Focus on Phosphorene and Related 2D Materials for Electronics and Optics

Black phosphorus, which is a relatively rare allotrope of phosphorus, was first discovered by Bridgman in 1914. Since the advent of two-dimensional (2D) black phosphorus (which is known as phosphorene due to its resembling graphene sheets) in early 2014, research interest in the arena of black phosphorus was reignited in the scientific and technological communities. Henceforth, a myriad of research studies on this new member of the 2D world have been extensively emerged. Fascinatingly, 2D black phosphorus exhibits a distinctive wrinkled structure with the high hole mobility up to 1000 cm² V⁻¹ s⁻¹, excellent mechanical properties, tunable band structures, anisotropic thermal, electrical and optical properties, thus leading to its marvelous prospects in device applications. This review firstly introduces the state-of-the-art development, structural properties and preparation routes of black phosphorus. In particular, anisotropy involved in mechanical properties, thermal conductivity, carrier transport as well as optical properties is comprehensively discussed. Apart from discussing the recent progress in black phosphorus which is applied to devices (i.e. field effect transistors and optoelectronic), the review also highlights the bottlenecks encountered by the society and finally casts an invigorating perspective and insightful outlook on the future direction of the next-generation 2D black phosphorus by harnessing its remarkable characteristics for energy production.

Few layer phosphorene (FLP) devices are extensively studied due to their unique electronic properties and potential applications on nano-electronics. Here we present magnetotransport studies which reveal electron–electron interactions as the dominant scattering mechanism in hexagonal boron nitride-encapsulated FLP devices. From weak localization measurements, we estimate the electron dephasing length to be 30 to 100 nm at low temperatures, which exhibits a strong dependence on carrier density n and a power-law dependence on temperature (∼T^−0.4). These results establish that the dominant scattering mechanism in FLP is electron–electron interactions.

We have conducted a comprehensive investigation into the magneto-transport properties of few-layer black phosphorus in terms of phase coherence length, phase coherence time, and mobility via weak localization measurement and Hall-effect measurement. We present magnetoresistance data showing the weak localization effect in bare p-type few-layer black phosphorus and reveal its strong dependence on temperature and carrier concentration. The measured weak localization agrees well with the Hikami–Larkin–Nagaoka model and the extracted phase coherence length of 104 nm at 350 mK, decreasing as ∼T^{−0.513+−0.053} with increased temperature. Weak localization measurement allows us to qualitatively probe the temperature-dependent phase coherence time τ_ϕ, which is in agreement with the theory of carrier interaction in the diffusive regime. We also observe the universal conductance fluctuation phenomenon in few-layer black phosphorus within moderate magnetic field and low temperature regime.

Phosphorene is a new two-dimensional material which is commonly prepared by exfoliation from black phosphorus bulk crystals that historically have been synthesized from white phosphorus under high-pressure conditions. The few layers of phosphorene have a direct band gap in the range of 0.3–2 eV and high mobility at room temperature comparable to epitaxial graphene. These characteristics  can be used for the design of high speed digital circuits, radio frequency circuits, flexible and printed systems, and optoelectronic devices. In this work, we synthesized black phosphorus from red phosphorus, which is a safer solid precursor, using sonochemistry. Furthermore, via a variety of microscopy and spectroscopy techniques, we report characterization results of the sonochemically synthesized black phosphorus in addition to the commercial black phosphorus. Finally, we describe the air stability of black phosphors and the crystalline structure of the synthesized material. This is the first result of sonochemical or solution-based synthesis of black phosphorus based on readily available low-cost red phosphorus. This solution-based synthesis of black phosphorus is suitable for printable applications of nanomaterial.

We report first-principles GW–Bethe–Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G₀W₀) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet–triplet splitting, and electron–hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron–hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.