Making a Switch


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Prof. David Cahen and Adi Salomon. Negative resistance


From simple light fixtures to the latest in cell phone technology or medical equipment, electrical switches are wired into the circuit. Whether made of metal contacts or engraved in silicon, their basic function is to stop and start the flow of electrons. But as scientists and inventors attempt to shrink new devices into the realm of nano-technology, the limitations of switches made of these materials are becoming apparent. What will replace them?
Prof. David Cahen of the Materials and Interfaces Department and his Ph.D. student and Clore Fellow Adi Salomon think that organic (that is, carbon-based) molecules may hold the answer. They have demonstrated a new kind of electrical switch created from organic molecules that could be used in future nanoscale electronic components.
Their approach involved rethinking a phenomenon that drives many of today’s high-speed semiconductors. Negative differential resistance (NDR) – for which its discoverer, Leo Esaki, won the 1973 Nobel Prize – works contrary to the standard laws of electricity. Normally, an increase in voltage translates into an increase in current. In NDR, as the voltage steadily increases, the current peaks and then drops off, essentially constituting a switch with no moving parts. Until now, how-ever, attempts to recreate NDR at the molecular scale were achieved only sporadically, mostly at extremely low temperatures or as an unstable, hard-to-reproduce phenomenon. “In hindsight, most efforts were probably aimed too squarely at trying to force molecules to behave like conventional materials, and too little at exploring the chemistry of the molecules,” say the researchers.
Some clues to practical nanoscale NDR emerged from earlier work at the Weizmann Institute conducted by Dr. Yoram Selzer (now at Tel Aviv University) and Salomon, under Cahen’s guidance, on connecting organic molecules to metal wires. They found that molecules and metals, like people, need chemistry between them for 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 de-signed 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 to the wire and, as the switchover occurs, the current drops off.
But the scientists still didn’t have a functional switch. Once the chemical bond to the wire was broken, the molecules tended to move apart, preventing them from switching back to the chemically bonded state. Prof. Abraham Shanzer of the Organic Chemistry Department, who worked with the team on the original molecular design, helped them create long add-on tails to hold the molecules in place. With this modification, the NDR then became stable, rever-sible and reproducible at room temperature.
Cahen and Salomon believe their work supports the notion that the future of miniaturized electronics may lie in methods that combine chemistry with nanoengineering. “We don’t take human-sized objects and try to scale them down; rather, from a different universe of possibilities, we create new things specifically designed to function in the nanoworld.”
Prof. David Cahen’s research is supported by Minerva Stiftung Gesellschaft fuer die Forschung m.b.H; the Wolfson Advanced Research Center; the Philip M. Klutznick Fund for Research; and the Delores and the Eugene M. Zemsky Weizmann-Johns Hopkins Research Program. Prof. Cahen is the incumbent of the Rowland Schaefer Professorial Chair in Energy Research.