Nano Masterpieces

23 July 2013 7:00 pm

C. Mirkin, Northwestern University

Light work. Using a digital micromirror to split beams of light and direct them through apertures in polymer pyramids, Northwestern University researchers drew a variety of molecular architectures and used those to make up the "land" in a map of the world.

Imagine trying to draw a maze on something the width of a human hair. Engineers do it all the time when they carve circuits onto semiconductors for our cell phones, tablets, and desktops. That doesn't mean it's easy: The process can require fabrication facilities costing billions of dollars. Now, modifications to an old technique could usher in a low-cost competitor. That could bring so-called nanomanufacturing out of the fabs and into the labs, making it easier for researchers to build and test prototypes of electronic circuits, DNA and protein arrays used in drug research, and other minute devices.

Today, the workhorse nano-patterning technology is photolithography, in which engineers beam light through slits in specially engineered stencils as a first step to patterning materials underneath. But photolithography requires those top-dollar facilities. A cheaper alternative, called polymer pen lithography (PPL), uses arrays of thousands to millions of tiny polymer pyramids essentially as quills to inscribe different materials onto a surface. PPL, however, has trouble creating varying patterns over large areas, as is needed for making computer chips and many other applications.

Now, researchers led by Northwestern University chemist Chad Mirkin have come up with a cheaper and better way—two, in fact. Both use arrays of polymer pyramids patterned on top of a transparent backing. In the first version, tiny heaters beneath the base of each pyramid make the polymer swell, pushing the tips out as much as 4 micrometers—far enough to touch a nearby surface and apply their ink. The Northwestern researchers demonstrated the approach, reported online this month in the Proceedings of the National Academy of Sciences, by writing a 600-square-micrometer version of the periodic table on a silicon wafer, in which the average size of each feature, such as the width of a letter or number, was 1.28 microns.

The second new technique created even smaller features—as little as 120 nanometers across—through a new take on photolithography. The researchers coat an array of transparent polymer pyramids with an opaque gold film, then etch a 100-nanometer hole into the tip of each pyramid. Using the a digital micromirror device similar to those found in media projectors, they split light from an LED into thousands of separate beams and steer each into the base of a separate pyramid. Light emerging from the tips acts as a tiny stylus to write on photosensitive material. Mirkin's team used the technique, reported online this month in Nature Communications, to make working electrical devices including inductors, capacitors, and surface acoustic wave transducers, and even drew a 9-millimeter-square map of the world.

Independent control over each tip opens the door to creating far larger and more complex nano patterns than previous versions of the technology, says Paul Weiss, a nanotechnology expert at the University of California, Los Angeles. "That is significant," he says. The work could make it easier for researchers to fabricate research tools such as complex DNA and protein arrays in which thousands of different biomolecules are dotted across surfaces in order to search for novel drugs or drug targets. But Weiss says that it remains to be seen how far the technology will spread. Previous versions of the technology are now found in labs around the world. But NanoInk, a company that made and sold such systems, recently went out of business. Mirkin says that he is now in discussions with other companies to license the new technology.


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