Left: Optical image of an encapsulated black phosphorus FET (field effect transistor). scale bar, 10 μm. (Right) Atomic force microscope image of the same

Graphene is known to be the thinnest material on earth, one atom thick, almost a million times slimmer than a strand of hair. The discovery of this material was awarded a Nobel Prize Nobel prize for physics in 2010. But, such layers can also be formed by black phosphorous.

Indeed, scientists at the Technische Universitat Munchen (TUM) have developed a semiconducting material, where individual phosphorous atoms are replaced by arsenic. Working with the German team, American scientists have built the first ever field effect transistors from the new material.

Silicon has been the basis of modern electronics, providing tinier transistors for smaller and smaller devices. While transistors work more efficiently when they are thin, with electrons moving in only two dimensions, they may have reached their physical limits.

To lower the operating voltage of transistors, and thereby reduce the heat they generate, one has to get closer to designing the transistor at the atomic level. Consumers are also seeking flexible devices that could be incorporated in electronics and wearables. Silicon, however, is hard and brittle.

There is therefore a need for new materials. Black arsenic phosphorous maybe the answer.

Benefits of black arsenic phosphorous

Like graphene, it can be formed in thin layers and it is flexible. But unlike graphene, which behaves more like metals with respect to electronic properties, black arsenic phosphorous behaves like semi conductors. Unlike graphene, which acts like a metal, black phosphorus is a natural semiconductor: it can be switched on and off.

An international cooperation from across the Atlantic has produced a field effect transistor made of black arsenic phosphorous. The German team (the TUM and the University of Regensburg) has synthesized the material using a new low-pressure fabrication method that demands less energy, is cheaper, and creates materials have some new electronic and optical properties not available with other 2D materials. The US team (The university of Southern California) produced the field effect transistors. The work was published in Advanced Materials, June 25, 2015.

What is extraordinary is that they are able to produce black arsenic phosphorous, with highly tunable chemical compositions and electronic and optical properties.

They can achieve extremely small band gaps, ranging from 0.3 to 0.15 eV. These band gaps fall into long-wavelength infrared (LWIR) regime and cannot be readily reached by other layered materials. In addition, they exhibit in-plane anisotropic properties, different characteristics along the x- and y-axes in the same plane.

This family of layered black arsenic phosphorous extends the electromagnetic spectra covered by 2D layered materials to the LWIR regime, and may find unique applications for future all 2D layered material based devices.

The array of possible applications ranges from transistors and sensors to mechanically flexible semiconductor devices. For example, the new black arsenic phosphorous material can sense long wavelength infrared, which is important for light radar (LIDAR) systems used in autonomous (and other new) vehicles, for infrared thermal imaging technology, flexible night vision glasses, and environmental sensing.

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