Nanowires Cross the Line

Before nanostructures can move from the lab to the assembly line, researchers must find efficient ways to organize them into more complex and useful devices. Now, a team at Harvard has found a “fast approach” for doing just that with nanowires.

In the January 26 issue of Science, researchers working in the lab of Harvard professor Charles Lieber detailed a new way to arrange nanowires-conductive structures only a few hundred atoms wide and up to 30 microns long. Researchers hope one day to use such structures in nanocomputers thousands of times smaller and more powerful than silicon-based processors (see “Molecular Computing” in the May/June 2000 TR).

“If you survey what’s going on in nanoelectronics, a lot of interesting small devices have been made recently,” Lieber told technologyreview.com. “One of the biggest challenges as one looks to the future is: How does one organize dense device arrays out of these single devices? What this paper illustrates is a fast approach to organize wire structures.”

Go with the Flow

Using a method called fluidic alignment, the researchers suspended nanowires in an ethanol solution, which they then flowed between a silicon surface and a temporary mold lined with channels. Like grass pointing downstream in a riverbed, the nanowires lined up on the silicon along the direction of the liquid’s flow, creating a parallel pattern. The faster the solution flowed, the more precisely the nanowires lined up.

To control the spacing of the nanowires, the Harvard team used surface patterning, a technology employed in making silicon chips, to create regularly spaced chemical patterns on the silicon. As the nanowires flowed along, the chemical patterns attracted them, leading to more precise spacing. “We believe that our approach… offers substantial advantages over current efforts, which have used random deposition, direct manipulation of individual [nanowires and nanotubes], and electric fields,” the researchers wrote.

While other researchers have worked separately with fluidic alignment and surface patterning, the Harvard team is the first to create regular arrays of parallel nanowires and control the spacing between them. The team includes Yu Huang and Xiangfeng Duan, who share credit as primary investigators, as well as Lieber and Qingqiao Wei.

Array for Us

The researchers also created intersecting arrays of nanowires by applying successive layers of nanowires, each time changing the direction of the fluidic alignment.

They studied three types of nanowires: gallium phosphide, indium phosphide, and silicon. By crossing nanowires of different materials, the team demonstrated the ability to make addressable junctions, the fundamental building blocks of semiconductors.

“We believe that our approach could be used to assemble high-density and individually addressable nano-LEDs and electronically more complicated nanodevices,” the researchers wrote.

Best of all, Lieber added, researchers can use the method in an ordinary laboratory. “Using benchtop assembly, you can build devices that you would have had to build in a clean room or under specialized assembly conditions,” he said.