Geometric Stability,Electronic Structure and Reactivity of Pt4 Cluster Supported on Defective Graphene

The geometric stability, electronic structure and magnetic properties of the Pt₄ clusters on graphene substrates are investigated using the first-principles methods. It is found that the Pt₄ clusters on the defective graphene with a single vacancy defect (SV-graphene) have larger binding energies than that on the pristine graphene. The different position of SV sites can modulate the geometric stability of Pt₄ clusters. The Pt₄ clusters provide more transferred electrons to the SV-graphene and exhibit the positively charged, which helps to weaken the CO adsorption. The results would be important for understanding that the adsorption behavior of metal clusters on graphene.     

基于密度泛函理论的油中溶解气体吸附分析

变压器油中溶解气体分析是监测变压器设备状态的重要手段。CO、CH 4和C 2 H 2是变压器油中溶解的代表性气体成分,基于密度泛函理论,首先确定过渡金属原子Pt在典型的层状过渡金属硫族化合物WSe 2表面的最佳吸附位置,获得3种气体在Pt-WSe 2表面的吸附行为,分析了气体吸附、电荷转移、吸附能、电子态密度、电荷密度形变和前沿轨道结构。作为一个电子受体,Pt-WSe 2从3种气体分子中吸引电子,CO和C 2 H 2分子的吸附类型为化学吸附,吸附效果较强,CH 4吸附是物理吸附,吸附效果较弱。气体分子的吸附导致Pt-WSe 2带隙增大,即电阻率增大。文中探索了将Pt掺杂的WSe 2改性材料应用于检测典型变压器油中溶解气体的潜力,提供了气敏吸附的理论依据。     

Computational study of transport properties of graphene upon adsorption of an amino acid: importance of including –$$\hbox {NH}_{2}$$ and –COOH groups

The effects of histidine and its imidazole ring adsorption on the electronic transport properties of graphene were investigated by first-principles calculations within a combination of density functional theory and non-equilibrium Greens functions. Firstly, we report adsorption energies, adsorption distances, and equilibrium geometrical configurations with no bias voltage applied. Secondly, we model a device for the transport properties study: a central scattering region consisting of a finite graphene sheet with the adsorbed molecule sandwiched between semi-infinite source (left) and drain (right) graphene electrode regions. The electronic density, electrical current, and electronic transmission were calculated as a function of an applied bias voltage. Studying the adsorption of the two systems, i.e., the histidine and its imidazole ring, allowed us to evaluate the importance of including the carboxyl (–COOH) and amine (–\(\hbox {NH}_{2}\)) groups. We found that the histidine and the imidazole ring affects differently the electronic transport through the graphene sheet, posing the possibility of graphene-based sensors with an interesting sensibility and specificity.     

First-principles density-functional calculations on HCr3O8: An exercise to better understand the ACr3O8(A = alkali metal) family

Accurate ab initio density-functional calculations are performed to understand structural stability, electronic structure, and magnetic properties of the ACr₃O₈(A = H, Li, Na, K, Rb, Cs) series. The ground-state structures for the compounds with A = Li−Rb take the same KCr₃O₈-type atomic arrangement (space group C2/m), whereas CsCr₃O₈adopts a modified atomic architecture (prototype; space group Pnma), in agreement with available experimental findings. The hypothetical compound HCr₃O₈is found to stabilize in an LiV₃O₈-type structure (space group P2₁/m), with an unexpectedly large equilibrium volume. The electronic structures of the ACr₃O₈compounds are analyzed using density-of-states, charge-density, and electron-localization-function plots, and all are found to exhibit semiconducting (insulating at 0 K) properties with very narrow band gaps. The Cr atoms occur in two different valence states and all compounds (except NaCr₃O₈) are found to exhibit antiferromagnetic ordering of magnetic moments at 0 K.     

Multiscale modeling of screening effects on conductivity of graphene in weakly bonded graphene-dielectric heterostructures

Graphene is often surrounded by different dielectric materials when integrated into realistic devices. The absence of dangling bonds allows graphene to bond weakly via the van der Waals interaction with the adjacent material surfaces and to retain its peculiar linear band structure. In such weakly bonded systems, however, the electronic properties of graphene are affected by the dielectric screening due to the long-range Coulomb interaction with the surrounding materials. Including the surrounding materials in the first principles density functional theory (DFT) calculations is computationally very demanding due to the large supercell size required to model heterogeneous interfaces. Here, we employ a multiscale approach combining DFT and the classical image-potential model to investigate the effects of screening from the surrounding materials (hBN, SiC, SiO₂, Al₂O₃, and HfO₂) on the dielectric function and charged impurity scattering limited conductivity of graphene. In this approach, the graphene layer is modeled using DFT and the screening from the surrounding materials is incorporated by introducing an effective dielectric function. The dielectric function and conductivity of graphene calculated using the simplified two-band Dirac model are compared with DFT calculations. The two-band Dirac model is found to significantly overestimate the dielectric screening and charged impurity scattering limited conductivity of graphene. The multiscale approach presented here can also be used to study screening effects in weakly bonded heterostructures of other emerging two-dimensional materials such as metal dichalcogenides.     

