Words Writ--Very--Small by a Nanopen

In a 1959 speech, physicist Richard Feynman wondered whether crafty researchers would one day find a way to write an encyclopedia on the head of a pin. Now, chemists at Northwestern University in Evanston, Illinois, have memorialized a paragraph of Feynman's speech in a most appropriate way, by writing it in an area just one-thousandth the size of a pinhead, using multiple "inks" that line up with one another to produce features as small as 5 nanometers. The work, reported in this week's Science (15 October 1999, p. 523), could pave the way for new nanotechnology applications, ranging from testing novel catalysts to creating nanoscale electronic devices.

Previously, researchers have used either electron beam lithography or, more recently, the tiny styluslike arm of an atomic force microscope (AFM) to create nanometer-sized features on a surface. But these techniques can damage the surface or leave behind molecular contaminants, making it hard to add new, pristine layers that line up in perfect registry with the ones below, a typical requirement for making electronic devices.

To get around these problems, a group led by Chad Mirkin came up with a technique called dip-pen nanolithography, which uses a water layer to transport organic ink on the AFM tip to a surface (Science, 29 January, p. 661). With just one ink, they could write simple structures, including letters. But making an electronically active nanostructure requires positioning different organic conductors, insulators, and semiconductors in different regions. The Mirkin team hasn't yet accomplished that, but it has taken a step in that direction by figuring out how to align a second set of ink marks with the first.

They began by coating one AFM tip with an ink consisting of 16-mercaptohexadecanoic acid (MHA), an organic molecule capped with a water-attracting carboxylic acid group. They then used this ink to write a set of parallel lines 70 nanometers apart. Because they feared that their second AFM pass would damage these lines if they used it to locate them directly, they also put in cross-shaped alignment marks, which sit 2 micrometers on either side of the lines.

Next, the researchers changed their AFM tip to one dipped in a second ink called 1-octadecanethiol (ODT), which is capped with a water-repelling methyl group, and scanned this tip across the surface to find the alignment marks. The computer then positioned the tip near the original set of parallel lines and wrote another set alongside the first. Finally, to view the patterns they created, the team switched to an uncoated AFM tip, which they used to scan the entire surface and create an image of the pattern.

The method "seems like a real enabler" of nanotechnology, says Clifford Kubiak, a chemist and nanotechnology expert at the University of California, San Diego. While nanowriting could generate some interest among spies, Kubiak believes its real value will be in making numerous nanoscale electronic devices in a highly reproducible fashion.