New Guide for Research On Multiblock Polymers Emerges

Polymers are large molecules composed of repeating sequences of monomers. When more than one monomer type is present and the dissimilar monomers are organized and chemically bound into "blocks," the resulting multiblock polymers can serve as the basis for a multitude of materials, to be used in applications as diverse as tennis shoes and solar cells. Since the genesis of polymer science in the 1950's, when scientists had only limited numbers of monomers, and, methods to choose from in creating multiblock polymers, the field has expanded. Scientists may now create materials using monomers from a variety of sources, from petroleum to renewable feedstocks such as sugar or cellulose.

"The Pandora's box is that you have so many monomers that you can put together and in so many block sequences," said Fredrickson, a professor of chemical engineering, explaining that the properties will vary according to sequence and by virtue of the interactions among the blocks. Because multiblock copolymers can "self-assemble" into nanometer-sized domains, these materials can exhibit remarkable combinations of properties, such as soft, strong, and elastic -- as in tennis shoe soles or skateboard wheels. For higher-tech applications, the researchers are currently partnering with the company Intel to develop multiblock polymers that will enable patterning of microelectronic devices at finer scales and lower cost.

The problem, say Fredrickson and Delaney, a project scientist in the Department of Engineering, has become the sheer number of possible combinations for these monomers. There are now so many, that choosing what multiblock polymer to make -- and what monomers to make it from -- has become an issue.
"It is a counting problem," said Fredrickson, referring to the potential for millions of different polymers that could be created with today's chemistry, a number that increases by leaps and bounds for every new block and monomer species added to the selection.

"Our simulation methods for predicting the self-assembled structures of multiblock polymers are quite advanced, and we are getting better at relating those nano-structures to the properties of the material," said Fredrickson. "Multiblock polymers are extremely versatile -- there is enormous latitude of design freedom, and it's very promising in terms of developing materials with truly unique properties."

F. S. Bates, M. A. Hillmyer, T. P. Lodge, C. M. Bates, K. T. Delaney, G. H. Fredrickson. Multiblock Polymers: Panacea or Pandora's Box? Science, 2012; 336 (6080): 434 DOI: 10.1126/science.1215368

Self-Assembling Highly Conductive Plastic Nanofibers

Researchers from CNRS and the Université de Strasbourg, headed by Nicolas Giuseppone and Bernard Doudin, have succeeded in making highly conductive plastic fibers that are only several nanometers thick. These nanowires, for which CNRS has filed a patent, "self-assemble" when triggered by a flash of light. Inexpensive and easy to handle, unlike carbon nanotubes, they combine the advantages of the two materials currently used to conduct electric current: metals and plastic organic polymers. In fact, their remarkable electrical properties are similar to those of metals. 

In collaboration with Doudin's team, the researchers then studied the electrical properties of these nanofibers in detail. This time, they placed their molecules in contact with an electronic microcircuit comprising gold electrodes spaced 100 nm apart. They then applied an electric field between these electrodes.
Their first important finding was that, when triggered by a flash of light, the fibers self-assemble solely between the electrodes. The second surprising result was that these structures, which are as light and flexible as plastics, turn out to be capable of transporting extraordinary current densities, above 2.10^6 Amperes per square centimeter (A.cm-2), approaching those of copper wire. In addition, they have very low interface resistance with metals (6) : 10,000 times below that of the best organic polymers.
The researchers now hope to demonstrate that their fibers can be used industrially in miniaturized electronic devices such as flexible screens, solar cells, transistors, printed nanocircuits, etc.

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Vina Faramarzi, Frédéric Niess, Emilie Moulin, Mounir Maaloum, Jean-François Dayen, Jean-Baptiste Beaufrand, Silvia Zanettini, Bernard Doudin, Nicolas Giuseppone. Light-triggered self-construction of supramolecular organic nanowires as metallic interconnects. Nature Chemistry, 2012; DOI: 10.1038/NCHEM.1332

Nanocrystal-Coated Fibers Might Reduce Wasted Energy

Researchers are developing a technique that uses nanotechnology to harvest energy from hot pipes or engine components to potentially recover energy wasted in factories, power plants and cars.

