New Graphene-Based Material Clarifies Graphite Oxide Chemistry

September 25, 2008

AUSTIN, Texas — A new "graphene-based" material that helps solve the structure of graphite oxide and could lead to other potential discoveries of the one-atom thick substance called graphene, which has applications in nanoelectronics, energy storage and production, and transportation such as airplanes and cars, has been created by researchers at The University of Texas at Austin.

To get an idea of the nanomaterial graphene, imagine a lightweight material having the strongest chemical bond in nature and, thus, exceptional mechanical properties. In addition it conducts heat better than any other material and has charge carriers moving through it at a significant fraction of the speed of light. Just an atom thick, graphene consists of a "chickenwire" (or honeycomb) bonding arrangement of carbon atoms—also known as a single layer of graphite.

Mechanical Engineering Professor Rod Ruoff and his co-authors have, for the first time, prepared carbon-13 labeled graphite. They did this by first making graphite that had every "normal" carbon atom having the isotope carbon-12, which is magnetically inactive, replaced with carbon-13, which is magnetically active. They then converted that to carbon-13 labeled graphite oxide and used solid-state nuclear magnetic resonance to discern the detailed chemical structure of graphite oxide.

The work by Ruoff's team will appear in the Sept. 26 issue of the journal Science.

"As a result of our work published in Science, it will now be possible for scientists and engineers to create different types of graphene (by using carbon-13 labeled graphene as the starting material and doing further chemistry to it) and to study such graphene-based materials with solid-state nuclear magnetic resonance to obtain their detailed chemical structure," Ruoff says. "This includes situations such as where the graphene is mixed with a polymer and chemically bonded at critical locations to make remarkable polymer matrix composites; or embedded in glass or ceramic materials; or used in nanoelectronic components; or mixed with an electrolyte to provide superior supercapacitor or battery performance. If we don't know the chemistry in detail, we won't be able to optimize properties."

Graphene-based materials are a focus area of research at the university because they are expected to have applications for ultra-strong yet lightweight materials that could be used in automobiles and airplanes to improve fuel efficiency, the blades of wind turbines for improved generation of electrical power, as critical components in nanoelectronics that could have blazing speeds but very low power consumption, for electrical energy storage in batteries and supercapacitors to enable renewable energy production at a large scale and in transparent conductive films that will be used in solar cells and image display technology. In almost every application, sensitive chemical interactions with surrounding materials will play a central role in understanding and optimizing performance.

Ruoff and his team proved they had made such an isotopically labeled material from measurements by co-author Frank Stadermann of Washington University in St Louis. Stadermann used a special mass spectrometer typically used for measuring the isotope abundances of various elements that are in micrometeorites that have landed on Earth. Then, 100 percent carbon-13 labeled graphite was converted to 100 percent carbon-13 labeled graphite oxide, also a layered material but with some oxygen atoms attached to the graphene by chemical bonds.

Co-authors Yoshitaka Ishii and Medhat Shaibat of the University of Illinois-Chicago then used solid state nuclear magnetic resonance to help reveal the detailed chemical bonding network in graphite oxide. Ruoff says even though graphite oxide was first synthesized more than150 years ago the distribution of oxygen atoms has been debated even quite recently.

"The ability to control the isotopic labeling between carbon-12 and carbon-13 will lead to many other sorts of studies," says Ruoff, who holds the Cockrell Family Regents Chair in Engineering #7.

He collaborates on other graphene projects with other university scientists and engineers such as Allan MacDonald (Departments of Physics and Astronomy), Sanjay Banerjee, Emanuel Tutuc and Bhagawan Sahu (Department of Electrical and Computer Engineering) and Gyeong Hwang (Department of Chemical Engineering), and some of these collaborations include industrial partners such as Texas Instruments, IBM and others.

Co-authors on the Science article include: Weiwei Cai, Richard Piner, Sungjin Park, Dongxing Yang, Aruna Velamakanni, Meryl Stoller and Jinho An (all of the Ruoff research group at The University of Texas at Austin); Sung Jin An, formerly of Pohang University of Science and Technology (POSTECH-Korea) and a visiting graduate student in the Ruoff group during the study; Dongmin Chen (Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences); Stadermann; and Ishii and Shaibat of the University of Illinois-Chicago.

A high-resolution photo of Ruoff is available. Learn more about Ruoff's work.

