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

Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

Inspired by rich physics and functionalities of graphenes, scientists have taken an intensive interest in two-dimensional (2D) crystals of h-BN (analogue of graphite, so-called "white" graphite). Recent calculations have predicted the exciting potentials of BN nanoribbons in spintronics due to tunable magnetic and electrical properties; however no experimental evidence has been provided since fabrication of such ribbons remains a challenge. Here, we show that few- and single-layered BN nanoribbons, mostly terminated with zigzag edges, can be produced under unwrapping multiwalled BN nanotubes through plasma etching. The interesting stepwise unwrapping and intermediate states were observed and analyzed. Opposed to insulating primal tubes, the nanoribbons become semiconducting due to doping-like conducting edge states and vacancy defects, as revealed by structural analyses and ab initio simulations. This study paves the way for BN nanoribbon production and usage as functional semiconductors with a wide range of applications in optoelectronics and spintronics.

"White graphenes": boron nitride nanoribbons via boron nitride nanotube unwrapping

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

/pdf/graphene-mos2-hybrid-technology-for-large-scale-two-3dg0f0mu9d.pdf

Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

Research into layered transition metal dichalcogenides (TMDCs), most notably those of molybdenum and tungsten disulfides, has become extensive, involving fields as diverse as optoelectronics, spintronics, energy storage, lubrication, and catalysis. The modification of TMDCs by transition metal doping can improve their performance in such applications and hence extend their potential for technological applications. This review concerns the synthetic strategies that have been used to incorporate transition metals into TMDCs and the applications of the resultant materials and relevant computational studies on the predicted properties of the doped materials.

http://pubsdc3.acs.org/doi/pdfplus/10.1021/acs.chemmater.6b00430

Synthesis, Properties, and Applications of Transition Metal-Doped Layered Transition Metal Dichalcogenides

Custom-made stacks of two-dimensional materials can be constructed to achieve specific optoelectronic properties in devices. This work uses density functional theory to investigate the electronic structures and transport properties of phosphorene--tin-dichalcogenide van der Waals heterostructures in an external electric field. The results show that these systems exhibit rare type-III (broken-gap) band alignment, which will facilitate the development of tunnel field-effect transistors. Moreover, the band alignment can be tuned effectively by the applied electric field.

Effects of Electric Field on the Electronic Structures of Broken-gap Phosphorene/Sn X 2 (X = S , Se ) van der Waals Heterojunctions

Based on first-principles calculations, the electronic structures and magnetism are investigated in 3d transition metal (TM)-embedded porous two-dimensional (2D) C2N monolayers. Numerical results indicate that except Mn and Co atoms, other TM atoms can be embedded stably in the 2D C2N monolayer. Moreover, the magnetic moments of the TM-embedded C2N monolayer depend highly on the atomic number of the TM atoms. The Sc, Ti, V, Cr, Mn, Fe, Co and Ni atom-embedded C2N monolayers possess a ferromagnetic ground state, while embedding Cu can induce paramagnetic characteristics in the 2D C2N monolayer. Meanwhile, the Zn-embedded C2N monolayer exhibits a nonmagnetic ground state. These results indicate that the magnetism of 2D C2N monolayers can be tuned via embedding TM atoms.

Tuning the electronic structures and magnetism of two-dimensional porous C2N via transition metal embedding

The electronic and magnetic properties of n- and p-type impurities by means of group V and VII atoms substituting sulfur in a MoS2 monolayer were investigated using first-principles methods based on density functional theory. Numerical results show that group V and VII atoms can induce magnetic properties, and the magnetic moment mainly originates from the dopant’s p orbital and neighbor Mo atoms’ d orbital. N-, P-, F-, and I-doped (or As-doped) MoS2 exhibits magnetic nanomaterial (or metallic) features, and Cl- and Br-doped systems exhibit half-metallic ferromagnetism (HMF). Moreover, the formation energy calculations also indicate that it is energetically favorable and relatively easier to incorporate group V and VII atoms into a MoS2 monolayer under Mo-rich experimental conditions. The formation energy of the F-doped system is the lowest, the next lowest formation energy is obtained in the Cl-doped system. By comparison, Cl-doped MoS2 is more suitable for spin injection. This finding is important for the achievement of spin devices on MoS2 nanostructures.

Xu Zhao

Papers

Effects of Electric Field on the Electronic Structures of Broken-gap Phosphorene/Sn X 2 (X = S , Se ) van der Waals Heterojunctions

Tuning the electronic structures and magnetism of two-dimensional porous C2N via transition metal embedding

Electronic and magnetic properties of n-type and p-doped MoS2 monolayers

Electronic and magnetic properties of X-doped (X = Ti, Zr, Hf) tungsten disulphide monolayer

Effect of structural defects on electronic and magnetic properties of pristine and Mn-doped MoS2 monolayer