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Scientists Create Single-Molecule Diode

A group of scientists led by Dr Latha Venkataraman of Columbia University has developed a single-molecule diode that may have real-world technological applications for nanodevices.

Molecular structure of thiophene-1,1-dioxide oligomers and schematic of the molecular junction created using asymmetric area electrodes. Image credit: Brian Capozzi et al.

Molecular structure of thiophene-1,1-dioxide oligomers and schematic of the molecular junction created using asymmetric area electrodes. Image credit: Brian Capozzi et al.

The idea of creating a single-molecule diode – a circuit element that directs current flow – was first suggested more than 40 years ago, in 1974, by researchers Arieh Aviram of IBM Thomas J. Watson Research Center and Mark Ratner of New York University.

Scientists have since been exploring the charge-transport properties of molecules. They have shown that single-molecules attached to metal electrodes can be made to act as a variety of circuit elements, including resistors, switches, transistors, and, indeed, diodes.

They have learned that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions.

“Constructing a device where the active elements are only a single molecule has long been a tantalizing dream in nanoscience,” Dr Venkataraman said.

“Our new approach created a single-molecule diode that has a high rectification and a high ‘on’ current.”

Since a diode acts as an electricity valve, its 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 simply designed molecules that have asymmetric structures.

“While such asymmetric molecules do indeed display some diode-like properties, they are not effective,” said team member Brian Capozzi, a PhD student at Columbia University.

In order to overcome the issues associated with asymmetric molecular design, the scientists focused on developing an asymmetry in the environment around the molecular junction.

They surrounded the active molecule (oligomer of thiophene-1,1-dioxide) with an ionic solution and used gold metal electrodes of different sizes to contact the molecule.

Their results, reported in the journal Nature Nanotechnology, achieved rectification ratios as high as 250: 50 times higher than earlier designs.

The ‘on’ current flow in their devices can be more than 0.1 microamps, which is a lot of current to be passing through a single-molecule.

And, because this new technique is so easily implemented, it can be applied to all nanodevices of all types, including those that are made with graphene electrodes.

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Brian Capozzi et al. Single-molecule diodes with high rectification ratios through environmental control. Nature Nanotechnology, published online May 25, 2015; doi: 10.1038/nnano.2015.97