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)