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Newswise
— Researchers at the Weizmann Institute of Science
have demonstrated a new kind of electrical switch,
formed of organic molecules, that could be used in
the future in nanoscale electronic components.
Their
approach involved rethinking a phenomenon that drives
many of today’s high-speed semiconductors. Negative
differential resistance (NDR), as the phenomenon is
called, works contrary to the normal laws of electricity,
in which an increase in voltage translates into a
direct increase in current. In NDR, as the voltage
steadily increases, the current peaks and then drops
off, essentially allowing one to create a switch with
no moving parts. But until now, those attempting to
recreate NDR at the molecular scale had only managed
it at extremely low temperatures.
Prof.
David Cahen of the Institute’s Materials and Interfaces
Department and graduate student Adi Salomon thought
research carried out by Salomon and others in Cahen’s
lab during her M.Sc. studies on connections between
metal wires and organic (carbon based) molecules might
hold part of the key to usable nanoscale NDR. They
had found that, like people, molecules and metal wires
need chemistry between them for barriers to be lowered
and the juice to really flow. For a given voltage,
if the molecules are held to the wire by chemical
bonds (in which the two are linked by shared electrons),
the current flowing through them will be many times
higher than if they are only touching a mere physical
bond.
Using
this insight, the team designed organic molecules
that pass electricity through chemical bonds at a
lower voltage, but through physical bonds at a higher
voltage. As the voltage approaches the higher level,
sulfur atoms at one end of the molecule loosen their
chemical bonds with the wire, and the current drops
off as the switchover occurs.
But
the molecules, once the chemical bond to the wire
was broken, tended to move apart, preventing them
from switching back to the chemically-bonded state.
Prof. Abraham Shanzer of the Organic Chemistry Department,
who had worked with the team on the original molecular
design, now helped them to create long add-on tails
to hold the molecules in place with a weak attraction.
Now, the NDR in their molecules was stable, reversible
and reproducible at room temperature.
Possible
applications include nanoscale electronic memory and
heat-sensing switches. The future of miniaturized
electronics may lie in methods that combine chemistry
with nanoscience, say the scientists. “We don’t take
human-sized objects and try to scale them down, but
create new things from the universe of possibilities
open to chemists that are specifically designed to
function in the nanoworld.”
Prof.
David Cahen’s research is supported by the Minerva
Stiftung Gesellschaft fuer die Forschun M. B. H.;
the Wolfson Advanced Research Center; the Philip M.
Klutznick Fund for Research; Delores and Eugene M.
Zemsky; and the Weizmann-Johns Hopkins Research Program.
Prof. Cahen is the incumbent of the Rowland Schaefer
Professorial Chair in Energy Research.
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