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Researchers Achieve First Electrowetting of Carbon Nanotubes If you can imagine the straw in your soda can being a million times smaller and made of carbon, you pretty much have a mental picture of a carbon nanotube. Scientists have been making them at will for years, but have never gotten the nanotubes to suck up liquid metal to form tiny wires. In fact, conventional wisdom and hundreds of refereed papers say that such is not even possible. Now, with the aid of an 1875 study of mercury's electrical properties, researchers from the California Institute of Technology (加州理工学院)have succeeded in forcing liquid mercury into carbon nanotubes. Their technique could have important applications, including nanolithography, the production of nanowires with unique quantum properties, nano-sized plumbing for the transport of extremely small fluid quantities, and electronic circuitry many times smaller than the smallest in existence today. Reporting in the December 2 issue of the journal Science, Caltech assistant professor of chemistry Patrick Collier and associate professor of chemical engineering Konstantinos Giapis describe their success in electrowetting carbon nanotubes. By "electrowetting" they mean that the voltage applied to a nanotube immersed in mercury causes the liquid metal to rise into the nanotube by capillary action and cling to the surface of its inner wall. Besides its potential for fundamental research and commercial applications, Giapis says that the result is an opportunity to set the record straight. "We have found that when measuring the properties of carbon nanotubes in contact with liquid metals, researchers need to take into account that the application of a voltage can result in electrically activated wetting of the nanotube. "Ever since carbon nanotubes were discovered in 1991, people have envisioned using them as molds to make nanowires or as nanochannels for flowing liquids. The hope was to have the nanotubes act like molecular straws," says Giapis. However, researchers never got liquid metal to flow into the straws, and eventually dismissed the possibility that metal could even do so because of surface tension. Mercury was considered totally unpromising because, as anyone knows who has played with liquid mercury in chemistry class, a glob will roll around a desktop without wetting anything it touches. "The consensus was that the surface tension of metals was just too high to wet the walls of the nanotubes," adds Collier, the co-lead author of the paper. This is not to say that researchers have never been able to force anything into a nanotube: in fact, they have, albeit by using more complex and less controllable ways that have always led to the formation of discontinuous wires. Collier and Giapis enter the picture because they had been experimenting with coating nanotubes with an insulator in order to create tiny probes for future medical and industrial applications. In attaching nanotubes to gold-coated atomic force microscope tips to form nanoprobes, they discovered that the setup provided a novel way of making liquid mercury rise in the tubes by capillary action. Casting far beyond the nanotube research papers of the last decade, the researchers found an 1875 study by Nobel Prize-winning physicist Gabriel Lippmann that described in detail how the surface tension of mercury is altered by the application of an electrical potential. Lippmann's 1875 paper provided the starting point for Collier and Giapis to begin their electrowetting experiments. After mercury entered the nanotubes with the application of a voltage, the researchers further discovered that the mercury rapidly escaped from the nanotubes immediately after the voltage was turned off. "This effect made it very difficult to provide hard proof that electrowetting occurred," Collier said. In the end, persistence and hard work paid off as the results in the Science paper demonstrate. Giapis and Collier think that they will be able to drive various other metals into the nanotubes by employing the process at higher temperature. They hope to be able to freeze the metal nanowires in the nanotubes so that they remain intact when the voltage is turned off. "We can pump mercury at this point, but it's possible that you could also pump nonmetallic liquids," Giapis says. "So we now have a way of pumping fluids controllably that could lead to nanofluidic devices. We envision making nano-inkjet printers that will use metal ink to print text and circuitry with nanometer precision. These devices could be scaled up to operate in a massively parallel manner. " The paper is titled "Electrowetting in Carbon Nanotubes." In addition to Collier and Giapis, the other authors are Jinyu Chen, a postdoctoral scholar in chemistry, and Aleksandr Kutana, a postdoctoral scholar in chemical engineering. Source: Caltech (转自PHYSORG.COM) |
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感谢分享！大家再看看这篇： Nanotube Speedway Gas and water zoom through the carbon nanotube pores of a membrane With their atomically smooth surfaces, carbon nanotubes can act like a Slip 'n Slide for gases and water, hastening the molecules through the nanotube's channel at speeds far greater than would be expected from classical models of transport. At least that's what theorists said. Now a group of researchers at Lawrence Livermore National Laboratory (LLNL) has demonstrated this phenomenon experimentally (Science 2006, 312, 1034).  Membrane Chip A chip made with the nanotube membrane is shown next to a quarter. Olgica Bakajin, Aleksandr Noy, Jason K. Holt, Hyung Gyu Park, and colleagues have determined the speed with which gas and water molecules travel through double-walled carbon nanotube pores in a silicon nitride membrane. The nanotubes measure less than 2 nm in diameter, or roughly six water molecules across. "The gas and water flows that we measured are 100 to 10,000 times faster than what classical models predict," Bakajin says. The permeability of the silicon nitride membrane is several orders of magnitude higher than those of commercial polycarbonate membranes, despite the fact that its pores are an order of magnitude smaller. "This is like having a garden hose that can deliver as much water in the same amount of time as a fire hose that is 10 times larger," Bakajin explains. "It's a unique nanoscale phenomenon." When the LLNL researchers first set up an experiment with water and the membrane, they left it overnight, thinking that water would not permeate the membrane. "Since water does not wet the outside surface of carbon nanotubes, we were skeptical that water would enter them, let alone flow really fast," Bakajin says. "But the molecular dynamics simulations in the literature predicted fast flow, so we wanted to test the predictions." When they returned to the lab the next morning, they found a puddle beneath the membrane, demonstrating its permeability. The researchers attribute the surprisingly fast transport to the nanotube's atomically smooth surface and to molecular interactions dictated by the nanotube's confined space. They are quick to point out, though, that further study is needed to determine the exact transport mechanisms. In a commentary accompanying the Science paper, chemical engineering professors David S. Sholl of Carnegie Mellon University and J. Karl Johnson of the University of Pittsburgh call the report "a fascinating step toward the development of highly efficient membranes." "These experiments show that carbon nanotube membranes can have spectacularly high fluxes, but to be useful as membranes, they must also show high selectivity," Scholl and Johnson note. "The experiments to date have only examined single-component transport, so no direct information on this crucial issue is available." The membranes have potential applications in desalination, demineralization, and gas separations, such as removing oxygen from air. However, Sholl and Johnson point out that moving from elegant membranes to devices suitable for large-scale application will not be easy. "The scope of this challenge is large, but the payoff is commensurately large," they say.  From Chemical and engineering news [ Last edited by westwolf on 2006-7-4 at 21:19 ] |
2楼2006-07-05 10:10:23