Nature Commun. 5, 4458 (2014)

Single-layer, two-dimensional materials such as graphene, molybdenum disulphide and tungsten diselenide are attracting attention for their usefulness in photonics. However, scientists in the USA believe that a stable allotrope of phosphorus called black phosphorus (BP) may also turn out to be useful as a layered, thin-film material. BP has a small bandgap of 0.3 eV, which is well-suited to applications in near- and mid-infrared optoelectronics and conveniently services the space between zero-gap graphene and large-gap transition metal dichalcogenides. It also features in-plane anisotropy that could prove useful for realizing thin-film infrared polarizers or plasmonic devices. The team fabricated samples of BP and then used polarization-resolved infrared spectroscopy and angle-resolved conductivity measurements to investigate its properties. Studies of a 15-nm-thick layer of BP indicated a Hall mobility of 1,000 and 600 cm2 V−1 s−1 in the x and y directions within the plane, respectively, at a temperature of 120 K. Field-effect transistors made using 5-nm-thick BP were fabricated and seen to provide an on–off current ratio of 105, a mobility of 205 cm2 V−1 s−1 and good current saturation characteristics at room temperature.