Northwestern chemists plot the next step in nanotechnology

October 14, 1999

EVANSTON, Ill. --- In a paper to be published in the Oct. 15 issue of the journal Science, researchers at Northwestern University demonstrate a new technology that may be used to miniaturize electronic circuits, put thousands of different medical sensors on an area much tinier than the head of a pin and develop an understanding of the intrinsic behavior of ultrasmall structures -- ones comprised of a small collection of molecules patterned on a solid substrate.

By miniaturizing existing writing and printing techniques, such as the 4,000-year-old quill pen, a research team led by Chad Mirkin, Charles E. and Emma H. Morrison Professor of Chemistry and director of Northwestern's Center for Nanotechnology, has paved the way for such possibilities.

In their paper, the researchers detail how they have transformed their world's smallest pen (Science, Jan. 29, 1999) into the world's smallest plotter, a device capable of drawing multiple lines of molecules -- each line only 15 nanometers or 30 molecules wide -- with such precision that only five nanometers, or about 200 billionths of an inch, separate each line. By contrast, a human hair is about 10,000 nanometers wide.

"Our dip-pen nanolithography, or what we call the world's smallest pen, allowed us to draw tiny lines with a single 'ink' or type of molecule," Mirkin said. "Now, with the nano-plotter, we can place multiple 'inks,' or different kinds of molecules, side by side with such accuracy that we can retain the chemical purity of each line. Solving the problem of nanostructure registration has taken us to a whole new level. In a sense, we have transitioned dip-pen nanolithography from a single ink process to a four-color printing type of process -- on a nanometer scale."

It is the nano-plotter's accuracy of registration when building nanostructures of different organic molecules that could dramatically impact molecule-based electronics, molecular diagnostics and catalysis, in addition to leading to new applications not yet imagined in nanotechnology.

Dip-pen nanolithography (DPN), which is described in the Jan. 29, 1999, issue of Science and is the basis for Mirkin's nano-plotter, turns a common laboratory instrument called an atomic force microscope (AFM ) into a writing instrument. First, an oily "ink" of octadecanethiol (ODT) is applied uniformly to the AFM's tip. When the tip is brought into contact with a thin sheet of gold "paper," the ODT molecules are transferred to the gold's surface via a tiny water droplet that forms naturally at the tip. Using this technique, the researchers can draw fine lines one molecule high and a few dozen molecules wide.

The nano-plotter, the subject of the Oct. 15 paper, multiplies this technique, laying down a series of molecular lines with precision never seen before.

The researchers first demonstrated DPN's registration prowess by putting down dots of 16-mercaptohexadecanoic acid (MHA), each 15 nanometers in diameter, on a surface of gold using an inked AFM tip. (MHA was selected because of its reactive properties with gold.) The same tip then images or "reads?" the pattern of dots and sends the dots' coordinates to the system's computer. Using this information, the computer calculates coordinates for a new pattern of dots, which it ships back to the AFM's tip. The inked tip then sets down the new pattern of MHA dots with such accuracy that only five nanometers of space stand between the second set of dots and the originals.

"While the registration using this technique was exceptional, there was one problem," Mirkin said. "Because we imaged the dots with the same inked AFM tip with which we drew them, there was a chance that, during the imaging process, we had scattered a few molecules where they shouldn't be," Mirkin said. "That could be unacceptable for electronic purposes and many other applications as it compromises the chemical integrity of the nanostructures, especially where multiple inks are used."

Mirkin and his team developed a solution to this problem that has not been matched by any currently available nanofabrication method. It required a straightforward but significant modification of the first experiment. Using DPN, they first drew cross-hair larger scale alignment marks with an MHA ink on either side of the area of gold to be patterned and imaged. Next, three parallel lines using the same MHA ink were set down at a precalculated position with respect to the alignment marks. The AFM tip was then replaced with a tip coated with ODT. The tip located the alignment marks and then, using precalculated coordinates based on the marks, drew three 50 nanometer ODT lines, each one exactly 70 nanometers to the left of an MHA line from the initial pattern. At that point, the entire area was imaged with an ink-free tip.

"Because the patterned lines are imaged only at the end of the process, cross-contamination of ink or molecules is prevented," Mirkin explained. "And that is one of the keys to this technology." What's more, Mirkin's multiple ink plotter can be automated, it uses a relatively inexpensive tool (an atomic force microscope) that is common in the laboratories of companies and universities, and it works under normal atmospheric conditions.

"This technology should become a real workhorse for the nanotechnologist," Mirkin said. "It will soon be possible to pattern one master plate with thousands of different organic nanostructures, each structure designed to react with a certain disease agent, for example. That's what is exciting about this -- no other method exists to do this on such a small scale."

While the microfabrication of electronic circuits and other products currently use solid-state or inorganic materials, innovations such as the nano-plotter will direct future technologies toward the use of organic and even biological materials. "Nature gives us a limited number of materials," Mirkin noted, "but, in the lab, there are an infinite number of organic molecules a chemist can make. And, by designing molecules carefully, one can use them as inks in DPN to custom design nanostructures for addressing critical issues in nanoscale science and technology. For example, the conductivity, thermal stability, and chemical reactivity of a circuit drawn via DPN could all be controlled through choice of inks used to generate such structures."

In the case of biomolecules like DNA, it will be possible to generate ultrahigh density arrays that could be quite useful in the genomics and medical diagnostics industries. Such arrays are currently generated via techniques with much lower resolution than DPN.

Nobel Prize-winning physicist Richard P. Feynman once said in a famous 1959 speech about manipulating things on a small scale, "I am not afraid to consider the final question as to whether, ultimately -- in the great future -- we can arrange the atoms the way we want; the very atoms, all the way down!" Forty years later, that's exactly what Mirkin and his colleagues are doing.
-end-
The research was funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation.

In addition to Mirkin, other authors on the Science article are graduate student Jin Zhu and postdoctoral researcher Seunghun Hong, both of Northwestern.

Northwestern University

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