Energy Band Calculations Research Articles

Tuning electronic and transport properties of MoS2/Ti2C heterostructure by external strain and electric field

We investigated the effects of external strain and electric field on the electronic, transport, and thermoelectric properties of MoS2/Ti2C heterostructure by using the first-principles method. Its electronic properties can be effectively tuned by applying uniaxial tensile strain, biaxial tensile strain, and vertical electric field, and exhibits different behaviour. An obvious band gap between the valence bands I and II is observed under external field. Upon application of tensile strain, energy bands become more dispersive, inducing the increase in conductivity. However, when the coupling of tensile strain and vertical electric field was imposed, energy bands become more flat, leading to the decrease in conductivity. Based on the calculated energy bands, the effect of external field on the carrier effective mass was also discussed. Furthermore, the change in electronic properties affects the transport and thermoelectric properties of MoS2/Ti2C heterostructure. Tensile strain enhances its transport properties. However, the transport properties of MoS2/Ti2C heterostructure is weakened under the application of vertical electric field. Comparing with no external field, the thermoelectric power factor is decreased under external field.

Strain effects on the electronic and thermoelectric properties of copper oxide

Using the density functional theory DFT with Tran-Blaha modified Becke-Johnson (TB-mBJ) and Boltzmann transport theory, we investigate the influence of strain on the electronic structure and the thermoelectric properties of Copper oxide (CuO). In this study, the strain is applied along a, b and c direction with strength from −2% to +2%. Based on the optimized structural parameters, the profiles of the structural properties, and the band gaps, the density of states (DOS) indicate that CuO is an antiferromagnetic semiconductor material with an indirect gap around 1.99 eV. It has been observed that this result matches well with experiment. Consequently, we use the calculated energy band structure in combination with the Boltzmann transport theory in order to explore the thermoelectric properties. For compressive and expansion strains on b and a parameter respectively, we find that the relative Seebeck coefficient is positive while the charge carrier concentration also increases with temperature compared to the unstrained one. Moreover, we explore the evolution of thermal and electrical conductivity for better understanding of the materials thermodynamics and efficiency (ZT). At room temperature and under normal conditions, the present study pushes us to suggest that CuO could be considered as a suitable potential thermoelectric candidate.

First-principles study of elastic, electronic, thermodynamic, and thermoelectric transport properties of TaCoSn

Abstract In this paper, we have performed a comprehensive set of first-principles calculations to study elastic, electronic, thermodynamic and thermoelectric properties of TaCoSn using density functional theory (DFT). Half-heusler, TaCoSn has been found to be elastically and thermodynamically stable, ductile and hard material. The Debye temperature of TaCoSn has been found to be 375.39 K. The calculated energy bands indicate that TaCoSn is an indirect band gap semiconductor and the value of gap is 1.107 eV using PBE functional and it is 1.153 eV by TB-mBJ potentials. Such small increase of band gap by TB-mBJ potential has no significant effect on the transport properties of TaCoSn. In TaCoSn, no significant spin-orbit interaction is found. The density of states at the Fermi energy is dominated by Ta-5d and Co-3d orbitals due to strong hybridization between them. We also calculate the relaxation time and lattice thermal conductivity. The lattice thermal conductivity of TaCoSn (4.95 W/mK at 300 K) is relatively small than that of other half-heusler compounds. The maximum Seebeck coefficient at 500 K is 249.41 μV/K. The obtained power factor (S2σ/τ) at 600 K is ∼12.5 × 1011 W/msK2. The calculated maximum figure of merit (ZT) is 0.73 at 600 K indicates that TaCoSn is a promising material for thermoelectric device applications.

Zn-doped SnO2 hierarchical structures formed by a hydrothermal route with remarkably enhanced photocatalytic performance

Zn-doped SnO2 hierarchical structures were synthesized by a facile and low-cost hydrothermal route using tin chloride and zinc nitrate as inorganic sources. The Zn-doped SnO2 hierarchical structures were pompon-like and composed of numerous homogenous nanocones which were radiated from one center. The length and diameter of nanocones were gradually decreased with the increase of doping amount, indicating the morphology of the samples could be well controlled by varying the Zn2+ concentration. Additionally, the feasible growth process of obtained samples was discussed on the basis of the habits of nucleation and crystal growth. The photocatalytic performances of Zn-doped SnO2 hierarchical structures were systematically investigated in the degradation of methylene blue (MB), methyl orange (MO), rhodamine B (RhB) and Congo red (CR). Compared with pure SnO2, the 4.0 wt% Zn-doped SnO2 hierarchical structure presented superior photocatalytic ability, which could be mainly attributed to both present oxygen vacancies and doped Zn2+ centers. At last, the possible photocatalytic mechanism was studied by means of the active species trapping experiments as well as the calculated energy band structures.

