A single-molecule diode with the highest on–off current (or rectification) ratio to date has been unveiled by a team of physicists and chemists in the US. While single-molecule diodes have been made in the past, they suffered from low conductance and very low rectification ratios. The new diode could be used to study the fundamental electronic properties of materials on the molecular scale, and might lead to the development of better nanoscale electronic devices.

Electronic devices made from single molecules, including single-electron transistors, memory elements and optical switches, have been around since the 1990s. However, making single-molecule diodes – the most basic of all electronic elements – has proved to be a difficult task.

A single-molecule diode is a two-terminal electronic component that allows current to flow in only one direction; the idea of such a device was first proposed more than 40 years ago in a theory paper. The concept involved an asymmetric “donor-bridge-acceptor” molecule, and was expected to work like the semiconductor p–n junction in a conventional diode. Since then, researchers have made several single-molecule diodes featuring asymmetric molecules. However, despite improvements in the properties of these devices over the decades, they still suffer from low conductance and low rectification ratios (of less than 11). They also require high operating voltages of around 1 V.

Symmetric molecule works

A molecular diode normally needs to have an asymmetric structure so that the current flow is also asymmetric in terms of direction. This is usually achieved by using an inherently asymmetric molecule or by using electrodes made from different materials. Now, a team of researchers led by Latha Venkataraman of Columbia University in New York has succeeded in building asymmetry into a molecular diode using a symmetric molecule and electrodes made from the same metal (gold). This was done by adjusting the electrostatic environments where the molecule is attached to each electrode, which involved having one end of the molecule in contact with a planar electrode with a large surface area. The other end of the molecule is in contact with a sharp-tipped electrode coated with wax, so it offers a much smaller surface area (see figure). The researchers also operated the device in a polar solvent and exposed different areas of the electrodes to this ionic medium.

Asymmetric charge distribution

The result of this interface asymmetry is that double layers of differing charge densities develop at the two electrodes–molecule interfaces. These double layers originate from ions in the solvent that propagate towards the interfaces to screen out the electric field generated by electrical charges in the gold. “This asymmetric charge distribution is responsible for the enhanced current rectification we observed,” explains Venkataraman.

“Our technique to enhance current rectification in these single-molecule structures is simple and robust. It also alleviates the need for complex synthesis strategies required to design asymmetric molecules,” says team member Brian Capozzi.

The researchers say they achieved rectification ratios of more than 200 at voltages as low as 370 mV using molecules comprising symmetric oligomers of thiophene-1,1-dioxide. The same junctions immersed in non-polar solvents do not show any rectification, which the team says proves that the environment around the electrodes plays a key role in the operation of the devices.

Fundamental electronic structure

“Combined with the high rectification and currents that we have measured, our technique might also be used to make real-world devices, and could be applied to other nanoscale device components, not just single-molecule junctions,” says Venkataraman. And that is not all: the method provides a way to experimentally probe how energy levels are aligned in single-molecule junctions – something that could be useful for studying the fundamental electronic structure of a variety of other device components.

The team, which includes groups lead by Luis Campos of Columbia University and Jeffrey Neaton of the University of California, Berkeley, says that it is now busy optimizing and developing even better single-molecule diodes.

The devices are described in Nature Nanotechnology.

• This article first appeared on nanotechweb.org