Black arsenic phosphorus

Black arsenic phosphorus, a reflection electron microscope image.

FOR many decades, silicon has formed the basis of modern electronics. But the size of silicon transistors is reaching its physical limit and silicon is hard and brittle, whereas a flexible material would be more suitable for new applications of semi- conductors. This has triggered a search for new materials. Graphene—the one-atom-thick sheet of carbon with desirable electrical properties, strength and flexibility—is fast emerging as the promising new material that might one day replace silicon. Chemists at the Technical University of Munich (TUM) have developed black arsenic phosphorus, a new material in which individual phosphorus atoms are replaced by arsenic, which could become a competing alternative to graphene. Like graphene, it forms extremely thin layers. Unlike graphene, it behaves like a semiconductor.

Cooperation between the TUM and the University of Regensburg in Germany and the University of Southern California and Yale University in the U.S. has now, for the first time, produced a field effect transistor made of black arsenic phosphorus. The new technology developed at the TUM allows the synthesis of black arsenic phosphorus without high pressure, which requires less energy and is cheaper. The gap between valence and conduction bands can be precisely tuned by adjusting the arsenic concentration. “This allows us to produce materials with previously unattainable electronic and optical properties in an energy window that was hitherto inaccessible,” said Tom Nilges of TUM.

With an arsenic concentration of 83 per cent the material exhibits an extremely small band gap of only 0.15 electronvolt, making it ideal for sensors for detecting far infrared radiation. Light Detection and Ranging sensors operate in this wavelength range, for example. They are used, among other things, in the measurement of dust particles and trace gases in environmental monitoring.