A theoretical study on the metal contacts of monolayer gallium nitride (GaN)

Low-Schottky barrier height (SBH) metal contacts to 2D materials is indispensable for achieving high performance in atomic layer 2D materials channel based optoelectronic devices. In this study, we systematically investigate the detailed face contact properties of monolayer (ML) hexagonal gallium nitride (GaN) with six different commonly used metals (Au, Ag, Pd, Pt, Ti, and Ni) in field-effect transistors (FETs) utilizing the first principles electronic structure calculations based on density functional theory (DFT). It is found that no tunnelling barriers (TB) exist in all the ML GaN-metal face contact systems by calculating and analyzing the average effective potentials (Veff). Moreover, ML GaN undergoes a vanishing of Schottky barriers in the vertical interfaces by analyzing the binding energy, electron localization function (ELF), and projected state density (PDOS) of six contact combinations. In terms of the energy band calculation, ML GaN forms n-type Schottky contacts with Au, Pd, Pt and Ti electrodes, and an n-type ohmic contacts with Ag electrode, while a p-type ohmic contact with Ni electrode is obtained in the lateral interfaces. The results would provide an insight into the ML GaN-metal interfaces, and be beneficial for developing low dimensional GaN-based devices with high performance.

Highly efficient direct Z-scheme WO3/CdS-diethylenetriamine photocatalyst and its enhanced photocatalytic H2 evolution under visible light irradiation

Cadmium sulfide (CdS) has demonstrated great promise in artificial photocataytic hydrogen (H2) production. However, the serious photocorrosion hinders its effective interaction and real-life application. In this work, a typical direct Z-scheme WO3/CdS-diethylenetriamine (CdS-DETA) composite has been designed via facile in-situ solvothermal method, which exhibits excellent H2 production properties in visible light region. In this system, the inorganic–organic CdS-DETA nanobelts (NBs) possess enough active sites and large surface area for the encouraging nanojunction structure information. Furthermore, we also systematically calculated energy band structure and investigated charge transfer of the WO3/CdS-DETA by PL and photocurrent test, the results demonstrated that the suitable band gap matching between CdS-DETA and WO3 and high redox potential improve the separation of photogenerated holes and electrons, restraining intrinsic photocorrosion of CdS as well as improving the photocatalytic activity. 5%WO3/CdS-DETA presented the most outstanding H2 evolution rate (15522 μmol g−1 h−1), which is twice higher than that of pure CdS-DETA. WO3/CdS-DETA composites also presented high stability after three recycle H2 production experiment. Finally, direct Z-scheme photocatalytic mechanism is calculated.

Synthesis and Characterization of Cadmium Chalcogenide Nanomaterial (CdE; E=Se/Te) from Novel Single Source Molecular Precursor

Background: Metal chalcogenide nanomaterials represent an important group of efficient materials, in which the subtle variations in shape, size and phase of nano-powders resulted in physical properties (e.g., electronic and optical) differing from their bulk counterparts, which makes them useful materials for various technological devices. Objective: Synthesis and growth of chalcogenide nano powders from novel single source molecular precursors (i.e., Cd(II) bis-(aminopropane) selenide, Cd(II) bis-(aminoethane) telluride) for the production of cadmium chalcogenide (CdE, E= Se/Te) at nano scale. Method: Single source molecular precursor inserted in quinoline, acting as a coordinating solvent at suitable temperature, yielded vacuum dried powders of CdSe and CdTe nanomaterials. Results: The average particle size was estimated as CdSe ≈ 3 nm, and CdTe ≈ 29 nm from powder X-ray diffraction pattern of synthesized nanoparticles. The produced nanomaterials possess optical properties and calculated energy band gap of nanoparticles as CdTe = 5.2 eV and CdSe = 4.0 eV from UV-Visible spectra. Conclusion: The economical, harmless single source molecular method may be a striking technique to fabricate metal chalcogenide nanoparticles.

