Revealing the secrets of chemical bath deposition

November 26, 2010 By Mark Wolverton

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Photo above: Drexel University Ph.D. student Kevin McPeak prepares the microreactor for XANES spectroscopy at the MR-CAT 10-ID beamline. Inset: Scanning electron micrograph of ZnO nanowire array and in situ, time-resolved Zn K-edge XANES spectra of ZnO nanowire growth at 90 ?C showing transition from Zn(H2O)62+ to ZnO.

X-ray absorption near-edge structure (XANES) spectroscopy is well known as a versatile and powerful technique for examining the microstructure of everything from crystalline solids to amorphous materials, even liquids. Its extreme sensitivity also makes it an ideal tool for probing the kinetics of various chemical reactions in situ.

Experimenters utilizing the U.S. Department of Energy Office of Science’s Advanced Photon Source at Argonne recently demonstrated a new wrinkle for XANES that has opened a window on a poorly-understood technique for deposition of materials. These insights will encourage the development of better-controlled and more precise chemical synthesis techniques for semiconductor and other nanomaterial applications, and are valuable as a demonstration of the extension of XANES spectroscopy into other realms of experimentation.

While chemical bath deposition (CBD) is widely used in the laboratory and industry for the creation of thin films and nanostructures for semiconductors and photovoltaics, its actual molecular workings have remained something of a mystery. This has somewhat limited its utility, because precise tailoring of CBD products is not possible without a clear understanding and thus control of CBD mechanics. Scientists from Drexel University and the University of Notre Dame have obtained the first detailed look at how CBD operates at the molecular level, using XANES spectroscopy to witness in situ the formation of zinc oxide nanowires. The work was published in October 2010 in Chemistry of Materials.

CBD begins with a water solution with chemical precursors containing the components from which the desired film structure will be formed. But because the precursor chemical species tend to be very dilute within the solution, identifying and isolating them to monitor their activity during the deposition process has been a daunting challenge. “It’s very difficult to find experimental techniques that will allow you to assess the different things that you need to measure,” said principal investigator Jason Baxter of Drexel University. “This has led to some criticism of CBD for being too recipe-based, where it can be difficult to take one set of conditions and say what might happen elsewhere.” XANES proved to be the ideal window into the CBD process. “It gives you very high sensitivity so you can measure species that are very dilute,” Baxter said. “So we were able to look at CBD with a degree of accuracy that people could not achieve before.”

The researchers subjected a solution of zinc nitrate and HMTA (hexamethylenetetramine) to different temperatures and pressures inside a custom-built microreactor device to induce ZnO nanowire growth, observing the reactions with XANES spectroscopy at the Materials Research Collaborative Access Team (MR-CAT) beamline 10-ID at the Advanced Photon Source. Baxter points out a particular advantage of XANES for the current work: “It also has good enough time resolution that we could actually watch the reaction proceeding in time. Every minute we could take a new set of data and look at the kinetics of the reaction.”

One open question the researchers sought to address was the specific role of HMTA in the ZnO CBD process. Previous work had suggested that HMTA might break down into intermediate forms that provided the raw materials for the ZnO film, perhaps even binding to zinc ions in the solution, or that it might act simply as a pH buffer to facilitate the reactions.

This first in situ view afforded by the XANES technique demonstrated that HMTA decomposes slowly under heating, releasing hydroxide ions that react with zinc ions in the formation of ZnO. This slow release of hydroxides also has the effect of minimizing ZnO saturation and thus controlling the solution pH.

“HMTA releases the hydroxide at the appropriate rate, just at the borderline where you’re primarily growing zinc oxide on the substrate with minimal precipitation,” says Baxter.

The team observed the growth of ZnO nanowires from zinc nitrate and HMTA precursors at 90° C after two hours, with typical hexagonal cross-sections and diameters of 300-500 nm.

They also employed principal component analysis (PCA) techniques to obtain quantitative data on the observed species during the CBD process. This showed that the ZnO nanowire growth occurred through direct crystallization from the precursor materials without any long-lived intermediates. The pH buffering provided by the HMTA helps to avoid overabundant precipitation of ZnO in the solution, allowing the controlled growth of the nanowire structures.

These new insights into the mechanisms of CBD will encourage the development of better-controlled and more precise chemical synthesis techniques for semiconductor and other nanomaterial applications.

The work is also valuable as a demonstration of the extension of XANES spectroscopy into other realms.

“I think the more widely useful part of this paper is actually in the application of XANES spectroscopy to a new type of system,” said Baxter.

He and his team plan to extend their work to study other CBD chemistries and processes. “You can actually see what’s happening as it is growing,” he said. “It gives one a lot of information about the process. I think that’s the exciting part.”

More information: Bruce A. Bunker2, and Jason B. Baxter1, “In Situ X-ray Absorption Near-Edge Structure Spectroscopy of ZnO Nanowire Growth During Chemical Bath Deposition,” Chem. Mater. 22, 6162 (2010). DOI:10.1021/cm102155m

Provided by Argonne National Laboratory (news : web)

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A greener way to grow carbon nanotubes

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Graphic: Christine Daniloff

Given their size, strength and electrical properties, carbon nanotubes — tiny, hollow cylinders made of carbon atoms — hold promise for a range of applications in electronics, medicine and other fields. Despite industrial development of nanotubes in recent years, however, very little is known about how they form or the environmental impacts of their manufacture.

It turns out that one process commonly used to produce carbon nanotubes, or CNTs, may release several hundred tons of chemicals, including greenhouse gases and hazardous air pollutants, into the air each year. In a paper published last week on the ACS Nano website, the researchers report that in experiments, removing one step in that process — a step that involves heating carbon-based gases and adding key reactive “ingredients” — reduced emissions of harmful by-products at least tenfold and, in some cases, by a factor of 100. It also cut the amount of energy used in the process by half.

“We were able to do all of this and still have good CNT growth,” says Desiree Plata, who led the research between 2007 and 2009 as a doctoral student in MIT’s joint program with the Woods Hole Oceanographic Institution. Now a visiting assistant professor in MIT’s Departments of Aeronautics and Astronautics and Civil and Environmental Engineering (CEE), Plata collaborated on the paper with several MIT and University of Michigan researchers, including Philip Gschwend, Ford Professor of Engineering in CEE, and John Hart, a mechanical engineering professor at the University of Michigan. The study is part of a long-term effort to change the approach to material development so that environmental chemists work with the young CNT industry to develop methods to prevent or limit undesirable environmental consequences.

In their study, Plata and her colleagues analyzed a common CNT manufacturing process known as catalytic chemical vapor deposition. In this method, manufacturers combine hydrogen with a “feedstock gas,” such as methane, carbon monoxide or ethylene. They then heat the combination in a reactor that contains a metal catalyst like nickel or iron, which then forms CNTs. The problem is that once the CNTs form, unreacted compounds (up to 97 percent of the initial feedstock) are often released into the air.

Turning off the heat

In a custom-made laboratory-scale reactor, the researchers heated hydrogen and ethylene, which is commonly used in high-volume CNT manufacturing, and then delivered it to a metal catalyst. They found that more than 40 compounds formed, including greenhouse gases like methane and toxic air pollutants like benzene.

The researchers suspected that not all of those compounds were essential for growing CNTs, and they knew that heating the feedstock gas plays a critical role in creating the dangerous compounds. So they combined unheated ethylene and hydrogen with several of the 40 compounds, one by one, to see which combination of compounds led to the best growth. They observed that certain alkynes, or molecules that have at least two carbon atoms stuck together with three distinct bonds, produced the best growth, while other compounds that are undesirable by-products, such as methane and benzene, did not.

Plata and her colleagues accomplished their dramatic reduction in both harmful emissions and energy consumption by impinging room-temperature alkynes, with ethylene and hydrogen, directly onto the metal catalyst, without heat. They also learned that they could reduce the amount of ethylene and hydrogen used by about 20 and 40 percent, respectively, and still achieve the same rate and quality of CNT growth. Plata says that while the results of lab experiments are hard to generalize, in a market that is expected to reach several billion dollars within several years, these changes could translate into “significant cost savings” for manufacturers.

Industry reaction

Although it’s too soon for manufacturers to adopt the method presented in the paper, David Lashmore, vice president and chief technology officer of Concord, N.H.-based Nanocomp Technologies, says the method is something his company is willing to try as it looks for ways to minimize the environmental effects of its production process. “This is of high interest to us and could have a broad impact on our process economics,” he says.

Plata points out that the MIT study analyzed only one of several feedstock gases used to make CNTs, and that the same analysis needs to be done for the others. But for her own part, she is now focusing on how CNTs form, trying to determine the precise interaction of the metal catalyst and the hydrocarbons in this process. Knowing the catalyst’s role could help researchers manipulate CNTs’ formation atom by atom — much more precisely than they can now, she says.
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Provided by Massachusetts Institute of Technology (news : web)

Scientists imitate nature to engineer nanofilms

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a, Schematic of PPX nanofilm deposition by OAP. b, Electron microscope cross-section of PPX nanofilm (insets show top view and high-resolution cross-section micrographs). c, Picture of the anisotropic adhesive wetting surface with water drops. d, Water adhesion and release in three configurations of the nanofilm. Schematics illustrate the nanorod inclination at each tilt angle and correspond to photographs showing the anisotropic wetting behaviour of the nanofilm. (Credit: Nature Materials)

In nature, water striders can walk on water, butterflies can shed water from their wings, and plants can trap insects and pollen. Scientists at the Naval Research Laboratory are part of a research team working to engineer surfaces that imitate some of these water repellency features found in nature.

