The team at the Columbia University School of Engineering and Applied Science in New York is the first to develop a single-molecule diode that may have real-world technological applications for nanoscale devices.
“Our new approach created a single-molecule diode that has a high rectification and a high ‘on’ current,” said Latha Venkataraman, associate professor of applied physics at Columbia Engineering.
“Constructing a device where the active elements are only a single molecule has long been a tantalising dream in nanoscience,” she added in a statement.
The idea of creating a single-molecule diode was suggested in 1974 by Arieh Aviram and Mark Ratner, who theorised that a molecule could act as a rectifier, a one-way conductor of electric current.
Since then, researchers have been exploring the charge-transport properties of molecules.
They have shown that single-molecules attached to metal electrodes (single-molecule junctions) can be made to act as a variety of circuit elements, including resistors, switches, transistors and diodes.
The diode’s structure needs to be asymmetric so that electricity flowing in one direction experiences a different environment than electricity flowing in the other direction.
In order to develop a single-molecule diode, researchers have designed molecules that have asymmetric structures.
“While such asymmetric molecules do indeed display some diode-like properties, they are not effective,” said Brian Capozzi, a PhD student and lead author of the paper Single-Molecule Diodes with High On-Off Ratios through Environmental Control, published in Nature Nanotechnology.
“A well-designed diode should only allow current to flow in one direction – the ‘on’ direction – and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have typically suffered from very low current flow in both ‘on’ and ‘off’ directions, and the ratio of current flow in the two has typically been low,” Capozzi said. “Ideally, the ratio of ‘on’ current to ‘off’ current, the rectification ratio, should be very high.”
In order to overcome the issues associated with asymmetric molecular design, the team focused on developing an asymmetry in the environment around the molecular junction.
They created environmental asymmetry through a simple method – surrounding the active molecule with an ionic solution and using gold metal electrodes of different sizes to contact the molecule.
Their results achieved rectification ratios as high as 250 – which is 50 times higher than earlier designs.
The ‘on’ current flow in their devices can be more than 0.1 microamps. This was a lot of current to be passing through a single-molecule, Venkataraman said.
Because the new technique is so easily implemented, it can be applied to nanoscale devices of all types, including those that are made with graphene electrodes.
The team is now working on understanding the fundamental physics behind the discovery, and trying to increase the rectification ratios they observed, using new molecular systems.
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