Milder synthesis takes the pressure off carbon-based nanothreads

Tiny nanothreads with carbon backbones, one hundred-thousandth the thickness of a human hair, have intrigued researchers since they were first made in 2014. Yet the extreme pressure required to make these high-strength polymers has been a major roadblock to exploiting their properties.

Now, researchers have found a way to produce carbon-based nanothreads at roughly one-half to one-third of the pressures required previously, enabling them to scale up their synthesis so that the threads can be investigated in more detail (ACS Nano 2021, DOI: 10.1021/acsnano.0c10400).

Carbon-based nanothreads are polymers in which subunit molecules are connected by two or three distinct bonds to form a chain. Unlike the carbon atoms in nanotubes, which are each bonded to three other atoms to form a honeycomb pattern, each carbon atom in a nanothread is generally bonded to four other atoms—a tetrahedral arrangement akin to the structure of diamond. “It can be considered a one-dimensional, diamond-like polymer material,” says Kenichiro Itami of Nagoya University, who studies nanocarbon structures and was not involved in the new research.

Until now, researchers have typically made carbon-based nanothreads by squeezing tiny samples of benzene or pyridine in diamond anvil cells. The molecules lock together under enormous pressures of 23–30 GPa—up to 300 times the pressure at the deepest point in the oceans—but this method can only produce nanogram batches of nanothreads.

A team led by Elizabeth Elacqua at Pennsylvania State University has now produced nanothreads at just 10 GPa using furan as the starting material. Furan has far less aromatic stabilization than benzene and more readily undergoes cycloaddition reactions to build the polymer. “I think it’s a very clever approach,” Itami says.

The lower reaction pressure enabled the team to use a piece of equipment called a Paris-Edinburgh press, which cannot reach the intense pressures of a diamond anvil cell but is larger and can produce a roughly 5,000 times greater mass of nanothreads in each batch, Elacqua says.

The Penn State researchers loaded liquid furan into the press and slowly increased the pressure to 15 GPa over 10–12 hours at ambient temperature. The furan solidified at about 1.4 GPa, and X-ray diffraction experiments showed that it started to polymerize at 10 GPa. The method produced about 5 mg of nanothreads in each run.

The nanothreads are a mixture of syn forms, in which the furan’s oxygen atoms all sit on the same side of the thread, and anti, in which the oxygen atoms alternate along the length of the thread. Mass spectrometry measurements showed that each thread contained about 100 furan rings.

Elacqua’s graduate student Steven Huss says that they now hope to use heat or light to help drive the reaction at just 5 GPa, the pressure used to make synthetic diamonds. That would enable them to use another piece of high-pressure equipment known as a multi-anvil press, which could produce hundreds of milligrams of nanothreads in each batch. The researchers also hope to incorporate different elements on the nanothread’s backbone, such as selenium, which could allow electrical charge to travel along the thread.

The team’s nanothread research was pioneered by John V. Badding of Pennsylvania State University, who led the National Science Foundation Center for Nanothread Chemistry until his death in October 2019. After Badding passed away, “I think it was hard for the students to pick everything up, because John worked in the lab with them and trained them individually,” Elacqua says. But the training has paid off, she adds: “It’s almost like we have four or five mini-Johns in the lab now, because they have that wealth of knowledge.”