Directing Electricity on the Molecular Scale

diode single molecule
Artistic rendering of the asymmetric current vs. voltage graph produced by a single molecular diode. When voltage is applied in one direction the electrical current shoots up. When it is applied in another, only a small amount of electrical current is produced. ( Image courtesy of Latha Venkataraman)

Scientists can measure and control the flow of electricity across a single molecule, paving the way for electronic devices so small they would fit inside a single cell. But these devices are still a long way off, because diodes–one of the most useful circuitry elements–are difficult to produce on a molecular scale.

Diodes are key for modern tech. The “D” in LED? That stands for diode. A diode is “an electricity valve,” explains Brian Capozzi, a graduate student in Applied Physics and Mathematics at Columbia University. “It lets electricity go one way and not the other.”

Capozzi is the lead author of a letter published Monday in Nature Nanotechnology. He and fellow scientists from the departments of Applied Physics and Chemistry at Columbia University* and the department of Physics at the University of California, Berkeley found a way to turn a single symmetrical molecule into a diode. Their discovery brings the field much closer to producing a functional molecular scale electronic device.

The search for single molecular diode “has been one of the central themes of molecular electronics,” says Gemma Solomon, an assistant professor in the Nano-Science Center and the Department of Chemistry at the University of Copenhagen who was not involved in the research.

All diodes, large and small, need to be asymmetric, so that the current flows in one direction but not the other. There are several examples of single molecular diodes, but up until now they required too much energy and leaked far too much current for a practical molecular electronic device. The diodes produced by Capozzi and his colleagues use 10 times less power and leak 100 times less current.

“For a long time people thought that in order to make a diode, you had to have asymmetry in the molecule or different kinds of metal electrodes,” explains Luis Campos, assistant professor of chemistry at Columbia and co-principal investigator on the study along with Professor Latha Venkataraman of Applied Physics and Professor Jeffrey Neaton of Berkeley. 

To make their diode, the scientists expose two gold electrodes to a solution of symmetrical molecules. When a molecule bridges the gap, electricity flows between them. The only difference in the system is that one of the electrodes was about 10,000 times smaller than the other. That’s enough to block current in one direction but not the other.

“They’re using the asymmetry of the junction itself,” explains Solomon. “So they’re working with the environment that you have to use for these types of devices.”

 “It really shows how important the environment of the single molecule junctions really is,” says Campos. “And the idea is that it’s not just for these single molecules, or the molecules we made. It’s a very general strategy.” Indeed, Capozzi says that, in principle, this type of environmental control could be expanded to nanoscale electronics containing all sorts of materials, from carbon nanotubes to metal nanoparticles.

However, beyond the future applications, the scientists say this system is most useful for studying the nature of electricity at such a small scale, which according to Capozzi, “is exciting for us.”

From: B. Capozzi, J. Xia, O. Adak, E.J. Dell, Z.F. Liu, J. C. Taylor, J.B. Neaton,L .M. Campos, and L.Venkataraman; “Single-molecule diodes with high rectification ratios through environmental control;” Nature Nanotech. (2015); doi:10.1038/nnano.2015.97

*Full disclosure: I am a lecturer and did my Ph.D. in the Chemistry Department at Columbia. Venkataraman and I also collaborated early on in my Ph.D. and Campos and I are friends.