This technology offers the possibility of significant advances for producing new generations of coatings that will be of great value for military, medical, and energy applications. The research is published in the December 2010 issue of Nature Materials.

Dr. Walter Dressick from NRL, working with Professor Melik Demirel of Penn State and Dr. Matthew Hancock of MIT, have collaborated to create an engineered water-repellant thin film. What sets this development apart from earlier technologies is that this newest film has the ability to control the directionality of liquid transport.

In this system, parylene nanorods are deposited on the surface by a simple, straightforward vapor deposition method. The single step usually takes less than 60 minutes, compared with the more complex, multi-step lithography processes often used in previous systems. This is the first time this kind of surface has been engineered at the nanoscale.

In the newly created surface, the nanorods that form the film are smooth on a micron scale. This size and smoothness in the posts means that when droplets are placed on the surface, they move without being distorted in any way. Also, they can be moved without pumps or optical waves. Previous systems caused the water droplets to be distorted, which could rupture, spill, or destroy the cargo in the droplet when used in medical or microassembly applications. As they continue the research, the team will focus on optimizing the droplet transport mechanism and tuning the preparation method.

Looking to the future, researchers are hopeful that this film could be used as a coating on the hull of ships where it would reduce the drag and slow the fouling. In industry applications, the film might have uses in directional syringes and fluid diodes, pump-free digital fluidic devices, increased efficiency of thermal cooling for microchips, and tire coatings.

Secrets of nanohair adhesion un-peeled by UA polymer scientists

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Fine hairs on the soles of gecko feet allow the lizards to climb vertical surfaces with ease. UA polymer researchers have discovered a synthetic glue (carbon nanotubes) with nearly four times the adhesion power of gecko hairs. Now the scientists reveal why the mimic version offers its remarkable staying power.

Not long after Dr. Ali Dhinojwala, chairman of The University of Akron Department of Polymer Science, unpeeled the secret (fine, clingy hairs) behind the remarkable adhesion of gecko feet, he and fellow researchers came up with a synthetic replica: carbon nanotubes. Now, five years after that initial discovery, the basis of the success of these nanotubes is published in the Oct. 12, 2010, issue of the American Chemical Society’s Nano Letters.

While the story of nanotubes is one of success, not all carbon nanotubes are equal, nor is the individual adhesion performance of each strand, according to Dhinojwala. Although Dhinojwala and UA polymer science graduate student Liehui Ge determined that these 8-nanometer-diameter carbon hairs — each 2,000 times smaller than the diameter of a human hair — adhere powerfully to glass and similar substrates, they furthered their research to learn why some strands have a firmer grip than others.

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Getting a grip on adhesion

Findings by the UA scientists, in collaboration with Lijie Ci and Anubha Goyal, researchers with the Department of Mechanical Engineering and Materials Science at Rice University; Rachel Shi, UA Research Experience for Undergraduates (REU) intern; and L. Mahadevan, professor of applied mathematics and professor of organismic and evolutionary biology at Harvard University, reveal that the softer the nanotube, the greater its adhesion.

Using a combination of mechanics, electrical resistance and scanning electron microscopy (SEM) to study the contact between hairs of a large number of vertically aligned carbon nanotubes with glass or silicon substrates, the researchers found that soft nanotubes clasp and curve when pressure is applied, contributing to their adhesive strength.

“We found out that the diameter of the tubes is an important parameter for adhesion because we have to balance the adhesion and bending rigidity of the tubes,” Ge says. “Also, if you apply a high pressure, the tubes bend and buckle and make a larger contact area with the surface, which is the reason for higher adhesion.”

The dry adhesive, unlike liquid glue counterparts, promises successful use in extreme atmospheric and temperature conditions and in other applications that present challenges.

“The carbon nanotube-based gecko adhesives are going to open up opportunities to using these materials on robots, to climb vertical walls, and could actually be used in outer space (vacuum condition) because these materials stick without any liquid glue,” Dhinojwala says.

Provided by University of Akron

New possibilities for solar energy with molecular 'stencils'

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This film of block copolymers shows the material's characteristic tendency to separate into distinct regions.

Scientists at the U.S. Department of Energy’s Argonne National Laboratory have begun to use molecular "stencils" to pave the way to new materials that could potentially find their way into future generations of solar cells, catalysts and photonic crystals.

Researchers at Argonne’s Center for Nanoscale Materials and Energy Systems Division have developed a technique known as sequential infiltration synthesis (SIS), which relies on the creation of self-assembled nanoscale chemical domains into which other materials can be grown. In this technique, a film composed of large molecules called block copolymers acts as a template for the creation of a highly-tunable patterned material.

This new method represents an extension of atomic layer deposition (ALD), a popular technique for materials synthesis that is routinely used by Argonne scientists. Instead of just layering two-dimensional films of different nanomaterials on top of one another, however, SIS allows scientists to construct materials that have much more complex geometries.

“This new technique allows us to create materials that just weren’t possible with ALD or block copolymers alone,” said Seth Darling, an Argonne nanoscientist who helped to develop SIS in collaboration with Argonne chemist Jeff Elam. “Having the ability to control the geometry of the material we’re making as well as its chemical composition opens the door to a whole universe of new materials.”

According to Darling, the success of the technique relies on the unique chemistry of block copolymers. Every block copolymer is composed of two chemically distinct subunits; for instance, one subunit might have an affinity for water while the other might repel water. In such a case, like would seek out like, creating a heterogeneous matrix of interspersed homogenous regions.

“You can think of a block copolymer as like a pair of molecular Siamese twins where one likes to talk and one likes to read quietly,” Darling said. “If you put a bunch of these twins together in a room, the talkative ones are going to try to be near the talkative ones and the readers are going to try to be near the readers, but they can’t simply all separate themselves to either side of the room, and it’s this action that gives us the geometries we’re looking for.”

Depending on the initial substrate, the block copolymers, and the processing that materials scientists use, regions can form that have many different shapes, from spherical to cylindrical to planar. While there are many types of block copolymers, in general they cannot serve as wide an array of purposes as inorganic materials. The challenge, according to Darling, is to bring the self-assembly of block copolymers together with the functionality of inorganic materials.

The physical and chemical properties of a material generated using SIS depend on how block copolymer chemistry and morphology interact with the chemistry of ALD techniques. “We can tailor our materials synthesis efforts in a much more precise way than we ever could before,” Darling said.

Darling and Elam have spent most of their careers at Argonne focused on the development of new types of materials, including the development of solar cells that combine organic and inorganic components. They believe that the types of materials that SIS can generate will drive fundamental solar energy technologies to greater efficiencies and lower cost.

“Our solar energy future does not have a one-size-fits-all solution,” Elam said. “We need to investigate the problem from many different angles with many different materials, and SIS will give researchers like us many new routes of attack.”

Provided by Argonne National Laboratory

DNA can act like Velcro for nanoparticles

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Argonne researcher Byeongdu Lee has determined that different shapes of gold nanoparticles, above and below, will self-assemble into different configurations when attached to single strands of DNA.

DNA can do more than direct how bodies our made -- it can also direct the composition of many kinds of materials, according to a new study from the U.S. Department of Energy’s Argonne National Laboratory.

Argonne researcher Byeongdu Lee and his colleagues at Northwestern University discovered that strands of DNA can act as a kind of nanoscopic "Velcro" that binds different nanoparticles together. "It’s generally difficult to precisely control the assembly of these types of nanostructures," Lee said. "By using DNA, we’re borrowing nature's power."

The "Velcro" effect of the DNA is caused by the molecule’s "sticky ends," which are regions of unpaired nucleotides — the building blocks of DNA — that are apt to bond chemically to their base-pair partners, just like in our genes. When sufficiently similar regions contact each other, chemical bonds form a rigid lattice. Scientists and engineers believe these complex nanostructures have the potential to form the basis of new plastics, electronics and fuels.

In 2008, Lee and his colleagues attached DNA to spherical nanoparticles made of gold, hoping to control the way the particles arrange themselves into compact, ordered crystals. This process is called nanoparticle "packing," and Lee believed that by affixing DNA to the nanoparticles, he could control how they packed together. "Materials that are packed differently — even if they are made from the same substance — have been shown to exhibit dramatically different physical and chemical properties," Lee said.

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While the 2008 experiment showed that DNA appeared to control that instance of nanosphere packing, it was not known whether the effect would occur with different nanoparticle geometries. The more recent experiment looked at different shapes of nanoparticles to determine whether their contours affected how they packed.

According to Lee, the spherical nanoparticles in the earlier experiment tended to arrange themselves into one of two separate types of cubic crystals: a face-centered cube (a simple cube with nanospheres at each vertex and additional ones located in the middle of each face) or a body-centered cube (a simple cube with an additional nanosphere located in the middle of the cube itself). The type of lattice that the nanoparticles formed was determined by how the "sticky ends" attached to the nanoparticles paired together.

