Researchers show the way forward for improving organic and molecular electronic devices

"We have shown that when the molecules in organic thin films are aligned in particular directions, there is much better conductance," says Miquel Salmeron, a leading authority on nanoscale surface imaging who directs Berkeley Lab's Materials Sciences Division and who led this study. "Chemists already know how to fabricate organic thin films in a way that can achieve such an alignment, which means they should be able to use the information provided by our methodology to determine the molecular alignment and its role on charge transport across and along the molecules. This will help improve the performances of future organic electronic devices."

Salmeron and Shaul Aloni, also of the Materials Sciences Division, are the corresponding authors of a paper in the journal Nano Letters that describes this work. The paper is titled "Electron Microscopy Reveals Structure and Morphology of One Molecule Thin Organic Films." Other co-authors were Virginia Altoe, Florent Martin and Allard Katan.

Organic electronics, also known as plastic or polymer electronics, are devices that utilize carbon-based molecules as conductors rather than metals or semiconductors. They are prized for their low costs, light weight and rubbery flexibility. Organic electronics are also expected to play a big role in molecular computing, but to date their use has been hampered by low electrical conductance in comparison to metals and semiconductors.

"Chemists and engineers have been using their intuition and trial-and-error testing to make progress in the field but at some point you hit a wall unless you understand what is going on at the molecular level, for example, how electrons or holes flow through or across molecules, how the charge transport depends on the structure of the organic layers and the orientation of the molecules, and how the charge transport responds to mechanical forces and chemical inputs," Salmeron says. "With our experimental results, we have shown that we can now provide answers for these questions."

In this study, Salmeron and his colleagues used electron diffraction patterns to map the crystal structures of molecular films made from monolayers of short versions of commonly used polymers containing long chains of thiophene units. They focused specifically on pentathiophene butyric acid (5TBA) and two of its derivatives (D5TBA and DH5TBA) that were induced to self-assemble on various electron-transparent substrates.

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Electron diffraction patterns provide a wealth of information about the morphology, structure, and quality of monolayer organic thin films. Credit: Image from Berkeley Lab's Molecular Foundry

Pentathiophenes - molecules containing a ring of four carbon and one sulfur atoms - are members of a well-studied and promising family of organic semiconductors.

Obtaining structural crystallographic maps of monolayer organic films using electron beams posed a major challenge, as Aloni explains.

"These organic molecules are extremely sensitive to high energy electrons," he says. "When you shoot a beam of high energy electrons through the film it immediately affects the molecules. Within few seconds we no longer see the signature intermolecular alignment of the diffraction pattern. Despite this, when applied correctly, electron microscopy becomes essential tool that can provide unique information on organic samples."

Salmeron, Aloni and their colleagues overcame the challenge through the combination of a unique strategy they developed and a transmission electron microscope (TEM) at the Molecular Foundry's Imaging and Manipulation of Nanostructures Facility. Electron diffraction patterns were collected as a parallel electron beam was scanned over the film, then analyzed by computer to generate structural crystallographic maps.

"These maps contain uncompromised information of the size, symmetry and orientation of the unit cell, the orientation and structure of the domains, the degree of crystallinity, and any variations on the micrometer scale," says first author Altoe. "Such data are crucial to understanding the structure and electrical transport properties of the organic films, and allow us to track small changes driven by chemical modifications of the support films."

In their paper, the authors acknowledge that to gain structural information they had to sacrifice some resolution.

"The achievable resolution of the structural map is a compromise between sample radiation hardness, detector sensitivity and noise, and data acquisition rate," Salmeron says. "To keep the dose of high energy electrons at a level the monolayer film could support and still be able to collect valuable information about its structure, we had to spread the beam to a 90 nanometer diameter. However a fast and direct control of the beam position combined with the use of fast and ultrasensitive detectors should allow for the use of smaller beams with a higher electron flux, resulting in a better than 10 nanometer resolution."

While the combination of organic molecular films and substrates in this study conduct electrical current via electron holes (positively-charged energy spaces), Salmeron and his colleagues say their structural mapping can also be applied to materials whose conductance is electron-based.

"We expect our methodology to have widespread applications in materials research," Salmeron says.

Provided by Lawrence Berkeley National Laboratory (news : web)

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Researchers help assess economic impact of nanotech on green & sustainable growth

Georgia Institute of Technology researchers Philip Shapira and Jan Youtie helped answer that question through research presented March 27th at the International Symposium on Assessing the Economic Impact of Nanotechnology held in Washington, D.C.  The researchers highlighted the importance of full lifecycle assessments to understand the impacts of nanotechnologies on green economic development in such areas as energy, the environment and safe drinking water.

