  Two-dimensional (2D) layered materials with diverse properties have attracted
significant interest in the past decade. The layered materials discovered so
far have covered a wide, yet discontinuous electromagnetic spectral range from
semimetallic graphene, insulating boron nitride, to semiconductors with
bandgaps from middle infrared to visible light. Here, we introduce new layered
semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical
compositions and electronic and optical properties. Transport and infrared
absorption studies demonstrate the semiconducting nature of b-AsP with tunable
bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into long-wavelength
infrared (LWIR) regime and cannot be readily reached by other layered
materials. Moreover, polarization-resolved infrared absorption and Raman
studies reveal in-plane anisotropic properties of b-AsP. This family of layered
b-AsP materials extend 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.