"The ugly truth is that 58 percent of the energy generated in the United States is wasted as heat," said Yue Wu, a Purdue University assistant professor of chemical engineering. "If we could get just 10 percent back that would allow us to reduce energy consumption and power plant emissions considerably."

Researchers have coated glass fibers with a new "thermoelectric" material they developed. When thermoelectric materials are heated on one side electrons flow to the cooler side, generating an electrical current.

Coated fibers also could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants. The fibers might be woven into a fabric to make cooling garments.
The glass fibers are dipped in a solution containing nanocrystals of lead telluride and then exposed to heat in a process called annealing to fuse the crystals together.

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Purdue University (2012, April 17). Nanocrystal-coated fibers might reduce wasted energy. ScienceDaily. Retrieved April 23, 2012, from http://www.sciencedaily.com­ /releases/2012/04/120417143857.htm

Silicon-Carbon Electrodes Snap, Swell, Don't Pop

A study that examines a new type of silicon-carbon nanocomposite electrode reveals details of how they function and how repeated use could wear them down. The study also provides clues to why this material performs better than silicon alone. With an electrical capacity five times higher than conventional lithium battery electrodes, silicon-carbon nanocomposite electrodes could lead to longer-lasting, cheaper rechargeable batteries for electric vehicles.

Published online in the journal Nano Letters last week, the study includes videos of the electrodes being charged at nanometer-scale resolution. Watching them in use can help researchers understand the strengths and weaknesses of the material.

"The electrodes expand as they get charged, and that shortens the lifespan of the battery," said lead researcher Chongmin Wang at the Department of Energy's Pacific Northwest National Laboratory. "We want to learn how to improve their lifespan, because silicon-carbon nanofiber electrodes have great potential for rechargeable batteries."

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Chong-Min Wang, Xiaolin Li, Zhiguo Wang, Wu Xu, Jun Liu, Fei Gao, Libor Kovarik, Ji-Guang Zhang, Jane Howe, David J. Burton, Zhongyi Liu, Xingcheng Xiao, Suntharampillai Thevuthasan, Donald R. Baer. In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries. Nano Letters, 2012; 12 (3): 1624 DOI: 10.1021/nl204559u

Nanopower: Avoiding Electrolyte Failure in Nanoscale Lithum Batteries

Researchers from the National Institute of Standards and Technology (NIST), the University of Maryland, College Park, and Sandia National Laboratories built a series of nanowire batteries to demonstrate that the thickness of the electrolyte layer can dramatically affect the performance of the battery, effectively setting a lower limit to the size of the tiny power sources. The results are important because battery size and performance are key to the development of autonomous MEMS -- microelectromechanical machines -- which have potentially revolutionary applications in a wide range of fields.

NIST researcher Alec Talin and his colleagues created a veritable forest of tiny -- about 7 micrometers tall and 800 nanometers wide -- solid-state lithium ion batteries to see just how small they could be made with existing materials and to test their performance.

Starting with silicon nanowires, the researchers deposited layers of metal (for a contact), cathode material, electrolyte, and anode materials with various thicknesses to form the miniature batteries. They used a transmission electron microscope (TEM) to observe the flow of current throughout the batteries and watch the materials inside them change as they charged and discharged.

The team found that when the thickness of the electrolyte film falls below a threshold of about 200 nanometers, the electrons can jump the electrolyte border instead of flowing through the wire to the device and on to the cathode. Electrons taking the short way through the electrolyte cause the electrolyte to break down and the battery to quickly discharge.

Dmitry Ruzmetov, Vladimir P. Oleshko, Paul M. Haney, Henri J. Lezec, Khim Karki, Kamal H. Baloch, Amit K. Agrawal, Albert V. Davydov, Sergiy Krylyuk, Yang Liu, JianY. Huang, Mihaela Tanase, John Cumings, A. Alec Talin. Electrolyte Stability Determines Scaling Limits for Solid-State 3D Li Ion Batteries. Nano Letters, 2012; 12 (1): 505 DOI: 10.1021/nl204047z

Metamaterials May Advance With New Femtosecond Laser Technique

Researchers in applied physics have cleared an important hurdle in the development of advanced materials, called metamaterials, that bend light in unusual ways. Working at a scale applicable to infrared light, the Harvard team has used extremely short and powerful laser pulses to create three-dimensional patterns of tiny silver dots within a material. Those suspended metal dots are essential for building futuristic devices like invisibility cloaks.

