Phosphorene, an emerging two-dimensional material, has received considerable attention due to its layer-controlled direct bandgap, high carrier mobility, negative Poisson's ratio and unique in-plane anisotropy. As cousins of phosphorene, 2D group-VA arsenene, antimonene and bismuthene have also garnered tremendous interest due to their intriguing structures and fascinating electronic properties. 2D group-VA family members are opening up brand-new opportunities for their multifunctional applications encompassing electronics, optoelectronics, topological spintronics, thermoelectrics, sensors, Li- or Na-batteries. In this review, we extensively explore the latest theoretical and experimental progress made in the fundamental properties, fabrications and applications of 2D group-VA materials, and offer perspectives and challenges for the future of this emerging field.

/pdf/recent-progress-in-2d-group-va-semiconductors-from-theory-to-340ay01tfh.pdf

Recent progress in 2D group-VA semiconductors: from theory to experiment

The recent isolation of atomically thin black phosphorus by mechanical exfoliation of bulk layered crystals has triggered an unprecedented interest, even higher than that raised by the first works on graphene and other two-dimensionals, in the nanoscience and nanotechnology community. In this Perspective, we critically analyze the reasons behind the surge of experimental and theoretical works on this novel two-dimensional material. We believe that the fact that black phosphorus band gap value spans over a wide range of the electromagnetic spectrum (interesting for thermal imaging, thermoelectrics, fiber optics communication, photovoltaics, etc.) that was not covered by any other two-dimensional material isolated to date, its high carrier mobility, its ambipolar field-effect, and its rather unusual in-plane anisotropy drew the attention of the scientific community toward this two-dimensional material. Here, we also review the current advances, the future directions and the challenges in this young research...

Black Phosphorus: Narrow Gap, Wide Applications

Phosphorene, a single- or few-layered semiconductor material obtained from black phosphorus, has recently been introduced as a new member of the family of two-dimensional (2D) layered materials. Since its discovery, phosphorene has attracted significant attention, and due to its unique properties, is a promising material for many applications including transistors, batteries and photovoltaics (PV). However, based on the current progress in phosphorene production, it is clear that a lot remains to be explored before this material can be used for these applications. After providing a comprehensive overview of recent advancements in phosphorene synthesis, advantages and challenges of the currently available methods for phosphorene production are discussed. An overview of the research progress in the use of phosphorene for a wide range of applications is presented, with a focus on enabling important roles that phosphorene would play in next-generation PV cells. Roadmaps that have the potential to address some of the challenges in phosphorene research are examined because it is clear that the unprecedented chemical, physical and electronic properties of phosphorene and phosphorene-based materials are suitable for various applications, including photovoltaics.

Phosphorene and Phosphorene‐Based Materials – Prospects for Future Applications

Quantum particles on a lattice with competing long-range interactions are ubiquitous in physics; transition metal oxides1,2, layered molecular crystals3 and trapped-ion arrays4 are a few examples. In the strongly interacting regime, these systems often show a rich variety of quantum many-body ground states that challenge theory2. The emergence of transition metal dichalcogenide moire superlattices provides a highly controllable platform in which to study long-range electronic correlations5-12. Here we report an observation of nearly two dozen correlated insulating states at fractional fillings of tungsten diselenide/tungsten disulfide moire superlattices. This finding is enabled by a new optical sensing technique that is based on the sensitivity to the dielectric environment of the exciton excited states in a single-layer semiconductor of tungsten diselenide. The cascade of insulating states shows an energy ordering that is nearly symmetric about a filling factor of half a particle per superlattice site. We propose a series of charge-ordered states at commensurate filling fractions that range from generalized Wigner crystals7 to charge density waves. Our study lays the groundwork for using moire superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional extended Hubbard model3,13,14 or spin models with long-range charge-charge and exchange interactions15,16.

Correlated insulating states at fractional fillings of moiré superlattices.

Rhenium disulfide (ReS2) is a two-dimensional (2D) group VII transition metal dichalcogenide (TMD). It is attributed with structural and vibrational anisotropy, layer-independent electrical and optical properties, and metal-free magnetism properties. These properties are unusual compared with more widely used group VI-TMDs, e.g., MoS2, MoSe2, WS2 and WSe2. Consequently, it has attracted significant interest in recent years and is now being used for a variety of applications including solid state electronics, catalysis, and, energy harvesting and energy storage. It is anticipated that ReS2 has the potential to be equally used in parallel with isotropic TMDs from group VI for all known applications and beyond. Therefore, a review on ReS2 is very timely. In this first review on ReS2, we critically analyze the available synthesis procedures and their pros/cons, atomic structure and lattice symmetry, crystal structure, and growth mechanisms with an insight into the orientation and architecture of domain and grain boundaries, decoupling of structural and vibrational properties, anisotropic electrical, optical, and magnetic properties impacted by crystal imperfections, doping and adatoms adsorptions, and contemporary applications in different areas.

