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segunda-feira, 23 de abril de 2012

Carbon Nanotubes Chart Their Own Course

Propelled by funding and pure scientific interest, research on carbon nanostructures—in all of its forms—is rapidly giving way to practical applications.

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The fluidized bed reactors at SouthWest NanoTechnologies Inc. were developed from technologies pioneered at Oklahoma State University. They can produce significant amounts of single-wall carbon nanotubes in less than one hour, keeping selectivity better than 90%. Image: SouthWest NanoTechnologies Inc.

Not since in the development of carbon fiber in the 1960s has a material stimulated so much research interest as carbon nanotubes. Presaged by the designs of Buckminster Fuller—who lent his name to the C-60 atom, or fullerene—the first glimpse of naturally occurring carbon nanotubes in the early 1990s by James Tour's laboratory at Rice University quickly generated the excitement of materials scientists worldwide.
Unlike most materials inventions, which have been developed for specific applications within a given industry (a new alloy grade for metals processing, for example), the carbon nanotube was quickly hailed as a potential replacement or complement for a wide range of well-established commodity products, from the alloys used to strengthen steel to the copper used to carry electric circuit.
An allotrope of carbon, and closely related to graphite, carbon nanotubes feature simple carbon-carbon bonds of varying length, and as a consequence have a remarkably high modulus of tensile strength. The conductivity of this form of carbon makes the nanotube a nearly perfect conductor of electricity when produced in a pure form. More importantly, the variability of the form allows a band gap, an energy range where no electrons can exist.
Prompted by such extraordinary performance, eight U.S. federal agencies formed the National Nanotechnology Initiative, which has grown to more than $10 billion in research and business development funding.
Unlocking the true power of the nanotube
As with any nanostructured material, the challenge in producing carbon nanotubes and its related forms depends on understanding the manner in which nanoscale building blocks and processes integrate and assemble into larger systems and how these processes can be precisely controlled to achieve predictable products.
Engineers are now learning how to design nanomaterials that can seamlessly and functionally integrate with tissues of the body to perform biological functions. They are wrestling with the development of "top-down" and "bottom-up" engineering approaches to control properties. This is a critical step prior to the development of nanodevices. Finally, analytical instruments and techniques must be adapted to allow precise characterization.
In their purest form, nanotubes are of the single-walled variety. Early on, developers produced multi-level tubes, also known as double- or multi-walled nanotubes. Easier to produce, this type of nanotube was billed as a bulk additive and dominated production levels for early nanotube materials.
Multi-walled nanotubes (MWNTs) consist of multiple rolled layers (concentric tubes) of graphite.
Restrictions on the relative diameters of the individual tubes typically means that MWNTs are zero-gap materials. They are typically used for bulk or composite applications, and are measured by percent/weight. The amount dictates how much a given amount of nanotubes is worth. For MWNTs, of which an estimated 3,400 tons were produced in 2010, this is often just 10 cents or less per gram. This low cost has led to some experimentation in combined bulk MWNTs with other types materials, particularly lightweight metals in aircraft.
Single-walled carbon nanotubes (SWNTs) are much more difficult to produce. Their unique properties are the direct result of a distinctive structure composed of carbon-carbon bonds more closely related to those in graphite than to those in diamond. Diamond has four-coordinated carbon, featuring an sp3 hybridization. Graphite involves three-coordinated carbons, in which three electrons are in sp2 hybridization and one is delocalized.
Fullerenes and nanotubes also have carbon bonds with sp2 hybridization such as graphite, but unlike the graphite structure, which is made up of flat planar honeycomb, the structures of fullerenes and nanotubes involve a high degree of curvature.
Single-wall carbon nanotubes exhibit unique properties due to their unusual structure. They consist of a hollow cylinder of carbon approximately 1 nm in diameter, up to 1,000 times as long as it is wide. This structure has remarkable optical and electronic properties, tremendous strength and flexibility, and high thermal and chemical stability. As a result, carbon nanotubes are expected to have dramatic impact on several industries, including displays, electronics, health care, and composites.
In recent years, other forms of carbon nanotubes have been researched, some of them boasting strange or substantial increases in certain properties. The nanotorus, for example, is a carbon nanotube bent into a ring, causing a large increase in its potential magnetic moment. Fullerenes have been attached to carbon nanotubes to form bud-like structures that show strong field-emissive properties.
Graphene is perhaps the most significant new R&D target for nanostructured carbon research. This single-atom thick layer of carbon has been celebrated for properties that resemble those of carbon nanotubes.
Research in graphene is growing and attracting investment, but carbon nanotubes have had a head start in the marketplace.
Undoubtedly, graphene will show its clout in the marketplace in the coming years. For now it has severely limited practical applications.

