Single-molecule diodes: A major step in miniaturizing electronics

John Hopton for redOrbit.com – @Johnfinitum

Colombia and Berkeley scientists have developed a new technique for creating single-molecule diodes, resulting in molecular diodes that perform 50 times better than all previous designs.

Latha Venkataraman, associate professor of applied physics at Columbia Engineering, has described the goal of applying the technique to real-world technology as a “tantalizing dream in nanoscience” and the “holy grail” of molecular electronics.

The hope is that single-molecule diode development will mean revolutionary miniaturization for electronic devices with high level functionality.

In fact, as the report points out, single molecules represent the ultimate limit of miniaturization.

The idea of creating a single-molecule diode was suggested by Arieh Aviram and Mark Ratner who theorized 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, indeed, diodes.

Too small to see, but working perfectly

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,” explains Brian Capozzi, a PhD student working with Venkataraman and lead author of the paper. “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. Ideally, the ratio of ‘on’ current to ‘off’ current, the rectification ratio, should be very high.”

Venkataraman and Capozzi worked with Chemistry Assistant Professor Luis Campos’ group at Columbia and Jeffrey Neaton’s group at the Molecular Foundry at UC Berkeley to develop an asymmetry in the environment around the molecular junction. They did this by 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: 50 times higher than earlier designs. The “on” current flow in their devices 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.

“It’s amazing to be able to design a molecular circuit, using concepts from chemistry and physics, and have it do something functional,” Venkataraman says. “The length scale is so small that quantum mechanical effects are absolutely a crucial aspect of the device. So it is truly a triumph to be able to create something that you will never be able to physically see and that behaves as intended.”

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Source: http://www.redorbit.com/news/technology/1113397006/single-molecule-diodes-a-major-step-in-miniaturizing-electronics-052615/