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Using first-principles calculations, we designed a new two-dimensional sp 2− sp 3 hybridized allotrope of carbon. This allotrope is made up of 5-6-8 carbon rings thus, known as C 568. C 568 is an indirect semiconductor with a band gap of 1.13 eV (HSE06) at Γ-M symmetric point in the Brillouin zone. The possibility of the existence of C 568 is evident from its energetic, dynamical, mechanical, and thermal stabilities. C 568 maintains its structural integrity up to 800 K, which we confirmed by employing ab-initio molecular dynamics (AIMD) simulations. C 568 shows extraordinary room temperature in-plane electronic mobility of order ∼104 cm2V−1s−1, which is significantly higher than the black phosphorus (∼103 cm2V−1s−1) monolayer. Energetically, this sheet is more stable than the other sp 2− sp 3-hybridized carbon allotropes such as pentagraphene and tetrahexcarbon. Moreover, C 568 can be stacked into three different configurations, which also exhibit interesting electronic as well as transport properties. These configurations are also dynamically stable and show in-plane mobilities of order ∼104 cm2V−1s−1. Image 1 [ABSTRACT FROM AUTHOR]

A two-dimensional structurally stable carbon allotrope is predicted using first-principles calculations. This unique network is composed of tetra and hexa-rings of carbon atoms known as Tetrahexcarbon. Tetrahexcarbon has a quasiparticle (QP) direct band gap of 3.70 eV at Γ , which is closer to the ZnS, a well-known direct bandgap semiconductor. Interestingly, it is dynamically stable and can withstand temperature up to 1000 K, which is confirmed by performing phonon and ab initio molecular dynamics (AIMD) simulations. The mobilities of both electrons and holes in this material are found to be anisotropic. It exhibits extraordinary room temperature in-plane electron mobility of order ∼ 10 4 cm 2 V − 1 s − 1 , which is an order of magnitude higher than the black phosphorus monolayer ( ∼ 10 3 cm 2 V − 1 s − 1 ) and two orders of magnitude higher than MoS 2 ( ∼ 200 cm 2 V − 1 s − 1 ) monolayer. The optical response of Tetrahexcarbon obtained by applying Bethe-Salpeter equation (BSE) to include excitonic effects on top of the partially self-consistent GW 0 calculation. The absorption onsets strongly depend on the light polarization directions indicating Tetrahexcarbon an anisotropic material. [ABSTRACT FROM AUTHOR]

Abstract: Under microgravity, thermocapillary flow becomes the dominant convection in float-zone crystal growth, it may lose stability with increasing Marangoni number, and the convection instability is detrimental to crystal quality. Because of the excellent electrical conductivity of semiconductor melt, the external rotating magnetic field (RMF) is applicable to control melt convection. In a typical simplified float-zone model, the influences of the non-uniform RMF, which is generated by the RMF inductor with three pairs of poles, on the three-dimensional thermocapillary flow are investigated numerically. Our results demonstrate that the axial velocities are reduced under the non-uniform RMF while the melt is stirred in azimuthal direction by RMF, as a result, the three-dimensional convection after the instability becomes to a two-dimensional axisymmetrical flow. It implies that the non-uniform RMF can control melt convection effectively for high-quality crystal growth. [Copyright &y& Elsevier]

Abstract: Ab initio molecular dynamics (MD) simulations are done to elucidate the electronic structure properties of a short-side-chain perfluorosulfonic acid (SSC PFSA) membrane at low humidity conditions. The artificial neural network (ANN) approach along with statistical methods is then employed to model and analyze these properties. The ANN method substantially speeds up the ab initio electronic structure calculations and has superior accuracy in mimicking the results of such calculations. The aim of this study is to understand the effects of hydration level, temperature, side chain flexibility of the SSC PFSA membranes, and backbone flexibility of the SSC PFSA membranes on the proton transfer in these membranes. Statistical analysis of results using analysis of means (ANOM) and analysis of variance (ANOVA) methods shows that no proton transfer from the SSC PFSA to the neighboring water molecules occurs at considerably low hydration levels. However, an increase in the probability of proton transfer is observed when the SSC PFSA membrane is sufficiently saturated. This process is more favorable at low temperatures. Moreover, the flexibility of either the side chains or the backbone of the SSC PFSA membrane has a great influence on the proton transfer phenomenon in such a way that allowing them to move freely causes an increase in the affinity of the SSC PFSA membrane to share its protons with water molecules. Further investigation is performed concerning the combined effect of the independent parameters (i.e., hydration level, temperature, side chain flexibility of the SSC PFSA membrane, and backbone flexibility of the SSC PFSA membrane) on the proton transfer process and the results are reported in detail. [Copyright &y& Elsevier]

