Led by Columbia Engineering, researchers built the longest and highly conductive molecular nanowires

New York, NY—July 7, 2022—As our devices get smaller, the use of molecules as key components in electronic circuits is becoming increasingly critical. Over the past 10 years, researchers have been trying to use single molecules as conducting wires because of their small scale, different electronic characteristics, and high tunability. But in most molecular wires, as the length of the wire increases, the efficiency of transmitting electrons across the wire decreases exponentially. These limitations make it very challenging to construct long molecular wires longer than nanometers. which actually conducts electricity well.

Columbia researchers announced today that they have constructed a nanowire that is 2.6 nanometers long, exhibits an unusual increase in conductance as the wire length increases, and has quasi-metallic properties. Its excellent conductivity holds great promise for the field of molecular electronics, enabling electronic devices to become smaller. The study was published today in Nature Chemistry.

Molecular wire design

The research team from Columbia Engineering and Columbia’s chemistry department, along with theorists from Germany and synthetic chemists in China, explored a molecular wire design that would support unpaired electrons at both ends, as the wire would form a one-dimensional analogue to a topological insulator (TI). ) which is highly conductive through the edges but isolated in the centre.

While the simplest 1D TIs are made of carbon atoms only where the terminal carbon supports the unpaired electron of the radical state, these molecules are generally very unstable. Carbon does not like having unpaired electrons. Replacing the terminal carbon, where the radical is located, with nitrogen increases the stability of the molecule. “This makes TI 1D fabricated with a carbon chain but terminated in nitrogen much more stable and we were able to work with this at room temperature under ambient conditions,” said team co-leader Latha Venkataraman, Lawrence Gussman Professor of Applied Physics and professor of chemistry.

Breaking the rules of exponential decay

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

“What’s really interesting is that our wire has conductance on the same scale as the gold-metal point contact, indicating that the molecule itself exhibits quasi-metallic properties,” said Venkataraman. “This work shows that organic molecules can behave like metals at the single-molecule level in contrast to what has been done in the past where they mainly conduct weak conduction.”

The researchers designed and synthesized a series of bis(triarylamines) molecules, which exhibit the one-dimensional properties of TI via chemical oxidation. They made the conductance measurements of single-molecule junctions where the molecules were connected to the source and drain electrodes. Through measurements, the team demonstrated that the longer molecules have higher conductance, which works up to wires longer than 2.5 nanometers, the diameter of the human DNA strand.

Laying the groundwork for more technological advances in molecular electronics

“The Venkataraman laboratory has always sought to understand the interactions of physics, chemistry, and engineering of single-molecule electronic devices,” adds Liang Li, a PhD student in the lab, and co-author of the paper. “So creating this particular cable will lay the groundwork for major scientific advances in understanding transport through this new system. We are very excited about our findings because they shed light not only on basic physics, but also on potential future applications.”

The group is currently developing new designs to make the molecular wires longer and still highly conductive.

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