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Optical image of a completed graphene integrated circuit (IC) including contact pads. The probes for testing the circuit (P1-P4) are also shown. The scale bar is 100 µm. Image: IBM
Scalable synthesis of nanotubes
Carbon nanotubes are still rare in the marketplace because they are challenging to produce in a predictable fashion. Bulk applications are less demanding with respect to purity or homogeneity, which has allowed applications to emerge for coatings and inks. Tesla NanoCoatings, for example, won an R&D 100 Award in 2011 for its corrosion-resistant coating for steel made with fullerene carbon nanotubes.
The product is an industry-first, which indicates how far along more tailored applications—like carbon nanotube wires or carbon nanotube solar cells—remain. The biggest problem with conventional manufacturing methods is that they haven’t been scalable. The predominant problem with most single-wall nanotube manufacturing processes is that they are basically "glorified arc-welding," according to David Arthur, CEO at SouthWest NanoTechnologies Inc. (SWeNT), Norman, Okla.
SWeNT emerged out of technologies pioneered at the University of Oklahoma by Daniel Resasco. After founding SWeNT in 2001, he commercialized the CoMoCat process, which is based upon a proprietary catalyst design.
CoMoCat allows single-wall nanotubes to be selectively grown in a fluidized bed reactor supported by a catalyst, the same process used to make commoditized materials like polyethyls. "Because of this, it’s inherently scalable. You simply need to achieve fluidization," says Arthur.
Unlike conventional chemical vapor deposition processes that grow the nanotubes vertically on a substrate, the fluidized bed reactor generates a material that appears to be a bed of powder. The flow gas expands the material to the boiling point, generating fluidization conditions.
According to Arthur, chirality of the SWNTs produced by this process can be controlled from synthesis. Chirality dictates the optical and electronic properties of carbon nanotubes, and depends on the diameter and orientation of carbon in the sidewall.
Chirality is typically described by a relationship between unit vector indices, which measure the spacing between carbon atoms in two directions on the sidewall. If a single index is zero, the nanotube is described as "zigzag". If one index equals the other, the material is an "armchair" nanotube. In other instances, the nanotube is chiral.
Originally, synthesis methods were unable to dictate the type of a chirality a nanotube can possess. As researchers realized how widely the materials properties can vary—band gaps can range from 0 to 2 eV, for example—efforts were made to not only produce SWNTs in relatively homogeneous lengths and distribution, but to manage chirality across the synthesis process.
"The only way to deliver against a customer’s expectation is to deliver the chirality they want," says Arthur.
Nearly five years of engineering work was required to perfect the manufacturing process, measure chirality, and ensure chirality was being controlled.
"The only other way to achieve the chirality and distribution you want is by way of sepraration methods. This is the needle-in-the-haystack approach," says Arthur. It's an effective solution, but the cost of "purifying" the product in this way drives up the price, from several hundreds of dollars per milligram up to $1,000/mg.
Worse, he continues, separation approaches are non-scalable and have been used primarily as a way to supply researchers with a small amount of a specific product. The limiting factor to the practical application of SWNTs has been just as much a question of production as it has been a question of application.
"We've demonstrated the ability to make a kilogram in a day. It depends on a number of factors, though, such as quality specifications," says Arthur, and whether the customer requires one type of product. Part of the reason so much effort was put into developing Resasco's production approach was that it offered the opportunity to tune the finished product, potentially meeting the needs of a several customers without losing time.
In addition to the usual microscopy techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) typically employed in nanotechnology research, several techniques are particularly suitable for characterizing the geometric and electronic structures of SWNT. Structural, electronic, and optical information about SWNT can be obtained from scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, optical absorption, and photoluminescence.
SWeNT's biggest customers over time have been in the R&D field, and like other nanotube manufacturers, SWeNT has allied with a major materials distributor and development partner Sigma-Aldrich (St. Louis).
Glimpses of end-use applications for SWNTs are on the horizon, particularly in thin-film applications. Now that single-wall structures can be produced in bulk with consistent chirality, and sometimes relatively consistent length, their optical properties can be more easily leveraged. But there's a competitor on the horizon for SWNTs.

