Abstract: Atomic vacancies and nanopores act as local scattering centers and modify the transport properties of charge carriers in phosphorene nanoribbons (PNRs). We investigate the influence of such atomic defects on the electronic transport of multi-terminal PNR. We use the non-equilibrium Green's function approach within the tight-binding framework to calculate the transmission coefficient and the conductance. Terminals induce band mixing resulting in oscillations in the conductance. In the presence of atomic vacancies and nanopores the conductance between non-axial terminals exhibit constructive scattering, which is in contrast to mono-axial two-terminal systems where the conductance exhibits destructive scattering. This can be understood from the spatial local density of states of the transport modes in the system. Our results provide fundamental insights into the electronic transport in PNR-based multi-terminal systems and into the ability of atomic defects and nanopores through tuning the transport properties.

Abstract: We investigate the energy levels and persistent currents of MoS2 quantum rings having different shapes and edge types in the presence of a perpendicular magnetic field by means of the tight-binding approach. We find states localized at the inner and outer boundaries of the ring. These energy levels exhibit different magnetic field dependences for the inner and outer ring states due to their different localization properties. They both exhibit the usual Aharanov-Bohm oscillations but with different oscillation periods. In the presence of spin-orbit coupling, we show distinct spin and charge persistent currents for inner and outer ring states. We find well-defined spin currents with negligibly small charge currents. This is because the local currents of spin-up and -down states flow in opposite directions.

Abstract: A novel two-dimensional (2D) structurally stable carbon allotrope is proposed using first-principles calculations, which is a promising material for water purification and for electronic devices due to its unique porous structure and electronic properties. Rectangular and hexagonal rings are connected with acetylenic linkages, forming a nanoporous structure with a pore size of 6.41 angstrom, which is known as T-4,T-4,T-4-graphyne. This 2D sheet exhibits a direct bandgap of 0.63 eV at the M point, which originates from the p(z)( )atomic orbitals of carbon atoms as confirmed by a tight-binding model. Importantly, T-4,T-4,T-4-graphyne is found to be energetically more preferable than the experimentally realized beta-graphdiyne, it is dynamically stable and can withstand temperatures up to 1500 K.

Abstract: 2D ternary systems provide another degree of freedom of tuning physical properties through stoichiometry variation. However, the controllable growth of 2D ternary materials remains a huge challenge that hinders their practical applications. Here, for the first time, by using a gallium/indium liquid alloy as the precursor, the synthesis of high-quality 2D ternary Ga2In4S9 flakes of only a few atomic layers thick (approximate to 2.4 nm for the thinnest samples) through chemical vapor deposition is realized. Their UV-light-sensing applications are explored systematically. Photodetectors based on the Ga2In4S9 flakes display outstanding UV detection ability (R-lambda = 111.9 A W-1, external quantum efficiency = 3.85 x 10(4)%, and D* = 2.25 x 10(11) Jones@360 nm) with a fast response speed (tau(ring) approximate to 40 ms and tau(decay) approximate to 50 ms). In addition, Ga2In4S9-based phototransistors exhibit a responsivity of approximate to 10(4) A W-1@360 nm above the critical back-gate bias of approximate to 0 V. The use of the liquid alloy for synthesizing ultrathin 2D Ga2In4S9 nanostructures may offer great opportunities for designing novel 2D optoelectronic materials to achieve optimal device performance.

Abstract: Two-dimensional (2D) carbon materials play an important role in nanomaterials. We propose a new carbon monolayer, named hexagonal-4,4,4-graphyne (H-4,H-4,H-4-graphyne), which is a nanoporous structure composed of rectangular carbon rings and triple bonds of carbon. Using first-principles calculations, we systematically studied the structure, stability, and band structure of this new material. We found that its total energy is lower than that of experimentally synthesized beta-graphdiyne and it is stable at least up to 1500 K. In contrast to the single Dirac point band structure of other 2D carbon monolayers, the band structure of H-4,H-4,H-4-graphyne exhibits double Dirac points along the high-symmetry points and the corresponding Fermi velocities (1.04-1.27 x 10(6) m/s) are asymmetric and higher than that of graphene. The origin of these double Dirac points is traced back to the nodal line states, which can be well explained by a tight-binding model. The H-4,H-4,H-4-graphyne forms a moire superstructure when placed on top of a hexagonal boron nitride substrate. These properties make H-4,H-4,H-4-graphyne a promising semimetal material for applications in high-speed electronic devices. (C) 2018 Elsevier Ltd. All rights reserved.

