The evolution of electronic structures in few-layer black phosphorus by infrared spectroscopy.

Black phosphorus (BP) is a newly discovered two-dimensional material with puckered lattice structures, exhibiting many intriguing properties, such as tunable direct bandgap, in-plane anisotropy and relatively high carrier mobility. In this work, we report the first systematic infrared study of mechanically exfoliated few-layer BP using FTIR (Fourier transform infrared spectrometer), with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer BP exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as an IR fingerprint. The evolution of electronic structures in few-layer BP is well understood within the framework of a tight binding model. Our study paves the way for BP applications in infrared optoelectronics.