First single-molecule device developed

Scientists, led by an Indian-origin researcher, have developed the first single-molecule diode that may have real-world technological applications for nanoscale devices.
The molecular diodes developed by a team led by Latha Venkataraman, from Columbia University, perform 50 times better than all prior designs.
"Our new approach created a single-molecule diode that has a high (over 250) rectification and a high "on" current (0.1 micro Ampere)," said Venkataraman.
"Constructing a device where the active elements are only a single molecule has long been a tantalising dream in nanoscience," said Venkataraman.
The idea of creating a single-molecule diode was suggested by Arieh Aviram and Mark Ratner who theorised in 1974 that a molecule could act as a rectifier, a one-way conductor of electric current.

Researchers have since 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.
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.
"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. Ideally, the ratio of 'on' current to 'off' current, the rectification ratio, should be very high," said Brian Capozzi, lead author of the paper.

In order to overcome the issues associated with asymmetric molecular design, Venkataraman and her colleagues focused on developing an asymmetry in the environment around the molecular junction.
They created an environmental asymmetry through a rather simple method - they surrounded the active molecule with an ionic solution and used gold metal electrodes of different sizes to contact the molecule.
Their results 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 microampers, which, Venkataraman notes, 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 nanoscale devices of all types, including those that are made with graphene electrodes.
The research was published in Nature Nanotechnology.