For more information, contact: Daniel Vargas, Cockrell School of Engineering, 512-471-7541; Rodney Ruoff, Department of Mechanical Engineering, Cockrell School of Engineering, 512-471-4691.

Source

By adding graphene, researchers create superior polymer

Posted: May 19, 2008

(Nanowerk News) Researchers at Northwestern University and Princeton University have created a new kind of polymer that, because of its extraordinary thermal and mechanical properties, could be used in everything from airplanes to solar cells.

The polymer, a nanocomposite that incorporates functionalized, exfoliated graphene sheets, even conducts electricity, and researchers hope to use that property to eventually create thermally stable, optically transparent conducting polymers.

The results of their research were published May 11 in the online version of Nature Nanotechnology ("Functionalized graphene sheets for polymer nanocomposites"). ******************************** Letter abstract Nature Nanotechnology Published online: 11 May 2008 | doi:10.1038/nnano.2008.96 Functionalized graphene sheets for polymer nanocomposites T. Ramanathan1, A. A. Abdala2,7, S. Stankovich3, D. A. Dikin1, M. Herrera-Alonso2, R. D. Piner1,6, D. H. Adamson4, H. C. Schniepp2, X. Chen1, R. S. Ruoff1,6, S. T. Nguyen3, I. A. Aksay2, R. K. Prud'Homme2 & L. C. Brinson1,5 Abstract Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications1. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content2, 3, 4, 5, 6, 7, 8, 9, 10, 11. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer–particle interactions. An unprecedented shift in glass transition temperature of over 40 °C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 °C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet– poly(methyl methacrylate) rivaling those for single-walled carbon nanotube–poly(methyl methacrylate) composites. Department of Mechanical Engineering, Northwestern University, Evanston, Illinois 60208, USA Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, USA Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA Present Address: Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712-0292, USA Present Address: Chemical Engineering Program, The Petroleum Institute, Abu Dhabi, United Arab Emirates Correspondence to: L. C. Brinson1,5 e-mail: cbrinson@northwestern.edu *********************************

Researcher at the McCormick School of Engineering originally teamed up with researchers at Princeton several years ago. McCormick researchers had experience working with polymer nanocomposites, and Princeton researchers had developed a way to exfoliate, or split apart, graphite sheets into very thin single layer, surface-functionalized graphene sheets.

Previous use of graphite in polymers did not garner significantly improved properties since researchers could never get the graphite exfoliated. That meant the graphite was rigid with a low surface area and could only minimally impact properties of the polymer.

But when researchers put even a small amount the newly exfoliated graphene sheets — enough to equal only .05 percent of the material — into the polymer, they found the graphene changed the polymer’s thermal stability temperature by 30 degrees. Even adding graphene sheets equal to .01 percent of the material increased stiffness by 33 percent — far beyond what researchers had predicted. The drastic changes in both the thermal stability and the stiffness after adding just a tiny percentage of functionalized graphene indicated that the graphene changes large regions of the polymer radiating out from the nanoparticle surfaces in a percolating network structure.

The new polymer nanocomposite based on graphene also exhibited the same or superior thermal and mechanical properties as using functionalized single-wall nanotubes in polymer — but was much easier and cheaper to create.

“This is the first time people have been able to demonstrate dramatically altered properties like this with really small quantities of graphite-based materials,” says Cate Brinson, Jerome B. Cohen Professor of Mechanical Engineering and corresponding author of the paper.

The graphene sheets also will inherently be able to block moisture and gases from penetrating the material as well as change the thermal stability temperature and improve mechanical properties, making the durable polymer a candidate for use in everything from aircrafts to sports equipment to solar cells

“I think it has enormous potential,” Brinson says. “With the ready availability of graphite and the properties we have demonstrated, this new material will enable significant structural scale use of carbon-based nanocomposites.”

Next researchers are studying the polymer’s electroconductivity, quantifying and optimizing the results with the goal of creating optically transparent conducting polymers that are thermomechanically stable.