Probing interfacial electronic properties of graphene/CH3NH3PbI3 heterojunctions: A theoretical study

Interfacial interactions and electronic properties of graphene/CH3NH3PbI3 heterojunctions were investigated by first-principles calculations incorporating semiempirical dispersion-correction scheme to describe van der Waals interactions. Two lattice match configurations between graphene and CH3NH3PbI3(001) slab were constructed in parallel contact and both of them were verified to form remarkable van der Waals heterojunctions with similar work functions. Our calculated energy band structures show that the Dirac-cone of graphene and the direct band gap of CH3NH3PbI3 are still preserved in the heterojunctions, thus graphene can be a promising candidate either as a capping or supporting layer for encapsulating CH3NH3PbI3 layer. It is identified that the Schottky barrier of graphene/CH3NH3PbI3 heterojunctions can be controlled by the interlayer distance and affected by the stacking pattern of graphene and CH3NH3PbI3. The 3D charge density differences present thebuild-ininternalelectric field from graphene to CH3NH3PbI3 after interface equilibrium and thus, a low n-type Schottky barrier is needed for high efficient charge transferring in the interface. The possible mechanism of the band edge modulations in the heterojunctions and corresponding photoinduced charge transfer processes are also described.

Visible light Bi2S3/BiFeO3 photocatalyst for effective removal of Rhodamine B

Bi2S3-sensitized BiFO3 (BFO) photocatalyst (Bi2S3/BFO) was successfully synthesized through a facile and environmental ion exchange method between BFO and Thiosurea (H2NCSNH2, TU). The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and UV-vis diffuse reflection spectroscopy (DRS). The obtained Bi2S3/BFO composites showed excellent photocatalytic performance for decomposing Rhodamine B (RhB) compared with pure BFO under visible light irradiation (λ>400nm). 5% Bi2S3/BFO exhibited the highest photocatalytic activity and excessive amount of Bi2S3 would result in the decrease of photocatalytic activity of BFO. The mechanism of enhanced photocatalytic activity was proposed on the basis of the calculated energy band positions.

Prediction of electronic structure, dielectric and thermodynamical properties of flurbiprofen by density functional theory calculation

The molecular structure of the pain killer drug Flurbiprofen has been fully optimized using Berny's optimization algorithm in Gaussian 09, using redundant internal coordinates. The dipole moments, polarizability tensors, average polarizability, anisotropy, first molecular hyperpolarizability, thermodynamics properties, dielectric constant, refractive index, dielectric susceptibility, molar refractivity, IR and Raman spectra have also been studied. A tentative assignment of the IR vibration frequencies was equally carried out. This was done by using Density functional theory calculations with three different functional B3LYB, X3LYP and BPBE by employing cc-pVDZ basis set. The calculated energy band gap (Egap) of the molecule obtained by using HOMO and LUMO molecular orbitals show that the molecule has an Egap value of 0.195 eV, 0.201 eV and 0.244 eV at the BPBE, B3LYP and X3LYP levels respectively. The first molecular hyperpolarizability value obtained by using the BPBE (6.79187 × 10−30 esu), B3LYP (5.07308 × 10−30 esu) and (2.74471 × 10−30 esu) methods are very large compared to that of Urea (0.1947 × 10−30 esu) reported in literature. The results also show that the molecule has a dielectric constant of 2.85 × 10−11, 3.03 × 10−11 and 2.99 × 10−11 obtained at the PBPE, B3LYP and X3LYP levels respectively. Equally, our IR and Raman spectra were in the same range as those reported in literature.

