Molecular Rulers Enabling Precise Nanoscale Construction
Source: Space Daily
February 13, 2001
University Park - Scientists at Penn State have
discovered an effective and precise way to make ultraminiature metal
wires in very close proximity to each other.
Their work -- important because nanoscale construction
methods have been limited to structures with larger, less controlled
spacings -- is expected to be useful in the effort to further
miniaturize electronic and opto-electronic devices used for circuits,
high-density data storage, and sensors.
In addition, their work is expected to serve as a
testbed in the rapidly developing field of molecular electronics.
The scientists' results, published in the 9 February
2001 edition of Science, describe the use of organic molecules as
"molecular rulers" that permit the fabrication of useful wires dozens
of times smaller than the period at the end of this sentence.
The scientists measure their results in nanometers,
equal to one billionth of a meter, and micrometers, equal to one
millionth of a meter. They have proven they can make extremely thin
wires from 15 to 70 nanometers wide and a few micrometers long that
are spaced 10 to 40 nanometers apart.
"We have known how to make smaller and smaller
structures by using techniques that have been developed for the
fabrication of computer chips, and we also have known how to make
molecules bigger and bigger," said Paul Weiss, associate professor of
chemistry at Penn State and coauthor of the paper describing the
group's results. "But that intermediate region between the two
approaches has been essentially inaccessible, and our technique of
using 'molecular rulers' represents a step toward bridging that gap."
The "molecular ruler" construction process requires some
existing nanoscale structures to "grow" in order to produce the even
smaller structures. Specifically, the scientists started with two
parallel gold nanostructures on a silicate substrate.
Those structures were formed by electron-beam
lithography, one of several widely used nanoscale construction
techniques limited to making structures tens of nanometers or larger.
Layers of organic molecules then were applied atop the
initial structures to make them bigger and wider, at the same time
reducing the gap between the structures.
Imagine two cookies rising beside each other while
baking, with the space between the cookies getting smaller as they
cook, and you get a sense of how each of the initial structures grows
by the addition of organic molecules and how the space between the
structures gets narrower.
Because the scientists knew the size and spacing of the
initial structures and the thickness of the layers of films created by
the molecules atop the structures, they could calculate the size of
the narrowing space between the structures.
As a result, the organic molecules, which selectively
bind to each other and to the substrate materials, provide "molecular
rulers" that precisely determine the size of the resulting space
between the initial structures. Scientists use the resulting space for
forming even smaller wires by filling the space with gold.
For their research, the scientists used silicate as the
substrate, gold for the prefabricated initial structures, and
mercaptoalkanoic acid as the organic molecule.
Those organic molecules, also are referred to as "resist
s" because they resist attack and protect the material underneath them
in various lithographic processing steps, were used by Weiss and his
team to improve the construction process for nanoscale structures.
"We had a lot of different ways we were trying to make
closely spaced, precise structures -- we had four people in the group
trying six different ways -- and we found this one has the precision
we need built into it," Weiss said.
"We know how to make the ends of organic molecules so
that they bind selectively, both to each other and to the substrate.
We also know they form films when they interact, and from that we can
determine a precise thickness of the film. That's what makes the whole
thing work. If they did not do that, the process would be just as
crude as the standard polymer resists."
Along with precision and increased miniaturization, the
construction process outlined by Weiss and Amat Hatzor, a
post-doctoral fellow at Penn State, includes a method to selectively
remove the molecular resists after the wires are cast, thereby
improving upon the efficiency and flexibility of existing methods.
Whereas other fabrication methods require scientists to build
structures individually, the "molecular ruler" method allows an entire
"cookie sheet" of structures or wires to be completed at once.
"It is a single fabrication process," Weiss said. "You
do not have to draw every single line one at a time. You simply do the
overall design and then in one set of steps you can complete the whole
surface. We can make a number of shapes and sizes that we cannot make
by other means."
This research was funded by the Army Research Office,
the Defense Advanced Research Projects Agency, the National Science
Foundation, and the Office of Naval Research. It was conducted at Penn
State's National Nanofabrication Users' Network Facility.
http://www.spacedaily.com/news/nanotech-01a.html