Source: Northwestern University

Source

'Sticky Nanotubes' Hold Key To Future Technologies

This composite image taken with an electron microscope shows a side and bottom image of a nanotube attached to a "microcantilever," a component in atomic force microscopes. Researchers at Purdue used the experimental setup to precisely measure the forces required to peel nanotubes off of other materials, opening up the possibility of creating standards for nano-manufacturing and harnessing a gecko's ability to walk up walls. The nanotube in this image has a length of about 6 microns, or millionths of a meter, and is 40 nanometers wide, roughly 500 times thinner than a human hair. This image also shows an artistic representation of how a nanotube peels away from surfaces. (Credit: Birck Nanotechnology Center, Purdue University)

ScienceDaily (Apr. 29, 2008) — Researchers at Purdue University are the first to precisely measure the forces required to peel tiny nanotubes off of other materials, opening up the possibility of creating standards for nano-manufacturing and harnessing a gecko's ability to walk up walls.

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So-called "peel tests" are used extensively in manufacturing. Knowing how much force is needed to pull a material off of another material is essential for manufacturing, but no tests exist for nanoscale structures, said Arvind Raman, an associate professor of mechanical engineering at Purdue.

Researchers are trying to learn about the physics behind the "stiction," or how the tiny structures stick to other materials, to manufacture everything from nanoelectronics to composite materials, "nanotweezers" to medical devices using nanotubes, nanowires and biopolymers such as DNA and proteins, he said.

Flexible carbon nanotubes stick to surfaces differently than larger structures because of attractive forces between individual atoms called van der Waals forces.

"Operating in a nanoscale environment is sort of like having flypaper everywhere because of the attraction of van der Waals forces," Raman said. "These forces are very relevant on this size scale because a nanometer is about 10 atoms wide."

Mechanical engineering doctoral student Mark Strus made the first peeling-force measurements for nanotubes in research based at the Birck Nanotechnology Center in Purdue's Discovery Park.

Findings were detailed in a research paper published in February in the journal Nano Letters. The paper was written by Strus; materials engineering doctoral student Luis Zalamea; Raman; Byron Pipes, the John Leighton Bray Distinguished Professor of Engineering; NASA engineer Cattien Nguyen; and Eric Stach, an associate professor of materials engineering.

The energy it takes to peel a nanotube from a surface was measured in "nanonewtons," perhaps a billion times less energy than that required to lift a cup of coffee. That peeling energy is proportional to the nanotube's "interfacial energy," which is one measure of how sticky something is, Strus said.

"This whole idea of measuring the stickiness of something is a standard material test in industry," he said. "There are certain tests that you need to have for measuring strength, toughness and adhesion."

But until now, no such test had been completed to successfully measure and quantify these forces on the nanoscale.

Nanotubes offer promise to produce a new class of composite materials that are stronger than conventional composites for use in aircraft and vehicles.

"This is a big area of research primarily because the strength of nanotubes can be much greater than that of carbon nanofibers," Raman said.

However, properly integrating high-strength nanotubes into polymers for composite materials requires a knowledge of how the nanotubes stick to polymers and to each other.

"One of the big areas in composites, in general, and nanocomposites, in particular, is how to coat a fiber with a material that makes it stick better to the matrix," Raman said. "So it's really important to know how to judge which coatings work best for specific types of fibers. For larger fibers, industry knows which coatings work best, but such knowledge is scarce for nanoscale fibers. It's all about how to make nanotubes 'sticky' to the surrounding matrix."

Nanotubes also must be dispersed uniformly in a solution before being mixed with the polymer to make composite materials, but the tiny rods tend to clump together. Learning precisely how the tubes adhere to each other could lead to a method for dispersing them.

The findings also promise to help researchers understand how geckos are able to stick to surfaces, a trait that could translate into practical uses for industrial and military applications.

Tiny branching hairs called setae on the animal's front feet use van der Waals adhesion. "The question is, how does it stick, and, equally important, if the adhesion force is strong enough to hold its weight onto a surface like a wall, then how does it then unstick, or peel, itself to move up a vertical surface?" Strus said.

Nanotubes also have possible medical applications, such as creating more effective bone grafts and biomolecular templates to replace damaged tissues, which requires knowing precisely how the nanotubes adhere to cells.

Yet another potential application is a "nanotweezer" that might use two nanorods to manipulate components for tiny devices and machines.

Raman and Strus plotted how much force it took to peel nanotubes from surfaces, discovering that the tubes lift off in fits and starts instead of smoothly.

"We saw these jumps in peeling forces, where the nanotubes would lift off suddenly and then snag, lift off suddenly and then snag. This behavior has a very deep physical significance and can only be appreciated by means of mathematical models," Raman said.

