A droplet of liquid and a few seconds are all that researchers need to produce neatly spaced ridges of molecules that cover a huge area--at least by the standards of nanotechnology. In a feat of so-called self-assembly, a group reports that disk-shaped molecules can stack themselves by the millions into lines of up to a millimeter in length and covering several square millimeters.
The process might help ease the fabrication of sensors such as liquid crystal displays (LCDs) that register the presence of offending chemicals. "You just drop a droplet of the solution on a surface and the molecules arrange themselves," says group member Johannes Elemans, a nano researcher at Radboud University Nijmegen in the Netherlands. "It takes five seconds to make the surface, which is not really labor intensive," he says.
Elemans and his colleagues were looking for a solution of molecules that would cling together just enough to form useful patterns as the solution evaporated but would not clump in the process. They struck on a flat molecule that has three lobes projecting from a central core. The cores have chemically sticky parts that project from the plane of the molecule. Ideally the molecules would line up because the cores would stick to each other and the stacked lobes would stabilize the strand [see image below].
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"We expected to get spaghetti all over the place," Elemans says. Instead, when they allowed a droplet to dry on mica, they found that the individual strings of molecules aligned themselves into regularly spaced parallel columns. These ridged domains grew as much as three square millimeters in area, and the spacing between columns varied from 0.5 to one micron depending on the domain.
Prior experiments relied on long prefabricated polymers to make such arrays, but these domains tended to cover an area only 10 square micrometers or less, the researchers note in their report published online November 30 by Science.
"This is a pretty impressive piece of work," says materials chemist Jeffrey Moore of the University of Illinois at Urbana-Champaign. "They've nicely demonstrated that they can get nearly defect-free organization over a few microns. This isn't just some highly localized, zoomed-in organization," he says.
Elemans says that the arrays might serve to grow LCDs. The researchers found they could align liquid crystal polymers over large areas by depositing their solution onto a glass slip and then adding the polymers. Currently LCDs are made by etching grooves into a polymer sheet and pouring liquid crystals on top, Elemans explains.
According to physical chemist Mohan Srinivasarao of Georgia Tech, the new technique might work for small sensors, but it would have to be dramatically expanded for an LCD monitor.
A further-off idea, Moore says, would be to exploit the columns as wires for transmitting electricity or light, and then link the wires into circuits.
For now the procedure is a step toward the dream of plopping a liquid down and letting it form a predetermined pattern, Moore notes, which would make industrial fabrication of many devices much easier. "It's really simplification of fabrication," he says. "That's what it boils down to."
