For the parade of ever-shrinking optoelectric devices to push on, engineers must be able to fashion ultraminiature wires with exacting precision. A new technique described today in Science should help. Paul Weiss of Pennsylvania State University and his colleagues have developed a "molecular ruler" construction process for making many thin wiresmeasuring from 15 to 70 nanometers wide and spaced only 10 to 40 nanometers apartall at once. "We had a lot of different ways we were trying to make closely spaced, precise structures," Weiss says. "We had four people in the group trying six different ways."
Fortunately one worked like a charm. The scientists started the process by creating parallel gold nanostructures on a silicate substrate using standard electron-beam lithography. Then they applied layers of an organic molecule called mercaptoalkonoic acid on top. This film coating systematically made the original structures widerand the gaps between them smaller. And because the researchers knew the size and spacing of the initial structures and the width of the organic layers, they could calculate the sizes of the narrowing gaps.
Using this "molecular ruler" like a mold, they could then fill the gaps with gold to form nanoscale wires with a known diameter. "It is a single fabrication process," Weiss remarks. "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."