Pipes and Zalamea, meanwhile, had already been developing theoretical models to describe how the nanotubes would peel away from a surface and from each other. The four researchers then worked together with the others to fine tune the model, which describes the physics of why nanotubes peel off unevenly.

The nanotubes used in the research had a length of about 6 microns, or millionths of a meter, and were 40 nanometers wide, roughly 500 times thinner than a human hair.

The researchers used an atomic force microscope to measure the peeling forces. The nanotube was attached to the end of a diving-board shaped part of the microscope called a microcantilever. As the nanotube was pulled away from a surface, the cantilever bent. This bending movement was tracked with a laser, revealing the forces required to peel the nanotube.

The carbon nanotubes for the research were provided by NASA. With the assistance of Stach, the structure of the nanotubes was characterized using a transmission electron microscope. The research was funded by the National Science Foundation and the Korean Center for Nanomanufacturing and Mechatronics. Strus' work is supported in part with a Bilsland Fellowship, which he was awarded this year.

Future work may focus on making measurements that apply to nanocomposites.

Adapted from materials provided by Purdue University.

Source

Cup-Stack Carbon Nanotube

The new material Cup-Stack Carbon Nanotube is applied in the racquet frame which enables to give the players right amount of flex to stay with the ball, and quick repulsion to make the full energy available for power return.


Carbon Nanotube
Stiff and Durable




Cup-Stack Carbon Nanotube
Flexible and Durable

Source

If these composites using CNTs of various characteristics can make these rackets so good - just think what they can (and will?) do for automobile frames and bodies - strength, flexibility and durability at reduced weight and cost, more than likely.

Nanotubes Show Their Strengths in Polymer Fibres

Researchers at Queen Mary, University of London and Nanoforce Technology Ltd. in the UK, have successfully produced single-walled nanotube reinforced polymer fibres and tapes that are as strong as theory predicts.

The work shows for the first time the true reinforcing potential of single-walled carbon nanotubes (SWNTs) with effective properties of nanotubes in composites, which are close to their theoretical values.

Prof. Ton Peijs, who heads the research team said: "The problem with carbon nanotubes has always been that despite their amazing potential of becoming the ultimate reinforcing fibre for the next generation of high-performance composites, their success in actually delivering these mechanical properties when embedded in polymer composites has been limited. Despite promises of tensile strengths of 100 GPa or more – 15 to 40 times higher than carbon fibres – their efficiency after embedding them in polymer matrices has often been poor with effective reinforcing properties not far better than those of carbon fibres”.

Dr. Zhujuang Wang, who processed and characterized the new nanocomposite fibres and tapes during her PhD study at QMUL, says that in order to get the most out of nanotubes the composite need to exhibit a good dispersion as well as good interfacial interaction of the nanotubes with the hosting matrix. Moreover, similar to polymer molecules the excellent intrinsic mechanical properties of nanotubes can only be expected if they are all fully aligned. The QMUL team has explored many different nanotube/polymer combinations but the best results were obtained for a system based on poly(vinyl alcohol) (PVA) and SWNTs. Using solid-state drawing technology the team showed that they can effectively align nanotubes along the polymer fibre axis and triple the tensile strength of the PVA fibre or tape with the addition of only 1 wt.% of SWNTs. Further analysis of the materials showed that the stress carried by the SWNTs in these oriented PVA composites was very close to the theoretical tensile strength of nanotubes, indicating the exceptionally high reinforcing efficiency of the SWNTs in these materials.

In order to make the research a commercial success still some significant further developments are needed. Peijs said: “Although our work shows that indeed the impressive mechanical properties of nanotubes can effectively be translated into high-performance composites, the challenge is still to make a nanocomposite fibre of record breaking strength. We expect that in order to make such a fibre we will need to incorporate at least 5 wt.% of perfectly aligned and dispersed SWNTs in a highly oriented PVA fibre. So far we have only achieved good dispersions up to 1 wt.% of nanotubes, which makes that our current nanocomposite fibres and tapes have still significantly lower strengths than ultra-strong carbon fibres possessing strengths up to 7 GPa. If dispersion problems at high nanotube loadings can be overcome, nanocomposite fibres with strengths exceeding those of the strongest carbon fibre are possible. Such fibres can find their way in a large range of advanced composite materials, ranging from structural materials in sports equipment and aircraft to anti-ballistics.

