Molecular electronics has advanced in leaps and bounds since the first molecular rectifiers were put forward by Aviram and Ratner back in 1974. These devices worked thanks to donor−σ−acceptor molecules connected symmetrically to two metal termini. Such donor-acceptor diodes, as they are known, are very sensitive to how the molecular energy levels are aligned with respect to each other, and with any connecting electrodes, which makes them very difficult to control. More importantly, a σ bridge is required to make these devices act as rectifiers – something that adds a large tunnel barrier to the device structure backbone and, which in turn, produces a very large junction resistance. The end result is that most functional single-molecule diodes available today all have resistances greater than 10 MΩ, which is far too big for practical device applications.

Although researchers have been busy looking for alternatives to donor−acceptor type diodes in the past few years, most of these efforts have resulted in “many-molecule” devices where one linker group is eliminated or weakened. Such devices contain an asymmetrically contacted junction that does not have a very well defined geometry, and a low conductance to boot. What is more, they work at relatively high voltage biases of more than1 V, which makes them particularly unstable at room temperature.

Highly conducting covalent gold−carbon bonds

A team led by Latha Venkataraman in the Applied Physics and Applied Mathematics Department at Columbia University may now have come up with a solution to these problems that involves exploiting the electronic properties of molecular junctions with highly conducting covalent gold−carbon bonds. The researchers say that this new family of molecular diodes has high electrical conductance and shows rectification at low bias. Most importantly, the behaviour of the devices can be efficiently and simply tuned.

The new molecular junction consists of a conjugated molecular backbone, connected to a gold electrode by a donor-acceptor bond and to another gold electrode by a covalent gold-carbon bond. The covalent bond acts like an electronic gateway between the metal electrode and the molecular backbone, explains Venkataraman, and is responsible for the special properties of this system.

Electronic transparency

The researchers designed a series of three devices based on their theoretical predictions. “The basic design for all three was the same: a conjugated molecular backbone connected by a gold-carbon gateway at one end, and a donor-acceptor bond at the other,” she told “The difference between them is the so-called electronic transparency (or coupling) of the donor-acceptor bond in each case. We found that rectification in a device increased when we reduced the coupling between the device backbone and the donor-acceptor bond.”

The broader message of this new work is that the gold-carbon gateway endows molecular junctions with properties that can be tuned to make functional devices, added team member Arunabh Batra. “In the present research we used these properties to create tuneable diodes. However, we believe that such ‘gateway states’ are just another element on the road to the rational design of molecular electronics systems in general.”

The team says that it is now busy trying to increase the observed rectification ratios in its devices, and make the junctions more stable at higher voltage biases.

The work is detailed in Nano Lett. DOI: 10.1021/nl403698m.

Further reading

QuIETs on the horizon (Mar 2012)
Pulling molecular junctions apart (Mar 2011)
Understanding current transport in molecular junctions (May 2011)
Molecular junctions make a switch (Mar 2009)