Topological insulators let molecular wires grow longer

Scientists have long dreamed of making molecular electronics, building one-dimensional strings atom by atom to create new circuits even smaller than today’s tiniest silicon structures. Shrinking silicon circuits has led to vast increases in computer chip power, and molecular wires could be the next advance. One hurdle has been that molecular wires become insulating and stop carrying current after a very short distance, shorter than the length they’d need to be for most practical devices. Now, scientists have shown they can overcome that limit, making wires that carry current along more useful lengths (J. Am. Chem. Soc. 2023, DOI: 10.1021/jacs.2c12059).

The researchers accomplished this using one-dimensional topological insulators as their wires. A topological insulator is a material that conducts electricity only along its edges. Topological insulators have unusual electronic structures that provide new ways to control the flow of current. But the distance electricity travels is limited by gaps in the electronic structure of organic molecules used to build such 1D wires, says Latha Venkataraman, a chemist and applied physicist at Columbia University, who led the research. The previous greatest conductance distance she and her colleagues had achieved with a topological insulator, made in that case of bis(triarylamine), was 2.6 nm.

At the suggestion of fellow Columbia University chemist Colin Nuckolls, the team built their new wire using emeraldine, a particular arrangement of carbon, hydrogen, and nitrogen atoms. The structure of the molecule allowed them to place a radical—an unpaired electron—on the nitrogen atoms. Those radicals, which come in sets of two, act essentially as relays, providing a series of connections over which current could flow from one end of the wire to another.

The new wire carried current over a length of almost 5 nm. “That’s almost getting to the length scales where current devices based on silicon are being made” she says. The wires could be made longer to carry current over greater distances, but the synthesis process for adding each new pair of radicals is complex with many steps, so the team stopped after demonstrating that the addition of radicals worked.

The chemists tested their molecular wire and found they could drive a current of more than a microampere over the almost 5-nm length, comparable to the ability of a carbon nanotube. “We can get really high currents through very long molecules, which hadn’t been done before,” Venkataraman says.

Other molecules will work as well, she says, as long as their structure allows them to hold radicals. In some cases, the radicals might be placed on carbon atoms rather than nitrogen.

Nuckolls says these molecular wires can form the basis of many electronic components, including switches, actuators, sensors, and light emitters. They could also be used to link molecular devices together, which would avoid the difficulty of forming those links with metal leads.

Judy Cha, a professor of materials science and engineering at Cornell University, says there’s a long road from this demonstration to actually making practical devices with such molecular wires. A device designer who wants to integrate such wires into nanoelectronic circuits, she says, “needs to think about processing temperatures, heat management, integrability with other materials, oxidation, etc.” That’s too much to ask for at this stage, she says, but those factors will need to be considered eventually.