The Dilute Charged Impurity Effects on Electronic Heat Capacity and Magnetic Susceptibility of Ferromagnetic Silicene Sheet

Starting from the Kane-Mele Hamiltonian, Dirac cone approximation and self-consistent Born approximation (SCBA), the effects of dilute charged impurity doping on electronic heat capacity (EHC) and magnetic susceptibility (MS) of a two-dimensional material ferromagnetic graphene’s silicon analog, silicene, are investigated within the Green’s function approach that allows the accurate assessment of dynamic of carriers. Also, we have studied the behavior of these quantities in the presence of applied external electric field (AEEF). Our results show that the inversion symmetry is broken by impurity doping. According to EHC behaviors, the band gap is decreased with impurity concentration (IC), impurity scattering strenght (ISS), and AEEF. As a remarkable point, a phase transition from ferromagnetic to paramagnetic and antiferromagnetic has been observed. On the other hand, there is a critical impurity concentration for maximum response to the temperature and magnetic field. The effect of AEEF leads to the decrease of band gap and phase transition depends on the direction of AEEF.     

MoS<Subscript>2</Subscript>/CoS<Subscript>2</Subscript> composites composed of CoS<Subscript>2</Subscript> octahedrons and MoS<Subscript>2</Subscript> nano-flowers for supercapacitor electrode materials

In pursuing excellent supercapacitor electrodes, we designed a series of MoS₂/CoS₂ composites consisting of flower-liked MoS₂ and octahedron-shaped CoS₂ through a facile one-step hydrothermal method and investigated the electrochemical performance of the samples with various hydrothermal time. Due to the coupling of two metal species and a big amount of well-developed CoS₂ and MoS₂, the results indicated that the MoS₂/CoS₂ composites electrodes exhibited the best electrochemical performance with a large specific capacitance of 490 F/g at 2 mV/s or 400 F/g at 10 A/g among all samples as the hydrothermal time reached 48 h (MCS₄₈). Furthermore, the retention of MCS₄₈ is 93.1% after 10000 cycles at 10 A/g, which manifests the excellent cycling stability. The outstanding electrochemical performance of MCS₄₈ indicates that it could be a very promising and novel energy storage material for supercapacitors in the future.     

MoS<Subscript>2</Subscript>/h-BN heterostructures: controlling MoS<Subscript>2</Subscript> crystal morphology by chemical vapor deposition

Tuning the properties of van der Waals heterostructures based on alternating layers of two-dimensional materials is an emerging field of research with implications for electronics and photonics. Hexagonal boron nitride (h-BN) is an attractive insulating substrate for two-dimensional materials as it may exert less influence on the layer’s properties than silica. In this work, MoS₂ layers were deposited by chemical vapor deposition (CVD) on thick h-BN flakes mechanically exfoliated deposited on Si/SiO₂ substrates. CVD affords the controllable, large-scale preparation of MoS₂ on h-BN alleviating shortcomings of manual mechanical assembly of such heterostructures. Electron microscopy revealed that in-plane and vertical to the substrate MoS₂ layers were grown at high yield, depending on the sample preparation conditions. Raman and photoluminescence spectroscopy were employed to assess the optical and electronic quality of MoS₂ grown on h-BN as well as the interactions between MoS₂ and the supporting substrate. Compared to silica, MoS₂ layers grown on h-BN are less prone to oxidation and are subjected to considerably weaker electronic perturbation.     

Synthesis and High-Temperature Heat Capacity of Sm<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Eu<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Sm₂Ge₂O₇ and Eu₂Ge₂O₇ germanates have been prepared by solid-state reactions via multistep firing of stoichiometric mixtures of Sm₂O₃ (Eu₂O₃) and GeO₂ in air at temperatures from 1273 to 1473 K. The molar heat capacity of the samarium and europium germanates has been determined by differential scanning calorimetry in the range 350–1000 K and the C _p (T) data have been used to evaluate their thermodynamic properties.     

Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

Electronic and Magnetic Properties of SnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe₂O₄ spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe₂O₄ inverse spinel phase and near half-metallic character for SnFe₂O₄ mixed spinel phase and metal character for SnFe₂O₄ normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe₂O₄ ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.     

Responsivity to solar irradiation and the behavior of carrier transports for MoS<Subscript>2</Subscript>/Si and MoS<Subscript>2</Subscript>/Si nanowires/Si devices

The behavior of carrier transports and the responsivity to solar irradiation are studied for MoS₂/n-type Si (n-Si) and MoS₂/Si nanowires (SiNWs)/n-Si device. The MoS₂ thin films were prepared by the sol–gel method. The MoS₂/n-Si and MoS₂/SiNWs/n-Si devices exhibit reliable rectification behavior. The thermionic emission-diffusion model is the dominant process in these fabricated MoS₂/n-Si and MoS₂/SiNWs/n-Si devices. By applying the insertion of the SiNWs, the responsivity to solar irradiation can be effectively increased. Because of the low reflectance values for the MoS₂/SiNWs/n-Si sample, the increased photocurrent density for the MoS₂/SiNWs/n-Si device is due to high external light injection efficiency. MoS₂/SiNWs/n-Si devices benefit from the high surface to volume ratio for SiNWs leading to a high responsivity of the device. The photo-response results confirm that the decay in the photocurrent is due to the dominance of short-lifetime electron trapping in the MoS₂ thin film.     

Effect of Dopant Concentration on Electronic and Magnetic Properties of Transition Metal-Doped ZrO<Subscript>2</Subscript>

Electronic and magnetic properties of the bulk monoclinic phase of pure and doped zirconia (ZrO₂) are calculated. Calculations have been performed using density functional theory based Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code. We have considered substitutional doping of transition metals (TM) V, Cr, Mn and Fe in zirconia corresponding to concentrations ranging from 3.125 to 25 %. Our results show that Cr, Mn-, and Fe doped oxides are half-metallic and the half-metallicity remains intact on reducing the dopant concentrations. The total magnetic moment is mainly due to d states of TM atoms and small induced magnetic moment exists on other nonmagnetic atoms as well. Also as the oxygen vacancy influences the performance of oxidebased devices, therefore we model the influence of oxygen vacancy on the magnetic moments in pure and doped zirconia. Our results show that pure oxide system remains nonmagnetic even on the introduction of oxygen vacancy whereas magnetic moment values for TM doped oxide changes. In the presence of oxygen vacancy, the total magnetic moment of V-, Cr-, and Mn doped cell increases whereas it decreases for Fe doping. This shows that oxygen vacancy (V _O) has a strong influence on the magnetic properties of the doped oxides. The results may be useful for further study on TM doped ZrO₂ system.     

The Heat Capacity and Electrophysical Properties of Neodymium and Lithium Chromite NdLiCr<Subscript>2</Subscript>O<Subscript>5</Subscript>

The method of dynamic calorimetry is used to investigate the heat capacity of chromite NdLiCr₂O₅ in the temperature range from 298 to 673 K. A λ-like peak is observed at 523 K.The equations of temperature dependence of the heat capacity of this chromite are derived.The temperature dependences of the thermodynamic functions are calculated.The results of electrophysical investigations indicate that this compound may be classed with semiconductors.     

High pressure effect on MoS<Subscript>2</Subscript> and MoSe<Subscript>2</Subscript> single crystals grown by CVT method

Single crystals of MoS₂ and MoSe₂ were grown by chemical vapour transport method using iodine as a transporting agent and characterized by optical microscopy, energy dispersive analysis (EDAX), X-ray powder diffraction (XRD) and Hall mobility at room temperature. The variation of electrical resistance under pressure was monitored in a Bridgman anvil set-up up to 6.5 GPa to identify occurrence of any structural transition. MoS₂ and MoSe₂ do not undergo any structural transitions under pressure.     

