MATERIALS ENGINEERING

New video showing the atom-by-atom growth of carbon nanotubes reveals they rotate as they grow, much like the halting motion of a mechanical clock's second hand.

Published online this month in the American Chemical Society's journal Nano Letters by researchers at France's Université Lyon1/CNRS and Houston's Rice University, the research provides the first experimental evidence of how individual carbon atoms are added to growing nanotubes.

"The key issue for realizing the potential of carbon nanotubes has always been better control of their growth," said team lead Stephen Purcell of the Université Lyon1/CNRS. "Our findings offer new insights for better measurement, modeling and control of nanotube growth."

Carbon nanotubes are long, hollow cylinders of pure carbon. They are hair-like in shape but are about 100,000 times smaller than human hair. They are also about six times stronger than steel, conduct electricity as well as copper and are almost impervious to radiation and chemical destruction. As a result, scientists are keen to use them in superstrong, "smart" materials, but they need to better understand how to produce them.

"The images from Dr. Purcell's lab show the atom-by-atom 'self assembly' of a nanotube," said Rice co-author Boris Yakobson, professor in mechanical engineering and materials science and of chemistry. "The video offers compelling evidence of the rotational motion that accompanies nanotube growth. It brings to mind Galileo's famous quote, 'And yet, it does turn.'"

In February, Yakobson offered a new theory suggesting that nanotubes grow like tiny, woven tapestries, with new atoms attaching to twisting atomic threads. The new video appears to support the theory, indicating that atoms are added in pairs as the tube spins and grows.

To create the images, Purcell's team at LPMCN (Laboratoire de Physique de la Matière Condensée et Nanostructures) used a field emission microscope (FEM). A few atoms of metal catalyst were placed on the tip of the FEM's needle-like probe, and carbon nanotubes grew atop the metal catalyst. An electric current was passed lengthwise through the probe and nanotube, and it projected a bright, top-down image of the nanotube onto a phosphor screen. The bright spot was filmed by a video camera, which revealed the nanotube's rotation during growth.

In one case, a nanotube turned approximately 180 times during its 11-minute growth. A frame-by-frame analysis of the video showed that the rotation proceeded in discrete steps -- much like the halting motion of the second hand on a mechanical clock -- with about 24 steps per rotation.

"The results support our predictions of how nanotubes grow," Yakobson said. "The video shows rotational movement during growth, as carbon atoms are added in pairs to the twisting, chiral network of carbon atoms that comprise the nanotube."

Co-authors include Mickaël Marchand, Catherine Journet, Dominique Guillot and Jean-Michel Benoit, all of Université Lyon. The research was supported by the Programme en Nanosciences et Nanotechnologies of France's L'Agence Nationale de Recherche, the National Science Foundation and the Air Force Research Laboratory.

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Journal reference:

1. Mickaël Marchand, Catherine Journet, Dominique Guillot, Jean-Michel Benoit, Boris I. Yakobson, Stephen T. Purcell. Growing a Carbon Nanotube Atom by Atom: 'And Yet It Does Turn'. Nano Letters, 2009; DOI: 10.1021/nl901380u

The description of compounds and interactions between atoms is one of the basic objectives of chemistry. Admittedly, chemical bonding models, which describe these properties very well, already exist. However, any deviation from the normal factors may lead to improving the models further. Chemists with Professor Thomas M. Klapötke at Ludwig-Maximilians-Universität (LMU) München have now analyzed a molecule, which has an extremely short bond length.

As reported by the researchers in Nature Chemistry, the carbon atom and the chlorine atom in the so-called chlorotrinitromethane molecule are only 1.69 Angstroms apart from one another. "Non-covalent interactions are one of the factors responsible for this short distance", declared Göbel, whose doctoral thesis revealed the new results. "A better understanding of these interactions is important and useful in all areas, where molecular recognition and self-assembly come into play."

Chemical bond models that have been successfully used for well over a century assume that a good description of the properties of a compound can be obtained while ignoring all but the nearest-neighbour bonding interactions. The idea that electrostatic interactions between second, third and even further neighbors are important and should not be ignored has not been a common notion so far. The team of Professor Thomas M. Klapötke of the Department of Chemistry and Biochemistry at LMU Munich, primarily concerned with the synthesis and investigation of new high-energy materials, has now demonstrated for the first time that even second and third neighbors can have a decisive effect on the properties of a chemical bond.

