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)

May 29, 2015

Over the last few decades, scientists have developed the capability to measure the flow of electricity across a single molecule, paving the way for electronic devices so small they would fit inside one of your cells.

However, one of the most fundamental and useful circuitry elements has proven difficult to produce on a molecular scale: the 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 have 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.

Producing a 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.

 There have been several examples of single molecular diodes, but up until now these miniscule electricity valves have been too leaky and have required far too much energy to make them practically useful in a molecular electronic device. The diodes produced by Capozzi and his colleagues are 100 times less leaky and use 10 times less power.

 All diodes, large and small, require a degree of asymmetry in their composition, in order to allow current to flow only in one direction. “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. 

The scientists use two gold electrodes to provide electricity to a completely symmetrical molecule dissolved in a liquid that would also respond to the direction of electricity flow. The only difference in the system is that one of the electrodes was about 10,000 times smaller than the other. That was 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.

This Is Your Brain on PDGFB

Don’t worry. It’s the big one.

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Comparison of brains from a mouse and a human (left and right, respectively). (Image courtesy of Jan Lui.)

March 3, 2015 

You and a mouse have a lot in common genetically. Deep down in your DNA 85% of the genes that make you, you, also make Mickey, Mickey. Why then are your brain and a mouse’s brain so very different?*

Unfortunately, all that genetic similarity makes it difficult for scientists to determine exactly why your brain is so sizable when compared with that of a mouse. To this end, neuroscientists at University of California, San Francisco have developed a new technique for studying the genetic differences in mouse and human brain development. Using this technique they have pinpointed a gene that contributes to human brain growth that is switched off in the mouse.

“If you look at the mouse you can learn a lot about the mechanisms that are evolutionarily conserved between the human and the mouse,” explains Dr. Jan Lui, lead author of the study, which appeared in Nature in November. “But these are not going to explain why the human is really big compared to the mouse.”

The researchers found their difference in radial glia, the stem cells that go on to produce neurons and some of the other myriad cell types that make up the largest part of the mature brains of both mice and men. To do so they first built a map of which genes are expressed in different parts of both human and mouse brains. They compared brains that were in their early stages of development—after they had begun to grow and take shape but before the radial glia had transformed into neurons.

The team then located 18 genes that were expressed by radial glia in humans but not in mice. One gene in particular stood out because it was already known to produce a protein responsible for stimulating growth, PDGFD.† They were then able to show that when this protein is removed from human neural stem cells in culture, their growth is halted, and when it is added to mouse neural stem cells in mice, their growth is promoted.

“We don’t want to claim is that this is the magic bullet that causes the human brain to be big and the mouse brain to be small,” adds Lui. There are many other genes and proteins that likely contribute to the size difference, and this is just one of them. Still they hope their method will be useful for locating other genes in these and other types of samples, and for comparing humans with other animals that are even more closely related. 

The scientists also only require a few unique specimens in order to build their genetic map. Lui believes that this means it will be useful for more than studying the brain. “I think that it is a method that can be useful to a lot of different people and for a lot of different tissues, particularly unique cancer samples. You can learn about the internal cell composition of a tumor just from one sample.”

From: J.H. Lui, T. J Nowakowski, A.A. Pollen, A. Javaherian, A.R. Kriegstein, and M.C. Oldham; “Radial glia require PDGFD-PDGFRβ signaling in human but not mouse neocortex;” Nature, (2014) 315, 264-268; doi: 10.1038/nature13973

 *And very different they are: in size, in shape, in the amount of time they take to develop, in the number of connections between their neurons, and in the number of connections between their different parts. 
† PDGFD stands for “platelet derived growth factor D.”