The researchers published their work in Nanotechnology in a paper entitled “Extraordinary reinforcing efficiency of single-walled carbon nanotubes in oriented poly (vinyl alcohol) tapes”.

http://www.iop.org/EJ/abstract/0957-4484/18/45/455709/

Publication Date: 11/03/2008

http://www.netcomposites.com/news.asp?4866

Composite of carbon nanotubes and graphene

Atsugi, Japan, March 3, 2008 — Fujitsu Laboratories Ltd. today announced the successful formation of a new nano-scale carbon composite featuring a self-organizing structure⁽¹⁾, by combining carbon nanotubes and graphene⁽²⁾ which are both nano-scale carbon structures. The newly-discovered composite structure is synthesized at a temperature of 510 °C, cooler than for conventional graphene formed at temperatures too high for electronic device applications, thereby paving the way for the feasible use of graphene as a material suitable for future practical use in electronic devices which are vulnerable to heat. Carbon nanotubes have properties including high thermal conductivity and high current-density tolerance⁽³⁾, while graphene is known for its high electron mobility. Carbon nanostructures combining these two materials hold the promise of creating new potential for material research and applications.

Details of this technology will be presented at the 34th Fullerene Nanotubes General Symposium to be held from March 3 to March 5 in Nagoya, Japan.

Larger View	Larger View
Figure 1. (a) Electron microscopic image (cross-sectional) of the new nano-scale carbon composite (b) Electron microscopic image of the graphene multi-layers	Figure 2. Schematic view of the new nano-scale carbon composite (Lower image: Diagram of anticipated structure)

Background

Carbon nanotubes and graphene are both nano-scale structures consisting of carbon atoms. Graphene is a sheet-like hexagonal lattice of carbon atoms, while nanotubes can be described as graphene wrapped into a cylinder with a nano-scale diameter.

Despite the fact that both are made from the same carbon atoms, each has very distinct characteristics. Of any material found in nature, carbon nanotubes feature the highest thermal conductivity and mechanical strength as well as the ability to withstand the highest current density, making them an attractive material for wiring, heat dissipation, field electron emitters⁽⁴⁾, and other potential applications. Research and development is underway to find technologies to synthesize carbon nanotubes at temperatures as low as approximately 400°C, a temperature that would enable its use in electronic devices vulnerable to heat. Since the discovery of its high electron mobility in 2004, graphene has become attractive as a channel material for future transistors. However, conventional methods for synthesizing graphene only work at temperatures over 700°C - considered too high for use in electronic devices - or involve a time-consuming and unreliable process of stripping away graphite crystals.

Fujitsu Laboratories is researching ways to develop electronic devices that take advantage of the superior properties of carbon nanostructures.

Overview of the new technology

In order to better understand the growth mechanism of carbon nanotubes, Fujitsu Laboratories conducted experiments using chemical vapor deposition, a technique in which a feedstock gas is heat-cracked in a vacuum chamber to synthesize film or structures on a substrate. This resulted in the formation and discovery of aligned growth⁽⁵⁾multi-walled carbon nanotubes⁽⁶⁾ featuring layers of graphene (from a few layers to a few dozen) on top formed in a self-organizing way, thereby forming a complex composite (see Figure 1).

Carbon-based materials come in a variety of different forms that depend on how their atoms link together, such as zero-dimensional fullerenes⁽⁷⁾, one-dimensional nanotubes, two-dimensional (2-D) graphene, and three-dimensional (3-D) diamonds. Complex structures consisting of zero-dimensional and one-dimensional elements, known as "peapod⁽⁸⁾" structures, have already been created. The new complex composite developed by Fujitsu Laboratories is the world's first composite featuring one-dimensional and two-dimensional elements based on graphene layers and nanotubes, which are perpendicularly connected. The composite was synthesized at the relatively low temperature of 510°C.

Results

Due to the fact that carbon nanotubes are linear, one-dimensional structures, in the two-dimensional directions perpendicular to the tube axis they have nearly no thermal or electrical conductivity between tubes. Graphene, on the other hand, possesses electrical and thermal conductivity across two dimensions. The newly-discovered carbon nanostructure is expected to have electrical conduction and thermal dissipation in all directions. Conventionally aligned-growth carbon nanotubes have had relatively poor uniformity in length, thus being inconsistent when joined in the upper areas and resulting in increased thermal and electrical resistance. As the new carbon nanostructures from Fujitsu Labs feature carbon nanotubes that nearly all connect to the graphene with good uniformity at their endpoints (see Figure 2), and since the graphene surface is planar, it is anticipated that the new carbon nanostructures will enable excellent electrical and thermal conductivity. This technology brings the application of graphene for electronic devices one step closer to practical use.

