09-02-2006
-- UPTON, NY - At the U.S. Department of Energy's
Brookhaven National Laboratory, researchers have
determined the structure of an experimental, organic
compound-based circuit component, called a “molecular
electronic junction,” that is only a few nanometers
(billionths of a meter) in dimension. This study
may help scientists understand how the structure
of molecular junctions relates to their performance
and function and, in the longer term, may help incorporate
these and other molecular-scale devices into a new
generation of remarkably small electronics-based
technologies.
The research is discussed in the February 8, 2006,
online edition of the Proceedings of the National
Academy of Sciences .
“Molecular electronics is a very exciting developing
field, since these extremely small circuits, based
on organic molecules rather than metal, have a potentially
greater circuit density than conventional silicon-based
technology. This means that more circuits could fit
on one circuit board, leading to electronic devices
that are much smaller than those currently produced,” said
one of the study's lead scientists, Brookhaven physicist
Julian Baumert. “We're interested in the structure
of the junction — how the molecules are oriented
and packed together — because it is linked to the
function and performance of the circuit.”
In conventional circuits, junctions are commonly
made of two different types of silicon that, when
layered together, allow electric current to flow
in one direction only. Here, the junction under study
consists of two very thin layers of two different
organic compounds — “alkyl-thiol” and “alkyl-silane.” They
are sandwiched on one side by a layer of solid silicon
and on
the other side by a layer of liquid mercury, which
serve as electrodes. Alkyl-thiol and alkyl-silane
molecules have simple structures (making them fairly
easy to study) and the potential to be good insulating
materials (a desirable property in many junctions).
The scientists created the junctions by filling
a small container with a drop of liquid mercury and
depositing a very small amount of alkyl-thiol onto
the mercury surface. They then topped the alkyl-thiol
layer with an alkyl-silane-coated silicon wafer.
This method yielded a junction with just a five-nanometer-thick
gap between the two electrodes.
“This technique is not limited to simple alkane
molecules,” said Columbia University graduate student
Michael Lefenfeld, the study's second lead scientist. “Many
other types of organic molecules could be used, such
as semi-conducting and conducting molecules. These
materials also have a structure and packing density
that plays a large role in their electrical performance.”
The research group studied the junction at Brookhaven's
National Synchrotron Light Source, a facility that
produces x-ray, ultraviolet, and infrared light for
research. They aimed high-energy x-rays — energetic
enough to penetrate the silicon wafer — at the junction
from several incident angles and measured how the
rays scattered off the organic molecules. Next, they
attached electric contacts to the silicon and mercury
electrodes and, for several different applied voltages,
measured both the reflected x-ray signal and the
electric current through the junction.
By analyzing the x-ray scattering data, the researchers
discovered that the organic molecules are densely
packed together, with most of the molecules positioned
vertically. Further, the combination of the electric-current
and x-ray measurements revealed that the current
does not deform that structure, even when the applied
voltage was very high. This implies that the molecular
structure is quite stable.
“These are important details to know in order to
fully understand the electronic properties of molecular-scale
junctions,” said Brookhaven Physicist Benjamin Ocko,
one of the paper's senior authors. “These investigation
methods should be able to provide us with a better
understanding of many other molecular junctions.”
This research was supported by the Office of Basic
Energy Sciences within the U.S. Department of Energy's
Office of Science, the National Science Foundation,
the New York State Office of Science, Technology,
and Academic Research, and the German-Israeli Foundation.
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