For a decade, materials scientists have dreamed of using cylinders of carbon with walls just one atom thick as the building blocks for a new generation of sensors, transistors, and other tiny devices. Before that happens, however, researchers must find better ways to grow and align these carbon nanotubes.
Jie Liu and his colleagues at Duke University in Durham, N.C., now report growing the longest individual carbon nanotubes ever and aligning them in a two-dimensional grid.
Other researchers have used strong electric fields to orient nanotubes, notes Liu. But the new technique may prove more useful, he suggests, because it doesn’t require strong external forces and can align tubes in multiple directions. Since the new process creates extraordinarily long nanotubes–up to 4 millimeters in length–researchers may also create many different nanoscale devices along a single tube.
“Simultaneous growth and alignment of ultralong, single-wall nanotubes is an important development,” comments Ray Baughman of the University of Texas at Dallas.
Liu’s work “joins an avalanche of recent advances on the assembly of nanotube architectures,” he adds.
The Duke researchers used a simple variation on a common nanotube-production method called chemical-vapor deposition. In the standard process, carbon nanotubes grow in a stream of carbon monoxide and hydrogen gas blown across catalysts on silicon wafers. In a furnace slowly warming from room temperature to 900C, the gases typically produce a tangled mass of nanotubes, each one no longer than about 20 micrometers.
Liu and his colleagues changed this procedure by preheating the furnace to 900C before placing the catalyst-bearing wafers inside. That way, the catalysts warmed to 900C within seconds, instead of the typical 10 minutes. The tweak was intended to reduce the aggregation of catalysts on the silicon wafers, but the researchers achieved something even better: 2-to-4-mm-long tubes anchored in the catalysts and aligned parallel to one another in the direction of the blowing gases.
After the wafers and their newly formed tubes cooled, the team rotated them 90 degrees and returned them to the oven. A second set of tubes grew perpendicularly to the first set, says Liu. He and his colleagues describe the technique in an upcoming Journal of the American Chemical Society.
Already, the researchers have used their new method to create experimental carbon nanotube-based devices including transistors and sensors for detecting chemical and biological agents.
“With all the nanotube work, one of the important problems we still have is how to make them in a controlled fashion,” says Otto Zhou of the University of North Carolina at Chapel Hill. “I would say that this is an important step toward the controlled fabrication of carbon-nanotube structures.”
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