"Quantum interference is a unique manifestation of the dual particle-wave nature of matter at the quantum scale," explains Latha Venkataraman of Columbia University in New York. "Technologically, such behaviour in electrons might be used to control their flow, resulting in transistor-like behaviour, but through an unconventional 'quantum knob'."

Quantum interference could also be a new way to control current in single-molecule junctions. Indeed, it has been found to lead to dramatic effects in these structures and in molecular wires. However, probing such effects is no easy task.

Until now, researchers only looked at one physical property, the junction conductance, when studying these systems. Such measurements are difficult to perform and the results tricky to analyse because the conductance can be very small. Theory predicts that single-molecule conductance will vary enormously in molecules like benzene rings and in molecular wires that show quantum interference.

A team led by Venkataraman and Colin Nuckolls is now saying that it can study the quantum mechanical behaviour in molecular electronic devices by simultaneously measuring both the force and conductance in these structures using a conducting atomic force microscope (AFM).

The researchers designed and synthesized three molecules in the so-called stilbene family of compounds that had the same overall chemical structure but different electronic transport properties. The team chose to study these molecules because they easily form molecular junctions. What is more, the thiomethyl terminal groups they possess can be exploited to form robust mechanical and electrical contact to gold electrodes for the sake of the measurements.

Statistically reliable study

“We also developed a new measurement and analysis technique to quantitatively obtain the rupture forces of single-molecule junctions in these molecules, independent of their conductance signatures, or lack thereof,” Venkataraman told nanotechweb.org. “Combining these different critical steps provided us with a statistically reliable study of quantum interference at the single molecule level.”

The work is the first experimental measurement of the dramatic changes that can be seen in the electronic conductance of single-molecule junctions thanks to subtle quantum interference effects occurring in the underlying molecular structures, she adds.

Such effects might allow for applications like quantum-interference effect transistors (QuIETs), she says. These are single-molecule devices that control electric current between two terminals by exploiting the wave- rather than particle-like properties of electrons. When the transistor is off, the special symmetry of certain aromatic molecules (like benzene, for example) causes destructive interference of the electron waves, which prevents current from flowing through the device. By changing the voltage on a third terminal, the molecular symmetry can be broken so that the electron waves no longer cancel out, thus allowing current to flow through the device.

“More generally, the experiments might provide a way to investigate intriguing quantum phenomena in a vast number of systems previously inaccessible to laboratory study,” explained Venkataraman.

The results were published in Nano Letters.