For their investigation, the researchers chose the so-called chlorotrinitromethane molecule, a compound, consisting of the halogen chlorine and the pseudohalogen trinitromethyl group. The latter is composed of one carbon atom and three nitro groups. The trinitromethyl unit belongs to the group of pseudohalogens, which has properties similar to those of the halogens. Both groups are composed of non-metals, which are generally present in the gaseous or liquid state and form salts with metals. Contrary to the halogens, however, the pseudohalogens, instead of being true chemical elements, are chemical groups composed of different elements.

Using X-ray structural analysis, the researchers succeeded for the first time in revealing the internal structure of the chlorotrinitromethane molecule and drawing conclusions concerning the distances between the individual atoms. In their analyses, the chemists came up against a particularly interesting property of the chlorotrinitromethane molecule, namely the distance between its chlorine atom and its carbon atom is only 1.69 Angstroms. An Angstrom is 10-7 millimeters. The distance, now detected between the atoms, is the shortest distance ever observed for comparable chlorine-carbon single bonds. All previously measured distances fall within the range of approximately 1.71 and 1.91 Angstroms.

By means of theoretical calculations, carried out in cooperation with Professor Peter Politzer and Dr. Jane S. Murray of the University of New Orleans in the USA, the researchers were able to reproduce the distribution of electrical charges within the molecule. It turned out that the chlorine atom has a completely positive electrostatic potential, a rare case, since chlorine usually has a negative electrostatic potential in other molecules. Together with the charge distributions of the remaining atoms, this finding explains why the chlorine and carbon atoms are linked so tightly to one another. The results impressively show that electrostatic interactions between atoms within a molecule can have a significant effect on bond lengths, even if these atoms are not linked directly to one of the two atoms that form the bond.

In the case of chlorotrinitromethane, this effect is particularly pronounced and leads to an unusually short chlorine-carbon bond. However, it could be of importance in various other cases, especially in areas, where molecules recognize one another and assemble to larger structures. These mechanisms play an important role, for example, in biological systems and in nanotechnology.

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Journal reference:

1. Göbel et al. Chlorotrinitromethane and its exceptionally short carbon-chlorine bond. Nature Chemistry, 2009; DOI: 10.1038/nchem.179

The ability to create tiny patterns is essential to the fabrication of computer chips and many other current and potential applications of nanotechnology. Yet, creating ever smaller features, through a widely-used process called photolithography, has required the use of ultraviolet light, which is difficult and expensive to work with.

John Fourkas, Professor of Chemistry and Biochemistry in the University of Maryland College of Chemical and Life Sciences, and his research group have developed a new, table-top technique called RAPID (Resolution Augmentation through Photo-Induced Deactivation) lithography that makes it possible to create small features without the use of ultraviolet light.

Photolithography uses light to deposit or remove material and create patterns on a surface. There is usually a direct relationship between the wavelength of light used and the feature size created. Therefore, nanofabrication has depended on short wavelength ultraviolet light to generate ever smaller features.

"The RAPID lithography technique we have developed enables us to create patterns twenty times smaller than the wavelength of light employed,"explains Dr. Fourkas, "which means that it streamlines the nanofabrication process. We expect RAPID to find many applications in areas such as electronics, optics, and biomedical devices."

"If you have gotten a filling at the dentist in recent years,"says Fourkas, "you have seen that a viscous liquid is squirted into the cavity and a blue light is then used to harden it. A similar process of hardening using light is the first element of RAPID. Now imagine that your dentist could use a second light source to sculpt the filling by preventing it from hardening in certain places. We have developed a way of using a second light source to perform this sculpting, and it allows us to create features that are 2500 times smaller than the width of a human hair."

Both of the laser light sources used by Fourkas and his team were of the same color, the only difference being that the laser used to harden the material produced short bursts of light while the laser used to prevent hardening was on constantly. The second laser beam also passed through a special optic that allowed for sculpting of the hardened features in the desired shape.

"The fact that one laser is on constantly in RAPID makes this technique particularly easy to implement,"says Fourkas, "because there is no need to control the timing between two different pulsed lasers."

Fourkas and his team are currently working on improvements to RAPID lithography that they believe will make it possible to create features that are half of the size of the ones they have demonstrated to date.