/pdf/advent-of-2d-rhenium-disulfide-res2-fundamentals-to-1nt3dgwkl7.pdf

Advent of 2D Rhenium Disulfide (ReS2): Fundamentals to Applications

The crystal structure of a material creates a periodic potential that electrons move through giving rise to its electronic band structure. When two-dimensional materials are stacked, the resulting moire pattern introduces an additional periodicity so that the twist angle between the layers becomes an extra degree of freedom for the resulting heterostructure. As this angle changes, the electronic band structure is modified leading to the possibility of flat bands with localized states and enhanced electronic correlations1–6. In transition metal dichalcogenides, flat bands have been theoretically predicted to occur for long moire wavelengths over a range of twist angles around 0° and 60° (ref. 4) giving much wider versatility than magic-angle twisted bilayer graphene. Here, we show the existence of a flat band in the electronic structure of 3° and 57.5° twisted bilayer WSe2 samples using scanning tunnelling spectroscopy. Our direct spatial mapping of wavefunctions at the flat-band energy show that the localization of the flat bands is different for 3° and 57.5°, in agreement with first-principles density functional theory calculations4. Using scanning tunnelling spectroscopy, the flat bands in twisted bilayer WSe2 are shown near both 0° and 60° twist angles.

/pdf/flat-bands-in-twisted-bilayer-transition-metal-2fx5jpvhza.pdf

Flat bands in twisted bilayer transition metal dichalcogenides

Black phosphorus has recently emerged as a promising material for high-performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap, and anisotropic electronic properties. Dynamical evolution of photoexcited carriers and the induced transient change of electronic properties are critical for materials’ high-field performance but remain to be explored for black phosphorus. In this work, we perform angle-resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photoexcitation. We find that the anisotropy of reflectivity is enhanced in the pump-induced quasi-equilibrium state, suggesting an extraordinary enhancement of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results raise attractive possibilities of creating high-field, angle-sensitive electronic, optoelectronic, and remote sensing devices exploiting the dynamical electronic anisotropy with black phosphorus.

Dynamical Evolution of Anisotropic Response in Black Phosphorus under Ultrafast Photoexcitation

The crystal structure of a material creates a periodic potential that electrons move through giving rise to the electronic band structure of the material. When two-dimensional materials are stacked, the twist angle between the layers becomes an additional degree freedom for the resulting heterostructure. As this angle changes, the electronic band structure is modified leading to the possibility of flat bands with localized states and enhanced electronic correlations. In transition metal dichalcogenides, flat bands have been theoretically predicted to occur for long moire wavelengths over a range of twist angles around 0 and 60 degrees giving much wider versatility than magic angle twisted bilayer graphene. Here we show the existence of a flat band in the electronic structure of 3° and 57.5° twisted bilayer WSe2 samples using scanning tunneling spectroscopy. Direct spatial mapping of wavefunctions at the flat band energy have shown that the flat bands are localized differently for 3° and 57.5°, in excellent agreement with first-principle density functional theory calculations.

The authors demonstrate the difference in the spatial dependence of the wave functions at different partial commensurate fillings of the flat bands in graphene. The paper shows that at partial filling the wave functions of the lower band become delocalized while the upper band changes becomes localized at different locations depending on filling.

Probing the wave functions of correlated states in magic angle graphene

The crystal structure of a material creates a periodic potential that electrons move through giving rise to the electronic band structure of the material. When two-dimensional materials are stacked, the twist angle between the layers becomes an additional degree freedom for the resulting heterostructure. As this angle changes, the electronic band structure is modified leading to the possibility of flat bands with localized states and enhanced electronic correlations. In transition metal dichalcogenides, flat bands have been theoretically predicted to occur over a range of twist angles below ~7 degrees. Here we show the existence of a flat band in the electronic structure of a 3{\deg} twisted bilayer WSe2 sample using scanning tunneling spectroscopy. Direct spatial mapping of wavefunctions at the flat band energy have shown that the flat band is localized in the form of a hexagonal network in excellent agreement with first-principle density functional theory calculations.

Zhiming Zhang

Papers

Flat bands in twisted bilayer transition metal dichalcogenides

Dynamical Evolution of Anisotropic Response in Black Phosphorus under Ultrafast Photoexcitation

Flat bands in twisted bilayer transition metal dichalcogenides

Probing the wave functions of correlated states in magic angle graphene

Flat bands in small angle twisted bilayer WSe2