a telephone pole is easy if you have the right tools,
say a power saw and some large chisels. And with
some much tinier tools you could even carve a design
into a paper clip if you wanted to. But shrink your
sights down to the nanoscale, to a nanowire that
is 1,000 times smaller than the diameter of a paper
clip, and you find there are no physical tools to
do the job properly.
So a team of Northwestern University scientists
turned to chemistry and developed a new method that can routinely and cheaply
produce nanowires with gaps as small as five nanometers wide -- a feat that is
unattainable using conventional lithographic techniques. The results will be
published in the July 1 issue of the journal Science.
Carved gaps are essential to a nanowire's function, and controlling those gaps
would allow scientists and engineers to design with precision devices ranging
from tiny integrated circuits to gene chips and protein arrays for diagnostics
and drug discovery.
"With miniaturization happening across so many fields, our existing tools --
our chisels of a sort -- can't control the shapes and spacing of these small
structures," said Chad
A. Mirkin , director of Northwestern's Institute
for Nanotechnology , who led the research team. "Our method allows us to
selectively introduce gaps into the wires. These gaps can be filled with molecules,
making them components for building small electronic and photonic devices or
chemical and biological sensors."
The development of sophisticated nanoelectronics, said Mirkin, depends on the
ability to fabricate and functionalize electrode gaps less than 20 nanometers
wide for precise electrical measurements on nanomaterials and even individual
molecules. While conventional techniques can't make gaps much smaller than 20
nanometers wide, Mirkin's method, called on-wire lithography, or OWL, has been
able to produce gaps as small as 2.5 nanometers wide.
Mirkin and his team made the notched structures by first depositing into a porous
template segmented nanowires made of two materials, one that is resistant to
wet-chemical etching (gold) and one that is susceptible (nickel). The template
is then dissolved, releasing the nanowires. Next, the wires are dispersed on
a flat substrate, and a thin layer of glass is deposited onto their exposed faces.
They are then suspended in solution, and a selective wet-chemical etching removes
the nickel, leaving gold nanowires with well-defined gaps where the nickel used
to be. (The glass is used as a bridging material, to hold the nanowire together.)
Using the OWL method, the researchers prepared nanowires with designed gaps of
5, 25, 40, 50, 70, 100, 140 and 210 nanometers wide. (A nanometer is one billionth
of a meter or roughly the length of three atoms side by side. A DNA molecule
is 2.5 nanometers wide.) In recent days, they have refined the technique to be
able to make gaps as small as 2.5 nanometers wide.
"With dip-pen nanolithography, we can then drop into these gaps many different
molecules, depending on what function we want the structure to have," said Mirkin,
also George B. Rathmann Professor of Chemistry. "This really opens up the possibility
of using molecules as components for a variety of nanoscale devices."
In addition to Mirkin, other authors on the Science paper are Lidong Qin (lead
author), Sungho Park and Ling Huang of Northwestern University.
About Northwestern University:
Northwestern University is a private institution founded in 1851 to serve the
Northwest Territory, an area that now includes the states of Ohio, Indiana,
Illinois, Michigan, Wisconsin, and part of Minnesota. In 1853 the founders
purchased a 379-acre tract of land on the shore of Lake Michigan 12 miles
north of Chicago. They established a campus and developed the land near it,
naming the surrounding town Evanston in honor of one of the University's
founders, John Evans. After completing its first building in 1855, Northwestern
began classes that fall with two faculty members and 10 students.
For more information, please visit www.northwestern.edu