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In the more recent experiment, the particles' shape did change the material's final structure, but only insofar as it altered how the DNA "sticky ends" attached to each other. In fact, the study showed that dodecahedral (12-sided) nanoparticles arranged into a face-centered cubic configuration while octahedral (8-sided) nanoparticles formed body-centered cubes — even when the nanoparticles were attached to identical strands of DNA. "We may be able to make all different types of nanoparticle packing structures, but the structure that will result will always be the one that maximizes the amount of binding," he said.

"The face-centered cubic structure is the most compact way for the nanoparticles to arrange themselves, while the body-centered cubic is slightly less compact. The DNA binding is really the true force controlling the construction of the lattice," he added.

More information: A paper based on the research, "DNA-nanoparticle superlattices formed from anisotropic building blocks", appeared in the October 3 issue of Nature Materials.

Provided by Argonne National Laboratory (news : web)

A new twist for nanopillar light collectors

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On the left a schematic of a germanium nanopillar array embedded in an alumina foil membrane; on the right are cross-sectional SEM images of a blank alumina membrane with dual-diameter pores; inset shows germanium nanopillars after growth. (Images courtesy of Ali Javey)

Sunlight represents the cleanest, greenest and far and away most abundant of all energy sources, and yet its potential remains woefully under-utilized. High costs have been a major deterrant to the large-scale applications of silicon-based solar cells. Nanopillars – densely packed nanoscale arrays of optically active semiconductors – have shown potential for providing a next generation of relatively cheap and scalable solar cells, but have been hampered by efficiency issues. The nanopillar story, however, has taken a new twist and the future for these materials now looks brighter than ever.

“By tuning the shape and geometry of highly ordered  nanopillar arrays of germanium or cadmium sulfide, we have been able to drastically enhance the optical absorption properties of our nanopillars,” says Ali Javey, a chemist who holds joint appointments with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley.

Javey, a faculty scientist with Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of electrical engineering and computer science, has been at the forefront of nanopillar research. He and his group were the first to demonstrate a technique by which cadmium sulfide nanopillars can be mass-produced in large-scale flexible modules. In this latest work, they were able to produce nanopillars that absorb light as well or even better than commercial thin-film solar cells, using far less semiconductor material and without the need for anti-reflective coating.

“To enhance the broad-band optical absorption efficiency of our nanopillars we used a novel dual-diameter structure that features a small (60 nanometers) diameter tip with minimal reflectance to allow more light in, and a large (130 nanometers) diameter base for maximal absorbtion to enable more light to be converted into electricity,” Javey says. “This dual-diameter structure absorbed 99-percent of incident visible light, compared to the 85 percent absorbtion by our earlier nanopillars, which had the same diameter along their entire length.”

Theoretical and experimental works have shown that 3-D arrays of semiconductor nanopillars – with well-defined diameter, length and pitch – excel at trapping light while using less than half the semiconductor material required for thin-film solar cells made of compound semiconductors, such as cadmium telluride, and about one-percent of the material used in solar cells made from bulk silicon. But until the work of Javey and his research group, fabricating such nanopillars was a complex and cumbersome procedure.

Javey and his colleagues fashioned their dual diameter nanopillars from molds they made in 2.5 millimeter-thick alumina foil. A two-step anodization process was used to create an array of one micrometer deep pores in the mold with dual diameters – narrow at the top and broad at the bottom. Gold particles were then deposited into the pores to catalyze the growth of the semiconductor nanopillars.

“This process enables fine control over geometry and shape of the single-crystalline nanopillar arrays, without the use of complex epitaxial and/or lithographic processes,” Javey says. “At a height of only two microns, our nanopillar arrays were able to absorb 99-percent of all photons ranging in wavelengths between 300 to 900 nanometers, without having to rely on any anti-reflective coatings.”

The germanium nanopillars can be tuned to absorb infrared photons for highly sensitive detectors, and the cadmium sulfide/telluride nanopillars are ideal for solar cells. The fabrication technique is so highly generic, Javey says,  it could be used with numerous other semiconductor materials as well for specific applications. Recently, he and his group demonstrated that the cross-sectional portion of the nanopillar arrays can also be tuned to assume specific shapes – square, rectangle or circle – simply by changing the shape of the template.

“This presents yet another degree of control in the optical absorption properties of nanopillars,” Javey says.

Javey’s dual-diameter nanopillar research was partially funded through the National Science Foundation’s Center of Integrated Nanomechanical Systems (COINS) and through Berkeley Lab LDRD funds.

More information: A paper describing this research appears on-line in the journal NANO Letters under the title “Ordered Arrays of Dual-Diameter Nanopillars for Maximized Optical Absorption.” Co-authoring the paper with Javey were Zhiyong Fan, Rehan Kapadia, Paul Leu,Xiaobo Zhang, Yu-Lun Chueh, Kuniharu Takei, Kyoungsik Yu, Arash Jamshidi, Asghar Rathore, Daniel Ruebusch and Ming Wu.

Provided by Lawrence Berkeley National Laboratory (news : web)

Fluorographene: The world's thinnest insulator

Kostya Novoselov and Andre Geim, working at the University of Manchester, UK, first isolated graphene in 2004. It was a tricky task, as one would expect if a Nobel Prize is among the rewards, even if it did involve using humble adhesive tape to peel away surfaces one layer at a time. They found graphene to be the thinnest and strongest form of carbon, and that it could conduct heat better than any other known material. As a conductor of electricity, it performs just as well as copper. Their most recent endeavours have led to a new derivative material that is just as strong and even more stable than the original graphene, but that does not conduct electricity at all: so-called fluorographene.

Graphene itself is a single atomic layer of the material graphite, commonly found in pencils. On a molecular level, it has a flat honeycomb structure of connecting hexagons with carbon atoms at the vertices. Clouds of electrons spread across the top and bottom surfaces, which is why the material conducts electricity so well.

The current achievement of the Manchester group, working closely with international collaborators, is to place a fluorine atom at every single carbon atom, thereby destroying the electron cloud and preventing electricity from flowing under normal conditions, but not impinging on the structural integrity of the carbon framework. In previous work, they had added hydrogen atoms instead of fluorine, but found the resulting material to be unstable at high temperatures.

The latest breakthrough is published this week in the journal Small. Rahul Raveendran-Nair is a postgraduate researcher at the University of Manchester and responsible for the publication. He describes fluorographene as “the thinnest possible insulator, made by attaching fluorine atoms to each of the carbon atoms in graphene. It is the first stoichiometric chemical derivative of graphene and it is a wide-gap semiconductor. Fluorographene is a mechanically strong and chemically and thermally stable compound. Properties of this new material are very similar to Teflon and we call this material 2D Teflon.”

Developing a suitable method for making this 2D Teflon was not simple. “Fluorine is a highly reactive element, and it reacts with all most everything. So the major challenge was to fully fluorinate graphene without damaging the graphene and its supporting substrates. Our fluorination of single-layer graphene membranes on chemically inert support grid and bulk graphene paper at elevated temperature overcomes this technical problem,” explains Raveendran-Nair.

The authors envisage that fluorographene will be used in electronics, but acknowledge that “for realistic electronic applications the electronic quality has to be improved. We hope this can be achieved very soon. Some possible electronic applications of fluorographene are its use as a tunnel barrier and as a high-quality insulator or barrier material for organic electronics.” Other fields of application are also possible. For example, as a wide-gap semiconductor that is fully transparent to visible light, fluorographene could well find use in LEDs (light-emitting diodes) and displays.

The Manchester group was not the only one involved, and collaborators from China (Shenyang National Laboratory for Materials Science), The Netherlands (Radboud University of Nijmegen), Poland (Institute of Electronic Materials Technology), and Russia (Nikolaev Institute of Inorganic Chemistry) added their expertise. According to Raveendran-Nair, having such a large team helped undertake a thorough investigation of fluorographene; “All of us worked very hard to make this project successful. We used a large variety of characterisation techniques and very detailed studies to understand the properties of this new material.”

During the course of the project the leaders were named as Nobel Laureates, but apparently life working in the group has not changed very much. “Even in their new busy life both professors still work very closely with all those in the group and are very much involved in the day-to-day research”, says Raveendran-Nair. “Working under them is a great inspiration. It is both a rewarding and enjoyable place to undertake research.”

Transparent conductive material could lead to power-generating windows

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Top: Scanning electron microscopy image and zoom of conjugated polymer (PPV) honeycomb. Bottom (left-to-right): Confocal fluorescence lifetime images of conjugated honeycomb, of polymer/fullerene honeycomb double layer and of polymer/fullerene honeycomb blend. Efficient charge transfer within the whole framework is observed in the case of polymer/fullerene honeycomb blend as a dramatic reduction in the fluorescence lifetime.

(PhysOrg.com) -- Scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Los Alamos National Laboratory have fabricated transparent thin films capable of absorbing light and generating electric charge over a relatively large area. The material, described in the journal Chemistry of Materials, could be used to develop transparent solar panels or even windows that absorb solar energy to generate electricity.

The material consists of a semiconducting polymer doped with carbon-rich fullerenes. Under carefully controlled conditions, the material self-assembles to form a reproducible pattern of micron-size hexagon-shaped cells over a relatively large area (up to several millimeters).

"Though such honeycomb-patterned thin films have previously been made using conventional polymers like polystyrene, this is the first report of such a material that blends semiconductors and fullerenes to absorb light and efficiently generate charge and charge separation," said lead scientist Mircea Cotlet, a physical chemist at Brookhaven's Center for Functional Nanomaterials.