The Electronic Structures and Magnetic Properties of Un-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript>: the First-Principle Calculation

The electronic structures and magnetic properties of native defects in cubic In₂O₃ are investigated systematically by the LDA + U first-principle calculations based on the density functional theory. It is found that the In₂O₃ system is a strongly correlated electron system; therefore, the coulomb potential of In-4d and O-2p should be considered. In this paper, the coulomb potential corrections are U _In?4d=3 eV and U _O?2p=5 eV. The magnetic moments of O interstitial, In vacancy, and In interstitial are 2 u _B , 3 u _B , and 1 u _B , respectively, which are consistent with the analysis of group theory and molecular orbital theory. Moreover, the distributions of these magnetic moments are both local and extended. The magnetic couplings of O interstitials, In vacancies, and In interstitials are ferromagnetic, anti-ferromagnetic, and paramagnetic respectively, which are determined by electronic structures of defects. The formation energy of O interstitial is high, while that of the O Frenkel defect (that is the O interstitial-vacancy) is ?4.98 eV. These results could provide the practical understanding to the room-temperature ferromagnetism in un-doped In₂O₃ system.     

Ballistic transport in single-layer MoS<Subscript>2</Subscript> piezotronic transistors

Because of the coupling between semiconducting and piezoelectric properties in wurtzite materials, strain-induced piezo-charges can tune the charge transport across the interface or junction, which is referred to as the piezotronic effect. For devices whose dimension is much smaller than the mean free path of carriers (such as a single atomic layer of MoS₂), ballistic transport occurs. In this study, transport in the monolayer MoS₂ piezotronic transistor is studied by presenting analytical solutions for two-dimensional (2D) MoS₂. Furthermore, a numerical simulation for guiding future 2D piezotronic nanodevice design is presented.

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Improved photocatalytic degradation of organic dye using Ag<Subscript>3</Subscript>PO<Subscript>4</Subscript>/MoS<Subscript>2</Subscript> nanocomposite

Highly efficient Ag₃PO₄/MoS₂ nanocomposite photocatalyst was synthesized using a wet chemical route with a low weight percentage of highly exfoliated MoS₂ (0.1 wt.%) and monodispersed Ag₃PO₄ nanoparticles (~5.4 nm). The structural and optical properties of the nanocomposite were studied using various characterization techniques, such as XRD, TEM, Raman and absorption spectroscopy. The composite exhibits markedly enhanced photocatalytic activity with a low lamp power (60 W). Using this composite, a high kinetic rate constant (k) value of 0.244 min^-1 was found. It was observed that ~97.6% of dye degrade over the surface of nanocomposite catalyst within 15 min of illumination. The improved photocatalytic activity of Ag₃PO₄/MoS₂ nanocomposite is attributed to the efficient interfacial charge separation, which was supported by the PL results. Large surface area of MoS₂ nanosheets incorporated with well dispersed Ag₃PO₄ nanoparticles further increases charge separation, contributing to enhanced degradation efficiency. A possible mechanism for charge separation is also discussed.     

Probing the intrinsic optical quality of CVD grown MoS<Subscript>2</Subscript>

Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance.The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is,therefore,highly desirable.Here,we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2).We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2.Low-temperature PL measurements are also used to evaluate the structural defects in MoS2,via defect-associated bound exciton emission,which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures.This work therefore provides a sensitive,noninvasive method to characterize the optical properties of TMDs,allowing the tuning of the growth parameters for the development of optoelectronic devices.     

The Effect of Substrate Temperature on Magnetic and Electrical Properties of (CeO<Subscript>2</Subscript>)<Subscript>14</Subscript>Fe<Subscript>86</Subscript> Films

(CeO₂)₁₄Fe₈₆ films were fabricated by a radio frequency magnetron sputtering method at different substrate temperature. The results reveal that the films deposited at substrate temperature lower than 773 K exhibit a strong perpendicular anisotropy, and the correlated dynamic permeability spectrum measured over the frequency range of 0.5–7 GHz shows a high resonance frequency. The study on the relation R～T shows that the resistivity of the thin film has a minimum near room temperature and tends to saturation as the temperature approaches zero, exhibiting a behavior reminiscent of Kondo scattering. However, as the substrate temperature increases to 973 K, the films possess an in-plane anisotropy and lower H _c. The resistivity exhibits a transition from metal to insulator characterized by a maximum of resistivity at 220 K.     

