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A group of researchers has verified the existence of a single-molecule electret, a device that could serve as a non-volatile memory unit and lead to the development of smaller computer storage devices.

Researchers from Nanjing University, Renmin University, Xiamen University, Southeast University, Rensselaer Polytechnic Institute and Yale University, including Yale professor of electrical engineering and applied physics Mark Reed, created the single-molecule structure. It consists of a gadolinium atom inside a carbon cage — the interaction between the atom and its cage allows the structure to hold an electric charge even after an electric field has been taken away. This device creates the possibility of single-molecule computer memory components. The paper was published in Nature Nanotechnology on Oct. 12.

“We were initially searching for magnetic dipoles,” said Fengqi Song, professor of physics at Nanjing University and a lead author on the paper. “But we actually found some electric dipoles. It’s just, you know. It’s an unexpected result.”

An electret is a semi-permanent polarizable material, which means that it can be switched between two stable states by applying an electric field. An electret can be thought of as analogous to a magnet — a magnet creates a persistent magnetic field, and an electret creates a persistent electric field.

Song explained that most electrets can hold their polarization because of the interactions between different molecules in their materials. Therefore, an electret with only one molecule would have to hold its charge in a novel way.

“You might say, ‘of course you can set the polarization state of something,’” said Fengnian Xia, a Yale professor of electrical engineering, when asked about his colleague’s paper. “I mean, that’s not surprising. But on the other hand, you have to do it at a single-molecule level. That sounds like something ‘Mission Impossible.’”

To do so, the lab introduced a single atom of gadolinium into a cage of carbon called a fullerene. Reed explained that when a voltage is applied to this structure, the atom interacts with one side of the cage to create a certain electric field. Then, when the voltage is applied in the other direction, the atom jumps to the other side and creates a different electric field. This device can therefore hold a semi-permanent electric charge without requiring interactions between molecules.

Reed’s previous work focused on creating other single-molecule electrical devices, such as a single-molecule transistor. He studied how to isolate a specific molecule for these devices using a technique called electromagnetic break junctions.

It was because of this research that he collaborated with professors from Nanjing University, including Song, who were studying the characteristics of molecular structures.

“We said, ‘OK. I know how to make single-molecule transistors. You know how to make small particles work. What can we do that’s interesting?’” Reed said.

With guidance from Reed, researchers from Nanjing University built the device. Then they applied an external electric field to it and studied its properties. Their initial analysis suggested that the device had two stable electric states. But observing this experimentally was not rigorous enough, so the researchers pulled in experts from other universities to validate their theory with calculations. The models agreed with their experimental results, and the team was able to verify the existence of the single-molecule electret.

Since single-molecule electrets semi-permanently store an on or off state, they could theoretically act as a form of non-volatile computer memory, which is preserved when the device is fully powered off. This could replace present storage devices with much smaller counterparts.

But the authors of the paper emphasized that engineers are far from using single-molecule electrets in actual computing. The manufacturing of the single-molecule electret was labor-intensive and unreliable, and the device works only in certain conditions. This paper’s contribution was proving that such a molecule can even exist.

“This is the part of my research portfolio that is basic science, but basic science with a long-term application,” Reed said. “The understanding of the basic mechanisms when you get down to that scale is potentially important for things that might happen in 2030, years from now.”

Reed and Song published their first paper together in 2015.


Olivia Fugikawa | olivia.fugikawa@yale.edu