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Journal reference:

1. Linjie Li, Rafael R. Gattass, Erez Gershgorem, Hana Hwang and John T. Fourkas. Achieving lambda/20 Resolution by One-Color Initiation and Deactivation of Polymerization. Science, April 10, 2009

The design of efficient systems for splitting water into hydrogen and oxygen, driven by sunlight is among the most important challenges facing science today, underpinning the long term potential of hydrogen as a clean, sustainable fuel. But man-made systems that exist today are very inefficient and often require additional use of sacrificial chemical agents. In this context, it is important to establish new mechanisms by which water splitting can take place.

Now, a unique approach developed by Prof. David Milstein and colleagues of the Weizmann Institute’s Organic Chemistry Department, provides important steps in overcoming this challenge. During this work, the team demonstrated a new mode of bond generation between oxygen atoms and even defined the mechanism by which it takes place. In fact, it is the generation of oxygen gas by the formation of a bond between two oxygen atoms originating from water molecules that proves to be the bottleneck in the water splitting process. Their results have recently been published in Science.

Nature, by taking a different path, has evolved a very efficient process: photosynthesis – carried out by plants – the source of all oxygen on Earth. Although there has been significant progress towards the understanding of photosynthesis, just how this system functions remains unclear; vast worldwide efforts have been devoted to the development of artificial photosynthetic systems based on metal complexes that serve as catalysts, with little success. (A catalyst is a substance that is able to increase the rate of a chemical reaction without getting used up.)

The new approach that the Weizmann team has recently devised is divided into a sequence of reactions, which leads to the liberation of hydrogen and oxygen in consecutive thermal- and light-driven steps, mediated by a unique ingredient – a special metal complex that Milstein’s team designed in previous studies. Moreover, the one that they designed – a metal complex of the element ruthenium – is a ‘smart’ complex in which the metal center and the organic part attached to it cooperate in the cleavage of the water molecule.

The team found that upon mixing this complex with water the bonds between the hydrogen and oxygen atoms break, with one hydrogen atom ending up binding to its organic part, while the remaining hydrogen and oxygen atoms (OH group) bind to its metal center.

This modified version of the complex provides the basis for the next stage of the process: the ‘heat stage.’ When the water solution is heated to 100 degrees C, hydrogen gas is released from the complex – a potential source of clean fuel – and another OH group is added to the metal center.

‘But the most interesting part is the third ‘light stage,’’ says Milstein. ‘When we exposed this third complex to light at room temperature, not only was oxygen gas produced, but the metal complex also reverted back to its original state, which could be recycled for use in further reactions.’

These results are even more remarkable considering that the generation of a bond between two oxygen atoms promoted by a man-made metal complex is a very rare event, and it has been unclear how it can take place. Yet Milstein and his team have also succeeded in identifying an unprecedented mechanism for such a process. Additional experiments have indicated that during the third stage, light provides the energy required to cause the two OH groups to get together to form hydrogen peroxide (H2O2), which quickly breaks up into oxygen and water. ‘Because hydrogen peroxide is considered a relatively unstable molecule, scientists have always disregarded this step, deeming it implausible; but we have shown otherwise,’ says Milstein. Moreover, the team has provided evidence showing that the bond between the two oxygen atoms is generated within a single molecule – not between oxygen atoms residing on separate molecules, as commonly believed – and it comes from a single metal center.

Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into oxygen and hydrogen is a major goal of renewable clean energy research. So far, Milstein’s team has demonstrated a mechanism for the formation of hydrogen and oxygen from water, without the need for sacrificial chemical agents, through individual steps, using light. For their next study, they plan to combine these stages to create an efficient catalytic system, bringing those in the field of alternative energy an important step closer to realizing this goal.

Participating in the research were former postdoctoral student Stephan Kohl, Ph.D. student Leonid Schwartsburd and technician Yehoshoa Ben-David all of the Organic Chemistry Department, together with staff scientists Lev Weiner, Leonid Konstantinovski, Linda Shimon and Mark Iron of the Chemical Research Support Department.

Prof. David Milstein’s research is supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research; and the Helen and Martin Kimmel Center for Molecular Design. Prof. Milstein is the incumbent of the Israel Matz Professorial Chair of Organic Chemistry.

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Journal reference:

1. Stephan W. Kohl, Lev Weiner, Leonid Schwartsburd, Leonid Konstantinovski, Linda J. W. Shimon, Yehoshoa Ben-David, Mark A. Iron, and David Milstein. Consecutive Thermal H₂ and Light-Induced O₂ Evolution from Water Promoted by a Metal Complex. Science, 2009; 324 (5923): 74 DOI: 10.1126/science.1168600

Radio frequency identification (RFID) devices have widely been used for tracking for years; recently, scientists from U.S. Department of Energy's (DOE) Argonne National Laboratory have developed a unique tracking technology that also monitors the environmental and physical conditions of containers of nuclear materials in storage and transportation.

