquitous silicon chip as the building block of electronics. The findings are published this week in Nature Communications.

    1
    Professor Yuval Yaish

    Carbon nanotubes (CNTs) have long fascinated scientists because of their unprecedented electrical, optical, thermal and mechanical properties, and chemical sensitivity. But significant challenges remain before CNTs can be implemented on a wide scale, including the need to produce them in specific locations on a smooth substrate, in conditions that will lead to the formation of a circuit around them.

    Led by Prof. Yuval Yaish of the Viterbi Faculty of Electrical Engineering and the Zisapel Nanoelectronics Center at the Technion, the researchers have developed a technology that addresses these challenges. Their breakthrough also makes it possible to study the dynamic properties of CNTs, including acceleration, resonance (vibration), and the transition from softness to hardness.

    The method could serve as an applicable platform for the integration of nano-electronics with silicon technologies, and possibly even the replacement of these technologies in molecular electronics.

    “The CNT is an amazing and very strong building block with remarkable electrical, mechanical and optical properties,” said Prof. Yaish. “Some are conductors, and some are semiconductors, which is why they are considered a future replacement for silicon. But current methods for the production of CNTs are slow, costly, and imprecise. As such, they generally cannot be implemented in industry.”

    2
    Preferential adsorption of p-nitrobenzoic acid on carbon nanotubes. (a) Top: Chemical structure of p-nitrobenzoic acid (pNBA). Bottom: Schematic illustration of the monoclinic unit cell of pNBA powder as extracted from X-ray diffraction analysis. (b,c) Dark field optical microscopy images of pNBA nanocrystals adsorbed along CVD grown carbon nanotubes (CNTs). Scale bar, 50 and 20 µm, respectively. (d) Amplitude image of AFM of a single CNT with a few pNBA nanocrystals along. Scale bar, 1 µm. Inset: height cross sections along the marked lines of the main figure. (e) Dark field optical microscopy image of pNBA nanocrystals after intensive deposition. Note the black voids along the CNT. Scale bar, 20 µm. (f) Dark field optical microscopy image of pNBA nanocrystals adsorb onto commercial dispersed CNTs. Scale bar, 20 µm. No image credit.

    Due to the nanometer size of the CNTs (100,000 times smaller in diameter than the thickness of a human hair) it is extremely difficult to find or locate them at specific locations. Prof. Yaish, and graduate students Gilad Zeevi and Michael Shlafman, developed a simple, rapid, non-invasive and scalable technique that enables optical imaging of CNTs. Instead of depending upon the CNT chemical properties to bind marker molecules, the researchers relied on the fact that the CNT is both a chemical and physical defect on the otherwise flat and uniform surface. It can serve as a seed for the nucleation and growth of small, but optically visible nanocrystals, which can be seen and studied using a conventional optical microscope (CNTs, because of their small size, are too small to be seen in this way). Since the CNT surface is not used to bind the molecules, they can be removed completely after imaging, leaving the surface intact, and preserving the CNT’s electrical and mechanical properties.

    “Our approach is the opposite of the norm,” he continued. “We grow the CNTs directly, and with the aid of the organic crystals that coat them, we can see them under a microscope very quickly. Then image identification software finds and produces the device (transistor). This is the strategy. The goal is to integrate CNTs in an integrated circuit of miniaturized electronic components (mainly transistors) on a single chip (VLSI). These could one day serve as a replacement for silicon electronics.”

    Prof. Yaish also noted that the ability to demonstrate this principle and create world-class devices was made possible by the unique infrastructures available at the Technion clean room facilities in the Wolfson Microelectronics Center, headed by Prof. Nir Tessler.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Technion Campus

    A science and technology research university, among the world’s top ten,
    dedicated to the creation of knowledge and the development of human capital and leadership,
    for the advancement of the State of Israel and all humanity.

     
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