tarot 2007-10-09 17:16
Walking Molecules Pick Up Cargo
Researchers have created a tiny machine made of just one moleculethat can carry other molecules on a surface. The technique can be usedto move atoms or molecules close to each other, controlling when theyreact. The new "molecule carrier," described online last week in [i]Science[/i] Express, could eventually lead to more-efficient catalysts and new methods for assembling molecular electronics.
The researchers, led by [url=http://research.chem.ucr.edu/groups/bartels/]Ludwig Bartels[/url],a chemistry professor at the University of California, Riverside, use amolecule called anthraquinone, an organic compound containing a chainof three benzene rings, with two oxygen atoms attached to the centralring, one on each side. When the researchers place anthraquinonemolecules on a flat copper surface at temperatures of around -223 ºC,the molecules move in a straight line. Carbon dioxide molecules, on theother hand, move randomly. But if the two molecules come close to eachother, carbon dioxide attaches to the anthraquinone's oxygen atom.
Becauseeach anthraquinone molecule has two oxygen atoms, it can effectivelypick up two carbon dioxides and move with them, Bartels says. Theresearchers can manipulate the molecules with the tip of a scanningtunneling microscope probe. They can, for example, slide carbon dioxidecloser to the anthraquinone to load, and they can unload the carbondioxide by "bumping the tip into the carbon dioxide molecule," Bartelssays.
"I think [this is] a really elegant study and very key to understanding transport at a molecular scale," says [url=http://www.ece.rice.edu/people/faculty/kkelly]Kevin Kelly[/url],an assistant professor of electrical and computer engineering at RiceUniversity. The study lays down the groundwork for designing newmolecules that can efficiently transport cargo molecules, says Kelly,who previously demonstrated controlled molecular motion using"nanocars." Kelly says that work in this area could have "realtechnological implications in the next three to five years" in terms ofdeveloping catalysts.
An automobile's catalyticconverter, for example, uses platinum as a catalyst: carbon monoxideand oxygen attach to the platinum surface and react when they get closeto each other. But moving the two closer with the help of anothermolecule would speed up the process and use less of the expensiveplatinum, says [url=http://physics.ucf.edu/%7Etalat/]Talat Rahman[/url], a physics professor at the University of Central Florida and coauthor of the [i]Science [/i]paper."We can wait for [carbon monoxide and oxygen] to jiggle around on thesurface, somehow break the bond with the surface, and find each other,"she says. "Or we could find something that can carry them from oneplace to another."
Experts say the work could alsohave implications for building molecular electronics, a fledgingresearch area in which organic molecules, rather than silicon, serve astransistors and other electronic devices. [url=http://www.nano.psu.edu/%7Epsw/Weiss.html]Paul Weiss[/url],a chemistry and physics professor at Penn State University, says thatthe researchers have made a significant breakthrough towardself-assembled molecular circuits.
To move along thecopper surface, anthraquinone has to overcome a small energy barrier.The researchers have found that this barrier doubles and triples,respectively, when the anthraquinone picks up one and then a secondcarbon dioxide molecule. Before now, no one has measured energy changesas molecules assemble and disassemble, Weiss says. By understanding howthe energy barrier changes, we can learn how to control it, which wouldbe crucial for assembling molecular circuits, he says. "That is thebeauty of the work."
[img]file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/moz-screenshot.jpg[/img]
[img]http://www.technologyreview.com/files/8913/Walking-Molecules.jpg[/img]
Anthraquinone molecules move in a straight line on a copper surface,while carbon dioxide moves randomly. But when the two molecules getclose together, the anthraquinone picks up the carbon dioxide and keepswalking. The “molecule carrier” is able to carry two carbon dioxides.
Credit: Ludwig Bartels, University of California, Riverside