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A paper describing the research results, titled "Molecular
Engineering of the Polarity and Interactions of Molecular Electronic
Switches," will be published in the Journal of the American Chemical
Society on 21 December 2005. "This research confirms our hypothesis of
how single-molecule switches work," says Penn State Professor of
Chemistry and Physics Paul S. Weiss, whose lab tested the molecules. "Molecular
switches eventually may become integrated into real electronics, but
not until after someone discovers a way to wire them." In addition to
Weiss, the research team includes Penelopie Lewis of Penn State, who
now is at Columbia University; James Tour and Francisco Maya at Rice
University; and James Hutchison and Christina Inman at the University
of Oregon.
The research is the latest achievement in the
team's ongoing studies of a family of stiff, stringy molecules known
as as OPEs - oligo phenylene-ethynylenes - which the scientists have
tailored in a number of ways to have a variety of physical, chemical,
and electronic characteristics. The potential for using these OPE
molecules as switches had been limited by their troublesome tendency
to turn on and off at random, but Weiss and his colleagues recently
discovered a way to reduce this random switching. In their current
research, the scientists demonstrated, with a number of definitive
experiments, how and why it is possible to control these molecular
switches.
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Credit: Lewis et
al. Sequential STM images of FAPPB/R1ATC9 obtained at alternating
sample biases of +1.0 and -1.0 V. The majority of the FAPPB
molecules (apparent protrusions, displayed as bright spots) switch
conductance states between OFF at +1.0 V and ON at -1.0 V sample
bias. The red and green boxes follow one FAPPB molecule that
exhibits this bias dependence. Imaging conditions: 4000 ‰ × 4000
‰, I = 2 pA. |
To study the properties of individual OPE molecules,
the scientists first inserted them into a hairbrush-like matrix of
similarly shaped molecules, which Weiss describes as a "self-assembled
amide-containing alkanethiol monolayer." One end of each molecular "bristle"
is attached to the thin gold base of the microscopic hairbrush. With
the individual OPE molecules surrounded by the matrix of alkanethiol
molecules, all anchored in gold, Weiss and his team were able to study
the properties of the OPE molecules with a powerful scanning tunneling
microscope (STM). The molecules were synthesized in Tour's lab at Rice
University and the matrix was synthesized in Hutchinson's lab at the
University of Oregon.
The team synthesized a variety of OPE molecules,
some with a large dipole - the difference in strength and polarity of
the electric charge between one end of the molecule and the other -
and others with a weaker dipole. Some of the OPE molecules were
designed to have a positive charge on the end facing away from the
gold base while others were designed to have a negative charge at that
end. Weiss's lab found that the tip of the microscope pulled an OPE
molecule up higher than the surrounding matrix - or "on" - if the OPE
molecule had a sufficiently strong dipole and if the charge of its
exposed end was opposite that of the STM tip, making the two
electrically attractive. "The OPEs that we engineered to have the
strongest dipoles are the most reliable," Weiss says.
The researchers also found that if the charge of
the STM tip was the same as that on the end of an OPE, and therefore
electrically repulsive, the molecule was pushed down - or "off" -
causing it to lean sideways into the matrix. They discovered that this
position alters the molecule's interaction with the system's gold base,
changing the system's electrical conductance. "When the molecule is
tilted over, electrons have a harder time going through this bond, so
the switch is more resistive," Weiss explains.
The scientists also demonstrated that it is
important to engineer the chemical environment, as well as the
electronic environment, that surrounds the OPE molecule. "We
repositioned a nitro group attached on the side of one of the
varieties of OPE switches so it had a strong-enough dipole and could
interact with the amide groups on the surrounding matrix molecules
through hydrogen bonding," Weiss says. The team also redesigned the
matrix so it would be able to interact better with the new
functionality of this repositioned group. The team's studies show that
interactions of the molecular switches with the surrounding matrix
molecules have a big effect on how long switches stayed in the on or
off state, which is critical for information storage. These states
remain stable and can be read back for hours in the systems that Weiss
and his colleagues designed, assembled, and measured. "These chemical
interactions stabilize the "on" and "off" states, reducing random
switching," Weiss reports.
"With these studies, we have been able to confirm
that we now have the predictive power to design molecular switches
that can be turned on or off at will, which was a critical test of our
understanding of their function." |
