Role of Interlayer Coupling on the Evolution of Band Edges in Few-Layer Phosphorene

Interlayer Quasi-Bonding Interactions in 2D Layered Materials: A Classification According to the Occupancy of Involved Energy Bands

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

InSe: a two-dimensional material with strong interlayer coupling

Atomically thin, two-dimensional (2D) indium selenide (InSe) has attracted considerable attention due to the large tunability in the band gap (from 1.4 to 2.6 eV) and high carrier mobility. The intriguingly high dependence of the band gap on layer thickness may lead to novel device applications, although its origin remains poorly understood, and is generally attributed to the quantum confinement effect. In this work, we demonstrate via first-principles calculations that strong interlayer coupling may be mainly responsible for this phenomenon, especially in the fewer-layer region, and it could also be an essential factor influencing other material properties of β-InSe and γ-InSe. The existence of strong interlayer coupling manifests itself in three aspects: (i) indirect-to-direct band gap transitions with increasing layer thickness; (ii) fan-like frequency diagrams of the shear and breathing modes of few-layer flakes; and (iii) strong layer-dependent carrier mobilities. Our results indicate that multiple-layer InSe may be deserving of attention from FET-based technologies and may also be an ideal system to study interlayer coupling, possibly inherent in other 2D materials.

Imidazolium Ionic Liquid Mediates Black Phosphorus Exfoliation while Preventing Phosphorene Decomposition

Forthcoming applications in electronics and optoelectronics make phosphorene a subject of vigorous research efforts. Solvent-assisted exfoliation of phosphorene promises affordable delivery in industrial quantities for future applications. We demonstrate, using equilibrium, steered and umbrella sampling molecular dynamics, that the 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄] ionic liquid is an excellent solvent for phosphorene exfoliation. The presence of both hydrophobic and hydrophilic moieties, as well as substantial shear viscosity, allows [EMIM][BF₄] simultaneously to facilitate separation of phosphorene sheets and to protect them from getting in direct contact with moisture and oxygen. The exfoliation thermodynamics is moderately unfavorable, which indicates that an external stimulus is necessary. Unexpectedly, [EMIM][BF₄] does not coordinates phosphorene by π-electron stacking with the imidazole ring. Instead, the solvation proceeds via hydrophobic side chains, while polar imidazole rings form an electrostatically stabilized protective layer. The simulations suggest that further efforts in solvent engineering for phosphorene exfoliation should concentrate on use of weakly coordinating ions and grafting groups that promote stronger dispersion interactions and on elongation of nonpolar chains.

Emerging Trends in Phosphorene Fabrication towards Next Generation Devices

The challenge of science and technology is to design and make materials that will dominate the future of our society. In this context, black phosphorus has emerged as a new, intriguing two-dimensional (2D) material, together with its monolayer, which is referred to as phosphorene. The exploration of this new 2D material demands various fabrication methods to achieve potential applications- this demand motivated this review. This article is aimed at supplementing the concrete understanding of existing phosphorene fabrication techniques, which forms the foundation for a variety of applications. Here, the major issue of the degradation encountered in realizing devices based on few-layered black phosphorus and phosphorene is reviewed. The prospects of phosphorene in future research are also described by discussing its significance and explaining ways to advance state-of-art of phosphorene-based devices. In addition, a detailed presentation on the demand for future studies to promote well-systemized fabrication methods towards large-area, high-yield and perfectly protected phosphorene for the development of reliable devices in optoelectronic applications and other areas is offered.

First-principles study on structural, thermal, mechanical and dynamic stability of T'-MoS2

Using first-principles density functional theory calculations, we investigate the structure, stability, optical modes and electronic band gap of a distorted tetragonal MoS₂ monolayer (T'-MoS₂). Our simulated scanning tunnel microscopy (STM) images of T'-MoS₂ are dramatically similar to those STM images which were identified as K _x (H₂O) _y MoS₂ from a previous experimental study. This similarity suggests that T'-MoS₂ might have already been experimentally observed, but due to being unexpected was misidentified. Furthermore, we verify the stability of T'-MoS₂ from the thermal, mechanical and dynamic aspects, by ab initio molecular dynamics simulation, elastic constants evaluation and phonon band structure calculation based on density functional perturbation theory, respectively. In addition, we calculate the eigenfrequencies and eigenvectors of the optical modes of T'-MoS₂ at [Formula: see text] point and distinguish their Raman and infrared activity by pointing out their irreducible representations using group theory. At the same time, we compare the Raman modes of T'-MoS₂ with those of H-MoS₂ and T-MoS₂. Our results provide useful guidance for further experimental identification and characterization of T'-MoS₂.

Infrared fingerprints of few-layer black phosphorus

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.

Few-layered black phosphorus offers an infrared bandgap, complementing that of graphene and transition metal dichalcogenides. Here, the authors investigate the thickness- and strain-dependent electronic structure of black phosphorus using polarised infrared spectroscopy.

Stacking dependence of carrier transport properties in multilayered black phosphorous

We present the effect of different stacking orders on carrier transport properties of multi-layer black phosphorous. We consider three different stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6 layers). We employ a hierarchical approach in density functional theory (DFT), with structural simulations performed with generalized gradient approximation (GGA) and the bandstructure, carrier effective masses and optical properties evaluated with the meta-generalized gradient approximation (MGGA). The carrier transmission in the various black phosphorous sheets was carried out with the non-equilibrium green's function (NEGF) approach. The results show that ACA stacking has the highest electron and hole transmission probabilities. The results show tunability for a wide range of band-gaps, carrier effective masses and transmission with a great promise for lattice engineering (stacking order and layers) in black phosphorous.

Preparation of large size, few-layer black phosphorus nanosheets via phytic acid-assisted liquid exfoliation

Ultrathin large BP nanosheets prepared via small molecule–assisted liquid phase exfoliation exhibited attractive electron accepting abilities from photosensitizers.