UNIVERSITY PARK, Pa., Nov. 28 -- A team of scientists at Penn State, Rice University and the University of Oregon have developed a way to control single-molecule switches by engineering their design and environment.
The discovery, which is an essential step in the emerging field of molecular electronics, could further the development of nanocomponents -- as small as molecules or atoms -- for use in future generations of computers and other electronic devices.
The research demonstrates that single-molecule switches can be tailored to respond in predictable and stable ways, depending on the direction of the electric field applied to them -- while some switches were engineered to turn on, others were engineered to turn off in response to the same applied electric field.
"This research confirms our hypothesis of how single-molecule switches work," said Paul S. Weiss, professor of chemistry and physics, 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."
Other team members are 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 discovery is part of their ongoing studies of a family of stiff, stringy molecules known as OPEs -- oligo phenylene-ethynylenes -- which they 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 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 how and why it is possible to control these molecular switches.
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; 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 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.
The team demonstrated that it is important to engineer the chemical environment, as well as the electronic environment, that surrounds the OPE molecule. They also redesigned the matrix so it would be able to interact better with the new functionality of this repositioned group. The 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 were designed, assembled and measured.
A paper describing the research results, "Molecular Engineering of the Polarity and Interactions of Molecular Electronic Switches," will be published next month in the Journal of the American Chemical Society. The research was funded, in part, by the Army Research Office, Defense Advanced Research Projects Agency, Department of Energy, National Science Foundation, National Institutes of Standards and Technology and Office of Naval Research.
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