Conduction and Gas–Surface Reaction Modeling in Metal Oxide Gas Sensors

A phenomenological approach to the operation of metal oxide gas sensors, the Integrated Reaction Conduction (IRC) model, is proposed which integrates the gas-surface reactions with the electrical conduction process in a weakly sintered, porous metal oxide. An effective medium approximation is employed to relate the mesoscopic microstructure and the carrier depletion at the granular surface to the macroscopic electrical conduction. For a given ambient gas concentration and temperature, the electron concentration in the depletion layer is calculated from the gas-surface reaction kinetics. The adsorption and oxidation reaction energies of the gas sensing reactions are extracted for a TiO_2-x CO sensor by comparing experimental data with three-dimensional plots of IRC model resistance as a function of the ambient [CO(g)] and temperature. The IRC model predicts novel properties of the gas sensor, including the sensitivity and the response range, which depend on the doping of the sensor material, the temperature, the grain size, and the geometry of the necks between grains.     

羧基修饰石墨烯吸附SF6分解组分的DFT计算

为研制高灵敏度SF6气体分解组分检测传感器,利用基于第一性原理的密度泛函理论(DFT)方法,采用分子模拟软件对SF6主要放电分解组分(SO2,SOF2,SO2F2,CF4)在羧基修饰石墨烯表面的吸附特性进行了模拟计算,从微观角度研究了羧基修饰石墨烯检测SF6气体放电分解组分的气敏机理。理论计算了单分子吸附过程中表征吸附性能特征参量的吸附能、吸附距离、净电荷转移量、分子前线轨道以及态密度。结果表明:羧基的修饰能够有效提高石墨烯的吸附性能,且羧基石墨烯效果更好,对4种分子吸附效果强弱依次为SO2>SOF2>SO2F2>CF4,其中SO2在羧基石墨表面发生较为强烈的化学吸附效应。     

Top electrode dependence of forming gas annealing effects on ferroelectric films

Abstract

The effects of forming gas annealing have been studied on PZT [Pb(Zr,Ti)O₃] and SBT (SrBi₂Ta₂O₉) samples with a variety of top electrode materials. It's been found that forming gas annealing causes changes in the remanent polarization, coercive field, and leakage current in both types of samples, and these changes depend strongly on the top electrode material. Among the 6 electrode materials that we have studied (Pt, Au, Ag, Cu, Ni, and In₂O₃), Pt tends to lead to the most severe degradation, while the oxide electrode tends to withstand better the forming gas anneal. To explain the results, a model has been proposed which is based on the catalytic activities of the top electrode to dissociate hydrogen molecules into hydrogen atoms, and the latter then migrate into PZT or SBT films to cause oxygen deficiency.     

Effects of water on the properties of Fe3O4@C superparamagnetic nanospheres

ABSTRACT

Fe₃O₄@C nanospheres have potential applications in the field of magnetic separation, magnetic hyperthermia, magnetic targeting and magnetic resonance imaging (MRI). The carbon layers formed around Fe₃O₄ nanopaticles determined the adsorption properties of this material. In this study, Fe₃O₄@C was prepared by a simple one-pot solvothermal method, in which different amount of water was added to investigate the influences of water on the properties of Fe₃O₄@C nanospheres. The resulted samples were characterized by techniques of TEM, FT-IR, UV–Vis and BET, and found that the thickness of carbon layer, the intensity of surface carboxyl group and the adsorption characters were significantly changed by introducing additional water in autoclave.     

Water molecule adsorption properties on surfaces of MVO4 (M = In, Y, Bi) photo-catalysts

Water molecule adsorption properties at the surface of InVO₄, YVO₄, and BiVO₄, which have indicated promising photocatalytic properties, have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V (3c-V) sites on the (001) surface and 4c-In sites on the (100) surface of InVO₄, however, in a non-dissociated molecular form on the 5c-Bi site on the (100) surface of BiVO₄ and the 7c-Y site on the (010) surface of YVO₄. The adsorption energies of water molecule and geometries of the systems with adsorbed water molecules are reported in detail.     

