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The 2.6-nm single-molecule wire has quasi-metallic properties and shows an unusual increase in conductivity as the length of the wire increases; its excellent conductivity holds great promise for the field of molecular electronics – ScienceDaily


As our devices become smaller and smaller, the use of molecules as basic components in electronic circuits becomes increasingly important. Over the past 10 years, researchers have tried to use single molecules as conductors because of their small size, distinct electronic characteristics, and high tunability. But in most molecular wires, as the length of the wire increases, the efficiency of electron transfer along the wire decreases exponentially. This limitation makes it particularly difficult to create a long molecular wire — well over a nanometer long — that conducts electricity really well.

Columbia researchers announced today that they have built a 2.6 nanometer-long nanowire that shows an unusual increase in conductivity as the length of the wire increases and has quasi-metallic properties. Its excellent conductivity holds great promise for molecular electronics, allowing electronic devices to become even smaller. The study was published today in Chemistry of nature.

Molecular structures of wires

A team of researchers from Columbia Engineering and Columbia’s Department of Chemistry, along with theorists in Germany and synthetic chemists in China, explored designs for molecular wires that would support unpaired electrons at both ends, as such wires would form one-dimensional analogs of topological insulators (TIs), which conduct well on the edges, but isolate in the center.

While the simplest 1D TI consists only of carbon atoms, where the terminal carbons support radical states – unpaired electrons, these molecules tend to be very unstable. Carbon does not like to have unpaired electrons. Replacing the terminal carbons, where the radicals are located, with nitrogen increases the stability of the molecules. “This makes 1D TIs made with carbon chains but nitrogen terminated much more stable, and we can work with them at room temperature under ambient conditions,” said team leader Latha Venkataraman, Lawrence Guzman Professor of Applied Physics and Professor of Chemistry .

Violation of the rule of exponential decay

Through a combination of chemical design and experimentation, the group created a series of one-dimensional TIs and successfully violated the law of exponential decay, the formula for the process of a quantity decreasing at a rate proportional to its current value. Using two radical edge states, the researchers created a highly conductive path through the molecules and achieved “reverse conductance decay,” ie. system that exhibits an increase in conductivity with increasing wire length.

“What’s really interesting is that our wire had the same conductivity as gold metal-to-metal point contacts, suggesting that the molecule itself exhibits quasi-metallic properties,” Venkataraman said. “This work demonstrates that organic molecules can behave like metals at the single-molecule level, unlike what has been done in the past, when they were mostly weak conductors.”

The researchers designed and synthesized a molecular series of bis(triarylamines) that exhibited one-dimensional TI properties through chemical oxidation. They performed conductivity measurements of single-molecule compounds where the molecules were connected to both source and drain electrodes. Through measurements, the team showed that longer molecules had higher conductivity, and this worked as long as the length of the wire did not exceed 2.5 nanometers, the diameter of a strand of human DNA.

Laying the groundwork for greater technological progress in molecular electronics

“The Venkataraman lab is always looking to understand the interplay of physics, chemistry, and single-molecule electronic device engineering,” added Liang Li, a graduate student in the lab and one of the paper’s authors. “Thus, the creation of these specific wires will lay the foundation for major scientific advances in understanding transport with these new systems. We are very excited about our findings because they shed light not only on the fundamental physics, but also on possible future applications.” .

The group is currently developing new designs to create molecular wires that are even longer and still highly conductive.

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Materials is provided School of Engineering and Applied Sciences, Columbia University. Original written by Holly Evarts. Note: Content can be edited for style and length.

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