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Molecular amplifier

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Zurich, Switzerland, 7 March 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.

"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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Nicole Strachowski
Media Relations
IBM Research - Zurich
Tel +41 44 724 84 45

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