What if we could make complex intermetallic compounds, simply and quickly, without resorting to sintering or melting metal powders? It sounds like a far-fetched idea, but it can be done, say chemists and physicists at Texas A&M University. Their startling discovery is based on nanotech methods, and offer "unprecedented" flexibility in the synthesis and processing of functional intermetallic compounds and alloys. It’s truly ‘metallurgy in a beaker.’
The method comes courtesy of chemists Raymond Schaak, Amandeep Sra, Brian Leonard, Robert Cable, John Bauer and Yi-Fan Han and Edward Funck, working with physicists Joel Means, Winfried Teizer and Yolanda Vasquez. Their starting point was the observation that under the right conditions, binary mixtures of weakly stabilized gold and copper nanoparticles will aggregate to form binary nanocomposites, which thermally transform into intermetallic nanocrystals at low temperatures. Following this conceptual trail, they found that the approach was extensible, and would also work with other bimetallic and trimetallic systems with a variety of compositions and crystal structures. Start with the right ratios of preformed nanoparticles, and you end up with multimetallic compounds with those ratios. What you get is a route that separates diffusion and nucleation steps, and removes solid-state diffusion as the rate-limiting step in getting to the bulk material.
The work is described in a relatively long paper in Journal of the American Chemical Society, first seen on the web on February 19 2005. It shows how one can produce multiple compounds from the Au-Cu bimetallic system, including AuCu, AuCu3, Au3Cu, and the Au-Cu-II superlattice. They also show how FePt3, CoPt, CuPt and the alloys Ag-Pt, Au-Pd and Ni-Pt can be made. The tricky low-temperature superconducting Ag2Pd3S can be made from Ag2S and palladium nanoparticles, they observed, while they also proved how to make an AuPd4 alloy that will catalyze the formation of hydrogen peroxide from hydrogen and water.
There’s no reason why the method should not also turn out morphologically diverse nanomaterials. With AuCu, they show how it came to make surface-confined thin films (or planar or nonplanar supports), free-standing monoliths, nanomeshes, inverse opals, and dense powders of a gram scale.
Intermetallics are of vital commercial importance, and are found in many industrial applications because of their surprising range of physical properties, which range across the gamut of ferromagnetism, superconductivity, shape-memory effects, catalytic activity, hydrogen storage, structural hardness and corrosion resistance, to name a few. The ability to make them without heating powders to over 1000 degree C and weeks of annealing is a remarkable plus, emulating recent solution routes such as sol-gel synthesis of oxides or spin-coating of polymer and chalcogenide films. A bonus is that some metastable, nonequilibrium forms that couldn’t survive high temperature processing are also possible.
If there’s a drawback, it rests in the use of various polymers and stabilizers, which means that highly pure products are out of reach. But the trade-off between ultrapurity and high cost versus cheap bulk production is one worth evaluating. The exciting work was funded by Texas A&M start-up funds, the Welch Foundation, and the American Chemical Society Petroleum Research Fund.
Details: Raymond E. Schaak, Assistant Professor, Dept. of Chemistry, Texas A&M University, College Station TX 77842-3012. Phone: 979-458-2858. Fax: 979-845-4719. E-mail: schaak@mail.chem.tamu.edu URL: www.chem.tamu.edu/faculty/faculty_detail.php?ID=779
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As we’ve moved the size of devices down from the macroscale, the use of electrodeposition masks has followed right along. At the nanoscale, we have a fairly broad range of options about what material to choose for mask making. Nanofabrication of patterns by e-deposition on metals, semiconductors and polymers on conductive substrates can be achieved using, for example, selforganized masks of anodic alumina, or with molecular crystals such as amphiphilic surfactants, or block copolymers. If there’s a problem with such methods, it would be the rather limited range of unit cell geometries that can be realized, especially in creating the kind of complex patterns we may actually need in sophisticated devices. For some time now, researchers have wondered whether proteins might not be better candidates for deposition masks. After all, we know from nature that they can organize into homo- or heterostructures that encompass a huge range of 2D forms, building on the chiral properties of the molecules. A further plus is that we also can get it right, thanks to advances in molecular biology, which can engineer features on a very short scale length. As a result, some successes have already been reported using crystalline protein architectures to make templates for nanoscale work, using them as shadow masks for vapor-phase deposition. Until recently, though, no one has tried to use such masks in electrodeposition, where conditions are mild, and other advantages also quickly become apparent.