Abstract: Methane needle-plane discharge has practical application prospect and scientific research significance since methane conversion heavy oil hydrogenation is formed by coupling methane needle-plane discharge with heavy oil hydrogenation, which can achieve high-efficient heavy oil hydrogenation and increase the yields of high value-added light olefins. In this paper, a two-dimensional fluid model is built up for numerically simulating the methane needle-plane discharge plasma at atmospheric pressure. Spatial and axial distributions of electric intensity, electron temperature and particle densities are obtained. Reaction yields are summarized and crucial pathways to produce various kinds of charged and neutral particles are found out. Simulation results indicate that axial evolutions of CH3+ and CH4+ densities, electric intensity and electron temperature are similar and closely related. The CH5+ and C2H5+ densities first increase and then decrease along the axial direction. The CH3 and H densities have nearly identical spatial and axial distributions. Particle density distributions of CH2, C2H4 and C2H5 are obviously different in the area near the cathode but comparatively resemblant in the positive column region. The CH3+ and CH4+ are produced by electron impact ionizations between electrons and CH4. The CH5+ and C2H5+ are respectively generated by molecular impact dissociations between CH3+ and CH4 and between CH4+ and CH4. Electron impact decomposition between electrons and CH4 is a dominated reaction to produce CH3, CH2, CH and H. The reactions between CH2 and CH4 and between electrons and C2H4 are critical pathways to produce C2H4 and C2H2, respectively. In addition, the yields of electron impact decomposition reactions between electrons and CH4 and reactions between CH2 and CH4 account for 52.15% and 47.85% of total yields of H-2 respectively.

Abstract: Recently, two stable allotropes (germancite and stancite) for the group IV elements (Ge and Sn) with a staggered layered dumbell structure were proposed to be three-dimensional (3D) topological Dirac semimetals [Phys. Rev. B 93, 241117 (2016)]. A pair of Dirac points is on the rotation axis away from the time-reversal invariant momentum, and the stability of the 3D bulk Dirac points is protected by the C-3 rotation symmetry. Here, we use the first principles calculations to investigate GexSny alloys which share the same rhombohedral crystal structure with the space group of D-3d(6). Six GexSny alloys are predicted to be energetically and dynamically stable, where (x, y) = (8, 6) and (6, 8) and the alpha and beta phases of (10, 4) and (4, 10). Our results demonstrate that all the six GexSny alloys are topological Dirac semimetals. The different nontrivial surface states and surface Fermi arcs are identified. Our work will substantially enrich the family of 3D Dirac semimetals which are within the reach of experimental realization. Published by AIP Publishing.

Abstract: By taking account of the electric-field-induced charge screening, a self-consistent calculation within the framework of the tight-binding approach is employed to obtain the electronic band structure of gated multilayer phosphorene and the charge densities on the different phosphorene layers. We find charge density and screening anomalies in single-gated multilayer phosphorene and electron-hole bilayers in dual-gated multilayer phosphorene. Due to the unique puckered lattice structure, both intralayer and interlayer charge screenings are important in gated multilayer phosphorene. We find that the electric-field tuning of the band structure of multilayer phosphorene is distinctively different in the presence and absence of charge screening. For instance, it is shown that the unscreened band gap of multilayer phosphorene decreases dramatically with increasing electric-field strength. However, in the presence of charge screening, the magnitude of this band-gap decrease is significantly reduced and the reduction depends strongly on the number of phosphorene layers. Our theoretical results of the band-gap tuning are compared with recent experiments and good agreement is found.

Abstract: Using the tight-binding approach, we investigate the energy spectrum of square, triangular, and hexagonal MoS2 quantum dots (QDs) in the presence of a perpendicular magnetic field. Novel edge states emerge in MoS2 QDs, which are distributed over the whole edge which we call ring states. The ring states are robust in the presence of spin-orbit coupling (SOC). The corresponding energy levels of the ring states oscillate as a function of the perpendicular magnetic field which are related to Aharonov-Bohm oscillations. Oscillations in the magnetic field dependence of the energy levels and the peaks in the magneto-optical spectrum emerge (disappear) as the ring states are formed (collapsed). The period and the amplitude of the oscillation decrease with the size of the MoS2 QDs.

