Black Phosphorus: The New And Improved Graphene

The fatal flaw inherent in graphene electronics has already been heavily discussed – graphene-based transistors lack an OFF state making them ineffective switches despite a myriad of other properties that seem so full of potential. Now scientists report a new two dimensional material that avoids this pitfall while maintaining many of graphene’s remarkable abilities. Black phosphorous is the next big thing in the 2D revolution.

Allotropes of phosphorus: the four main forms of phosphorus -- white, red, violet and black. Image from Materialscientist.Allotropes of phosphorus: the four main forms of phosphorus -- white, red, violet and black. Image from Materialscientist.

Phosphorus is the 15th element of the periodic table and can exist in several allotropes, ie. different structural arrangements of the same element. The most common of these are white and red phosphorus, though violet and the newly popular black variations are also prevalent. Black phosphorus is obtained by heating white phosphorus under the massively high pressure of 12,000 atmospheres – much greater than what is found at even the greatest ocean depths. The black product is similar in appearance to graphite thereby prompting researchers to investigate its electronic possibilities. Like graphite, which can be exfoliated using simple Scotch tape to produce valuable graphene, black phosphorous given the Scotch tape treatment yields ‘phosphorene’. And phosphorene, it turns out, could be revolutionary. As with graphene, it is atomically thin, cheap to produce, and shows considerable potential as an electronic material. Unlike graphene, it does not have a major stumbling block preventing its use in transistors.

Black phosphorus: the appearance and chemical structure of the newest electronic materials' star: black phosphorus. Image from Alshaer666.Black phosphorus: the appearance and chemical structure of the newest electronic materials' star: black phosphorus. Image from Alshaer666.

The reason graphene has no OFF state is because the material lacks an electronic band gap, a range of forbidden energy levels. The size of a material’s band gap dictates its behavior. Those with large band gaps are insulating, small band gaps are semiconducting, and those without band gaps are conducting. Graphene lacks a band gap and is classified a semimetal rather than the more electronically relevant semiconductor. Silicon, for example, is a semiconductor. Here’s where black phosphorous is really special. Not only does it naturally have a small band gap, making it semiconducting, its band gap can be tuned! A simple procedure allows the user to dictate the electronic properties of the material. To carry out the electronic manipulation, the surface of the phosphorene is decorated with potassium atoms which produce an electric field at the phosphorous surface and thus the resultant adjustable band gap.

The research, published last week by one of the most prestigious journals, Science, was conducted by a Korean team at the Pohang University of Science and Technology led by Professor Keun Su Kim. Hoping to find a material akin to graphene, but with a more workable band gap, they surprised even themselves in discovering the utility of black phosphorous. By developing a material that can be easily manipulated, significantly greater flexibility is added in the production of, for example, next generation telecommunication lasers and solar panels.

Both the scientific community and the public seem to be waiting with bated breath for the true value of graphene to be exposed. Research like this, and other developments using boron nitride, suggests that maybe graphene’s legacy will not be in the material itself, but rather in the way it seems to have kicked off a two dimensional materials revolution. Regardless, Moore's Law seems to have a new lease on life.

Via Science Daily and Science.