Linker-Free Molecular Wires

Stringing conjugated organic compounds between two electrodes via direct metal-carbon bonds rather than via linking groups will likely change the way molecular electronic circuits are “wired.” Although circuits made of molecular building blocks are not yet a reality outside research laboratories, they promise to shrink electronics beyond what is possible with silicon-based devices.

A research team led by Ronald Breslow and Latha Venkataraman of Columbia University and Mark S. Hybertsen of Brookhaven National Laboratory bonded the terminal methylene groups of oligophenyl compounds directly to two gold electrodes. To form the Au–C bonds, the scientists synthesized the methylene-terminated oligophenyls with trimethyltin end caps. When the molecules come into contact with the electrodes, the compounds shed their caps, and each methylene forms a covalent bond with one of the gold electrode surfaces.

Since the first measurements of electron transport in single molecules in the late 1990s, researchers have been searching for ways to make highly conductive connections between molecular wires and metal electrodes, says Kristian S. Thygesen, a physicist at the Technical University of Denmark. Typical linking groups such as thiols and amines help organic compounds form strong bonds to metal electrodes, but at the same time suppress electron transport, he says.

By eliminating the linkers, the new method enhanced electrical conductance through the methylene-linked oligophenyls by two orders of magnitude (J. Am. Chem. Soc., DOI: 10.1021/ja208020j). In addition, the researchers showed that a single dimethylenebenzene molecule spanning the junction between the gold electrodes achieved 90% of the conductance of a circuit with a gold atom between the electrodes.

When the scientists developed the direct Au–C bonding chemistry and tested it on alkanes earlier this year (Nat. Nanotech., DOI: 10.1038/nnano.2011.66), they observed some conductance enhancement, but it was not as large as what they attained with the methylene-terminated oligophenyls.

“This is the first demonstration of truly strong coupling dictated by well-defined bonding in a molecular electronic junction,” says Douglas Natelson, a nanotechnology researcher at Rice University. “That’s quite exciting.”