Abstract: Air pollutants from fossil fuel fired power plants harm the environment and human health. More than 91% of Irans electricity production is from thermal power plants that use natural gas, diesel, and fuel oil. We apply the impact pathway approach to estimate the health impacts arising from Iranian fossil-based electricity generation emission, and in a next step, we calculate monetary costs of the estimated damages, for a one-year period starting from 20 March 2016 through 2017. We use the new version of SIMPACTS (International Atomic Energy Agency, Vienna, Austria) to investigate the health effects from 61 major Iran fossil-based power plants separately. The selected plants represent 95.6% of total Iran fossil-based power generation. Using the individual and different power plant estimates, we avoid extrapolation and our results can be considered more reliable, taking into account spatial differences. The total damage cost is 723.42 million USD (2000). The damage cost per generated electricity varies from 0.06 to 22.41 USD/MWh and average plant damage cost is 2.85 USD/MWh. Accounting for these external costs indicates the actual costs of fossil energy. The results are useful for policy makers to compare the health costs from these plants and to decide on cleaner energy sources and to take measures to increase benefits for society.

Abstract: Nanocomposite systems and nanoparticle (NP) films are crucial for many applications and research fields. The structure-properties correlation raises complex questions due to the collective structure of these systems, often granular and porous, a crucial factor impacting their effectiveness and performance. In this framework, we investigate the optical and morphological properties of Ag nanoparticles (NPs) films and of Ag NPs/TiO₂ porous matrix films, one-step grown by supersonic cluster beam deposition. Morphology and structure of the Ag NPs film and of the Ag/TiO₂ (Ag/Ti 50-50) nanocomposite are related to the optical properties of the film employing spectroscopic ellipsometry (SE). We employ a simple Bruggeman effective medium approximation model, corrected by finite size effects of the nano-objects in the film structure to gather information on the structure and morphology of the nanocomposites, in particular porosity and average NPs size for the Ag/TiO₂ NP film. Our results suggest that SE is a simple, quick and effective method to measure porosity of nanoscale films and systems, where standard methods for measuring pore sizes might not be applicable.

Abstract: Using the tight-binding approach, we investigate the electronic properties of bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular electric and magnetic fields. Since BLP consists of two coupled phosphorene layers, it is of interest to examine the layer-dependent electronic properties of BLP QDs, such as the electronic distributions over the two layers and the so-produced layer-polarization features, and to see how these properties are affected by the magnetic field and the bias potential. We find that in the absence of a bias potential only edge states are layer polarized while the bulk states are not, and the layer-polarization degree (LPD) of the unbiased edge states increases with increasing magnetic field. However, in the presence of a bias potential both the edge and bulk states are layer polarized, and the LPD of the bulk (edge) states depends strongly (weakly) on the interplay of the bias potential and the interlayer coupling. At high magnetic fields, applying a bias potential renders the bulk electrons in a BLP QD to be mainly distributed over the top or bottom layer, resulting in layer-polarized bulk Landau levels (LLs). In the presence of a large bias potential that can drive a semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different magnetic-field dependences, i.e., the zeroth LLs exhibit a linearlike dependence on the magnetic field while the other LLs exhibit a square-root-like dependence.

Abstract: Ultrathin metal nanoparticles coatings, synthesized by gas-phase deposition, are emerging as go-to materials in a variety of fields ranging from pathogens control, sensing to energy storage. Predicting their morphology and mechanical properties beyond a trial-and-error approach is a crucial issue limiting their exploitation in real-life applications. The morphology and mechanical properties of Ag nanoparticles ultrathin films, synthesized by supersonic cluster beam deposition, are here assessed adopting a bottom-up, multi-technique approach. A virtual film model is proposed merging high resolution scanning transmission electron microscopy, supersonic cluster beam dynamics and molecular dynamics simulations. The model is validated against mechanical nanometrology measurements and is readily extendable to metals other than Ag. The virtual film is shown to be a flexible and reliable predictive tool to access morphology-dependent properties such as mesoscale gas-dynamics and elasticity of ultrathin films synthesized by gas-phase deposition.

Abstract: In epitaxial thin film systems, the crystal structure and its symmetry deviate from the bulk counterpart due to various mechanisms such as epitaxial strain and interfacial structural coupling, which is accompanyed by a change in their properties. In perovskite materials, the crystal symmetry can be described by rotations of sixfold coordinated transition metal oxygen octahedra, which are found to be altered at interfaces. Here, it is unraveled how the local oxygen octahedral coupling at perovskite heterostructural interfaces strongly influences the domain structure and symmetry of the epitaxial films resulting in design rules to induce various structures in thin films using carefully selected combinations of substrate/buffer/film. Very interestingly it is discovered that these combinations lead to structure changes throughout the full thickness of the film. The results provide a deep insight into understanding the origin of induced structures in a perovskite heterostructure and an intelligent route to achieve unique functional properties.