Electrons clean wire machine

By Chhavi Sachdev, Technology Research News

Order and uniformity are the keys to making useful microscopic wires. From electron beam etching to vapor deposition, researchers have tried many methods in a quest to make perfectly formed microscopic wires in very large batches.

A group of researchers at Brown University has made more uniform, ordered arrays of nanowires by altering the mix of materials they start with, and keeping microscopic molds from clogging by finely controlling the electrical current that coaxes metal ions in solution to grow into wires.

The Brown technique allows the researchers to exactly control the deposition rate, or growth, said Jimmy Xu, an engineering and physics professor at Brown University. "By timing the deposition activity, we can grow ordered arrays of nanowires all of the same length. Since many physical properties of nanoscale systems are length dependent, it is valuable to be able to control these properties precisely," he said.

The resulting nanowires are "very long and skinny," said Xu. They can be as long as 60 microns, but only .03 to .08 microns in diameter. A micron is a thousandth of a millimeter.

The method allowed the researchers to cram up to a trillion nanowires on a one-inch square array, spaced 80 to 120 nanometers apart, said Xu.

The researchers made nickel and bismuth nanowires by depositing liquid containing the metal into the very tiny pores of an aluminum foil template. When current passed through the device, wires formed in the holes. The researchers extracted the nanowire array from the template by selective wet etching, which dissolves the aluminium template but leaves the nanowires intact, Xu said.

Common electrochemical deposition uses direct current (DC) to coax nanowires to form from water that contains metal molecules. Wires made using this technique manifest the skyscraper effect: some wires tower over the others, said Xu.

Some wires can stop growing after a certain point because many pores of the substrate, or template, become blocked due to the chemical reactions in the liquid. "The DC deposition methods operate well when electroplating films on large area electrodes, but they fail when used for electrodeposition into narrow pores. In many cases, the deposition material accumulates on the walls of the pores and stops the wire growth at the bottom of the pore," Xu said.

Xu's team solved the skyscraper problem by experimenting with both direct and alternating current (AC), and using an organic electrolyte rather than a water-based solution. Electrons in direct current flow steadily in a single direction, while in alternate current, the direction of electrons' flow switches back and forth.

Alternating the two types of current purged the pores of stray ions and minimized deposition on the pore walls, resulting in a uniform array of wires, Xu said.

"During the forward portion of the AC cycle, ions move into the pore and accrete at the bottom of the pore, discharging, becoming neutral, and forming the wire. During the negative portion of the cycle, the remaining ions which have not reached the bottom of the pore are flushed out of the pore to prevent them from [collecting] on the walls and stopping wire growth," he said.

The Brown method could be used to make nanowires from metallic, semiconductor, and magnetic materials as well as superconductors, said Xu. The method allows nanowire arrays to grow on curved surfaces, which is hard to do with conventional semiconductor processing techniques, he said. This could lead to applications such as "coating an aircraft surface with these nano arrays to form distributed sensors and electromagnetic shields," said Xu.

Because the method is relatively simple, it is inexpensive. It can produce objects with dimensions of 50 nanometers or less, which is beyond the reach of standard optical lithography methods that are used for making electrical components like semiconductors, Xu said.

These lithographic methods cut or etch away unwanted materials from the top, somewhat "like cutting a big flat piece of wood into billions of tooth picks standing straight up," said Xu. "This is doable... but it gets harder as the etching gets deeper, and becomes impossible [when] the wires are to be very thin, straight up, and uniform in diameter because any etching will inevitably eat away materials sideways while going downward."

While the use of templates for nanowire fabrication is not new, the research has some novel aspects, said Mark Tuominen, an associate professor of physics at the University of Massachusetts at Amherst. The paper presents "some interesting explorations of the effects of tuning some of the electrochemical deposition parameters," he said.

To make smaller and cheaper storage media, sensors, and finer displays, nanopores must become smaller and the process of growing the wires must be better controlled. The Brown process "makes new important headway" in precise electrodeposition, Tuominen said.

"It's clear from the field of work on template-based nanowire array fabrication that electrodeposition is an important enabling tool of nanotechnology," he added.

The researchers next plan to study the material properties of the nanowires and the collective magnetic, electronic, and optical behavior of the tiny arrays, Xu said. "Collective behaviors of certain biologic systems and physical systems are known to manifest extraordinary properties, such as ones capable of processing information, that are not likely to be attainable through wiring nanoelements together say, into a digital circuitry," Xu said.

The approach could produce magnetic data storage and biomedical sensing and sorting devices in about five years, Xu said. "The possibility of information processing applications using collective electronic or magnetic behavior in these nanoarrays is particularly intriguing to us," he said.

Xu's research colleagues were Aijun (Nick) Yin, Jing Li, Tola Jian, and Jack Bennett at Brown University. The project was funded by the National Science Foundation (NSF), the Canadian Institute for Advanced Research (CIAR), Motorola, Nortel, and the Defense Advanced Research Projects Agency (DARPA). The researchers published their findings in the August 13 issue of the journal Applied Physics Letters.

Timeline:  <5 years
Funding:   Corporate; Government
TRN Categories:  Nanotechnology; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Fabrication of highly ordered metallic nanowire arrays by electrodeposition," Applied Physics Letters, August 13, 2001.


October 31, 2001

Page One

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Electrons clean wire machine


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