The realization of a molecular device with a unimolecular rectifying function is one of the most important requirements in nanotechnology. In the present study, the geometric and electronic structure of the alkyl derivative molecule C37H50N4O4 has been investigated theoretically using ab initio quantum mechanical calculations. This molecule has a donor–spacer–acceptor structure, and is a leading candidate for the creation of a molecular rectifying device. The results suggest that in donor–acceptor molecular complexes such as this, while the lowest unoccupied orbital is concentrated around the acceptor sub-unit, the highest occupied molecular orbital is localized on the donor sub-unit. The approximate potential differences for an optimized PNX molecule have been estimated to be 2.683 eV at the B3LYP/6-311g++(d,p) level of theory. [Copyright &y& Elsevier]

Many fullerene-based supramolecules have been proposed as potential organic photovoltaic devices, with their electrochemical and photo-electrochemical properties measured under light illumination. Phthalocyanine possesses good electron-donating properties due to its large easily ionised π-electron system, whereas fullerene is good π-electron acceptor which can be connected with other organic molecules. A phthalocyanine–fullerene-based supramolecular system is therefore a potential material candidate for a photovoltaic cell due to its large and flexible absorption combined with electrical properties similar to an inorganic semiconductor. We investigated the geometric and electronic structure of phthalocyanine–fullerene supramolecule using an ab initio quantum mechanical calculation. The results suggest that the lowest unoccupied molecular orbital (LUMO) state of this supramolecule localized on the fullerene and the highest occupied molecular orbital (HOMO) state is localized on half of the phthalocyanine. The energy difference of localized LUMO levels strongly depended on the functional group attached to the phthalocyanine and the structure of the supramolecule. [Copyright &y& Elsevier]

Abstract: The formation energies and transition energy levels of intrinsic defects in hexagonal BN (h-BN) bilayer and monolayer have been studied by first-principles calculations based on density functional theory. Our calculated results predict that excellent intrinsic p-type and n-type conductivities are very difficult to be realized in h-BN bilayer and monolayer. This is because of the high formation energies of acceptor-like defects (≥4.6 eV ) and the rather deep transition energy levels of donor-like defects (≥2.0 eV ). In order to obtain h-BN layers with more efficient p-type and n-type conductivity, extrinsic doping using foreign impurities is necessary. [Copyright &y& Elsevier]

Abstract: Langatate (La3Ta0.5Ga5.5O14, LTG) has been widely used in piezoelectric sensors in high temperature applications because of its structural and piezoelectric reliability at high temperature. However, in the low oxygen partial pressure and elevated temperature ranging from 300 to 700°C, an increase of electrical conductivity originated from intrinsic oxygen vacancies have been observed in a previous experimental result. In this study, to elucidate the oxygen vacancies effects on LTG, formation energy of each electronic state of oxygen vacancies (, and ) at non-equivalent Wyckoff positions (2d and 6g) and their electronic structures were calculated by combining GW quasi-particle scheme and conventional density functional theory (DFT). The formation energy of VO(6 g ) was lower than the formation energy of VO(2 d ) in each electronic state. Thus, relatively stable oxygen vacancies at 6g sites (VO(6 g )) can be major oxygen vacancies in LTG system. Also, calculated band gaps of LTG with thermodynamically stable defects such as or were higher than band gaps of LTG with or at Γ-point. By the band interpolation based on Boltzmann transport theory, calculated electrical conductivities per unit relaxation time (σ/τ) of the LTG with and were similar at the same electron energy, however, σ/τ of the LTG with was higher than that of the LTG with . [Copyright &y& Elsevier]

Abstract: Single crystalline langatate (LTG, La3Ta0.5Ga5.5O14) has been widely used as a sensor material in high temperature applications because of its thermally stable piezoelectric properties. In this research, to elucidate the relationship between piezoelectric tensors and local ionic configurations, first-principles calculations based on density functional perturbation theory (DFPT) were performed on various local ionic structures. The results indicate that two independent relaxed-ion piezoelectric coefficients, e 11 and e 14, increased with increases in La( 3e )–O and Ta( 1a )–O distances or decreases in Ga( 3f , 2d )–O distances. Thus, to obtain high piezoelectric constants in this crystal, ions larger than La3+ should be incorporated at 3e sites to open the distance between 3e ions and oxygen ions, and ions smaller than Ga3+ should be introduced at 2d and 3f sites to reduce the distance between Ga and O ions. Finally, from this design rule, a new crystal, BTAS (Ba3TaAl3Si2O14), which belongs to the same P321 group, is proposed. The calculated relaxed-ion piezoelectric coefficient e 11 of BTAS was 17.7% higher than the coefficient of a LTG crystal. This significant increase confirms BTAS as a useful new piezo-material, especially in applications where there is also a need to reduce the use of more expensive elements. [Copyright &y& Elsevier]