German - US team make competitor to graphene
Chemists at the Technical University of Munich (TUM) have developed a 2D semiconducting material in which individual phosphorus atoms are replaced by arsenic. In a collaborative international effort, American colleagues have also built the first field-effect transistors from the new material.
Like graphene, which consists of a single layer of carbon atoms, black arsenic phosphorus forms extremely thin layers. The array of possible applications ranges from transistors and sensors to mechanically flexible semiconductor devices. Unlike graphene, whose electronic properties are similar to those of metals, black arsenic phosphorus behaves like a semiconductor.
The compounds were synthesised by Marianne Koepf at the laboratory of the research group for Synthesis and Characterisation of Innovative Materials at the TUM. The field effect transistors were built and characterised by a group headed by Chongwu Zhou and Bilu Liu at the Department of Electrical Engineering at USC.
The new technology developed at TUM allows the synthesis of black arsenic phosphorus without high pressure. This 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," saysTom Nilges, head of the research group for Synthesis and Characterisation of Innovative Materials.
Detectors for infrared
With an arsenic concentration of 83 percent the material exhibits an extremely small band gap of only 0.15 electron volts, making it suitable for sensors which can detect long wavelength infrared radiation. LiDAR (Light Detection and Ranging) sensors operate in this wavelength range, for example. They are used, among other things, as distance sensors in automobiles. Another application is the measurement of dust particles and trace gases in environmental monitoring.
A further interesting aspect of these new, 2D semiconductors is their anisotropic electronic and optical behaviour. The material exhibits different characteristics along the x- and y-axes in the same plane. To produce graphene like films the material can be peeled off in ultra thin layers. The thinnest films obtained so far are only two atomic layers thick.
This work was supported by the Office of Naval Research (ONR), the Air Force Office of Scientific Research (AFOSR), the Center of Excellence for Nanotechnologies (CEGN) of King Abdul-Aziz City for Science and Technology (KACST), the German Research Council (DFG) and the TUM Graduate School.
'Black Arsenic-Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties' by Bilu Liu et al, Adv. Mater., 2015, DOI: 10.1002/adma.201501758