Introducing borophane, a stable material made from sheets one atom thick

March 11, 2021

Borophane is set to become a popular multi-use synthetic material. (Mark Hersam)

U.S. researchers have created a new material called borophane that consists of atomically thin sheets of boron and hydrogen atoms and is more durable than similar compounds, potentially leading to innovations in important electronics. 

Borophane and its predecessor borophene, which was first produced in 2015, both have a number of properties that make them appealing for use in batteries, sensors and quantum computers — but the latter quickly degrades outside the confines of ultrahigh vacuum chambers. Borophane, on the other hand, is stable at room temperature and standard air pressure and oxygen levels, the team reported March 11 in Science.

"Since borophane is relatively stable in ambient conditions, it allows two-dimensional boron to be explored in real-world conditions, which is likely to accelerate application development," Mark Hersam, the director of the Materials Research Center at Northwestern University and last author of the paper, told The Academic Times.

Borophene and borophane share some similarities to graphene, which was first grown in 2003 and is made from honeycomb-shaped carbon lattices that are a single atom thick. Graphene is many times stronger than steel and conducts electricity better than copper. Scientists hope to use it to make bendable phones and faster computers, among other things. 

Borophene is even stronger than graphene, and more flexible. It's also lightweight and great at conducting electricity. Under the right conditions, it's also a superconductor, which means that it can transport electricity with no resistance. Borophene's superconductivity is potentially useful for quantum computing, Hersam says. Superconductors are used in Google's quantum computer, which in 2019 took just minutes to perform a mathematical calculation that would have taken a traditional supercomputer 10,000 years, he notes.

"However, borophene is highly chemically reactive, which implies that it irreversibly oxidizes and degrades in air," Hersam said. This reactivity has hindered the exploration of borophene's potential real-world applications, he says, making borophane an intriguing alternative. 

Back in 2015, Hersam and his colleagues created borophene by condensing hot boron atoms onto a silver surface. To make borophane, the researchers exposed crystalline sheets of borophene to hydrogen atoms in an ultrahigh vacuum chamber. 

The resulting borophane lattices were as thin as borophene. However, the added hydrogen prevented the boron atoms from reacting with the oxygen in the air. In contrast to borophene, which degrades within minutes, the borophane persisted unchanged for several days and had degraded only slightly after one week. 

"With ambient stability on the order of days, borophane provides a sufficient time window for most ambient characterization and processing methods," Hersam and his colleagues wrote in the paper.

The team identified eight different versions of borophane, with various arrangements of boron atoms showing up at different temperatures. Further research will be needed to determine how their properties may differ, Hersam said.

To their surprise, the researchers also discovered that they could separate borophane back into its initial ingredients, borophene and hydrogen, by heating it up. 

"In this manner, we can convert between borophene and borophane on demand, which provides a lot of flexibility for future work," Hersam said. "In some applications, you might want to switch back to borophene after performing some processing/handling in air where you need it to be in the borophane state to avoid damage."

However, the fact that borophene and borophane must be grown on metals such as silver could make them challenging to work with. Developing techniques to remove two-dimensional boron from these surfaces will make it easier to integrate the new materials into batteries and other devices, Hersam says. 

As a next step, the researchers will also expose borophene sheets to other elements beyond hydrogen to see whether they can create more new materials with slightly different properties from borophane. 

The study, "Synthesis of borophane polymorphs through hydrogenation of borophene," published March 11 in Science, was authored by Qiucheng Li, Matthew S. Rahn, Shaowei Li and Mark C. Hersam, Northwestern University; Venkata Surya Chaitanya Kolluru, Argonne National Laboratory and University of Florida; Eric Schwenker, Northwestern University and Argonne National Laboratory; Richard G. Hennig, University of Florida; and Pierre Darancet and Maria K. Y. Chan, Argonne National Laboratory and Northwestern-Argonne Institute of Science and Engineering.

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