Two-dimensional black phosphorus—a black, flaky graphite-like material made by heating phosphorus under high pressure—has mustered tremendous interest from materials scientists since 2014 for electronics and photonics applications. But it has been a challenge to produce atomically thin flakes of the material in bulk quantities. Researchers now report a synthesis method that gives the highest yield and thinnest flakes yet. By making suspensions of black phosphorus in an aqueous solution, they have made flakes of the material that are just a few atomic layers thick.

The scalable and environmentally friendly method reported in the journal Proceedings of the National Academy of Sciences could be used to mass-produce 2D-phosphorus, also called phosphorene. "We can imagine using inkjet printing with these dispersions to lay down the material on a substrate," says Mark Hersam, a professor of materials science and engineering at Northwestern University.

Phosphorene is attractive for devices because unlike its carbon cousin graphene, it has an electronic bandgap, which is necessary to switch the flow of electrons on and off in digital logic devices, and for creating photons in light-emitting diodes and lasers.

But while graphene can be grown in bulk using chemical vapor deposition, which has enabled commercial success, phosphorus cannot be vapor-deposited since it is chemically reactive and quickly oxidizes in air. In order to mass-produce phosphorene, researchers have looked at liquid-based synthesis methods. This involves mixing black phosphorus in anhydrous organic solvents such as N-methyl-2-pyrrolidone or dimethylformamide, and then applying sound waves to agitate the material and separate few layers-thick phosphorene flakes. But the process gives a small number of flakes that are relatively thick and small in area, Hersam says.

Hersam and his colleagues instead made a black phosphorus suspension in water through which they had bubbled argon gas to remove oxygen. "Because phosphorene is unstable in air, most people thought you wouldn't want to put [it] in water because it will degrade," Hersam says. "But what we learned is that you need both water and oxygen to degrade. That's why it degrades in air. But if you put phosphorene in deoxygenated water, it's perfectly stable."

After sonicating the aqueous suspension, which also contained surfactants, the researchers sorted, filtered out, and dried the flakes. They were able to get flakes that were a single atom thick, about five to 10 times thinner than previously made flakes.

To test the flakes' electronic properties, the team transferred the largest flakes, which were a few hundred nanometers wide, to a silicon substrate. They made gold/nickel electrodes on top such that the flakes span the distance between the electrodes. This resulted in a field-effect transistor, the basic building block of digital logic. The transistor has current and voltage characteristics comparable to those others have made before from mechanically exfoliated phosphorene.

The biggest challenge for using phosphorene in practical devices is the realization of wafer-scale high quality, crystalline material with desirable thickness, says Fengnian Xia, a professor of electrical engineering at Yale University. "Fortunately, there are multiple promising routes to get there. This work shows the potential of the chemical approach. This demonstration might enable the realization of devices on a large scale."

Read the article in Proceedings of the National Academy of Sciences.