Two years ago, Mark Hersam at Northwestern University discovered a way to stabilize exfoliated black phosphorus – or phosphorene – a layered two-dimensional (2D) semiconductor that chemically degrades in open air but shows great promise in advanced electronics. By encapsulating it in aluminum oxide, he was able to stabilize phosphorene's reactivity to oxygen and water.

"The problem is that now the phosphorene is buried underneath the aluminum oxide coating, which limits what we can do with it," said Hersam, professor of materials science and engineering at Northwestern's McCormick School of Engineering. "Wouldn't it be better if we could stabilize phosphorene without occluding its surface?"

And that is exactly what Hersam and his team have now done.

By coating a single-molecule-thick layer of the organic compound aryl diazonium onto phosphorene, the team effectively imparted the same passivation it achieved with alumina back in 2014. But this time the layer is thin enough to still provide access to the material's surface.

"If it's going to be useful for applications such as sensors, then whatever you want to detect needs to be able to interact with the material," Hersam said. "The thick layer of aluminum oxide prevented any atmospheric species from reaching the phosphorene surface, so it could not be used as a detector."

"We can imagine many possibilities. The future will teach us exactly where phosphorene has a competitive advantage."Mark Hersam, Northwestern University

Supported by the US Office of Naval Research and the US Department of Energy, the research is described in a paper in Nature Chemistry. Christopher Ryder, a graduate student in Hersam's laboratory, served as the paper's first author. Tobin Marks, professor of catalytic chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering, and George Schatz, professor of chemistry and professor of chemical and biological engineering, also co-authored the paper.

In recent years, phosphorene has captured attention as a powerful semiconductor with great potential for use in thin, flexible electronics. Its instability in open air, however, has prevented it from being tested in potential applications such as transistors, optoelectronics, sensors or even batteries. Hersam and his team have now solved this stability problem with their covalently-bonded, single-molecule-thick layer of aryl diazonium. But they also discovered that this layer improves phosphorene’s electronic properties, making it even more suitable for these applications.

"The chemistry influenced the flow of charge through phosphorene," Hersam said. "We achieved improvement in charge mobility, which is related to the speed of the transistor, and how well it switches in an integrated circuit."

Now that Hersam's team has created a stable version of phosphorene, it plans to explore these potential applications. The next step is to create optimized devices based on phosphorene and compare them to devices made with alternative materials.

"We can imagine many possibilities," Hersam said. "The future will teach us exactly where phosphorene has a competitive advantage."

This story is adapted from material from Northwestern University's McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.