Furthermore, the material remains largely transparent because the polymer chains pack densely only at the edges of the hexagons, while remaining loosely packed and spread very thin across the centers. "The densely packed edges strongly absorb light and may also facilitate conducting electricity," Cotlet explained, "while the centers do not absorb much light and are relatively transparent."

"Combining these traits and achieving large-scale patterning could enable a wide range of practical applications, such as energy-generating solar windows, transparent solar panels, and new kinds of optical displays," said co-author Zhihua Xu, a materials scientist at the CFN.

"Imagine a house with windows made of this kind of material, which, combined with a solar roof, would cut its electricity costs significantly. This is pretty exciting," Cotlet said.

The scientists fabricated the honeycomb thin films by creating a flow of micrometer-size water droplets across a thin layer of the polymer/fullerene blend solution. These water droplets self-assembled into large arrays within the polymer solution. As the solvent completely evaporates, the polymer forms a hexagonal honeycomb pattern over a large area.

"This is a cost-effective method, with potential to be scaled up from the laboratory to industrial-scale production," Xu said.

The scientists verified the uniformity of the honeycomb structure with various scanning probe and electron microscopy techniques, and tested the optical properties and charge generation at various parts of the honeycomb structure (edges, centers, and nodes where individual cells connect) using time-resolved confocal fluorescence microscopy.

The scientists also found that the degree of polymer packing was determined by the rate of solvent evaporation, which in turn determines the rate of charge transport through the material.

"The slower the solvent evaporates, the more tightly packed the polymer, and the better the charge transport," Cotlet said.

"Our work provides a deeper understanding of the optical properties of the honeycomb structure. The next step will be to use these honeycomb thin films to fabricate transparent and flexible organic solar cells and other devices," he said.

Structure of new form of super-hard carbon identified

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Views along [100]/[001], and [010] directions of 2x2x2 supercell of bct-Carbon,the dotted-dashed in (b) indicate the perpendicular graphene-like structure of bct-Carbon. Image credit: Xiang-Feng Zhou, http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.1569v4.pdf

(PhysOrg.com) -- An experiment in 2003 formed what was believed to be a new form of carbon, but the findings were controversial. Now two teams of scientists have used different means to identify a three-dimensional network structure called "bct-carbon," which they say could have been the structure formed in 2003.

Pure carbon exists in a variety of structures, including graphite and diamond. The new structure, body-centered tetragonal carbon or bct-carbon, is unexpectedly simple and consists of sheets of squares of four carbon atoms each, joined by “short” bonds perpendicular to the sheets. This form of carbon is created when graphite is exposed to high pressure at normal temperatures.

It has been known for nearly 50 years that graphite subjected to cold compression (high pressure at ambient temperatures) undergoes a transformation that is reversible, and in 2003 researchers at Stanford University compressed graphite in a diamond anvil press, while simultaneously obtaining the x-ray diffraction pattern to help them study the bonds within the structure. They found that when the pressure exceeded 17 gigapascals (GPa) (170,000 atmospheres) the carbon atoms in the normally soft graphite formed a material hard enough to crack diamond, but its structure remained unclear.

Now a team of scientists led by Hui-Tian Wang of Nankai University at Tianjin, China, have shown through computer simulations that the super-hard carbon may be at least partly composed of bct-carbon, since this takes the least energy to form. Bct-carbon has a structure part-way between diamond’s cubes of carbon atoms and graphite’s linked sheets of carbon atoms in a hexagonal lattice. Bct-carbon consists of sheets of four-atom carbon rings linked together by strong bonds.

The team studied 15 possible structures and found the transparent bct-carbon not only required lower energies to form but that its shear strength is 17 percent greater than diamond’s. If the results are confirmed, this means it may be possible to produce a material stronger than diamond at normal temperatures.

Another group of scientists, including Renata Wentzcovitch of the University of Minnesota and Takashi Miyake from the National Institute of Advanced Industrial Science and Technology in Japan, came to similar conclusions earlier this year, but by a different method. This group analyzed the proposed bct-carbon structure using quantum mechanical simulations. They found bct-carbon was more stable than graphite at 18.6 GPa, and that when mixed with M-carbon it would produce an x-ray diffraction pattern closely matched to that found in 2003. (M-carbon is a structure consisting of layers of carbon in rings of five and seven members.)

The paper from Hui-Tian Wang’s team was published in the journal Physical Review B, while the US/Japan research was reported in Physical Review Letters in March this year.

Berkeley lab scientists generate low-cost, hybrid thermoelectrics

November 8, 2010 by Aditi Risbud

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Using simple water-based chemistry to wrap a polymer that conducts electricity around a nanorod of tellurium, this composite nanoscale thermoelectric is easily spin cast or printed into a film.

Although climate-controlled car seats don't spring to mind when you think of energy efficiency, the latest technology underpinning this luxury automobile feature is based on thermoelectrics—materials that convert electricity directly into heating or cooling. Conversely, thermoelectrics can also funnel excess heat from energy inefficient systems, such as car engines or power plants, by recovering this 'waste heat' and turning it into electricity. As a result, these materials offer a potentially clean source of energy to reduce fuel consumption and CO2 emissions.

Currently, this thermal energy is converted with high-efficiency, expensive thermoelectric materials. In automotive exhaust systems, for example, solid-state thermoelectrics recover waste heat that can result in fuel savings of up to five percent, but their high cost bars them from being used in smaller-scale settings. Boosting these savings through lower-cost materials could make a significant impact in power generation for batteries or electronic components in computers.

Now, Lawrence Berkeley National Laboratory (Berkeley Lab) scientists are tackling this challenge by “changing the budget for thermal energy management,” said Jeff Urban, Deputy Director of the Inorganic Nanostructures Facility at the Molecular Foundry, a nanoscience user facility.

“Historically, high-efficiency thermoelectrics have required high-cost, materials-intensive processing,” said Urban. “By engineering a hybrid of soft and hard materials using straightforward flask chemistry in water, we’ve developed a route that provides respectable efficiency with a low cost to production.”

In their approach, Urban and colleagues constructed a nanoscale composite material by wrapping a polymer that conducts electricity around a nanorod of tellurium—a metal coupled with cadmium in today’s most cost-effective solar cells. This composite material is easily spin cast or printed into a film from a water-based solution. Along with its ease of manufacture, this hybrid material also has a thermoelectric figure of merit thousands of times greater than either the polymer or nanorod alone—a crucial factor in boosting device performance.

“In recent years, we’ve seen tremendous gains in thermoelectric efficiency, but there is a need for low-cost, moderate efficiency materials that are easy to process and pattern over large areas,” said Rachel Segalman, a faculty scientist at Berkeley Lab and professor of Chemical and Biomolecular Engineering at University of California, Berkeley. “We had a lot of intuition about what would work using polymers and nanocrystals, and will now explore materials space to optimize these systems and switch to more earth-abundant materials.”

More information: A paper reporting this research titled, “Water-processable polymer-nanocrystal hybrids for thermoelectrics,” appears in Nano Letters and is available to subscribers online. Co-authoring the paper with Urban and Segalman were Kevin See, Joseph Feser, Cynthia Chen and Arun Majumdar.

Provided by Lawrence Berkeley National Laboratory (news : web)

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Water could hold answer to graphene nanoelectronics

October 26, 2010

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Researchers at Rensselaer Polytechnic Institute developed a new method for using water to tune the band gap of the nanomaterial graphene, opening the door to new graphene-based transistors and nanoelectronics. In this optical micrograph image, a graphene film on a silicon dioxide substrate is being electrically tested using a four-point probe. Credit: Rensselaer/Koratkar

Researchers at Rensselaer Polytechnic Institute developed a new method for using water to tune the band gap of the nanomaterial graphene, opening the door to new graphene-based transistors and nanoelectronics.

By exposing a graphene film to humidity, Rensselaer Professor Nikhil Koratkar and his research team were able to create a band gap in graphene – a critical prerequisite to creating graphene transistors. At the heart of modern electronics, transistors are devices that can be switched "on" or "off" to alter an electrical signal. Computer microprocessors are comprised of millions of transistors made from the semiconducting material silicon, for which the industry is actively seeking a successor.

Graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chain-link fence, has no band gap. Koratkar's team demonstrated how to open a band gap in graphene based on the amount of water they adsorbed to one side of the material, precisely tuning the band gap to any value from 0 to 0.2 electron volts. This effect was fully reversible and the band gap reduced back to zero under vacuum. The technique does not involve any complicated engineering or modification of the graphene, but requires an enclosure where humidity can be precisely controlled.

"Graphene is prized for its unique and attractive mechanical properties. But if you were to build a transistor using graphene, it simply wouldn't work as graphene acts like a semi-metal and has zero band gap," said Koratkar, a professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer. "In this study, we demonstrated a relatively easy method for giving graphene a band gap. This could open the door to using graphene for a new generation of transistors, diodes, nanoelectronics, nanophotonics, and other applications."

Results of the study were detailed in the paper "Tunable Band gap in Graphene by the Controlled Adsorbtion of Water Molecules," published this week by the journal Small.

In its natural state, graphene has a peculiar structure but no band gap. It behaves as a metal and is known as a good conductor. This is compared to rubber or most plastics, which are insulators and do not conduct electricity. Insulators have a large band gap – an energy gap between the valence and conduction bands – which prevents electrons from conducting freely in the material.

