Scientists Create "Molecular Rulers" Enabling Precise Construction Of Nanoscale Structures

The scientists' results, published in the 9 February 2001 edition of Science, describe the use of organic molecules as "molecular rulers" that permit the fabrication of useful wires dozens of times smaller than the period at the end of this sentence. The scientists measure their results in nanometers, equal to one billionth of a meter, and micrometers, equal to one millionth of a meter. They have proven they can make extremely thin wires from 15 to 70 nanometers wide and a few micrometers long that are spaced 10 to 40 nanometers apart.

"We have known how to make smaller and smaller structures by using techniques that have been developed for the fabrication of computer chips, and we also have known how to make molecules bigger and bigger," said Paul Weiss, associate professor of chemistry at Penn State and coauthor of the paper describing the group's results. "But that intermediate region between the two approaches has been essentially inaccessible, and our technique of using 'molecular rulers' represents a step toward bridging that gap."

The "molecular ruler" construction process requires some existing nanoscale structures to "grow" in order to produce the even smaller structures. Specifically, the scientists started with two parallel gold nanostructures on a silicate substrate. Those structures were formed by electron-beam lithography, one of several widely used nanoscale construction techniques limited to making structures tens of nanometers or larger. Layers of organic molecules then were applied atop the initial structures to make them bigger and wider, at the same time reducing the gap between the structures.

Imagine two cookies rising beside each other while baking, with the space between the cookies getting smaller as they cook, and you get a sense of how each of the initial structures grows by the addition of organic molecules and how the space between the structures gets narrower.

Because the scientists knew the size and spacing of the initial structures and the thickness of the layers of films created by the molecules atop the structures, they could calculate the size of the narrowing space between the structures. As a result, the organic molecules, which selectively bind to each other and to the substrate materials, provide "molecular rulers" that precisely determine the size of the resulting space between the initial structures. Scientists use the resulting space for forming even smaller wires by filling the space with gold.

For their research, the scientists used silicate as the substrate, gold for the prefabricated initial structures, and mercaptoalkanoic acid as the organic molecule. Those organic molecules, also are referred to as "resists" because they resist attack and protect the material underneath them in various lithographic processing steps, were used by Weiss and his team to improve the construction process for nanoscale structures.

"We had a lot of different ways we were trying to make closely spaced, precise structures—we had four people in the group trying six different ways—and we found this one has the precision we need built into it," Weiss said. "We know how to make the ends of organic molecules so that they bind selectively, both to each other and to the substrate. We also know they form films when they interact, and from that we can determine a precise thickness of the film. That's what makes the whole thing work. If they did not do that, the process would be just as crude as the standard polymer resists."

Along with precision and increased miniaturization, the construction process outlined by Weiss and Amat Hatzor, a post-doctoral fellow at Penn State, includes a method to selectively remove the molecular resists after the wires are cast, thereby improving upon the efficiency and flexibility of existing methods. Whereas other fabrication methods require scientists to build structures individually, the "molecular ruler" method allows an entire "cookie sheet" of structures or wires to be completed at once.

"It is a single fabrication process," Weiss said. "You do not have to draw every single line one at a time. You simply do the overall design and then in one set of steps you can complete the whole surface. We can make a number of shapes and sizes that we cannot make by other means."

This research was funded by the Army Research Office, the Defense Advanced Research Projects Agency, the National Science Foundation, and the Office of Naval Research. It was conducted at Penn State's National Nanofabrication Users' Network Facility.