"RFID technology is ideally suited for management of nuclear materials during both storage and transportation," said Dr. Yung Liu, Argonne senior nuclear engineer and RFID project manager. "Key information about the nuclear materials is acquired in real-time," he explained.

Data on the status and history of each individual container are available with a click of the mouse and can be used to augment and modernize DOE's existing management systems for nuclear materials.

"The Argonne system can simultaneously monitor thousands of drums 24 hours a day, seven days a week. Any abnormal situation, such a loss of seal, a sudden shock, a rise in temperature or humidity, can trigger an alarm for immediate action," Liu explained.

The monitoring of tens of thousands of radioactive and fissile material packages has been a challenge for DOE to ensure accountability, safety, security and worker and public health.

"The RFID system that Dr. Liu and his group developed with collaborators will help DOE overcome this challenge," said Dr. James Shuler, Manager of DOE Packaging Certification Program, Office of Environmental Management.

The system is comprised of active transponders, or tags with long-life batteries (>10 years), on each package, readers that collect information from the tags, control computer, and application software. The information is constantly updated and communicated via a secured network, thus decreasing the need for manned surveillance. Explained Liu, "information can be retrieved promptly by local and authorized off-site users via a secured network for action."

This RFID technology also has applications outside the nuclear field and may be used for other hazardous materials or any valued material, according to Liu.

"This new Argonne RFID technology, expected to be patented, has applications in many industries and as the technology is further developed, its usefulness is bound to grow," Liu said.

Funding for this project was made by the U.S. Department of Energy, Office of Environmental Management. The Office of Environmental Management (EM) is responsible for the risk reduction and safe cleanup of the environmental legacy of the Nation's nuclear weapons program and government-sponsored nuclear energy research and is one of the largest, most diverse, and technically complex environmental programs in the world.

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Researchers have developed a critical part of a hydrogen storage system for cars that makes it possible to fill up a vehicle's fuel tank within five minutes with enough hydrogen to drive 300 miles.

The system uses a fine powder called metal hydride to absorb hydrogen gas. The researchers have created the system's heat exchanger, which circulates coolant through tubes and uses fins to remove heat generated as the hydrogen is absorbed by the powder.

The heat exchanger is critical because the system stops absorbing hydrogen effectively if it overheats, said Issam Mudawar, a professor of mechanical engineering who is leading the research.

"The hydride produces an enormous amount of heat," Mudawar said. "It would take a minimum of 40 minutes to fill the tank without cooling, and that would be entirely impractical."

Researchers envision a system that would enable motorists to fill their car with hydrogen within a few minutes. The hydrogen would then be used to power a fuel cell to generate electricity to drive an electric motor.

The research, funded by General Motors Corp. and directed by GM researchers Darsh Kumar, Michael Herrmann and Abbas Nazri, is based at the Hydrogen Systems Laboratory at Purdue's Maurice J. Zucrow Laboratories. In February, the team applied for three provisional patents related to this technology.

"The idea is to have a system that fills the tank and at the same time uses accessory connectors that supply coolant to extract the heat," said Mudawar, who is working with mechanical engineering graduate student Milan Visaria and Timothée Pourpoint, a research assistant professor of aeronautics and astronautics and manager of the Hydrogen Systems Laboratory. "This presented an engineering challenge because we had to figure out how to fill the fuel vessel with hydrogen quickly while also removing the heat efficiently. The problem is, nobody had ever designed this type of heat exchanger before. It's a whole new animal that we designed from scratch."

The metal hydride is contained in compartments inside the storage "pressure vessel." Hydrogen gas is pumped into the vessel at high pressure and absorbed by the powder.

"This process is reversible, meaning the hydrogen gas may be released from the metal hydride by decreasing the pressure in the storage vessel," Mudawar said. "The heat exchanger is fitted inside the hydrogen storage pressure vessel. Due to space constraints, it is essential that the heat exchanger occupy the least volume to maximize room for hydrogen storage."

Conventional automotive coolant flows through a U-shaped tube traversing the length of the pressure vessel and heat exchanger. The heat exchanger, which is made mostly of aluminum, contains a network of thin fins that provide an efficient cooling path between the metal hydride and the coolant.

