May 18, 2011
Understanding current transport in molecular junctions
Understanding how current flows through circuits that consist of single organic molecules attached to metal electrodes is crucial for making improved nanoscale electronic devices. Now, researchers at Columbia University in New York and the Lawrence Berkeley National Lab have shown that the environment around such single-molecule circuits can modulate current transport. They explain the mechanism behind this phenomenon using theoretical calculations.
Most electronic devices are made from semiconductor materials, such as silicon. However, some organic molecules seem to have similar electronic properties to these traditional materials. Organic molecules, which are extremely small compared with semiconductor structures, might be used to make electronic devices that contain many more circuits on a single chip.
Latha Venkataraman and colleagues used a modified scanning tunnelling microscope (STM) and a break-junction-based technique, where they repeatedly formed and broke single molecule junctions using gold electrodes. Conductance measurements were carried out on the "target" molecule 1,4-benzenediamine dissolved in a variety of solvents, including chlorobenzene, bromobenzene and iodobenzene.
"We found that the conductance of 1,4 benzenediamine increases by almost 50% when the solvent is changed from chlorobenzene to iodobenzene," Venkataraman told nanotechweb.org.
To explain the mechanism behind this effect, the researchers then used standard density functional theory calculations to show that the solvents bind to the electrodes around the target molecule and alter the metal's work function. This results in the observed increase in conductance, says team member Jeff Neaton.
Going into more detail, increasing the gold's work function reduces the separation between the metal's Fermi energy and the highest occupied molecular orbital of 1,4 benzenediamine in the junction, he adds.
The work provides a new tool for investigating molecular conductance, and the mechanism revealed by the researchers could even be used to make chemical sensors. "This sort of mechanism could be exploited to enhance the conductance signatures of some molecules so that they can be detected above the noise level of standard instruments," said Venkataraman.
The work was reported in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org