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Making electronic devices in which performance is controlled by a single molecule seemed like a far-fetched idea just a few years ago. But since then, scientists have created circuits and simple devices based on a single carbon nanotube, a C60 molecule, and other molecules. The scientists fabricated and tested transistors in which one molecule of a transition-metal organic complex bridges the nanometer-scale gap between the devices’ electrodes and dictates their electronic properties. Authors compared the electronic properties of two coordination complexes in which a cobalt ion, whose charge state is readily switched between 2+ and 3+, is bonded to two terpyridinyl molecules with thiol end groups. The pair of molecules differ by a five-carbon alkyl chain that serves as a spacer between the terpyridinyl units and the end groups. Because there is no simple method for directly imaging a lone molecule sitting between a pair of electrodes, both research groups measured electrical conductance properties of their specimens and showed that the devices exhibit single-molecule signatures. The teams also showed that the nanostructures indeed behave as transistors in that the flow of electrical current can be turned on or off by controlling the voltage on an electrode known as a gate. Group found that the alkyl-chain spacers included in one of their molecules weaken the molecule’s electronic coupling to the electrodes. The molecule with the spacer conducts via a single-electron tunneling mechanism. By contrast, in the other molecule, conductance involves a Kondo resonance—a strong correlation between the spin on the cobalt ion and the spins of the electrons in the electrodes.
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