Frisbie Group. Credit: C D Frisbie

Understanding the electrical transport properties of molecules is crucial for molecular electronics. Scientists have devised a variety of schemes for placing molecules between electrical contacts so they can measure their current-voltage characteristics. Until now, however, it has not been possible to examine how the resistance of a molecular wire changes with its length. This length dependence of resistance depends on the charge transport mechanism in the wire.

From tunnelling to hopping
Daniel Frisbie and colleagues have succeeded in growing molecular nanowires around 7 nm long using step-wise chemistry and systematically measuring the wire's resistance as it gets longer. The researchers showed that there is a clear change in the transport mechanism from tunnelling to hopping conduction when the wire is about 4 nm long.

"Our results confirm the widely anticipated change in transport mechanism with increasing wire length and also open opportunities for examining the impact of chemical structure on hopping transport," Frisbie told nanotechweb.org. "Ultimately, more experiments like these may help our understanding of electrical transport in thin films of highly conjugated polymer semiconductors."

The Minnesota team obtained its results thanks to imine chemistry. The researchers grew molecular wires one repeat unit at a time from a gold surface. Next, they verified the chemistry in several ways, including infrared spectroscopy, which allowed them to carefully track new chemical bonds forming during wire growth.

They made wires of different lengths ranging from 1 to 7 nm and then used a conducting AFM tip to make a second electrical contact to the top of the wire "film" (consisting of around 100 wires). The current-voltage characteristics of the wires were measured by applying a voltage between the tip and gold substrate.

The scientists say that the result will help us to better understand electrical transport in molecular systems in general. "It also provides additional important data to answer the question: 'what is the electrical resistance of a molecule?' " added Frisbie.

"Ultimately, understanding electrical transport on the nanoscale in molecular systems may help us to describe and optimize transport in more macroscopic molecular films that are being used for plastic electronics applications," he said.

The team now plans to vary the chemical "backbone" of the wire and examine the effect that this has on hopping transport. "We will also likely study transport in even longer wires to see if there are any other effects with increasing length," revealed Frisbie.

The work was published in Science.