Abstract
Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Although highly desirable, a comprehensive infrared characterization is still lacking. Here we report a systematic infrared study of mechanically exfoliated few-layer black phosphorus, with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as its infrared fingerprints. Surprisingly, unexpected absorption features, which are associated with the forbidden optical transitions, have been observed. Furthermore, we unambiguously demonstrate that controllable uniaxial strain can be used as a convenient and effective approach to tune the electronic structure of few-layer black phosphorus. Our study paves the way for black phosphorus applications in infrared photonics and optoelectronics.
Highlights
Black phosphorus is an infrared layered material
It has been predicted that the bandgap of black phosphorus (BP) is always direct regardless of layer (L) number and ranges from 0.3 to 2 eV6,8, bridging the gap between zero-gap graphene and large-gap transition metal dichalcogenides[9]
With the majority of the optical transitions expected in the mid- to near-infrared frequency range for few-layer BP, Fourier transform infrared spectrometer (FTIR)-based infrared spectroscopy is believed to be the superior characterization tool
Summary
Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Compared with the bulk counterpart, one of the most intriguing distinctions for single or few-layer 2D materials is the highly tunable physical properties, through various techniques This tunability is typically associated with the modification of the electronic band structure. We systematically investigate the evolution of electronic structures in few-layer BP with layer number ranging from 2 up to 15, and report the experimental demonstration of highly tunable electronic structures in few-layer BP via controllable uniaxial strain[21,22,23,24,25], using polarized infrared spectroscopy. The infrared absorption shows strong polarization dependence, with strong optical resonances showing up in the AC direction This dependence provides us a reliable way to determine the crystallographic orientation, which complements polarized Raman spectroscopy.
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