Transformation of monolayer MoS<Subscript>2</Subscript> into multiphasic MoTe<Subscript>2</Subscript>: Chalcogen atom-exchange synthesis route

Molybdenum ditelluride (MoTe2),which is an important transition-metal dichalcogenide,has attracted considerable interest owing to its unique properties,such as its small bandgap and large Seebeck coefficient.However,the batch production of monolayer MoTe2 has been rarely reported.In this study,we demonstrate the synthesis of large-domain (edge length exceeding 30 μm),monolayer MoTe2 from chemical vapor deposition-grown monolayer MoS2 using a chalcogen atom-exchange synthesis route.An in-depth investigation of the tellurization process reveals that the substitution of S atoms by Te is prevalently initiated at the edges and grain boundaries of the monolayer MoS2,which differs from the homogeneous selenization of MoS2 flakes with the formation of alloyed Mo-S-Se hybrids.Moreover,we detect a large compressive strain (approximately-10％) in the transformed MoTe2 lattice,which possibly drives the phase transition from 2H to 1T'at the reaction temperature of 500 ℃.This phase change is substantiated by experimental facts and first-principles calculations.This work introduces a novel route for the templated synthesis of two-dimensional layered materials through atom substitutional chemistry and provides a new pathway for engineering the strain and thus the intriguing physics and chemistry.     

Impurity Effects on Energy Gap in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+<Emphasis Type="Italic">y</Emphasis></Subscript> Investigated by Electronic Raman Scattering

The Raman scattering experiments were carried out in Zn-doped and Zn-free Bi₂Sr₂CaCu₂O_8+y with optimal hole concentration below and above T _c . The energy of pair-breaking peak in the B _1g Raman spectrum, corresponding to the magnitude of superconducting gap 2Δ ₀, is suppressed by 1% Zn-doping. In the normal state, the B _1g Raman spectrum for Zn-doped sample shows no pseudogap behaviour, suggesting that the pseudogap is strongly smeared by Zn-doping.     

Growth of MoS<Subscript>2</Subscript> layers on the surface of multiwalled carbon nanotubes

Molybdenum disulfide has been deposited on the surface of multiwalled carbon nanotubes synthesized through arc vaporization of graphite. As shown by transmission electron microscopy, extended MoS₂ layers have been formed on the surface of the carbon nanoparticles. According to x-ray diffraction results, the crystallinity of the MoS₂ layers in the composites improves with increasing annealing temperature. Free MoS₂ particles can be removed from the composites by centrifugation in bromoform.     

Structural and Magnetic Properties of La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript> Hard-Soft Magnetic Systems

We report on the growth and magnetic properties of La_2/3Sr_1/3MnO₃/SrTiO₃/CoFe₂ hard-soft magnetic systems prepared by pulsed laser deposition on SrTiO₃(001) substrates. In situ reflection high-energy electron diffraction along the [100]SrTiO₃ substrate azimuth and atomic force microscopy measurements reveal that La_2/3Sr_1/3MnO₃ and SrTiO₃ grow both in a three dimensional mode and that the roughness of the lower and upper magnetic/non-magnetic interfaces ranges between 2 and 4 Å. Cross-section transmission electron microscopy observations show that the layers are continuous, with an homogeneous thickness, and that the interfaces are mostly sharp and correlated. The magnetization curves show a two step reversal of the magnetization, with very distinct coercive fields. A small anisotropy is observed for the CoFe₂ layer with an in plane easy magnetization axis along the [110]SrTiO₃ direction. Minor magnetization loops indicate that the coupling between the magnetic layers is negligible.