Purdue University scientists have developed a way to bring chemical structures known as ions in for a "soft-landing" on surfaces, providing a new way to trap and study ions and modify the outermost layer of materials.
The new technique, outlined in today's (Friday, 3/7) issue of the journal Science , provides a way to alter surfaces of even the tiniest components, opening doors for new applications in computing, microelectronics and storing information at a nanoscale level.
"Many properties of materials depend on their surfaces for such traits as electrical conductivity and resistance to degradation," says R. Graham Cooks, the Henry Bohn Hass Distinguished Professor of Chemistry at Purdue.
"Our method differs from other deposition methods in that the ions do not react chemically with the surface on which they are being deposited. This not only allows us to preserve the ions for future use, but also allows us to modify the surface of a material in a way that is reversible."
Chemically modified surfaces are used in a variety of consumer items ranging from Saran Wrap to Post-it Notes. But traditional methods of modification rely on a limited range of chemicals that react after they are deposited on the surface, Cooks says.
His approach is done with a completely different chemical species, molecular ions, which can be selected on the basis of mass, allowing for a higher degree of chemical selection. Ions are electrically charged atoms or molecules, formed when an atom gains or loses an electron. Ions play a role in many everyday chemical reactions, such as those in electrical batteries.
To study ions, scientists use instruments called mass spectrometers, which generate the charged particles by turning neutral atoms or molecules into a gas, or vaporizing them, and passing them through a beam of high-speed electrons. Electrical and magnetic fields are used to control and measure the paths of the ions as they accelerate in the vacuum in the machine.
Though mass spectrometers make it possible to study ions, the particles are only stable while in flight. When they hit a surface, crash landings take a toll on the tiny structures, making it impossible to preserve them or deposit them onto a surface without destroying them, Cooks says. "Even the most successful instruments can trap ions for only a few seconds," he says.
Cooks and his students Scott Miller, now a researcher at Princeton University, and Hai Luo developed a way to "parachute" ions onto a surface and bring them in for a soft landing.
"Our study shows for the first time that polyatomic ions can be transferred from the gas phase to a surface and held there for periods of days," Cooks says.
They deposited ions onto gold surfaces covered with a single layer of organic molecules, which stood vertically like flagpoles from the surface and were strongly bonded to the gold. These molecules, called fluorocarbons, are inert compounds that resemble the chemicals that make up Teflon.
"These features of fluorocarbons may be why the ions can land on the surface without reacting with it," Cooks says.
Like tiny parachutes, bulky groups of trimethylsilicon molecules were attached to the end of each ion to slow its landing and ensure that it did not hit the gold surface. Instead, the ions wedged between the fluorocarbon structures. Once the ions landed, the fluorocarbons serve as shields to keep the chemically reactive particles from interacting with the gold surface or with molecules in the air.
The findings represent the first successful attempt to "soft-land" molecules onto a surface, and hold promise for new ways of modifying the outermost layer of surfaces, Cooks says.
"This experiment allows virtually any chemical species to be generated in a mass spectrometer as an ion, and injected into a surface," he says.
Using this method to deposit ions on a surface also will allow scientists to expand studies of isolated ions, Cooks says. "Because the fluorocarbon molecules keep the ions from reacting with other atoms and molecules, scientists can for the first time perform studies of isolated ions outside a mass spectrometer."
The method also allows scientists to trap undamaged ions on a surface for days at a time, then release them back into the mass spectrometer for further study.
"Our knowledge of ions can be greatly increased by the opportunity to study them at relative leisure while they are trapped and immobile in a surface," he says. "Such studies could boost progress in the many areas to which mass spectrometry is applied, from testing for drugs to determining the age of archaeological samples and monitoring environmental pollutants in the air and water."
The Purdue group collaborated with Steven J. Pachuta, a research scientist at the 3M Co. in St. Paul, Minn. The surfaces prepared with ions were first analyzed at Purdue and then sent to 3M for an independent analysis with a higher resolution instrument.
The study was funded by the National Science Foundation.
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