Between the two are semiconductors, which can function as both a conductor and an insulator. Semiconductors have a narrow band gap, and application of an electric field can provoke electrons to jump across the gap. The ability to quickly switch between the two states – "on" and "off" – is why semiconductors are so valuable in microelectronics.

"At the heart of any semiconductor device is a material with a band gap," Koratkar said. "If you look at the chips and microprocessors in today's cell phones, mobile devices, and computers, each contains a multitude of transistors made from semiconductors with band gaps. Graphene is a zero band gap material, which limits its utility. So it is critical to develop methods to induce a band gap in graphene to make it a relevant semiconducting material."

The symmetry of graphene's lattice structure has been identified as a reason for the material's lack of band gap. Koratkar explored the idea of breaking this symmetry by binding molecules to only one side of the graphene. To do this, he fabricated graphene on a surface of silicon and silicon dioxide, and then exposed the graphene to an environmental chamber with controlled humidity. In the chamber, water molecules adsorbed to the exposed side of the graphene, but not on the side facing the silicon dioxide. With the symmetry broken, the band gap of graphene did, indeed, open up, Koratkar said. Also contributing to the effect is the moisture interacting with defects in the silicon dioxide substrate.

"Others have shown how to create a band gap in graphene by adsorbing different gasses to its surface, but this is the first time it has been done with water," he said. "The advantage of water adsorption, compared to gasses, is that it is inexpensive, nontoxic, and much easier to control in a chip application. For example, with advances in micro-packaging technologies it is relatively straightforward to construct a small enclosure around certain parts or the entirety of a computer chip in which it would be quite easy to control the level of humidity."

Based on the humidity level in the enclosure, chip makers could reversibly tune the band gap of graphene to any value from 0 to 0.2 electron volts, Korarkar said.

Gold nanoparticles that make leaves glow in the dark

October 25, 2010 by Lin Edwards Image credit: Nanoscale, DOI:10.1039/C0NR00330A . For more details, please see the original publication.

Light-emitting diodes (LEDs) are much more energy efficient than traditional light globes, but researchers in Taiwan led by Yen Hsun Su and colleagues at Academia Sinica in Taipei and the National Cheng Kung University in Tainan wanted to find a way of making LEDs that were even more efficient than those currently available. They succeeded by synthesizing gold nanoparticles and implanting them into leaves of the Bacopa caroliniana plant to induce bio-luminescence in them.

Bacopa caroliniana is a perennial aquatic or semi-aquatic creeping herb commonly used as an aquarium plant. The color of the leaves varies in proportion to the amount of light, turning bronze to almost red when exposed to high light levels.

The green pigment in leaves, chlorophyll, is bioluminescent when exposed to high wavelength (400 nanometers (nm)) ultra violet excitation, but the wavelength is much shorter for the photoluminescence of gold nanoparticles, and they emit light at 400 nm. The light is localized at a nanoscale and the nanoparticles made by the Taiwan team suppresses emission blinking, which is a problem already known in gold nanoparticles. Using their sea-urchin-shaped nanoparticles (dubbed nano-sea-urchins or NSUs), Su was able to excite chlorophyll in the leaves to emit red light.

Su said the bio-LEDs (light emitting diodes) might eventually be used to make trees lining roads luminescent at night, and since the light causes the chloroplast to conduct photosynthesis no energy source is needed and the plant will absorb CO2 for the process, which does not normally occur at night.

Su and his colleagues are looking at applying the same technique to other plant molecules and on trying to improve the efficiency of the process. Their work was reported in the journal Nanoscale.

Birth of nanoparticles seen by Argonne scientists for the first time

October 14, 2010 By Louise Lerner

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These silver nanoplates are decorated with silver oxy salt nanoparticles along the edges. These nanostructures were grown under irradiation of high-energy x-rays, which allowed scientists to "watch" them grow in real time. The image is from a scanning electron microscope.

A team of scientists at the U.S. Department of Energy's Argonne National Laboratory and the Carnegie Institution of Washington has succeeded in "watching" nanoparticles grow in real time.

The revolutionary technique allows researchers to learn about the early stages of nanoparticle generation, long a mystery due to inadequate probing methods, and could lead to improved performance of the nanomaterials in applications including solar cells, sensing and more.

"Nanocrystal growth is the foundation of nanotechnology," said lead researcher Yugang Sun, an Argonne chemist. "Understanding it will allow scientists to more precisely tailor new and fascinating nanoparticle properties."

The way that nanoparticles look and behave depends on their architecture: size, shape, texture and surface chemistry. This, in turn, depends very much on the conditions under which they are grown.

"Accurately controlling nanoparticles is very difficult," Sun explained. "It's even harder to reproduce the same nanoparticles from batch to batch, because we still don't know all the conditions for the recipe. Temperature, pressure, humidity, impurities—they all affect growth, and we keep discovering more factors."

In order to understand how nanoparticles grow, the scientists needed to actually watch them in the act. The problem was that electron microscopy, the usual method for seeing down into the atomic level of nanoparticles, requires a vacuum. But many kinds of nanocrystals have to grow in a liquid medium—and the vacuum in an electron microscope makes this impossible. A special thin cell allows a tiny amount of liquid to be analyzed in an electron microscope, but it still limited the researchers to a liquid layer just 100 nanometers thick, which is significantly different from the real conditions for nanoparticle synthesis.

To solve this conundrum, Sun found he needed to use the very high-energy X-rays provided at Sector 1 of Argonne’s Advanced Photon Source (APS), which adjoins the laboratory’s Center for Nanoscale Materials, where he works. The pattern of X-rays scattered by the sample allowed the researchers to reconstruct the earliest stages of nanocrystals second-by-second.

"This technique yields a treasure trove of information, especially on the nucleation and growth steps of the crystals, that we had never been able to get before," said Sun.

The intensity of the X-rays does affect the growth of the nanocrystals, Sun said, but the effects only became significant after an especially long reaction time. "Getting a clear image of the growth process will allow us to control samples to get better results, and eventually, new nanomaterials that will have a wide range of applications,” Sun explained.

The nanomaterials could be used in photovoltaic solar cells, chemical and biological sensors and even imaging. For example, noble metal nanoplates can absorb near-infrared light, so they can be used to enhance contrast in images. In one possible case, an injection of specially tailored nanoparticles near a cancer patient's tumor site could increase the imaging contrast between normal and cancerous cells so that doctors can accurately map the tumor.

"The key to this breakthrough was the unique ability for us to work with scientists from the Advanced Photon Source, the Center for Nanoscale Materials and the Electron Microscopy Center—all in one place," Sun said.

Embedded rods: Chitin-silicon dioxide nanocomposite made by self-organization and sol-gel chemistry

October 11, 2010

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(PhysOrg.com) -- Self-organization processes involving chemical building blocks are the basis for many biological processes and are increasingly of interest in the field of materials synthesis, for example in the production of highly ordered nanocomposites or high-porosity materials with special properties.

In the journal Angewandte Chemie, Bruno Alonso and Emmanuel Belamie from the Charles Gerhardt Institute in Montpellier (France) have introduced a novel, highly versatile approach to the large-scale synthesis of a new family of bioorganic–inorganic nanocomposites -- with a previously unattainable degree of control over the composition and structure of the materials produced.

Nanocomposites are solid materials made of different substances, one of which is in the form of nanoparticles. The properties of the composites differ significantly from those of the pure individual components. Nanocomposites can also serve as “molds” for the production of porous substances. These have potential application in the areas of gas storage, catalysis, or materials separation.

For their synthesis, the researchers chose to use a sol–gel process, a popular technique for the production of inorganic network structures. In the first step they needed to generate a sol: a suspension of finely divided nanoscopic particles in a solvent. Their challenge was to obtain co-suspension of the two different components, silicon dioxide precursors (siloxane oligomers) and chitin nanorods from shrimp shells (a renewable resource). However, these two components require different conditions to remain in stable suspensions without uncontrolled precipitation. The researchers produced an alcohol suspension by slowly replacing water with ethanol. Through slow removal of the solvent, a gel formed. Gels are gelatinous substances; they contain solid but loose, cross-linked, three-dimensional polymer structures.

The sol can be “poured” into a desired mold and dried or it can be spray-dried into spherical particles. This process results in a nanocomposite made of chitin rods that are fully embedded in a silicon dioxide matrix. The mechanism by which this occurs is based on a self-organized aggregation of the chitin molecules and weak attractive forces between chitin and siloxane oligomers.

The stability of the alcohol suspensions opens up a wide range of possibilities for the production of materials with controllable volume ratios, spatial arrangements, and morphologies. If a magnetic field is applied during preparation of the material, the chitin rods line up in parallel. If the nanocomposite is heated, the chitin rods can be burned off to leave behind cavities. This forms a highly porous material with interesting properties.

More information: Emmanuel Belamie, Chitin–Silica Nanocomposites by Self-Assembly, Angewandte Chemie International Edition, http://dx.doi.org/ … ie.201002104

Provided by Wiley (news : web)

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Extreme darkness: Carbon nanotube forest covers NIST's ultra-dark detector

August 18, 2010

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This is a colorized micrograph of the world's darkest material -- a sparse "forest" of fine carbon nanotubes -- coating a NIST laser power detector. Image shows a region approximately 25 micrometers across. Credit: Aric Sanders, NIST

Harnessing darkness for practical use, researchers at the National Institute of Standards and Technology have developed a laser power detector coated with the world's darkest material -- a forest of carbon nanotubes that reflects almost no light across the visible and part of the infrared spectrum.

