We report the quasiparticle band gap, excitons, and highly anisotropic optical responses of fewlayer black phosphorous (phosphorene). It is shown that these new materials exhibit unique manyelectron effects; the electronic structures are dispersive essentially along one dimension, leading to particularly enhanced self-energy corrections and excitonic effects. Additionally, within a wide energy range, including infrared light and part of visible light, few-layer black phosphorous absorbs light polarized along the structure's armchair direction and is transparent to light polarized along the zigzag direction, making them viable linear polarizers for applications. Finally, the number of phosphorene layers included in the stack controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy-window across a wide range.PACS numbers:

Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.

For decades, two-dimensional electron gases (2DEG) have allowed important experimental discoveries and conceptual developments in condensed-matter physics. When combined with the unique electronic properties of two-dimensional crystals, they allow rich physical phenomena to be probed at the quantum level. Here, we create a 2DEG in black phosphorus--a recently added member of the two-dimensional atomic crystal family--using a gate electric field. The black phosphorus film hosting the 2DEG is placed on a hexagonal boron nitride substrate. The resulting high carrier mobility in the 2DEG allows the observation of quantum oscillations. The temperature and magnetic field dependence of these oscillations yields crucial information about the system, such as cyclotron mass and lifetime of its charge carriers. Our results, coupled with the fact that black phosphorus possesses anisotropic energy bands with a tunable, direct bandgap, distinguish black phosphorus 2DEG as a system with unique electronic and optoelectronic properties.

Lately rediscovered orthorhombic black phosphorus (BP) exhibits promising properties for near- and mid-infrared optoelectronics. Although recent electrical measurements indicate that a vertical electric field can effectively reduce its transport bandgap, the impact of the electric field on light-matter interaction remains unclear. Here we show that a vertical electric field can dynamically extend the photoresponse in a 5 nm-thick BP photodetector from 3.7 to beyond 7.7 μm, leveraging the Stark effect. We further demonstrate that such a widely tunable BP photodetector exhibits a peak extrinsic photo-responsivity of 518, 30, and 2.2 mA W−1 at 3.4, 5, and 7.7 μm, respectively, at 77 K. Furthermore, the extracted photo-carrier lifetime indicates a potential operational speed of 1.3 GHz. Our work not only demonstrates the potential of BP as an alternative mid-infrared material with broad optical tunability but also may enable the compact, integrated on-chip high-speed mid-infrared photodetectors, modulators, and spectrometers.

ABSTRACT:We report the electronic structure and optical absorption spectra of monolayer black phosphorus (phosphorene) nanoribbons (PNRs) via first-principles simulations. The band gap of PNRs is strongly enhanced by quantum confinement. However, differently orientated PNRs exhibit distinct scaling laws for the band gap vs the ribbon width ( ). The band gaps of armchair PNRs scale as ⁄ , while zigzag PNRs exhibit a ⁄ behavior. These distinct scaling laws reflect a significant implication of the band dispersion of phosphorene: electrons and holes behave as classical particles along the zigzag direction, but resemble relativistic particles along the armchair direction. This unexpected merging of classical and relativistic properties in a single material may produce novel electrical and magnetotransport properties of few-layer black phosphorus and its ribbon structures. Finally, the respective PNRs host electrons and holes with markedly different effective masses and optical responses, which are suitable for a wide range of applications. KEYWORDS:Phosphorene nanoribbon, band gap, relativistic particle Graphene-inspired two-dimensional (2D) structures have garnered tremendous interest in fundamental science and have inspired broad applications. [1][2][3][4][5][6][7][8] These layered structures can be etched or patterned 2 along a specific lattice direction, forming one-dimensional (1D) strips, called nanoribbons. Graphene nanoribbons (GNRs) and MoS 2 nanoribbons are quintessential examples of these 1D strips. 9-11 Because of unique quantum confinement and edge effects, nanoribbons exhibit many exploitable electrical, optical, and magnetic properties. [12][13][14][15][16] Recently, few-layer black phosphorus (phosphorene), a novel 2D direct band gap semiconductor, was successfully exfoliated with promising electric and opticalproperties. [17][18][19][20] Although phosphorene nanoribbons (PNRs) have yet to be fabricated, the research history of graphene and other 2D materials strongly suggests that theoretical predictions of the electronic structures and optical responses of PNRs will be essential for informing and motivating future research of phosphorene-based nanoelectronics.Unlike other widely studied 2D structures, the band structure, electrical conductivity, and optical responses of few-layer black phosphorus are all highly anisotropic. 18,[21][22][23] These anisotropies can be associated with the orientation of PNRs, and can be used to dramatically tune the material's transport behaviors and even unexpected electronic properties. This is a distinguishing feature of PNRs that makes them particularly promising for potential applications. Moreover, as seen in other nanoribbons, 24-26 the relaxations and passivations of edge structures will provide additional ways of modifying the PNR's electronic structure.In this work, we employ first-principles simulations to study the electronic structure and optical absorption spectra of two typical types of PNRs, armchair PNRs (APNRs) and zigzag PNRs (ZPNRs).All of the studie...