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Analytical Chemistry

Nanoscale Copies

Parallel method makes 55,000 copies of minuscule pattern in minutes

by Mitch Jacoby
October 2, 2006 | A version of this story appeared in Volume 84, Issue 40

Gearing Up
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Credit: Mitch Jacoby/C&EN
Northwestern's Wang (left) and Mirkin prepare a dip-pen patterning experiment.
Credit: Mitch Jacoby/C&EN
Northwestern's Wang (left) and Mirkin prepare a dip-pen patterning experiment.

Scanning probes are often characterized as powerful tools for building nanostructures but limited to making those structures one at a time. The perceived limitation may soon be a thing of the past.

Researchers at Northwestern University have demonstrated that 55,000 copies of a complex nanoscale pattern can be drawn simultaneously in just minutes (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200603142). The square-centimeter-sized array of patterns is generated in a dot matrix fashion using a dip-pen nanolithography (DPN) method that transfers a molecular ink to a solid surface via microscopic cantilever tips.

Over the years, countless experts have observed that although scanning tunneling microscopes (STM) and atomic force microscopes (AFM) are uniquely able to probe and manipulate matter on the atomic scale, the one-at-a-time production rate precludes their use in large-scale fabrication of nanostructures.

"Since the inception of STM and AFM, researchers have talked about using such tools to develop new types of molecule-based nanofabrication processes," says Chad A. Mirkin, a professor of chemistry and materials science who led the study. "But development of such techniques has hit a brick wall: throughput."

Printing Money
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80-nm-wide features in the face of Thomas Jefferson that appears on a 2005 U.S. nickel are drawn by a new dip-pen method that can make 55,000 copies of the pattern in minutes.
80-nm-wide features in the face of Thomas Jefferson that appears on a 2005 U.S. nickel are drawn by a new dip-pen method that can make 55,000 copies of the pattern in minutes.

To overcome some of the commonly encountered problems, Mirkin's research group designed a tiny chip that features 55,000 custom-made cantilevers and developed a simple procedure that prepares the array for use in a commercial DPN instrument.

A combination of unique features makes the array a simple and robust patterning tool, Mirkin explains. To begin with, the cantilevers are slightly curved, and the tips are unusually tall. These features provide both clearance between the cantilever and the surface that will be patterned and increased tolerance of variations in surface morphology. In addition, the tips are aligned via a gravity-driven procedure, making the new wire-free design much simpler than wired multitip tools designed by other research groups, Mirkin says.

Demonstrating the new tool's abilities, Mirkin, postdoc Yuhuang Wang, and their coworkers formed 55,000 copies of a 2005 nickel coin from a pattern of 80-nm dots of 1-octadecanethiol (about 9,000 dots per nickel) in less than 30 minutes.

The method provides "a new way of generating nanostructures on a macroscopic scale," says Harald Fuchs, a physics professor at the University of Münster, in Germany.

Noting that the technique has been used with phospholipids and other types of "inks," Fuchs points to the possibilities of combinatorial methods and remarks that there is "fantastic potential for further developments in biochemical and physical-chemical applications."

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