Printing technique makes nanotransistors
By Davide Castelvecchi
From Science News, September 22nd, 2007; Vol.172 #12 (p. 182)
With a combination of existing methods, some new tricks, and a drop of water, researchers have found a way of stamping carbon-nanotube circuits onto virtually any surface. The technique might lead to bright, flexible displays and to more-powerful wireless-communications chips.
Carbon nanotubes are chicken wire–style networks of carbon atoms rolled up to form hollow rods that are only nanometers across. The alignment of the chicken wire hexagons relative to the axis of a nanotube determines its electronic properties. Correspondingly, when nanotubes form, some turn out to be semiconductors, while others emerge as better conductors than any metal.
Nanotubes are also extremely strong and can withstand temperatures of many hundreds of degrees. These properties make them ideal as molecular-scale components of electronic circuits that could be faster than ordinary, silicon-based chips. But so far, nanotubes have been tricky to assemble into circuits in a way that might be reproduced on industrial scales.
Yael Hanein and her team at Tel Aviv University have now taken steps toward being able to mass-produce carbon-nanotube circuitry. The researchers first made arrays of silicon pillars spaced tens of microns apart. Using a standard technique, they then grew a grid of carbon nanotubes strung from one pillar to another.
The researchers used optical measurements to examine the nanotubes’ structures and identify which were conductors and which were semiconductors. Next, they pressed their grid onto a glass surface. After months of trial and error, the team discovered that wetting the glass helped bind a network of nanotubes to the surface, and that the materials would stick together even after the water had evaporated.
The novelty of the method lies in combining optical mapping with the researchers’ newfound water-printing trick, Hanein says. “When the tubes are suspended, you can easily map them.” Because the locations of conducting and semiconducting tubes are known and the network is accurately transferred onto a surface, it is relatively easy to create a desired circuit by adding metal connections at certain spots or by etching away excess nanotubes, Hanein says. The results appear in the September Nano Letters.
Hanein’s team has demonstrated a simple arrangement of nanotubes that acts like a transistor and shown that it or similar devices could find applications in high-speed electronics for wireless communications.
Hanein adds that the method can print circuits on a variety of surfaces. “You could even think of flexible materials with [light-emitting diodes] on them,” she says. Such sheets could then act as energy-efficient lighting fixtures or even as color displays.
Cees Dekker of Delft University of Technology in the Netherlands calls the new method “very original” and “exciting,” in that it can both determine the nanotubes’ conductivity and arrange them in an orderly fashion.
According to Boris Yakobson of Rice University in Houston, the results suggest that it might be possible to use a single template of pillars to produce multiple copies of a circuit, “à la Gutenberg.” However, he adds, the new technique isn’t yet fully reproducible, since no one has figured out how to control which kind of conductor a growing nanotube becomes. “It is kind of like printing a predefined pattern with random inks.”