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Molecular amplifier
Rüschlikon/Switzerland, March 7, 1997 -- Scientists of IBM Research - Zurich in Switzerland and the National Center
of Scientific Research (CNRS) in Toulouse, France, have demonstrated
an experimental electronic device with a tiny active part: it consists
of a single molecule actuated by the ultrasharp tip of a scanning
tunneling microscope (STM) and represents a fully functional electromechanical
amplifier.
Both experimental and theoretical studies revealed that the electrical
conductance of C60 molecules, the famous soccer ball-like
"bucky balls" made of 60 carbon atoms, can be changed
continuously and reversibly by applying a mechanical force to a
single molecule using an STM tip. From the STM principle it is well
understood that electrons flow or tunnel between the tip and a sample
surface when a voltage is applied. For the C60 experiments,
researchers positioned the STM tip on top of a bucky ball adsorbed
to a copper surface and let a tunneling current flow between tip
and sample through the bucky ball, which exhibits an electrical
contact resistance.
Gently "squeezing" the bucky ball by lowering the tip
only one-tenth of a nanometer (one millionth of a millimeter) deformed
the molecular structure slightly, which in turn changed the electrical
properties of the bucky ball. In fact, the researchers observed
that the resistance of the bucky ball was one hundred times lower,
which allowed the electrons to tunnel more easily through the molecule.
A small voltage of only 10 mV applied to the piezoelectric element
of the tip to squeeze the molecule resulted in a fivefold voltage
gain measured across the load resistor of the circuit.
The graphical view illustrates the C60 amplifier: an
electrical signal through STM tip, C60 molecule and
copper surface (left) is amplified by about five times when the
tip is lowered by the tenth of a nanometer, which requires only
10 mV (right)
"Our device, while of no practical use at the moment, is nevertheless
a realization of a three-terminal, single-molecular device,"
write Christian Joachim of CNRS and James K. Gimzewski of IBM Research - Zurich in the scientific journal Chemical Physics
Letters.* "The next step in further decreasing the size of
this nanoscopic amplifier is to replace the macroscopic piezo-actuated
electromechanical gate (the STM tip today) by an additional nanoscopic
component. It is readily foreseeable that a micromechanical electrode
actuated by electrostatic, thermomechanical, magnetic, or even photo-
or electrochromic means can be used to vary the pressure on the
molecule. This would then reduce the overall dimensions of the complete
amplifier to less than 1 micrometer or even smaller."
Theoretical calculations of the device's behavior were performed
at CNRS and the experiments were performed at IBM Research - Zurich. The work was sponsored by the Swiss Federal Office of
Education and Science within the European Strategic Program for
Research in Information Technology (ESPRIT) of the European Union.
*The scientific report "An electromechanical amplifier using
a single molecule" was published in Chemical Physics Letters,
Vol. 265, Nos. 3-5, page 353, February 7, 1997.
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