NIST will use the new ultra-dark detector, described in a new paper in Nano Letters,* to make precision laser power measurements for advanced technologies such as optical communications, laser-based manufacturing, solar energy conversion, and industrial and satellite-borne sensors.

Inspired by a 2008 paper by Rensselaer Polytechnic Institute (RPI) on "the darkest man-made material ever,"** the NIST team used a sparse array of fine nanotubes as a coating for a thermal detector, a device used to measure laser power. A co-author at Stony Brook University in New York grew the nanotube coating. The coating absorbs laser light and converts it to heat, which is registered in pyroelectric material (lithium tantalate in this case). The rise in temperature generates a current, which is measured to determine the power of the laser. The blacker the coating, the more efficiently it absorbs light instead of reflecting it, and the more accurate the measurements.

The new NIST detector uniformly reflects less than 0.1 percent of light at wavelengths from deep violet at 400 nanometers (nm) to near infrared at 4 micrometers (µm) and less than 1 percent of light in the infrared spectrum from 4 to 14 µm. The results are similar to those reported for the RPI material and in a 2009 paper by a Japanese group. The NIST work is unique in that the nanotubes were grown on pyroelectric material, whereas the other groups grew them on silicon. NIST researchers plan to extend the calibrated operating range of their device to 50 or even 100 micrometer wavelengths, to perhaps provide a standard for terahertz radiation power.

NIST previously made detector coatings from a variety of materials, including flat nanotube mats. The new coating is a vertical forest of multiwalled nanotubes, each less than 10 nanometers in diameter and about 160 micrometers long. The deep hollows may help trap light, and the random pattern diffuses any reflected light in various directions. Measuring how much light was reflected across a broad spectrum was technically demanding; the NIST team spent hundreds of hours using five different methods to measure the vanishingly low reflectance with adequate precision. Three of the five methods involved comparisons of the nanotube-coated detector to a calibrated standard.

Carbon nanotubes offer ideal properties for thermal detector coatings, in part because they are efficient heat conductors. Nickel phosphorous, for example, reflects less light at some wavelengths, but does not conduct heat as well. The new carbon nanotube materials also are darker than NIST's various Standard Reference Materials for black color developed years ago to calibrate instruments.

New super strong alloy discovered

September 8, 2010

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Dr Peter Liddicoat next to the atom probe.

(PhysOrg.com) -- International team of researchers has discovered a new super-strength light alloy and had their key findings published in Nature Communications.

A North Carolina State University researcher and colleagues have figured out a way to make an aluminum alloy, or a mixture of aluminum and other elements, just as strong as steel.

That's important, says Dr. Yuntian Zhu, professor of materials science and the NC State researcher involved in the project, because the search for ever lighter - yet stronger - materials is crucial to devising everything from more fuel-efficient cars to safer airplanes.

In a paper published in the journal Nature Communications, Zhu and his colleagues describe the new nanoscale architecture within aluminum alloys that have unprecedented strength but also reasonable plasticity to stretch and not break under stress. Perhaps even more importantly, the technique of creating these nanostructures can be used on many different types of metals.

Zhu says the aluminum alloys have unique structural elements that, when combined to form a hierarchical structure at several nanoscale levels, make them super-strong and ductile.

The aluminum alloys have small building blocks, called "grains," that are thousands of times smaller than the width of a human hair. Each grain is a tiny crystal less than 100 nanometers in size. Bigger is not better in materials, Zhu says, as smaller grains result in stronger materials.

Zhu also says the aluminum alloys have a number of different types of crystal "defects." Nanocrystals with defects are stronger than perfect crystals.

The unexpectedly high level of strengthening appears to be due to two factors. Firstly, the way that the alloying elements are arranged within the grains is thought to increase the dislocation-storage capacity of the alloy. Secondly, the clustering of elements between the grains could limit nanocrystal growth, increase the cohesion of the grains, and resist embrittlement and defect generation.

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Imaging of nano-sized grains inside an aluminum alloy. Colored blobs show the grains: colored dots show clusters of zinc and magnesium atoms that strengthen the material. (Yonghao Zhao/UC Davis graphic)

Now, Zhu plans on working on strengthening magnesium, a metal that is even lighter than aluminum. He's collaborating with the Department of Defense on a project to make magnesium alloys strong enough to be used as body armor for soldiers.

Zhu's colleagues on the Nature Communications paper are affiliated with the University of Sydney in Australia; the University of California, Davis; and Ufa State Aviation Technical University in Russia.

Radically simple technique developed to grow conducting polymer thin films

November 1, 2010 By Mike Rodewald

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Sequence of images illustrating growth of polymer film in tubes over 35 seconds

(PhysOrg.com) -- Oil and water don't mix, but add in some nanofibers and all bets are off.

A team of UCLA chemists and engineers has developed a new method for coating large surfaces with nanofiber thin films that are both transparent and electrically conductive. Their method involves the vigorous agitation of water, dense oil and polymer nanofibers. After this solution is sufficiently agitated it spreads over virtually any surface, creating a film.

"The beauty of this method lies in its simplicity and versatility," said California NanoSystems Institute (CNSI) researcher Richard B. Kaner, a professor of chemistry and biochemistry and a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science. "The materials used are inexpensive and recyclable, the process works on virtually any substrate, it produces a uniform thin film which grows in seconds and the entire thing can be done at room temperature."

Conducting polymers combine the flexibility and toughness of plastics with electrical properties. They have been proposed for applications ranging from printed electronic circuits to supercapacitors but have failed to gain widespread use because of difficulties processing them into films.

"Conducting polymers have enormous potential in electronics, and because this technique works with so many substrates, it can be used in a broad spectrum of applications, including organic solar cells, light-emitting diodes, smart glass and sensors," said Yang Yang, a professor of materials science and engineering at the Samueli School of Engineering and Applied Science and faculty director of the Nano Renewable Energy Center at the CNSI.

One of the potential applications is smart, or switchable, glass that can change between states when an electric current is applied — for example, switching between see-through and opaque states to let light in or block it. The UCLA research group is applying the technique to other nanomaterials in addition to polymer nanofibers in the hopes of expanding the number of available applications.

The team's solution-based technique, published in the peer-reviewed journal Proceedings of National Academy of Sciences, was discovered serendipitously when a transparent film of polymer spread up the walls of a container while nanofibers in water were being purified with chloroform.

"What drew me in immediately was the eerie phenomenon of what appeared to be self-propelled fluid flow," said Julio M. D'Arcy, lead author on the PNAS paper and a senior graduate student in the Kaner's UCLA lab.

"Now I can tell people that I make films in L.A.," he joked.

When water and oil are mixed, a blend of droplets is formed, creating a water–oil interface that serves as an entry point for trapping polymer nanofibers at liquid–liquid interfaces. As droplets unite, a change in the concentration of blended solids at the water–oil interface leads to a difference in surface tension. Spreading up a glass wall occurs as result of an attempt to reduce the surface-tension difference. Directional fluid flow leads to a continuously conductive thin film comprised of a single monolayer of polymer nanofibers. The uniformity of the film surface is due to the particles being drawn out of the water–oil interface, sandwiched between two fluids of opposing surface tensions.

Development of the technology is occurring in collaboration with Fibron Technologies Inc., with support from the National Science Foundation through a Small Business Technology Transfer grant. Fibron is a small company that has licensed the technology from UCLA. It was founded by Kaner, who serves as chief scientific adviser, and two of his former Ph.D. students — Christina Baker and Henry Tran, who have gone on to take leadership roles in the company.

Fibron's CEO, Christian Behrenbruch, said "working with UCLA to develop this technology has been a win-win. It enables us to access incredibly innovative people, but also, the NSF has helped enable the establishment of a formal and transparent IP releationship with the university. The good news is that this technology is moving rapidly into commercial development."

Other techniques exist for creating thin films of conducting polymers, but each technique tends to work only a limited number of applications, or they are not feasible for scaling up. A method has long been sought which would overcome the limitations of each of the previous methods. The water and oil technique, with a bit of nanotechnology thrown in, might provide just that — a scalable universal method for creating large thin films of conducting polymers.

Organic solvent system may improve catalyst recycling and create new nanomedicine uses

October 28, 2010

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Wei Lin holds a gold/organic aqua regia solution while Rongwei Zhang holds a silicon substrate coated with 200-nanometer gold. The image on the monitor shows gold recovered from the solution using calcinations. Credit: Credit: Gary Meek

Noble metals such as platinum and palladium are becoming increasingly important because of growth in environmentally friendly applications such as fuel cells and pollution control catalysts. But the world has limited quantities of these materials, meaning manufacturers will have to rely on efficient recycling processes to help meet the demand.

Existing recycling processes use a combination of two inorganic acids known as "aqua regia" to dissolve noble metals, a class of materials that includes platinum, palladium, gold and silver. But because the metals are often dissolved together, impurities introduced in the recycling process may harm the efficiency of catalysts produced from the recycled materials. Now, researchers at the Georgia Institute of Technology have developed a new organic solvent process that may help address the problem – and open up new possibilities for using these metals in cancer therapeutics, microelectronics and other applications.