Future Developments

Fujitsu Laboratories will continue to explore the mechanisms by which complex carbon nanostructures form and elucidate their physical characteristics, in order to develop electronic device application technologies that take advantage of those characteristics. In addition, in the field of material sciences Fujitsu Laboratories will pursue the development of technologies to enable the formation of high-quality carbon nanostructures at a lower temperature.

Glossary and notes

1 Self-organizing structure:

Refers to a desired structure that self-forms naturally, without the need for complex controls.

2 Graphene:

A hexagonal lattice of carbon atoms. Graphite consists of layers of graphene stacked on top of each other.

3 Current-density tolerance:

The limit of high-density current that can flow through a material without changing its physical structure.

4 Field electron emitter:

A device that extracts electrons from a fixed surface employing an electrical field. Displays referred to as field-emission displays (FED) operate on the principle of electrons bumping up against fluorescent bodies, and causing them to emit light.

5 Aligned growth:

A growth pattern that grows perpendicular to a substrate.

6 Multi-walled carbon nanotube:

A type of carbon nanotube in which multiple graphene layers are arranged concentrically as a cylinder. Sizes vary in diameter from a few nanometers, to a few tens of nanometers.

7 Fullerene:

A molecule consisting of 60 carbon atoms arranged in a soccer-ball like structure.

8 Peapod:

A complex nanostructure consisting of fullerene laid out in a row inside a carbon nanotube. Named for its resemblance to a peapod.

For more information:

• 34th Fullerene Nanotubes General Symposium
• Fujitsu Laboratories

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About Fujitsu Laboratories

Founded in 1968 as a wholly owned subsidiary of Fujitsu Limited, Fujitsu Laboratories Ltd. is one of the premier research centers in the world. With a global network of laboratories in Japan, China, the United States and Europe, the organization conducts a wide range of basic and applied research in the areas of Multimedia, Personal Systems, Networks, Peripherals, Advanced Materials and Electronic Devices.
For more information, please see:http://jp.fujitsu.com/group/labs/en/

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Press contacts:

Fujitsu Limited
Public and Investor Relations

Inquiries

Technical contacts:
Fujitsu Laboratories Ltd.
Nanotechnology R&D Center
Tel: +81-46-250-8234
E-mail:nano-mate@labs.fujitsu.com

http://www.fujitsu.com/global/news/pr/archives/month/2008/20080303-01.html

Ref:
http://www.japancorp.net/Article.Asp?Art_ID=17179

Unique nanotube composites constructed for organic solar cells

Somenath Mitra and Cheng Li

Single-wall carbon nanotubes improve the performance of organic photovoltaics and could bring them closer to practical implementation.

Harvesting energy directly from the abundant resource of solar radiation through the use of solar cells is increasingly becoming a major component of future global energy production. Other renewable energy sources, like wind and hydroelectric power, can require large scale infrastructure. Solar energy, on the other hand, only needs solar cells and sunshine. Technologically feasible solutions are available today for solar electricity generation. They are predominantly based on the use of silicon conversion cells. The most efficient cells, however, use relatively expensive high-quality single-crystal or amorphous silicon wafers. Unless there are major breakthroughs, current silicon-based thin-film technologies may be reaching their limit in terms of their ratio of cost to efficiency.

Organic photovoltaics (OPVs) are made of polymers and have the advantage that they can be painted on a surface, such as on the outside walls of a building or on rooftops. Accordingly, there is a great deal of interest in putting them to use in large-scale applications. Compared with existing technologies, OPVs promise moderate power conversion efficiencies. By the same token, they have the very attractive feature that they can be made by highly scalable, high-speed coating and printing processes such as spray painting and inkjet printing to cover large areas on flexible plastic substrates. They provide a low-cost alternative for the future.

In an OPV, solar radiation is harnessed in an unusual way. Incoming radiation excites the photoactive polymer, which functions atomically as a loosely bounded electron-hole pair, referred to as an exciton. The key to OPV technology is the mechanism of effective separation and transport of the electrons and holes (charge carriers). Otherwise, energy is wasted. Examining certain classes of molecules can help in understanding the mechanism's importance.

