Electronic and optical properties of boron-based hybrid monolayers.

Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 1012cm-2which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in thex-direction is accompanied with that in the visible region along they-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications.

Strain modulated carrier mobility and optical properties of graphene nanowiggles.

Recently, synthesized Chevron graphene nanoribbons (CGNRs) and its laterally extended counterpart known as extended CGNRs (ECGNR) are constructed by making alternated regular cuts in pristine graphene nanoribbons (GNRs). First-principles calculations demonstrate that these GNRs are super-ductile and possess width-dependent mechanical properties. The Young's modulus is calculated to be 389.4 GPa and 414.6 GPa for CGNR and ECGNR, respectively. The bandgap of these nanoribbons decreases on the application of tensile strain. The carrier effective masses are found to be highly sensitive towards mechanical strains. The holes (electrons) mobility of ECGNR is calculated to be 7.68 × 104 cm2 V-1 s-1 (1.69 × 104 cm2 V-1 s-1), which is higher than that of CGNR can be further enhanced by elongation. The prominent peaks of the imaginary part of dielectric function and electron energy loss spectra show redshift on increasing the tensile strain. The electron energy loss spectra show intense plasmonic structure in low energy spectrum indicating GNRs to be more sensitive to the visible region than ultra violet spectrum. Our results provide insight about the possible applications of GNRs in the fields of high-speed transistors, sensors, photonics, and optoelectronics.

Two dimensional allotropes of arsenene with a wide range of high and anisotropic carrier mobility.

Considering the rapid development of experimental techniques for fabricating 2D materials in recent years, various monolayers are expected to be experimentally realized in the near future. Motivated by the recent research activities focused on the honeycomb arsenene monolayers, the stability and carrier mobility of non-honeycomb and porous allotropic arsenene are determined using first principles calculations. In addition to five honeycomb structures of arsenene, a total of eight other structures are considered in this study. An extensive analysis comprising energetics, phonon spectra and mechanical properties confirms that these structures are energetically and dynamically stable. All these structures are semiconductors with a broad range of band gaps varying from ∼1 eV to ∼2.5 eV. Significantly, these monolayer allotropes possess anisotropic carrier mobilities as high as several hundred cm2 V-1 s-1 which is comparable with well-known 2D materials such as black phosphorene and monolayer MoS2. Combining such broad band gaps and superior carrier mobilities, these monolayer allotropes can be promising candidates for the superior performance of the next generation nanoscale devices. We further explore these monolayer allotropes for photocatalytic water splitting and find that arsenene monolayers have potential for usage in visible light driven photocatalytic water splitting.

Pressure and electric field-induced metallization in the phase-engineered ZrX2 (X = S, Se, Te) bilayers.

The phase-, pressure- and electric field-induced changes in the electronic properties of Zr dichalcogenide, ZrX2 (X = S, Se, Te), bilayers are investigated using density functional theory. On going from the trigonal (T) to hexagonal (H) phase, a significant modulation in the electronic structure of bilayer dichalcogenides is predicted. This is mainly due to the distinct stacking nature of the bilayer in two phases leading to a delicate difference in the interplanar interaction, which is concurrently affected by the nature of X-X bonding. Application of the pressure reduces the band gap of layered dichalcogenides leading to the metallization of the ZrTe2 bilayer for ≈6 GPa. Similarly, application of the transverse electric field (0.05-0.25 V Å(-1)) induces a complete metallization in dichalcogenide bilayers. Our results show that band gap engineering by changing the phase, applying pressure and electric field can be an effective strategy to modulate the electronic properties of bilayer dichalcogenides for the next-generation device applications.

Electronic stability and electron transport properties of atomic wires anchored on the MoS2 monolayer.

The stability, electronic structure, and electron transport properties of metallic monoatomic wires anchored on the MoS2 monolayer are investigated within the density functional theory. The anchoring of the atomic wires on the semiconducting monolayer significantly modifies its electronic properties; the metallic characteristics of the assembled monolayers appear in the density of states and band structure of the system. We find that Cu, Ag and Au wires induce the so-called n-type doping effect, whereas Pt wires induce a p-type doping effect in the monolayer. The distinctly different behavior of Pt-MoS2 compared to the rest of the metallic wires is reflected in the calculated current-voltage characteristics of the assembled monolayers with a highly asymmetric behavior of the out-of-the-plane tunneling current with respect to the polarity of the external bias. The results of the present study are likely to extend the functionality of the MoS2 monolayer as a candidate material for the novel applications in the areas of catalysis and optoelectronic devices.

Ferromagnetism in Defected TMD (MoX2, X = S, Se) Monolayer and Its Sustainability under O2, O3, and H2O Gas Exposure: DFT Study.

Spin-polarized density-functional theory (DFT) has been employed to study the effects of atmospheric gases on the electronic and magnetic properties of a defective transition-metal dichalcogenide (TMD) monolayer, MoX2 with X = S or Se. This study focuses on three single vacancies: (i) molybdenum "VMo"; (ii) chalcogenide "VX"; and (iii) di-chalcogenide "VX2". Five different samples of sizes ranging from 4 × 4 to 8 × 8 primitive cells (PCs) were considered in order to assess the effect of vacancy-vacancy interaction. The results showed that all defected samples were paramagnetic semiconductors, except in the case of VMo in MoSe2, which yielded a magnetic moment of 3.99 µB that was independent of the sample size. Moreover, the samples of MoSe2 with VMo and sizes of 4 × 4 and 5 × 5 PCs exhibited half-metallicity, where the spin-up state becomes conductive and is predominantly composed of dxy and dz2 orbital mixing attributed to Mo atoms located in the neighborhood of VMo. The requirement for the establishment of half-metallicity is confirmed to be the provision of ferromagnetic-coupling (FMC) interactions between localized magnetic moments (such as VMo). The critical distance for the existence of FMC is estimated to be dc≅ 16 Å, which allows small sample sizes in MoSe2 to exhibit half-metallicity while the FMC represents the ground state. The adsorption of atmospheric gases (H2O, O2, O3) can drastically change the electronic and magnetic properties, for instance, it can demolish the half-metallicity characteristics. Hence, the maintenance of half-metallicity requires keeping the samples isolated from the atmosphere. We benchmarked our theoretical results with the available data in the literature throughout our study. The conditions that govern the appearance/disappearance of half-metallicity are of great relevance for spintronic device applications.

Tunnelling characteristics of Stone-Wales defects in monolayers of Sn and group-V elements.

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby strongly influencing the electronic properties of layered systems. For the monolayers of Sn and group-V elements, we report the results based on density functional theory determining the role of Stone-Wales (SW) defects in modifying their electronic properties. The calculated results find the electronic properties of the Sn monolayer to be strongly dependent on the concentration of SW defects, e.g. defective stanene has nearly zero band gap (≈0.03 eV) for the defect concentration of 2.2 × 1013 cm-2 which opens up to 0.2 eV for the defect concentration of 3.7 × 1013 cm-2. In contrast, SW defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current-voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles. These distinctive features can serve as fingerprints to identify the topological defects in experiments for the monolayers of group-IV and group-V elements.