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Tin crystals promenade across copper, stopping now and then to trade places with a counterpart, using choreography akin to the "camphor dance"--a phenomenon first observed in 1686.
The discovery of dancing tin, reported 24 November 2000 in Science's Nanotechnology Issue, may promise surprisingly efficient nanomotors, if researchers can harness this chemical locomotion system.
By manipulating the surface energies that drive tin crystals to move across copper, it might also be possible to control such movements, thereby forcing alloys to form desired nanoshapes, according to researchers with the Sandia National Laboratories in Livermore, California.
Scientists have known for centuries that free energy on liquid surfaces can stimulate movement by particles. Camphor particles were spotted dancing across water more than 300 years ago. In the 19th century, British scientist Lord Rayleigh expanded such observations to provide one of the first reliable measurements of the surface tension of water.
Science authors Andreas K. Schmid, Norm C. Bartelt and Robert Q. Hwang of Sandia have shown that tin behaves in a similar fashion as it diffuses across copper to form bronze.
Within two seconds, tin deposited onto copper at room temperature converges to form two-dimensional crystalline clusters or "islands," the Sandia researchers said. These tin islands zip along the copper surface, picking up copper atoms in exchange for tin atoms left in their wake. Kidnapped copper atoms are then ejected from the tin islands, having morphed into 2-D bronze crystals. After a few moments, the copper surface is covered by the smaller bunches of bronze and the tin islands dissolve.
This "completely unanticipated cooperative process" occurs because roving tin is repelled by tin already embedded within the copper, researchers said. When the dancing tin encounters no competition for a particular surface slot, however, it quickly dumps an atom and grabs copper.
Thus, Schmid reported, tin islands "lower the surface free energy by moving toward unalloyed regions of the surface." In other words, tin islands are efficient: Once they have ruled out a surface area as occupied by other tin, they keep moving. The crystalline clumps will sometimes even paint themselves into a corner to avoid covering the same ground twice, researchers found.
Schmid's research "can be viewed as a direct observation of a nanomotor," according to a Science Perspectives essay. Tin islands convert chemical energy into forward motion, thereby overcoming the friction between tin and the copper surface, explained Flemming Besenbacher of Denmark's University of Aarhus and Jens K. Noskov of the Technical University of Denmark.
How powerful are these natural nanomotors? Tin islands crank out roughly 0.3 horsepower per kilogram of weight, Besenbacher and Noskov estimated. By comparison, a car's power-to-weight ratio is about 0.1 hp/kg--making the natural nanomotors more efficient, in theory.
"The challenge," they concluded, "is to devise nanomotors whose motion can be controlled externally (so that they can be used to move things around at will) and that can be refueled."
To watch bronze-formation in real-time, the Sandia researchers, sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, used two modern imaging technologies: Scanning tunneling microscopy (STM) and low-energy electron microscopy (LEEM).
The LEEM process let the researchers "see" objects on the copper surface and follow tin's rapid movements in real-time, based on the diffraction of electrons. Complementing this information, a topographical map of the material's surface was generated using high-sensitivity STM. In this process, a fine-tipped probe delivers electrical current to the mounted sample. Any Protuberance causes a current surge, as the tip's sudden proximity to the surface prompts electrons to "tunnel" through the sample. Tin atoms left by the crystalline islands were revealed as bumps in the image.
Science's Nanotechnology Issue also includes the following research reports:
MICROSCOPIC MACHINES: Tiny helicopters--powered by biomolecular motors and complete with rotating nanopropellers--may suggest self-propelled machines for delivering medicine to specific regions inside the human body, Cornell University researchers reported. Invisible without a microscope but mighty enough to muster eight propeller rotations per second, the new mini-copters combine fabricated components with biological molecules. An enzyme-based biomotor (ATPase) drives a nickel propeller when powered by the biochemical fuel, ATP (adenosine triphosphate). In nature, ATPase transforms food into an energy source for people, plants, and other living systems by breaking atomic bonds in ATP to generate ADP (adenosine diphosphate). The reaction cranks a cylindrical, rotor-like protein inside ATPase, thereby spinning an attached propeller. The bioengineered nanomachines may someday serve as "a pharmacy inside a cell," perhaps even functioning in concert with the physiology of living cells, according to Carlo D. Montemagno of Cornell.
NANOTUBE DIODES DEBUT: By creating a simple electronic device from a carbon tube just two nanometers in diameter, Stanford University researchers have set the stage for wire devices small enough to work inside molecules. Chongwu Zhou, Hongjie Dai and colleagues demonstrated a method for "doping" or chemically modifying the properties of nanotubes to make them serve as junctions between semiconductors, capable of electrical signal manipulation. The researchers covered half of a single-walled carbon nanotube with a plastic-type material, polymethylmethacrylate (PMMA). Subjecting the uncovered region to a dose of potassium atoms then triggered an electron transfer, which left the exposed area with a negative potential. Thus, a carbon nanotube was transformed into a p-n (positive-negative) junction device.
EARNING THEIR STRIPES: Fundamental studies of fabricated patterns similar to fingerprints or zebra stripes, reported by researchers with Princeton University, may provide new information to support high-density information storage and laser technologies. More immediately, the study by Christopher Harrison and colleagues promises a better understanding of what happens during copolymer lithography, a technique for creating highly intricate patterns. Copolymer lithography may prove especially useful for developing nanoscale semiconducting devices from, say, certain liquid crystals, but defect-formation events slow the process. Using time-lapse atomic force microscopy while annealing synthetic fingerprints, Harrison's team investigated defects called disclinations. They found that groups of disclinations (three or four) could come together and annihilate themselves, suggesting that it may be possible to minimize defects and speed up patterning processes.
KONDO-IN-A-BOX: Many well-known effects in solid-state physics are being revisited in nanoenvironments. One example of this trend is the Kondo Effect, named for Jun Kondo, who realized in 1964 that magnetic impurities slow electron movement through metals--even at low temperatures. Such slowing could find use in device switching, especially in single-walled carbon nanotubes (SWNTs). Harvard University researchers reported that magnetic cobalt clusters on SWNTs literally throw electrons for a loop, increasing resistivity. Conductivity can be enhanced by lengthening the tubes, which seems to give electrons more space to recover, Teri W. Odom, Charles Lieber and colleagues reported. Shorter tubes, on the other hand, exhibited discrete states characteristic of the quantum mechanics for "particle-in-a-box" scenarios, which in this case would be the nanotube.
In addition to these research reports, Science's Nanotechnology Issue includes a special section containing three Reviews, a News article and five News laboratory profiles, addressing the latest in nanotechnology and the attention (both positive and negative) that the field has received in the last decade. The Review articles provide a glimpse of the future for tiny electronics, labs-on-chips, and microrobots, and the challenges inherent in building and moving machinery at this miniature scale. News coverage examines nanotechnology's rise from science fiction to reality, and the recent breakthroughs in the field that have brought nanotechnology to the attention of policymakers, funding agencies and naysayers who believe the technology itself is dangerous.
For copies of any of these articles, please call (202) 326-6440, or send e-mail to scipak@aaas.org
A brief movie clip is available to illustrate the "dancing tin" research. For more information about this artwork, please contact Francesca Carpenter at (202) 326-6634, or send e-mail to fcarpent@aaas.org
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