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The finished product: single-wall carbon nanotubes, imaged by electron microscope. Image: SouthWest NanoTechnologies Inc.
The graphene wave
Just how important is graphene? Apart from research investment figures that now number in the billions of dollars, a quick glance at journal publications reveals the impact of this material. Citation index manager Thomson Reuters, in a 2011 survey of nanotechnology research, examined six core research papers published from 2006 to 2010 that dealt with the study of the electronic properties of graphene. Those six research papers generated 9,524 citations, outpacing all other leading research nanoscale materials research topics. In terms of citation impact, in fact, the papers proved more than three times as influential as the next leading topic: polymer solar cells.
In the past few years, graphene has outpaced carbon nanotubes as a target for laboratory R&D. As companies like SWeNT anticipate demand for their products to accelerate, companies that manufacture commodity graphene have yet to see the technology take hold.
Despite the dearth of manufacturers, however, a number of large companies are invested heavily in developing ways to manufacture graphene for practical product, both in the industrial arena and for consumers. Earlier this year, researchers at Samsung and Sungkyunkwan University in Korea, produced a continuous layer of pure graphene the size of a large television, spooling it out through rollers on top of a flexible, see-through, 63-cm-wide polyester sheet.
In June, XG Sciences Inc., a Lansing, Mich.-based spinoff of Michigan State University, signed an agreement with POSCO, a Korean corporation and one of the world's largest steel producers, to advance of graphene manufacturing and product development. XG Sciences makes an evolutionary variation of graphene called xGnP graphene nanoplatelets, an inexpensive material that can be used to improve the strength and performance properties of materials ranging from plastics to electronic components and batteries. The morphology of these platelets, or particles, is thin but wide, with aspect ratios in the thousands. Each particle consists of several sheets of graphene with an overall thickness ranging from 5 to about 15 nm, depending on the grade. Particle diameters can range from sub-micrometer to more than 50 µm.
According to the company's CEO Michael R. Knox, the fabrication process for this material was specifically designed to allow the company to tailor the finished product for three anticipated application areas: advanced composites, electronics, and energy storage.
Electronics could be the golden ticket for graphene pioneers. Like carbon nanotubes, graphene exhibits superb conductivity, and can be layered so thin that it is essentially optically transparent. Unlike CNTs, however, graphene does not have a bandgap. So researchers are coming up with way to produce graphene that is "semiconductor-ready", but does not involve too many complicated steps that could drive up costs.
The first hurdle to clear is the production of the graphene. When the material was first identified and studied around the year 2000, the only known process for "making" it was the Scotch tape method.
A thin sample of graphite was prepared and placed on sheet. Then, a sticky film, essentially tape, was applied to the graphite and peeled back with just enough force to lift single atom-thin layer of graphene. For a number of years, researchers were stuck with this method. Recently, researchers have made substantial advances in manufacturing pure, single-layer sheets of graphene. At IBM Research, Yorktown Heights, N.Y., this process was perfected with a clear goal in sight: the semiconductor industry.
Yu-Ming Lin, a research staff member at IBM's T.J. Watson Center, has been leading a project to produce graphene for wafer-scale VLA applications in computer electronics.
"Until recently, there has been no good way to produce graphene in a scalable fashion," says Lin. "What we've been able to do at IBM is develop a new chemical vapor deposition method that grows graphene on 8-in metal film."
Graphene produced for R&D is typically made via either CVD processes, like Lin's, or through an epitaxial method in which silicon carbide is subjected to intense heat of 1500 C or more, driving away silicon atoms to leave pure carbon.
The latest epitaxial approach can produce a 2-in wide wafer, but to achieve the scales necessary for the standardized world of semiconductors—which can feature 300-mm diameter wafer sizes—Lin's group pioneered this new method.
"For as long as 40 years, researchers have had the ability to grow carbon nanotubes using CVD," says Lin, though for much of that time they did not have the ability to detect them. All that is necessary is a flow gas and a catalyst, such as iron, nickel, or some other type of metallic nanoparticle.
The proof-of-principle for IBM's process was suggested for more than a decade, but in the last couple of years the process has produced the correct quality. As the temperatures rise, the gas fed into the system reacts with the metal, which absorbs elements other than carbon. As the temperature falls, the solubility of carbon allows carbon atoms to precipitate to the surface first, spontaneously forming graphene because it is the lowest energy form of carbon.
The difference between IBM's method and previous efforts is in the morphology of the metal and how it reacts with the solubility of the carbon. Also, the final material must have the ability to accept other materials such as oxides and gate dielectrics.
After the CVD process, the metal is removed to reveal the insulating substrate. The silicon wafer is covered with silicon dioxide and cut into pieces using oxygen. Lin's team has experimented with the upper limits of their process and have achieved up to 0.5-m in size.
The effort, sponsored in part by the Defense Advanced Research Projects Agency (DARPA), is geared toward the production of raw materials for high-performance electronic devices. IBM has already produced a 100 GHz transistor made from graphene, and earlier this year built the first integrated circuit from graphene.
Fonte: R&D