The new Georgia Tech solvent system uses a combination of two chemicals – thionyl chloride and a variety of organic reagents such as pyridine, N,N-dimethylformamide (DMF), pyrimidine or imidazole. The concentrations can be adjusted to preferentially dissolve gold or palladium, and more importantly, no combination of the organic chemicals dissolves platinum. This ability to preferentially dissolve noble metals creates a customized system that provides a high level of control over the process.

"We need to be able to selectively dissolve these noble metals to ensure their purity in a variety of important applications," said C.P. Wong, a Regents professor in the Georgia Tech School of Materials Science and Engineering. "Though we don't fully understand how it works yet, we believe this system opens a lot of new possibilities for using these metals."

A paper describing the research was published recently in the journal Angewandte Chemie.

Catalyst systems that make use of more than one metal, such as palladium with a gold core, are becoming more widely used in industrial processes. To recycle those, the new solvent system – dubbed "organic aqua regia" – could first use a combination of thionyl chloride and DMF to dissolve out the gold, leaving hollow palladium spheres. Then the palladium spheres could be dissolved using a different combination.

So far, the researchers have demonstrated that the solvent system can selectively dissolve gold and palladium from a mixture of gold, palladium and platinum. They have also used it to remove gold from a mixture of gold and palladium.

Beyond recycling, the new solvent system could also provide new ways of producing nanometer-scale cancer chemotherapy agents that involve these metals. And the new solvent approach could have important implications for the electronics industry, which uses noble metals that must often be removed after specific processing steps. Beyond selectivity, the new approach also offers other advantages for electronics manufacturing – no potentially harmful contamination is left behind and processing is done under mild conditions.

"In semiconductor production, people want to avoid having a metal catalyst remaining in devices, but in many cases, they cannot use existing water-based processes because these can damage the semiconductor oxides and introduce contamination with free ions in the aqueous solution," explained Wei Lin, a graduate research assistant in Wong's laboratory. "Use of this organic system avoids the problem of moisture."

Use of the selective process could also facilitate recycling of noble metals used in electronics manufacturing. Wire-bonding, metallization and interconnect processes currently use noble metals.

Noble metals are also the foundation for widely-used chemotherapy agents, but the chemistry of synthesizing them involves a complex process of surfactants and precursors. Wong believes the new Georgia Tech solvent process may allow creation of novel compounds that could offer improved therapeutic effects.

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Wei Lin holds a gold/organic aqua regia solution. The image on the monitor shows gold recovered from the solution using calcinations. Credit: Credit: Gary Meek

"We hope this will open up some new ways of making these important pharmaceutical compounds as well as novel gold and palladium catalytic systems," he said.

Lin discovered the new solvent system by accident in 2007 while using thionyl chloride in an unrelated project that involved bonding carbon nanotubes to a gold substrate. "I left my sample in the solution and went to lunch," he recalled. "Then I received a couple of phone calls and the sample stayed in the solution for too long. When I got it out, the gold was gone."

The researchers were intrigued by the discovery and pursued an explanation as they had time over the past three years. They tested other reagents mixed with the thionyl chloride, and learned the proportions necessary for selective dissolution of palladium and gold. They worked with other researchers at Georgia Tech, including nanotechnology pioneer Zhong Lin Wang, to develop a fundamental understanding of the process – research that is continuing.

The chemicals used by the Georgia Tech research team are well known in organic chemistry, and are used today in polymer synthesis. Beyond their selectivity, the new solvent system is more environmentally friendly than traditional aqua regia – which is a combination of concentrated nitric and hydrochloric acids – and can operate at mild conditions. Potential disadvantages compared to traditional aqua regia include higher costs and slower dissolution rates.

"We have opened up a new approach to noble metals using organic chemistry," Wong added. "We don't yet thoroughly understand the mechanism by which this works, but we hope to develop a more complete understanding that may lead to additional applications."

Pivoting hooks of graphene's chemical cousin could revolutionize work of electron microscopes

November 1, 2010

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This is a sample seen attached to the graphene oxide. Credit: University of Warwick/ Nano Letters

The single layer material Graphene was the subject of a Nobel prize this year but research led by a team of researchers at the University of Warwick has found molecular hooks on the surface of its close chemical cousin, Graphene Oxide, that will potentially provide massive benefits to researchers using transmission electron microscopes. They could even be used in building molecular scale mechanisms.

The research team, which includes Drs. Jeremy Sloan, Neil Wilson and PhD student Priyanka Pandey from the Department of Physics and Dr. Jon Rourke from the Department of Chemistry together with the groups of Drs. Kazu Suenaga and Zheng Liu from AIST in Japan and Drs. Ian Shannon and Laura Perkins in Birmingham were looking at the possibility of using Graphene as a base to mount single molecules for imaging by transmission electron microscopy. As Graphene forms an electron transparent sheet just one atom thick it would enable high precision, high contrast imaging of the molecules being studied as well as the study of any interactions they have with the supporting graphene.

While this idea is great in theory, Graphene is actually very difficult to create and manipulate in practice. The researchers therefore turned to Graphene's easier to handle cousin, Graphene Oxide. This choice turned out to be a spectacularly better material as they found extremely useful properties, in the form of ready-made molecular hooks that could make Graphene Oxide the support material of choice for future transmission electron microscopy of any molecule with oxygen on its surface.

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This is a graphic of sample binding to a graphene oxide "hook". Credit: University of Warwick / Nano Letters

Graphene Oxide's name obscures the fact that it is actually a combination of carbon, oxygen and hydrogen. For the most part it still resembles the one atom thin sheet of pure Graphene, but it also has "functional groups" consisting of hydrogen paired with oxygen. These functional groups can bind strongly to molecules with external oxygens making them ideal tethers for researchers wishing to study them by transmission electron microscoscopy.

This feature alone will probably be enough to persuade many researchers to turn to Graphene Oxide as a support for the analysis of a range of molecules by transmission electron microscopy, but the researchers found yet another intriguing property of these handy hooks – the molecules attached to them move and pivot around them.

Dr Jeremy Sloan said: "Under the right conditions the functional groups not only provide molecular tethers that hold molecules in an exact spot they also allow the molecule to be spun in that position. This opens up a range of new opportunities for the analysis of such molecules but could also be a useful mechanism for anyone seeking to create molecular sized "machinery"."

Intricate, curving 3-D nanostructures created using capillary action forces

October 19, 2010

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Twisting spires are one of the 3-D shapes researchers at the University of Michigan were able to develop using a new manufacturing process. Credit: A. John Hart

ANN ARBOR, Mich.---Twisting spires, concentric rings, and gracefully bending petals are a few of the new three-dimensional shapes that University of Michigan engineers can make from carbon nanotubes using a new manufacturing process.

The process is called "capillary forming," and it takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and travel up a drinking straw of their own accord.

The new miniature shapes, which are difficult if not impossible to build using any material, have the potential to harness the exceptional mechanical, thermal, electrical, and chemical properties of carbon nanotubes in a scalable fashion, said A. John Hart, an assistant professor in the Department of Mechanical Engineering and in the School of Art & Design.

They could lead to probes that can interface with individual cells and tissues, novel microfluidic devices, and new materials with a custom patchwork of surface textures and properties.

A paper on the research is published in the October edition of Advanced Materials, and is featured on the cover.

"It's easy to make carbon nanotubes straight and vertical like buildings," Hart said. "It hasn't been possible to make them into more complex shapes. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing."

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By using unique two-dimensional templates, researchers at the University of Michigan could coax carbon nanotubes to grow in intricate, curving three-dimensional structures. Credit: A. John Hart

Hart's method starts by stamping patterns on a silicon wafer. His ink in this case is the iron catalyst that facilitates the vertical growth of the carbon nanotubes in the patterned shapes. Rather than stamp a traditional, uniform grid of circles, Hart stamp hollow circles, half circles and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings. One such grouping is a pentagon of half circles with their flat sides facing outward.

He uses the traditional "chemical vapor deposition" process to grow the nanotubes in the prescribed patterns. Then he suspends the silicon wafer with its nanotube forest over a beaker of a boiling solvent, such as acetone. He lets the acetone condense on the nanotubes, and then lets the acetone evaporate.

As the liquid condenses, capillary action forces kick in and transform the vertical nanotubes into the intricate three-dimensional structures. For example, tall half-cylinders of nanotubes bend backwards to form a shape resembling a three-dimensional flower.

"We program the formation of 3D shapes with these 2D patterns," Hart said. "We've discovered that the starting shape influences how the capillary forces change the structures' geometry. Some bend, others twist, and we can combine them any way we want."

The capillary forming process allows the researchers to create large batches of 3D microstructures---all much smaller than a cubic millimeter---over essentially limitless areas, Hart said. In addition, the researchers show that their 3D structures are up to 10 times stiffer than typical polymers used in microfabrication. Thus, they can be used as molds for manufacturing of the same 3D shapes in other materials.

"We'd like to think this opens up the idea of creating custom nanostructured surfaces and materials with locally varying geometries and properties, " Hart said. "Now, we think of materials as having the same properties everywhere, but with this new technique we can dream of designing the structure and properties of a material together."

Turning down the noise in graphene

August 6, 2010 by Lynn Yarris

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New noise model shows all single layer graphene samples with an M-shaped pattern of noise (top) and all bi-layer graphene samples with a V-shaped noise pattern.