“Nanotechnology promises to foster green and sustainable growth in many product and process areas,” said Shapira, a professor with Georgia Tech’s School of Public Policy and the Manchester Institute of Innovation Research at the Manchester Business School in the United Kingdom. “Although nanotechnology commercialization is still in its early phases, we need now to get a better sense of what markets will grow and how new nanotechnology products will impact sustainability. This includes balancing gains in efficiency and performance against the net energy, environmental, carbon and other costs associated with the production, use and end-of-life disposal or recycling of nanotechnology products.”

But because nanotechnology underlies many different industries, assessing and forecasting its impact won’t be easy. “Compared to information technology and biotechnology, for example, nanotechnology has more of the characteristics of a general technology such as the development of electric power,” said Youtie, director of policy research services at Georgia Tech’s Enterprise Innovation Institute. “That makes it difficult to analyze the value of products and processes that are enabled by the technology. We hope that our paper will provide background information and help frame the discussion about making those assessments.”

The symposium is sponsored by the Organization for Economic Cooperation and Development and by the U.S. National Nanotechnology Initiative. Support for Georgia Tech research into the societal impacts of nanotechnology has come from the National Science Foundation through the Center for Nanotechnology in Society based at Arizona State University.

For their paper, co-authors Shapira and Youtie examined a subset of green nanotechnologies that aim to enable sustainable energy, improve environmental quality, and provide healthy drinking water for areas of the world that now lack it. They argue that the lifecycle of nanotechnology products must be included in the assessment.

“In examining the economic impact of these green nanotechnologies, we have to consider the lifecycle, which includes such issues as environmental health and safety, as well as the amount of energy required to produce materials such as carbon nanotubes,” said Shapira.

Environmental concerns have been raised about what happens to nanomaterials when they get into water supplies, he noted. In addition, some nanostructures use toxic elements such as cadmium. Energy required for producing nano-enabled products is also an important consideration, though it may be balanced against the energy saved – and pollution reduced – through the use of such products, Shapira said.

Research into these societal issues, which is being conducted in parallel with the research and development of nanotechnology – may allow the resulting nano-enabled products to avoid the kinds of the controversies that have hindered earlier technologies.

“Scientists, policy-makers and other observers have found that some of the promise of prior rounds of technology was limited by not anticipating and considering societal concerns prior to the introduction of new products,” Youtie said. “For nanotechnology, it is vital that these issues are being considered even during the research and development stage, before products hit the market in significant quantities.”

The nanotechnology industry began with large companies that had the resources to invest in research and development. But that is now changing, Youtie said.

“A lot of small companies are involved in novel nanomaterials development,” she said. “Large companies often focus on integrating those nanomaterials into existing products or processes.”

Among the goals of the OECD symposium are development of methodologies and approaches for estimating the impacts of green nanotechnology on jobs and new product sales. Existing forecasts have come largely from proprietary models used by private-sector firms.

“While these private forecasts have high visibility, their information and methods are often proprietary,” Shapira noted. “We also need to develop open and peer-reviewed models in which approaches are transparent and everyone can see the methods and assumptions used.”

In their paper, Youtie and Shapira cite several examples of green nanotechnology, discuss the potential impacts of the technology, and review forecasts that have been made. Examples of green nanotechnology they cite include:

• Nano-enabled solar cells that use lower-cost organic materials, as opposed to current photovoltaic technologies that require rare materials such as platinum;
• Nanogenerators that use piezoelectric materials such as zinc oxide nanowires to convert human movement into energy;
• Energy storage applications in which nanotechnology materials improve existing batteries and nano-enabled fuel cells;
• Thermal energy applications, such as nano-enabled insulation;
• Fuel catalysis in which nanoparticles improve the production and refining of fuels and reduce emissions from automobiles;
• Technologies used to provide safe drinking water through improved water treatment, desalination and reuse.

Provided by Georgia Institute of Technology (news : web)

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Researchers theorize cold compression of graphite results in new superhard carbon allotropes

An allotrope is a substance that is essentially the same as another, with just minor differences in structure. Thus, both graphite and diamonds are allotropes of carbon. In their paper, the research team shows, via mathematical calculations, that subjecting a graphite allotrope to varying degrees of both cold and high pressure, would result in small changes to the structure, resulting in two new carbon allotropes.

Prior to this work, other researchers have theorized that applying pressure at room temperature (more than 10 GigaPascals) to graphite would also result in structural changes, creating new allotropes (M10-carbon, monoclinic M-carbon, orthorhombic W-carbon or cubic body center C4 carbon) though thus far it isn’t clear if those changes would remain in effect after the pressure is removed.

