clean wire machine
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,
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
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,"
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
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.
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