The first work to prove this concept comes from a team at University of Washington in Seattle. The researchers, chemical engineers Daniel Allred and François Baneyx, worked with Daniel Schwartz and Mehmet Sarikaya, who are with both the chemical engineering and materials science and engineering faculty. As they point out in a recent paper, due for publication in Nano Letters, electrodeposition has something special going for it, because "material growth proceeds from the substrate outward, and need not follow a line-of-sight path through the mask, as do many vapor phase deposition methods." As a result, "[it] offers the unique prospect of being able to grow dense materials through a tortuous multilayer crystalline protein mask," and making the job of optimizing the protein-surface interface more simple, aiding the goal of perfect monolayer coverage. In their experiments, they worked with a popular bionanotech material, surface-layer proteins (S-layer proteins), which form a 2D layer, cladding and protecting bacterial cells. The S-layer proteins are very stable, they resist low pH conditions, and are heat stable. They also selfassemble into all known 2D rotational symmetries, and have 1 nm to 4 nm solvent-accessible openings, and a typical lattice parameter of 10 nm to 20 nm. In their work, they chose the hexagonally packed intermediate layer (HPI layer) of the renowned tough bacterium Deinococcus radiodurans, which they peel off with surfactant, and can be kept in stable form in solution for at least six months.
The S-layer proteins were used as masks on ultrathin gold-palladium films, and observed by draping the film across the grid of a transmission electron microscope. Among the materials they deposited were cuprous oxide (Cu2O), nickel, platinum, palladium and cobalt, with patterns of long-range order emerging, with an 18 nm periodicity corresponding to the HPI layer’s own structure. As they point out, the work has broader implications. The D. radiodurans HPI layer is only one possibility, and the concept should extend to other class members, S-layer proteins from other organisms, and maybe other self-assembling proteins, opening up a range of superlattice symmetry possibilities for nanotech.
The research is funded by the US Army research Office Defense University Research Initiative in Nanotechnology (ARO-DURINT), National Science Foundation (NSF) and the Boeing-Sutter Endowment.
Details: Daniel T. Schwartz, Professor, Chemical Engineering Dept. & Materials Science & Engineering Dept., 353 Benson Hall, University of Washington, Seattle WA 98195-1750. Phone: 206-685-4815. Fax: 206-685-3451. E-mail: dts@u.washington.edu
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We don’t discover drugs by feeding organic molecules to humans and watching what happens. Eventually, they have to be tested that way, but the thrust of pharmaceuticals and genetics research is much more focused. Essentially, it’s a search for identifying specific molecules that bind in a chosen way to proteins. Going that route, we can not only find drugs to treat specific diseases, but also get a better understanding of the complex pathways that rule the biochemistry of complex biological systems. All manner of means are used in this quest, but it seems that nanotech has much to offer; after all, it is the science of prodding around at the nanoscale. It has long been recognized that if we could come up with a suitably miniaturized device, we could directly analyze the binding of organic molecules and proteins, thereby by-passing the kind of commonplace ‘labeling’ methods that are rather difficult to perform in practice. Some recent work by Charles Lieber’s versatile team at Harvard shows that nanowire sensors may be ideal for the task.
Lieber, working with Wayne Wang, Chuo Chen, Keng-hui Lin and Ying Fang, focused his efforts on a specific problem, of identifying inhibitors to protein tyrosine kinases. What are they? Medical researchers are fascinated by them, because they are central to the process of signal transduction in mammalian cells, and work by phosphorylation of a tyrosine residue of a substrate protein, using the common cellular ‘fuel’ adenosine triphosphate (ATP) as a phosphate source. It’s thought that mutation or over-expression of the protein tyrosine kinases may be a factor in a number of diseases including cancer. That promotes the quest for a potential multibillion dollar opportunity. So far, going the conventional discovery route, researchers have found a small molecule called STI-571 (‘Gleevec’) that inhibits the ATP binding to a specific tyrosine kinase, named Abl. As a result, there’s some hope for chronic myelogenous leukemia sufferers. Unfortunately, Gleevec doesn’t seem to work as well in late-stage patients, probably because yet more mutations are occurring as the disease progresses. To push the envelope, we need to test a lot more small-molecule ATP or substrate protein inhibitors. And that’s where Lieber’s crafty discovery comes in.
