World's Smallest Pen May Help Draw Tiny Circuits

The finding, reported in the Jan. 29 issue of the journal Science, shows that an atomic force microscope, or AFM, already a commonplace lab tool, can be used to transfer molecules with extremely high precision onto substrate materials in a way that could be useful in the manufacture of nanoelectronic circuitry -- components 1,000 times smaller than microcircuits.

"This should open up many ways to explore the nano-world of electronics based on molecules," said Chad A. Mirkin, Charles E. and Emma H. Morrison Professor of Chemistry, who directed the study. "It's engineering, but when you get down to the Œnano' scale, it's really chemistry."

An AFM uses an extremely fine stylus of silicon nitride to trace the contour of a surface, much as a phonograph needle traces the bumps and grooves on a record. Feedback circuitry in the tip allows it to follow the molecular topography and render a three-dimensional image of the surface in atomic detail. AFMs have also been used to etch tiny grooves in the surface of a material.

But an annoying problem in atomic force microscopy is the tendency of the tip to attract moisture from the air and form a tiny meniscus of water at the point of contact. Mirkin's team realized that this water was always moving, either from tip to surface or the reverse. They reasoned that they might be able to use this flow to float other molecules onto the surface like a nanometer-scale dip-pen.

"The dip-pen is a 4,000-year-old technology," Mirkin said. "This is a little different, because our Œink' is not just flowing from the tip right onto the surface, it's going through the water. The water forms a nanocapillary, which lets us write a very narrow line."

The researchers tried several "inks," but focused on an oily, sulfur-containing compound called octadecanethiol, or ODT. The sulfur allows the ODT molecule to adsorb to a "paper" of granular gold particles fused to silica.

The ink and paper used for these experiments were not ideal for drawing the thinnest possible lines with the AFM, but were chosen to allow easy chemical identification and measurement of the ink lines. Nevertheless, the researchers were able to draw, in a wide variety of patterns, lines as thin as 30 nanometers, or about one millionth of an inch. Movement of the pen is controlled by computer.

Mirkin expects his "dip-pen nanolithography," or DPN, to take its place alongside other lithographic techniques that are vital for nanotechnology and molecular electronics.

"This technique will be even more technologically useful once we convert our dip-pen to a fountain pen, and once we can draw multiple lines in parallel rather than serial fashion," Mirkin said. Northwestern University filed a provisional patent covering the technique last month.

Mirkin expects DPN to complement, rather than displace, microcontact printing and other lithographic methods used in micro- and nanofabrication.

"The DPN technique would be immediately useful for the detailed functionalization of a nanochip," Mirkin said. "Suppose I have a computer chip that will form the basis for a chemical sensor, and I need to put onto its nanocomponents some chemical that will tell me whether or not some chemical agents are around. I could use this type of technique to do that. I can go in and just paint those components with different types of molecules."

The research was funded by the U.S. Air Force Office of Scientific Research.

In addition to Mirkin, other authors on the Science article are graduate students Jin Zhu and Feng Xu, and postdoctoral researcher Seunghun Hong, all of Northwestern; and postdoctoral researcher Richard D. Piner, who is now at Washington University.