(PhysOrg.com) -- Graphene is a two-dimensional crystalline sheet of carbon atoms - meaning it is only one atom thick - through which electrons can race at nearly the speed of light - 100 times faster than they can move through silicon. This plus graphene's incredible flexibility and mechanical strength make the material a potential superstar for the electronics industry. However, whereas the best electronic materials feature a strong signal and weak background noise, attaining this high signal-to-noise ratio has been a challenge for both single and bi-layers of graphene, especially when placed on a substrate of silica or some other dielectric. One of the problems facing device developers has been the lack of a good graphene noise model.

Working with the unique nanoscience capabilities of the Molecular Foundry at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory, a multi-institutional team of researchers has developed the first model of signal-to-noise-ratios for low frequency noises in graphene on silica. Their results show noise patterns that run just the opposite of noise patterns in other electronic materials.

Berkeley Lab materials scientist Yuegang Zhang led a study in which it was determined that for graphene on silica, the background signal noise is minimal near the region in the graphene where the electron density of states (the number of energy states available to each electron) is lowest. For semiconductors, such as silicon, in the region where electron density states is low the background noise is at its highest. However, there were distinct differences in the noise patterns of single and bi-layer graphene.

"In this work, we present the four-probe low frequency noise characteristics in single- and bi-layer graphene samples, using a back-gated device structure that helps simplify the physics in understanding the interactions between the graphene and the silica substrate," says Zhang. "For single-layer graphene we found that the noise was reduced either close to or far away from the lowest electron density of states, sometimes referred to as the Dirac point for graphene, forming an M-shaped pattern. For the bi-layer graphene, we found a similar noise reduction near the Dirac point but an increase away from that point, forming a V-shaped pattern. The noise data near the Dirac point correlated to spatial-charge inhomogeneity."

The results of this research are reported in the journal Nano Letters in a paper titled "Effect of Spatial Charge Inhomogeneity on 1/f Noise Behavior in Graphene." Co-authoring the paper with Zhang were Guangyu Xu, Carlos
Torres Jr., Fei Liu, Emil Song, Minsheng Wang, Yi Zhou, Caifu Zeng and Kang Wang.

Lead author Guangyu Xu, a physicist with the Department of Electrical Engineering at the University of California (UC) Los Angeles, says the spatial charge inhomogeneity responsible for the graphene's unique noise patterns was probably caused by the charge impurities near the graphene-substrate interface.

"Our experiment carefully rules out other possible extrinsic factors that might influence the result," Xu says. "We conclude the correlation between the anomalous noise feature and the spatial charge inhomogeneity, is one
of the main carrier scattering mechanisms for unsuspended graphene samples."

Xu says this model of low frequency noise characteristics in graphene should be a significant help for fabricating electronic devices because biasing at the low noise regime can be designed into the device.

"This will benefit the high signal-to-noise ratio in graphene," Xu says.

New self-assembling photovoltaic technology that repairs itself

September 5, 2010 by David L. Chandler

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This proof-of-concept version of the photoelectrochemical cell, which was used for laboratory tests, contains a photoactive solution made up of a mix of self-assembling molecules (in a glass cylinder held in place by metal clamp) with two electrodes protruding from the top, one made of platinum (the bare wire) and the other of silver (in a glass tube). Photo: Patrick Gillooly

Plants are good at doing what scientists and engineers have been struggling to do for decades: converting sunlight into stored energy, and doing so reliably day after day, year after year. Now some MIT scientists have succeeded in mimicking a key aspect of that process.

One of the problems with harvesting sunlight is that the sun?s rays can be highly destructive to many materials. Sunlight leads to a gradual degradation of many systems developed to harness it. But plants have adopted an interesting strategy to address this issue: They constantly break down their light-capturing molecules and reassemble them from scratch, so the basic structures that capture the sun?s energy are, in effect, always brand new.

That process has now been imitated by Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering, and his team of graduate students and researchers. They have created a novel set of self-assembling molecules that can turn sunlight into electricity; the molecules can be repeatedly broken down and then reassembled quickly, just by adding or removing an additional solution. Their paper on the work was published on Sept. 5 in Nature Chemistry.

Strano says the idea first occurred to him when he was reading about plant biology. ?I was really impressed by how plant cells have this extremely efficient repair mechanism,? he says. In full summer sunlight, ?a leaf on a tree is recycling its proteins about every 45 minutes, even though you might think of it as a static photocell.?

One of Strano?s long-term research goals has been to find ways to imitate principles found in nature using nanocomponents. In the case of the molecules used for photosynthesis in plants, the reactive form of oxygen produced by sunlight causes the proteins to fail in a very precise way. As Strano describes it, the oxygen ?unsnaps a tether that keeps the protein together,? but the same proteins are quickly reassembled to restart the process.

This action all takes place inside tiny capsules called chloroplasts that reside inside every plant cell ? and which is where photosynthesis happens. The chloroplast is ?an amazing machine,? Strano says. ?They are remarkable engines that consume carbon dioxide and use light to produce glucose,? a chemical that provides energy for metabolism.

To imitate that process, Strano and his team, supported by grants from the MIT Energy Initiative, the Eni Solar Frontiers Center at MIT and the Department of Energy, produced synthetic molecules called phospholipids that form disks; these disks provide structural support for other molecules that actually respond to light, in structures called reaction centers, which release electrons when struck by particles of light. The disks, carrying the reaction centers, are in a solution where they attach themselves spontaneously to carbon nanotubes ? wire-like hollow tubes of carbon atoms that are a few billionths of a meter thick yet stronger than steel and capable of conducting electricity a thousand times better than copper. The nanotubes hold the phospholipid disks in a uniform alignment so that the reaction centers can all be exposed to sunlight at once, and they also act as wires to collect and channel the flow of electrons knocked loose by the reactive molecules.

The system Strano?s team produced is made up of seven different compounds, including the carbon nanotubes, the phospholipids, and the proteins that make up the reaction centers, which under the right conditions spontaneously assemble themselves into a light-harvesting structure that produces an electric current. Strano says he believes this sets a record for the complexity of a self-assembling system. When a surfactant ? similar in principle to the chemicals that BP has sprayed into the Gulf of Mexico to break apart oil ? is added to the mix, the seven components all come apart and form a soupy solution. Then, when the researchers removed the surfactant by pushing the solution through a membrane, the compounds spontaneously assembled once again into a perfectly formed, rejuvenated photocell.

?We?re basically imitating tricks that nature has discovered over millions of years? ? in particular, ?reversibility, the ability to break apart and reassemble,? Strano says. The team, which included postdoctoral researcher Moon-Ho Ham and graduate student Ardemis Boghossian, came up with the system based on a theoretical analysis, but then decided to build a prototype cell to test it out. They ran the cell through repeated cycles of assembly and disassembly over a 14-hour period, with no loss of efficiency.

Strano says that in devising novel systems for generating electricity from light, researchers don?t often study how the systems change over time. For conventional silicon-based photovoltaic cells, there is little degradation, but with many new systems being developed ? either for lower cost, higher efficiency, flexibility or other improved characteristics ? the degradation can be very significant. ?Often people see, over 60 hours, the efficiency falling to 10 percent of what you initially saw,? he says.

The individual reactions of these new molecular structures in converting sunlight are about 40 percent efficient, or about double the efficiency of today?s best solar cells. Theoretically, the efficiency of the structures could be close to 100 percent, he says. But in the initial work, the concentration of the structures in the solution was low, so the overall efficiency of the device ? the amount of electricity produced for a given surface area ? was very low. They are working now to find ways to greatly increase the concentration.

Philip Collins ?90, associate professor of experimental and condensed-matter physics at the University of California, Irvine, who was not involved in this work, says, ?One of the remaining differences between man-made devices and biological systems is the ability to regenerate and self-repair. Closing this gap is one promise of nanotechnology, a promise that has been hyped for many years. Strano's work is the first sign of progress in this area, and it suggests that ?nanotechnology? is finally preparing to advance beyond simple nanomaterials and composites into this new realm.?

Microwave oven key to self-assembly process meeting semiconductor industry need

October 25, 2010 Thanks to a microwave oven, the fundamental nanotechnology process of self assembly may soon replace the lithographic processing use to make the ubiquitous semi-conductor chips.

By using microwaves, researchers at Canada's National Institute for Nanotechnology (NINT) and the University of Alberta have dramatically decreased the cooking time for a specific molecular self-assembly process used to assemble block copolymers, and have now made it a viable alternative to the conventional lithography process for use in patterning semiconductors.

When the team of chemists and electrical engineering researchers replaced convective heat with a microwave oven, nano-sized particles were encouraged to organize themselves into very regular patterns extremely quickly – reducing the processing time from days to less than one minute.

The processing time is very important if this self-assembly process is to be introduced to industrial semi-conductor fabrication. In the International Technology Roadmap for Semiconductors, the promise of self-assembly to address the need to put more and more functionality onto chips was recognized. The block co-polymer method, which directs nanomaterials to create molds and then fills them in with a target material, was known to be capable of creating very detailed patterns many times smaller than current technology. But previously the time needed for molecules to organize themselves was too long to be useful for the industry. The change of the heat source has brought that processing time well under the suggested target of 4 minutes.

"This is one of the first examples of the self-assembly process being used to address a real world problem for the semi-conductor industry," said Dr. Jillian Buriak "We've got the process; the next step is to exploit it to make something useful."