The new fabrication process, described in the journal Applied Physics Letters, advances nanoscale metal lithography into three dimensions -- and does it at a resolution high enough to be practical for metamaterials.
"If you want a bulk metamaterial for visible and infrared light, you need to embed particles of silver or gold inside a dielectric, and you need to do it in 3D, with high resolution," says lead author Kevin Vora, a graduate student at the Harvard School of Engineering and Applied Sciences (SEAS).
"This work demonstrates that we can create silver dots that are disconnected in x, y, and z," Vora says. "There's no other technique that feasibly allows you to do that. Being able to make patterns of nanostructures in 3D is a very big step towards the goal of making bulk metamaterials."
Vora works in the laboratory of Eric Mazur, Balkanski Professor of Physics and Applied Physics at SEAS. For decades, Mazur has been using a piece of equipment called a femtosecond laser to investigate how very tightly focused, powerful bursts of light can change the electrical, optical, and physical properties of a material.When a conventional laser shines on a transparent material, the light passes straight through, with slight refraction. The femtosecond laser is special because it emits a burst of photons as bright as the surface of the sun in a flash lasting only 50 quadrillionths (5 × 10-14) of a second. Instead of shining through the material, that energy gets trapped within it, exciting the electrons within the material and achieving a phenomenon known as nonlinear absorption.
Inside the pocket where that energy is trapped, a chemical reaction can take place, permanently altering the internal structure of the material. The process has previously been exploited for 2D and simple 3D metal nanofabrication.
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Kevin Vora, SeungYeon Kang, Shobha Shukla, Eric Mazur. Fabrication of disconnected three-dimensional silver nanostructures in a polymer matrix. Applied Physics Letters, 2012; 100 (6): 063120 DOI: 10.1063/1.3684277

Single-Atom Transistor Is End of Moore's Law; May Be Beginning of Quantum Computing

The smallest transistor ever built -- in fact, the smallest transistor that can be built -- has been created using a single phosphorus atom by an international team of researchers at the University of New South Wales, Purdue University and the University of Melbourne.

The single-atom device was described Sunday (Feb. 19) in a paper in the journal Nature Nanotechnology.
Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales, says the development is less about improving current technology than building future tech.
"This is a beautiful demonstration of controlling matter at the atomic scale to make a real device," Simmons says. "Fifty years ago when the first transistor was developed, no one could have predicted the role that computers would play in our society today. As we transition to atomic-scale devices, we are now entering a new paradigm where quantum mechanics promises a similar technological disruption. It is the promise of this future technology that makes this present development so exciting."
The same research team announced in January that it had developed a wire of phosphorus and silicon -- just one atom tall and four atoms wide -- that behaved like copper wire.
Simulations of the atomic transistor to model its behavior were conducted at Purdue using nanoHUB technology, an online community resource site for researchers in computational nanotechnology.
Gerhard Klimeck, who directed the Purdue group that ran the simulations, says this is an important development because it shows how small electronic components can be engineered.
"To me, this is the physical limit of Moore's Law," Klimeck says. "We can't make it smaller than this."
Although definitions can vary, simply stated Moore's Law holds that the number of transistors that can be placed on a processor will double approximately every 18 months. The latest Intel chip, the "Sandy Bridge," uses a manufacturing process to place 2.3 billion transistors 32 nanometers apart. A single phosphorus atom, by comparison, is just 0.1 nanometers across, which would significantly reduce the size of processors made using this technique, although it may be many years before single-atom processors actually are manufactured.
The single-atom transistor does have one serious limitation: It must be kept very cold, at least as cold as liquid nitrogen, or minus 391 degrees Fahrenheit (minus 196 Celsius).
"The atom sits in a well or channel, and for it to operate as a transistor the electrons must stay in that channel," Klimeck says. "At higher temperatures, the electrons move more and go outside of the channel. For this atom to act like a metal you have to contain the electrons to the channel.
"If someone develops a technique to contain the electrons, this technique could be used to build a computer that would work at room temperature. But this is a fundamental question for this technology."
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Journal Reference:

1. Martin Fuechsle, Jill A. Miwa, Suddhasatta Mahapatra, Hoon Ryu, Sunhee Lee, Oliver Warschkow, Lloyd C. L. Hollenberg, Gerhard Klimeck, Michelle Y. Simmons. A single-atom transistor. Nature Nanotechnology, 2012; DOI: 10.1038/nnano.2012.21

Self-Assembling Nanorods

Researchers Obtain 1-, 2 And 3-D Nanorod Arrays and Networks.