Nanostructured TiO2-based mixed metal oxides prepared using microemulsions for carbon monoxide detection

Nanostructured powders of Nb-doped TiO₂ (TN) and SnO₂ mixed with Nb-doped TiO₂ in two different atomic ratios—10 to 1 (TSN 101) and 1 to 1 (TSN 11)—were synthesized using the reverse micelle microemulsion of a nonionic surfactant (brine solution/1-hexanol/Triton X-100/cyclohexane). The powders were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Thick films were fabricated for gas sensors and characterized by XRD analysis and field emission scanning electron microscopy (FE-SEM). The effects of the film morphology and firing temperature in the range 650–850 °C on CO sensitivity were studied. The best gas response, expressed as the ratio between the resistance in air and the resistance under gas exposure (R _air/R _gas), was measured for TSN 11 at 11 for 1,000 ppm CO exposure. All types of sensors showed good thermal stability. The electrochemical impedance spectroscopy (EIS) measurements were performed in different gas atmospheres (air, O₂, CO and NO₂) to better understand the electrical properties of the nanostructured mixed metal oxides.     

Tuning electronic,magnetic, and transport properties of blue phosphorene by substitutional doping: a first-principles study

Using first-principles density functional theory, we investigated the geometrical structure and magnetic, electronic, and transport properties of blue phosphorene doped with a multitude of substitutional impurities, including both metallic and semiconducting elements. Substitutional dopants modified the properties of blue phosphorene. B, Al, Ga, Sb, Bi, and Sc substitutional dopants led to an indirect- to direct-gap transition. Blue phosphorene with C, Si, Ge, Sn, O, S, Se, and Fe substitutional dopant atoms showed dilute magnetic semiconducting properties. Furthermore, the effective mass as well as zero-bias transmission spectrum of this material support the fact that the transport properties of blue phosphorene are modified by the above-mentioned impurity atoms. The effective mass of holes for the Bi- and Sb-doped systems was about \(0.138m_{0}\), implying that these systems have high hole mobility. Meanwhile, the Sb-doped system exhibited the smallest effective mass for electrons of \(0.244m_{0}\). The results of this study illustrate that doped blue phosphorene exhibits different electronic, magnetic, transport, and optical properties from pristine blue phosphorene, which may enable many useful applications in nanoelectronics, gas sensing, optoelectronics, and spintronics.     

Magnetic interactions and spin-reversal mechanism in ErMn1-xCoxO3-�� samples

Manganese-based oxides present a very complex magnetic phase diagram, and many of their basic physical properties and magnetic interaction need more exploration. At the crystallographic level, the magnetic properties of rare-earth oxide manganites are strongly affected by changes in the rare-earth ion present in the structure, the nature of the transition metals, and the possibility of electronic valence fluctuations. This work studied the synthesis procedure and structural and magnetic characterization of high-quality polycrystalline samples of the ErMn_1-xCo_xO_3-?? family. Crystallographic analyses show a decrease in lattice parameters as cobalt substitution increases, accompanied by a reduction in the interatomic distances and a small increase in MT-O-MT angles (MT= cobalt and manganese) considering the c-axis. Magnetic measurements indicated a phase separation picture, and show the coexistence of different magnetic interactions with the samples considering the Mn/Co ratio. In addition, the spin-inversion mechanism is interpreted as having a crucial role in the family??s magnetic response.     

High Selective deNO x Electrochemical Cell with Self-Assembled Electro-Catalytic Electrode

A new family of electrochemical cells for decomposition of NO gas in the presence of excess O₂, in which the cathode was covered with mixed oxide layer of NiO and YSZ (electro-catalytic electrode) were designed and investigated. The deNO_x properties were increased by microstructural and compositional control of the electro-catalytic electrode. Nano-size Ni grains were self-assembled at NiO/YSZ interfaces by oxidation-reduction reaction of the NiO during the cell operation. In order to use the reduced Ni for NO decomposition reaction effectively, the authors investigated multilayering of the electro-catalytic electrode. Through this process, the adsorption of coexisting O₂ was prevented and the deNO_x properties of the electrochemical cell were improved.     

One step synthesis of Fe3O4/GO nanocomposites at 100°C and its magnetic properties

ABSTRACT

In the research work, we had planned a one-step procedure to prepare magnetite/graphene oxide (Fe₃O₄/GO) nanocomposites by using chemical co-precepitation method. The polycrystalline phase formation of the magnetite nanoparticles was confirmed by using X-ray diffraction setup. Structural and surface morphology properties of Fe₃O₄/GO nanocomposites are analysed by scanning electron microscopy and transmission electron microscopy. The Raman spectrum of Fe₃O₄/GO nanocomposites shows the enhancement of D and G band, which are associated with GO sheets due to the presence of Fe₃O₄ nanoparticles. Magnetic property measurement was carried out by physical property measurement system (PPMS), which shows its superparamagnetic behaviour.     

