Semiconducting sheets of phosphorus just a few atoms thick show promise as a material for speedy, low-power, flexible electronics. But one major hurdle has been that making high-quality, electronic-grade black phosphorus is painstakingly slow. Now, chemists have demonstrated a solution-based method that can more readily produce large amounts of the material (ACS Nano 2015, DOI: 10.1021/acsnano.5b01143).
Black phosphorus, like other ultrathin electronic materials such as graphene and molybdenum disulfide, is an attractive material for use in durable, flexible, high-performance electronics for smartphones and other devices. Compared with other two-dimensional materials, black phosphorus combines good semiconducting properties with a relatively high charge mobility. Electrons move through it quickly, which means speedier switching and faster computation. Researchers started working on electronic applications of black phosphorus in 2014, and so far, they have constructed individual transistors out of the material. However, today’s microprocessors integrate millions or billions of these switches in complex circuits.
As electrical engineers figure out how to build more sophisticated devices with the material, chemists are working on the more fundamental problem of producing swaths large enough for building circuits. The method that currently produces the best quality black phosphorus is the same as the one originally used to make graphene—mechanical exfoliation, says Mark C. Hersam, a chemist at Northwestern University. Researchers crush a chunk of black phosphorus, then use adhesive tape to peel apart the layers until they achieve films just a few layers thick. With this “Scotch-tape method” researchers can produce only limited amounts of the material, Hersam says, which slows the progress of research and makes manufacturing it impractical.
Hersam specializes in using solution chemistry to produce, sort, and print novel electronic materials, including carbon nanotubes and graphene. He figured that similar techniques should work to exfoliate black phosphorus in solution, as long as they could keep out water and oxygen, which would degrade the material.
He and his colleagues place a crystal of black phosphorus and a solvent in the bottom of an ultrasonication tube, which uses a rapidly vibrating metal tip to agitate a liquid. The combined action of the solvent and the sonication separates the black phosphorus into sheets just nanometers thick suspended within the liquid. They then spin-coat this “ink” onto a surface, covering it with a random distribution of thin black phosphorus flakes, each of which can then be fashioned into a transistor. Hersam’s group tried seven different organic solvents and found that the best one was N-methylpyrrolidone, a high surface-tension liquid that also excels at exfoliating graphene in solution.
The researchers made transistors from the solvent-exfoliated black phosphorus and found that the devices had a charge mobility similar to ones made from mechanically exfoliated black phosphorus.
So far, researchers haven’t had much success producing black phosphorus suitable for electronics at larger scales, says Fengnian Xia, an electrical engineer at Yale University who has been developing black phosphorus electronics. Hersam’s demonstration is just the first step, he says, pointing out that with spin-coating, researchers cannot precisely control where the black phosphorus sheets land on a surface. Hersam is now working on a way to deposit the flakes in predetermined locations.