Single-Molecule Potentiometers

The conductance through a single-molecule molecular wire can be controlled by changing where an electrode contacts it, chemist Colin P. Nuckolls of Columbia University reported Sunday at the American Chemical Society national meeting in Anaheim and in a recent Nano Letters paper (DOI: 10.1021/nl104411f). The resulting device behaves as a single-molecule potentiometer, a variable resistor that can control electronic devices.

Nuckolls, physicist Latha Venkataraman, and their coworkers connected both ends of oligoacetylene molecular wires to gold electrodes and measured the system's conductance. They found that the conductance through the molecule depends on where the electrode binds to it. The electrodes can contact the molecules at their terminal sulfides or at any point along the conductive conjugated backbone.

Nuckolls likens the double bonds in the oligoacetylene chain to the rungs of a ladder. Each time the electrode steps past one of the rungs, "it changes the resistance because you've changed the effective length of the molecule you're measuring through," he told C&EN. Resistance is the reciprocal of conductance.

The Columbia team performed the experiments using a scanning tunneling microscope. When the gold STM tip moves repeatedly toward and away from a gold substrate, it exposes gold atoms on the electrodes where oligoacetylene chains can bind. The tip then steps along and binds at different points along the conjugated backbone. The system's conductance decreases stepwise as the tip binds the molecular wire farther from the substrate.

"Very often, papers in the molecular electronics community present simplified, idealized diagrams of molecular junctions, showing two sharp metal electrodes bridged neatly by a fully extended molecule, nicely bound at both ends to the apexes of the electrodes," said Douglas Natelson, a molecular electronics expert at Rice University. "This work is particularly important because it highlights that this does not have to be the case. Molecules may be contacted asymmetrically, even somewhere in the middle of a long molecule, and contribute in a well-defined way to the overall conductance."

In the future, Nuckolls plans to use single-molecule measurements to study the effects of dopants on single-molecule electronics.