Scribbling On Ionic Liquids

Capitalizing on a chance discovery, chemists at the University of Nottingham, in England, found that they could write and erase patterns on the surface of a frozen ionic liquid (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700144).

Peter Licence and coworkers Frank J. M. Rutten and Haregewine Tadesse believe the method could one day be applied to high-density data storage, photolithography, and a variety of other materials applications.

The chemists were studying thin layers of solid 1-ethyl-3-methylimidazolium ethylsulfate at its melting point (???85 ^oC) by time-of-flight secondary-ion mass spectrometry as part of their ultra-high-vacuum research on ionic liquids. They figured out that the gallium ion beam source of the spectrometer could be used to etch patterns into the surface of frozen samples.

When they solidify, ionic liquids lose their conductivity and become insulators, the researchers note. But as the ion beam passes over and interacts with the solid sample surface, lines of positive charges are created. These written patterns can be erased by warming the sample until it liquefies or by flooding the etched surface with low-energy electrons to neutralize the surface charges, Licence explains.

The micrometer resolution of the patterns is "only just a start," Licence notes, and is merely a circumstance of the instrument his group has been using. In planned experiments using a focused ion beam, pattern resolution down to 10 nm should be possible, he says. And the low temperature of the current experiments should not be a detriment, he adds, because an ionic liquid with a melting point at or above room temperature could be used. In that case, the surface could be patterned at room temperature and erased by slight warming or by washing with low-energy electrons.

The ionic liquid write-erase phenomenon "is fascinating," says Kenneth R. Seddon, an ionic liquids expert at Queen's University, Belfast, in Northern Ireland. Ionic liquids function as catalysts, solvents, and materials for an increasing number of applications that are beginning to be commercialized, Seddon notes. But this work is the first application that takes advantage of the surface chemistry of an ionic liquid, he says.

The Nottingham research is "a great example of the untapped potential for ionic liquids as tunable multipurpose materials," adds Robin D. Rogers, an ionic liquids specialist at the University of Alabama, Tuscaloosa. Rogers believes there has been "too much emphasis" on the solvent properties of ionic liquids and "definitely not enough emphasis" on materials properties. "This paper is a nice example of where an ionic liquid, even unoptimized for a given technology, shows much promise," he says.