Abraham Mauleon Amieva

A relatively fast, easy and inexpensive technique for inducing nanorods -- rod-shaped semiconductor nanocrystals -- to self-assemble into one-, two- and even three-dimensional macroscopic structures has been developed by a team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab). This technique should enable more effective use of nanorods in solar cells, magnetic storage devices and sensors. It should also help boost the electrical and mechanical properties of nanorod-polymer composites.


Leading this project was Ting Xu, a polymer scientist who holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California (UC) Berkeley's Departments of Materials Sciences and Engineering, and Chemistry. Xu and her research group used block copolymers -- long sequences or "blocks" of one type of monomer bound to blocks of another type of monomer -- as a platform to guide the self-assembly of nanorods into complex structures and hierarchical patterns. Block copolymers have an innate ability to self-assemble into well-defined arrays of nano-sized structures over macroscopic distances.

"Ours is a simple and versatile technique for controlling the orientation of nanorods within block copolymers," Xu says. "By varying the morphology of the block copolymers and the chemical nature of the nanorods, we can provide the controlled self-assembly in nanorods and nanorod-based nanocomposites that is critical for their use in the fabrication of optical and electronic devices."

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Tandem Polymer Solar Cells That Set Record for Energy-Conversion

In the effort to convert sunlight into electricity, photovoltaic solar cells that use conductive organic polymers for light absorption and conversion have shown great potential. Organic polymers can be produced in high volumes at low cost, resulting in photovoltaic devices that are cheap, lightweight and flexible.

In the last few years, much work has been done to improve the efficiency with which these devices convert sunlight into power, including the development of new materials, device structures and processing techniques.
In a new study, available online this week in the journal Nature Photonics, researchers at the UCLA Henry Samueli School of Engineering and Applied Science and UCLA's California Nanosystems Institute (CNSI) report that they have significantly enhanced polymer solar cells' performance by building a device with a new "tandem" structure that combines multiple cells with different absorption bands. The device had a certified power-conversion efficiency of 8.62 percent and set a world record in July 2011.

Further, after the researchers incorporated a new infrared-absorbing polymer material provided by Sumitomo Chemical of Japan into the device, the device's architecture proved to be widely applicable and the power-conversion efficiency jumped to 10.6 percent -- a new record -- as certified by the U.S. Department of Energy's National Renewable Energy Laboratory.

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Atomic Antenna Behavior in Graphene

Atomic-level defects in graphene could be a path forward to smaller and faster electronic devices, according to a study led by researchers at the Department of Energy's Oak Ridge National Laboratory.

With unique properties and potential applications in areas from electronics to biodevices, graphene, which consists of a single sheet of carbon atoms, has been hailed as a rising star in the materials world. Now, an ORNL study published in Nature Nanotechnology suggests that point defects, composed of silicon atoms that replace individual carbon atoms in graphene, could aid attempts to transfer data on an atomic scale by coupling light with electrons.

"In this proof of concept experiment, we have shown that a tiny wire made up of a pair of single silicon atoms in graphene can be used to convert light into an electronic signal, transmit the signal and then convert the signal back into light," said coauthor Juan-Carlos Idrobo, who holds a joint appointment at ORNL and Vanderbilt University.

An ORNL-led team discovered this novel behavior by using aberration-corrected scanning transmission electron microscopy to image the plasmon response, or optical-like signals, of the point defects. The team's analysis found that the silicon atoms act like atomic-sized antennae, enhancing the local surface plasmon response of graphene, and creating a prototypical plasmonic device.
"The idea with plasmonic devices is that they can convert optical signals into electronic signals," Idrobo said. "So you could make really tiny wires, put light in one side of the wire, and that signal will be transformed into collective electron excitations known as plasmons. The plasmons will transmit the signal through the wire, come out the other side and be converted back to light."
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