"This milestone paves the way for practical on-board hydrogen storage systems that can be charged multiple times in much the same way a gasoline tank is charged today," said Kumar, a researcher at GM's Chemical & Environmental Sciences Laboratory and the GM R&D Center in Warren, Mich. "As newer and better metal hydrides are developed by research teams worldwide, the heat exchanger design will provide a ready solution for the automobile industry."

The researchers have developed the system over the past two years. Because metal hydride reacts readily with both air and moisture, the system must be assembled in an airtight chamber, Pourpoint said.

Research activities at the hydrogen laboratory involve faculty members from the schools of aeronautics and astronautics, mechanical engineering, and electrical and computer engineering.

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For the first time, MIT researchers have shown they can genetically engineer viruses to build both the positively and negatively charged ends of a lithium-ion battery.

The new virus-produced batteries have the same energy capacity and power performance as state-of-the-art rechargeable batteries being considered to power plug-in hybrid cars, and they could also be used to power a range of personal electronic devices, said Angela Belcher, the MIT materials scientist who led the research team.

The new batteries, described in the April 2 online edition of Science, could be manufactured with a cheap and environmentally benign process: The synthesis takes place at and below room temperature and requires no harmful organic solvents, and the materials that go into the battery are non-toxic.

In a traditional lithium-ion battery, lithium ions flow between a negatively charged anode, usually graphite, and the positively charged cathode, usually cobalt oxide or lithium iron phosphate. Three years ago, an MIT team led by Belcher reported that it had engineered viruses that could build an anode by coating themselves with cobalt oxide and gold and self-assembling to form a nanowire.

In the latest work, the team focused on building a highly powerful cathode to pair up with the anode, said Belcher, the Germeshausen Professor of Materials Science and Engineering and Biological Engineering. Cathodes are more difficult to build than anodes because they must be highly conducting to be a fast electrode, however, most candidate materials for cathodes are highly insulating (non-conductive).

To achieve that, the researchers, including MIT Professor Gerbrand Ceder of materials science and Associate Professor Michael Strano of chemical engineering, genetically engineered viruses that first coat themselves with iron phosphate, then grab hold of carbon nanotubes to create a network of highly conductive material.

Because the viruses recognize and bind specifically to certain materials (carbon nanotubes in this case), each iron phosphate nanowire can be electrically "wired" to conducting carbon nanotube networks. Electrons can travel along the carbon nanotube networks, percolating throughout the electrodes to the iron phosphate and transferring energy in a very short time.

The viruses are a common bacteriophage, which infect bacteria but are harmless to humans.

The team found that incorporating carbon nanotubes increases the cathode's conductivity without adding too much weight to the battery. In lab tests, batteries with the new cathode material could be charged and discharged at least 100 times without losing any capacitance. That is fewer charge cycles than currently available lithium-ion batteries, but "we expect them to be able to go much longer," Belcher said.

The prototype is packaged as a typical coin cell battery, but the technology allows for the assembly of very lightweight, flexible and conformable batteries that can take the shape of their container.

Last week, MIT President Susan Hockfield took the prototype battery to a press briefing at the White House where she and U.S. President Barack Obama spoke about the need for federal funding to advance new clean-energy technologies.

Now that the researchers have demonstrated they can wire virus batteries at the nanoscale, they intend to pursue even better batteries using materials with higher voltage and capacitance, such as manganese phosphate and nickel phosphate, said Belcher. Once that next generation is ready, the technology could go into commercial production, she said.

Lead authors of the Science paper are Yun Jung Lee and Hyunjung Yi, graduate students in materials science and engineering. Other authors are Woo-Jae Kim, postdoctoral fellow in chemical engineering; Kisuk Kang, recent MIT PhD recipient in materials science and engineering; and Dong Soo Yun, research engineer in materials science and engineering.

The research was funded by the Army Research Office Institute of the Institute of Collaborative Technologies, and the National Science Foundation through the Materials Research Science and Engineering Centers program.

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Journal reference:

1. Yun Jung Lee, Hyunjung Yi, Woo-Jae Kim, Kisuk Kang, Dong Soo Yun, Michael S. Strano, Gerbrand Ceder, and Angela M. Belcher. Fabricating Genetically Engineered High-Power Lithium Ion Batteries Using Multiple Virus Genes. Science, 2009; DOI: 10.1126/science.1171541