The new allotropes that theoretically would be produced by exerting pressure under cold conditions, which the team have called H-carbon and S-carbon, would also apparently be more stable than the allotropes produced without the cold, and even more stable, they say, than graphite under pressure, which means they would be more likely to survive in their compressed state after being returned to normal conditions.

By using mathematical models to predict the creation of new carbon allotropes, researchers pave the way for real world experiments to find out if the new materials would truly exist, and if so, to what purpose they might be used. New carbon allotropes would have different optical properties, such as their degree of transparency, for example or how well they reflect light, than already well understood allotropes that are already being used in real world applications,. Such properties in new allotropes, if they can be caused to persist under reasonable conditions, might lead to new and better products.

But before researchers begin trying to produce these new allotropes, more theoretical work will need to be done to see if there are others out there still waiting to be discovered.

More information: New Superhard Carbon Phases Between Graphite and Diamond, arXiv:1203.5509v1 [cond-mat.mtrl-sci] http://arxiv.org/abs/1203.5509

Abstract
Two new carbon allotropes (H-carbon and S-carbon) are proposed, as possible candidates for the intermediate superhard phases between graphite and diamond obtained in the process of cold compressing graphite, based on the results of first-principles calculations.Both H-carbon and S-carbon are more stable than previously proposed M-carbon and W-carbon and their bulk modulus are comparable to that of diamond. H-carbon is an indirect-band-gap semiconductor with a gap of 4.459 eV and S-carbon is a direct-band-gap semiconductor with a gap of 4.343 eV. S-carbon is even more stable than the Z-carbon which is the most table carbon phase proposed recently. The transition pressure from cold compressing graphite is 10.08 GPa and 5.93 Gpa for H-carbon and S-carbon,respectively, which is in consistent with the recent experimental report.

via Arxiv Blog

? 2012 PhysOrg.com

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Researchers find simple and cheap way to mass-produce graphene nanosheets

Graphene, which is made from graphite, the same stuff as "lead" in pencils, has been hailed as the most important synthetic material in a century. Sheets conduct electricity better than copper, heat better than any material known, are harder than diamonds yet stretch.

Scientists worldwide speculate graphene will revolutionize computing, electronics and medicine but the inability to mass-produce sheets has blocked widespread use.

A description of the new research will be published the week of March 26 in the online Early Edition of the Proceedings of the National Academy of Sciences.

Jong-Beom Baek, professor and director of the Interdisciplinary School of Green Energy/Advanced Materials & Devices, Ulsan National Institute of Science and Technology, Ulsan, South Korea, led the effort.

"We have developed a low-cost, easier way to mass produce better graphene sheets than the current, widely-used method of acid oxidation, which requires the tedious application of toxic chemicals," said Liming Dai, professor of macromolecular science and engineering at Case Western Reserve and a co-author of the paper.

Here's how:

Researchers placed graphite and frozen carbon dioxide in a ball miller, which is a canister filled with stainless steel balls. The canister was turned for two days and the mechanical force produced flakes of graphite with edges essentially opened up to chemical interaction by carboxylic acid formed during the milling.

The carboxylated edges make the graphite soluble in a class of solvents called protic solvents, which include water and methanol, and another class called polar aprotic solvents, which includes dimethyl sulfoxide.

Once dispersed in a solvent, the flakes separate into graphene naonsheets of five or fewer layers.

To test whether the material would work in direct formation of molded objects for electronic applications, samples were compressed into pellets. In a comparison, these pellets were 688 times better at conducting electricity than pellets yielded from the acid oxidation of graphite.

After heating the pellets at 900 degrees Celsius for two hours, the edges of the ball-mill–derived sheets were decarboxylated, that is, the edges of the nanosheets became linked with strong hydrogen bonding to neighboring sheets, remaining cohesive. The compressed acid-oxidation pellet shattered during heating.

To form large-area graphene nanosheet films, a solution of solvent and the edge-carboxylated graphene nanosheets was cast on silicon wafers 3.5 centimeters by 5 centimeters, and heated to 900 degrees Celsius. Again, the heat decarboxylated the edges, which then bonded with edges of neighboring pieces. The researchers say this process is limited only by the size of the wafer. The electrical conductivity of the resultant large-area films, even at a high optical transmittance, was still much higher than that of their counterparts from the acid oxidation.

By using ammonia or sulfur trioxide as substitutes for dry ice and by using different solvents, "you can customize the edges for different applications," Baek said. "You can customize for electronics, supercapacitors, metal-free catalysts to replace platinum in fuel cells. You can customize the edges to assemble in two-dimensional and three-dimensional structures."

Provided by Case Western Reserve University (news : web)

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