Spherical fullerenes or C₆₀ (also known as buckyballs) are allotropes (different forms) of carbon that are capable of trapping electrons. They can be used in OPVs for separating charges to prevent recombination of electrons and holes. However, the allotropes are neither good conductors of electricity nor optimal for charge transport. A single-wall carbon nanotube (SWNT), a cylindrical variation of a fullerene, offers a solution owing to its shape. SWNTs have a nanometer-scale diameter and exhibit ballistic electrical conductivity (many times better than copper) that can serve as tiny wires.

Figure 1. (a) Attachment of C₆₀ clusters on the sidewall of carbon nanotubes. Under light irradiation, electrons captured by C₆₀ molecules will be injected into and then transported via SWNTs. (b) Photograph of devices fabricated on flexible plastic. (c) Scanning electron microscope image of the C₆₀-SWNT complex showing decoration of the nanotube surface with C₆₀ clusters. SWNT: Single-wall nanotube.

The key component of the OPVs developed in our group is a C₆₀-SWNT complex. The SWNTs offers superior electron transport properties, and the spherical C₆₀, with its large surface-to-volume ratio, is extremely efficient at separating photogenerated charge carriers. The charge partitioning at the polymer/C₆₀ interface is followed by efficient electron transport through the nanotubes. Together, these lead to higher quantum efficiencies.

Recently, we developed the chemistry related to the synthesis of the C₆₀-SWNT complex and the associated OPV fabrication technology.^1,2 Figure 1(a) shows in schematic form nanotubes decorated with clusters of C₆₀ molecules and the mechanism of charge transport. Figure 1(b) is a photograph of a solar cell made by coating a flexible plastic substrate. Figure 1(c) presents a scanning electron microscope image of the SWNT-C₆₀ complex. The surface of the tubes is dotted with clusters of C₆₀.

Adding SWNTs to a photoactive coating improves the performance of OPVs. The coating is composed of a conducting polymer: poly(3-hexylthiophene). We tested both the C₆₀-SWNT complex and the pristine C₆₀ in our lab under simulated AM1.5-G solar irradiation at 95 mW/cm². When the SWNTs were introduced into the photoactive composite layer via binding with C₆₀, the short circuit current and fill factor improved significantly with power conversion efficiency, by as much as 78%.

In photovoltaic cells without SWNTs and after charge separation at the polymer/C₆₀ interface, electrons can move toward the cathode only by hopping between C₆₀ molecules. In contrast, SWNTs can form a network throughout the composite layer and provide a direct pathway for enhanced electron transport. Electrons captured by C₆₀ molecules or clusters are transferred to SWNTs for rapid current flow. The C₆₀-SWNT composite appears to be an excellent candidate for constructing low-cost OPVs. C₆₀ is significantly less expensive than other fullerene derivatives, and only a small amount of the more expensive SWNT is needed in the photoactive composite. Further optimization of material synthesis and device fabrication is necessary to optimize the performance of our solar cell.

This work was supported at New Jersey Institute of Technology by the US Army Armament Research, Development, and Engineering Center.

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Somenath Mitra, Cheng Li

Department of Chemistry and Environmental Science

New Jersey Institute of Technology

Newark, NJ

Somenath Mitra chairs the Department of Chemistry and Environmental Science at the New Jersey Institute of Technology (NJIT). His research interests include organic photovoltaics, nanotechnology, and sensor development.

Cheng Li is a research fellow. He completed his PhD in materials science and engineering at NJIT in 2003 before joining Somenath Mitra's group. His research interests include device physics of organic solar cells, organic thin-film transistors, and thin-film sensors for damage detection.

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References:

1. C. Li, Y. Chen, Y Wang, Z. Iqbal, M. Chhowalla, S. Mitra, Fullerene-single wall carbon nanotube complex for polymer bulk heterojunction photovoltaic cells, J. Mater. Chem. 17, pp. 2406, 2007.

2. C. Li, S. Mitra, Processing of fullerene-single wall carbon nanotube complex for bulk heterojunction photovoltaic cells, Appl. Phys. Lett. 91, pp. 253112, 2007.

http://spie.org/x19641.xml?highlight=x2358#B1

DOI: 10.1117/2.1200802.1045