Electrical characterization of TiO2-based ceramics for VOCs

Cr doped TiO₂ and TiO₂/Cr₂O₃ powders are prepared by sol–gel technique and solid state reaction respectively. The structural and electronic properties of these powders are investigated. The responses of the materials made from the powders to benzene and formaldehyde vapors are studied. The research results show that Cr³⁺ can enter into the lattice as substitutional metal dopant by sol gel route at lower temperature. The segregation of Cr₂O₃ happens when Cr content exceed 8 at.%. The resistance of TiO₂ can be remarkable decreased by doping Cr³⁺ as substitutional metal dopant. Besides Cr³⁺, the segregation of Cr₂O₃ also contributes to the alteration of conduction from n- to p-type. With some other catalyzer, both of Cr doped TiO₂-based and Cr₂O₃/TiO₂-based ceramics can detect benzene and formaldehyde vapors at 21 ppm.     

Synthesis of monodispersed pH-sensitive Ni0.5Zn0.5Fe2O4 magnetic nanoparticles and the potential medical application of adsorption for bovine serum album

We synthesized Ni_0.5Zn_0.5Fe₂O₄ magnetic nanoparticles (MNPs) by co-precipitation method and performed its adsorption of bovine serum albumin V(BSA (V)). The Ni_0.5Zn_0.5Fe₂O₄ MNPs in nitrogen atmosphere annealed at 400, 500, 600, 700 and 800°C, respectively. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer(VSM) were used to characterize the resulting powders of the structure and properties. XRD results show that the Ni_0.5Zn_0.5Fe₂O₄ powder is a typical single-phase cubic spinel structure, and has no impurity phases. TEM results show that the Ni_0.5Zn_0.5Fe₂O₄ crystalline size is approximately 30–50 nm, which is similar with the XRD estimate results. The magnetic measurements show that Ni_0.5Zn_0.5Fe₂O₄ samples, super paramagnetic material, have high saturation magnetization and small coercivity properties. Moreover, We adopted ultraviolet spectrophotometer to analysis adsorption of BSA(V) on the Ni_0.5Zn_0.5Fe₂O₄ MNPs at room temperature, which show that annealed temperatures, pH, and stirring time can affect BSA(V) adsorption on the Ni_0.5Zn_0.5Fe₂O₄ MNPs. Most importantly, when ultrasonic stirring 2 hours in the liquor BSA(V) at pH = 7.0, the Ni_0.5Zn_0.5Fe₂O₄ samples annealed at 600°C has a highest value of 39.81mgg^?1. From above these results, Ni_0.5Zn_0.5Fe₂O₄ MNPs reveals a great adsorbing ability for BSA(V) that probable are a potential BSA-carrier candidate.     

EMC/FDTD/MD simulation of carrier transport and electrodynamics in two-dimensional electron systems

We present the implementation and application of a multiphysics simulation technique to carrier dynamics under electromagnetic excitation in supported two-dimensional electronic systems. The technique combines ensemble Monte Carlo (EMC) for carrier transport with finite-difference time-domain (FDTD) for electrodynamics and molecular dynamics (MD) for short-range Coulomb interactions among particles. We demonstrate the use of this EMC/FDTD/MD technique by calculating the room-temperature dc and ac conductivity of graphene supported on SiO₂.     

Selective Gas Detection of SnO2-TiO2 Gas Sensors

The composite, consisting of two materials with different sensing temperatures, may show the selectivity for a particular gas. In this study, the microstructural and compositional effects on the electrical conductivity and the CO and the H₂ gas sensing properties of SnO₂-TiO₂ composites were examined. SnO₂-TiO₂ composites in entire (0–100 mol%) composition range were fabricated in the form of porous pellet by sintering at 800C for 3 h. The effects of CuO-coating (or doping) on the electrical conductivity and the sensing properties to 200 ppm CO and H₂ gases were examined.With CuO-coating, SnO₂-TiO₂ composites showed the increased sensitivity to CO gas and a large difference in the sensing temperatures between CO and H₂ gases. As a result, CuO-coated SnO₂-TiO₂ composites showed the selectivity for CO gas between 100C and 190C and the selectivity for H₂ gas between 280C and 380C.     

Synthesis and Characterization of Ni0.7Zn0.3Fe2O4 and Mn0.7Zn0.3Fe2O4 Nanoparticles by Water-in-oil Microemulsion

Ferrite is an important ceramic material with magnetic properties that are useful in many types of electronic devices. In this paper we synthesized nanostructured Ni_0.7Zn_0.3Fe₂O₄ and Mn_0.7Zn_0.3Fe₂O₄ with spinel structure in the system of water-in-oil microemulsion of water/Triton X–100/n-hexanol/cyclohexane. The result showed that nanoparticles with size in the range of 50–80 nm could be obtained by microemulsion method. FT-IR spectrum, XRD patterns and DTA curves revealed that the nanoparticles could be obtained via calcining its precursor at 500°C. The VSM plots showed that the ferrimagnetic behavior was expected for this type of magnetic material.