The basic tool will be quite familiar to Nanotech Alert readers--real-time readout from a silicon nanowires (SiNW) field-effect transistor (FET) is the key. Lieber’s team has quite a track record in biochemical applications with SiNW FETs, detecting binding and unbinding of proteins to ligands on the nanowires in solution, and using them as detectors for proteins and nucleic acids. Eliminating the molecular label removes one possible confounding factor in the tests. In their experimental work, described in Proceedings of the National Academy of Sciences, and first web-released on February 14 2005, they link the Abl kinase to the SiNW FET surface and see the inhibition in binding of ATP via an increase or decrease on the FET’s gate voltage.
Aside from its immediate interest, the method may be more general in application, and can be scaled up using large arrays of FETs. The team concludes, "the results suggest that it should be straightforward to use our SiNW detection method to probe small-molecule-mediated inhibition of protein-protein interactions," and believe, "these SiNW nanosensors could broadly impact drug discovery and chemical genetics." Funding came from the National Cancer Institute, the Ellison Medical Foundation and DARPA.
Details: Charles M. Lieber, Professor, Dept. of Chemistry & Chemical Biology/Division of Engineering & Applied Sciences, Harvard University, 12 Oxford St., Cambridge MA 02138. Phone: 617-496-3169. Fax: 617-496-5442. E-mail: cml@cmliris.harvard.edu
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The idea of a quantum computer made from quantum dots is a goal that we’ll no doubt obtain. But until we find a way to make the behavior of quantum dots predictable, the computer concept remains only an idea, not reality. Researchers in Germany have been able to hit quantum dots with light to induce the quantum mechanical state needed to run a quantum computer. But the German researchers couldn’t consistently control that state. A team of researchers from Ohio University believe they have discovered the reason why.
The Ohio team--Jose Villas-Bôas, a postdoctoral fellow, Sergio Ulloa, a professor of Physics, and Alexander Govorov, an associate professor of physics--developed theoretical models to find out why quantum dots weren’t giving consistent results. The researchers thought the problem had something to do with the creation of the type of quantum dots being studied. Using a molecular beam epitaxy chamber, the Ohio team created an atomic coating by spraying atoms on a surface under high temperatures. As more layers were added, quantum dots appeared on the surface. The team discovered that a fine residue or wetting layer that was left behind on the surface was causing interference when the dots were hit with a beam of light. The wetting layer prevented the light from triggering the quantum state, says Ulloa.
The Ohio team says that the quantum dots can be triggered if the beam is refocused or the duration of the light pulses is changed. Either of these techniques will negate the effects of the wetting layer. Theory has been proved in practice with one physicist reporting that a quantum dot has been successfully manipulated in the lab.
To learn more about the work, see Physical Review Letters 94, 057404 (2005). The work is supported by the US Dept. of Energy, the Indiana 21st Century Fund, the Ohio University Postdoctoral Fellow Program and the FAPESP fellowship.
Details: J.M. Villas-Bôas, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, OH 45701-2979. Phone: 740-593-9611. E-mail: villasb@phy.ohiou.edu
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A University of Illinois Urbana-Champaign (UIUC) team has created a way to produce hollow nanospheres and nanocrystals using high-intensity ultrasound.
The UIUC team--Kenneth Suslick, the Marvin T. Schmidt Professor of Chemistry, and former postdoctoral research associate Arul Dhas--worked with molybdenum disulfide and molybdenum oxide. Using high-intensity ultrasound, the team was able to generate nanoparticles that bind to the surface of nanosized silica spheres. After the spheres are heated to produce uniform coatings, the team uses hydrofluoric acid to etch away the silica. What’s left behind are hollow spheres of molybdenum.
The team found that if the hollow molybdenum oxide nanospheres were further processed by heating again, the spheres were transformed into single-crystal boxes with spherical hollow voids.
The nanosphere discovery could benefit microelectronics, drug delivery, and can be used as catalysts for environmentally friendly fuels, which remove sulfur-containing compounds from gasoline and other fossil fuels. The team also believes that the sonochemical method can be used on other materials to create additional types of hollow, nanostructured particles.
To learn more, see the Journal of the American Chemical Society, web-published on February 4, 2005. The work is supported by the National Science Foundation and the UIUC Center for Microanalysis of Materials.
Details: Kenneth S. Suslick, Professor, School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL 61801. Phone: 217-333-2794. E-mail: ksuslick@uiuc.edu
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