Assessing the thermoelectric properties of ScRhTe half-heusler compound

We investigate structural, elastic, electronic and thermoelectric properties of ScRhTe with frame work of density functional theory and Boltzmann equations. The physical properties such as elastic constants, bulk modulus, shear modulus, Young's modulus and Poisson's ratio of ScRhTe compound are calculated. The elastic properties of ScRhTe reveal that this compound is mechanically stable. Further, the phonon calculation of ScRhTe indicates that it also dynamically stable. The band structure as well as DOS spectra indicates that ScRhTe compound has a semiconductor nature. The calculated energy band gap of ScRhTe compound is 0.8eV.The thermoelectric properties such as Seebeck coefficient, electrical conductivity scaled by relaxation time, power factor scaled by relaxation time and electronic thermal conductivity has been investigated with variation of carrier concentration and temperature. The highest power factor obtained at 1200 K is 11.9 × 1011V2SK−2ms−1 at optimum carrier concentration n∼1.3 × 1021cm−3 for n-type ScRhTe. The maximum dimensionless figure of merit for n-type ScRhTe is 0.63. Using the slack equation, the lattice thermal conductivity of ScRhTe is also estimated. At room temperature, ScRhTe attains low thermal conductivity as compared to well know ZrNiPb half heusler compound. The lower lattice thermal conductivity of ScRhTe suggest that this compound may be act as good thermoelectric material. Our results suggested that power factor can't improve further when electron carrier concentration reaches to n∼1.3 × 1021cm−3 and hole carrier concentration reaches to n∼7.5 × 1020cm−3. This is the first quantitative theoretical estimation of these properties.

Insights into the photocatalytic mechanism of mediator-free direct Z-scheme g-C3N4/Bi2MoO6(010) and g-C3N4/Bi2WO6(010) heterostructures: A hybrid density functional theory study

Abstract Graphite-like carbon nitride (g-C3N4)-based heterostructures have received much attention due to their prominent photocatalytic activity. The g-C3N4/Bi2WO6 and g-C3N4/Bi2MoO6 heterostructures, which follow a typical hetero-junction charge transfer mechanisms show a weak potential for hydrogen evolution and reactive radical generation under visible light irradiation. A mediator-free Z-scheme g-C3N4/Bi2MoO6(010) and g-C3N4/Bi2WO6(010) heterostructures photocatalyst are designed for the first time using first-principles studies. Moreover, theoretical understanding of the underlying mechanism, the effects of interfacial composition and the role the interface play in the overall photoactivity is still unexplained. The calculated band gap of the heterostructures is reduced compared to the bulk Bi2WO6 and Bi2MoO6. In this study, we systematically calculated energy band structure, optical properties and charge transfer of the g-C3N4/Bi2MoO6(010) and g-C3N4/Bi2WO6(010) heterostructures using the hybrid density functional theory approach. The results show that the charge transfer at the interface of the heterostructures induces a built-in potential, which benefits the separation of photogenerated charge carriers. The g-C3N4/Bi2MoO6(010) heterostructure with more negative adhesion energy (−1.10 eVA−2) is predicted to have a better adsorptive ability and can form more easily compared to the g-C3N4/Bi2WO6(010) interface (−1.16 eVA−2). Therefore, our results show that the g-C3N4 interaction with Bi2MoO6 is stronger than Bi2WO6, which is also verified by the smaller vertical separation (3.25 A) between Bi2MoO6 and g-C3N4 compared to the g-C3N4/Bi2WO6(010) interface (3.36 A). The optical absorption verifies that these proposed Z-scheme heterostructures are excellent visible light harvesting semiconductor photocatalyst materials. This enhancement is ascribed to the role of g-C3N4 monolayer as an electron acceptor and the direct Z-scheme charge carrier transfer at the interface of the heterostructures. This work is useful for designing new types of Z-scheme photocatalyst and offers new insight into Z-scheme charge transfer mechanism for applications in the field of solar energy conversion.

Probing π-π stacking modulation of g-C3N4/graphene heterojunctions and corresponding role of graphene on photocatalytic activity

The photoelectrochemical properties of g-C3N4 sheet are modified by the π-π stacking interaction with graphene, and the corresponding role of graphene on the surface chemical reactions is investigated by density functional theory. The calculated cohesive energies and the lattice mismatch energies indicate that g-C3N4 and graphene are in parallel contact and can form a stable heterojunction. According to our calculated energy band structures and work functions of g-C3N4/graphene heterojunctions, the band edge modulations by graphene are discussed and corresponding photoinduced charge transfer processes are analyzed in detail. It is found that the incorporating of graphene into g-C3N4 facilitates the separation of photoinduced e–/h+ pairs and the oxidation capacity enhancement of the photoinduced holes with the downshifting of the valence band edge of g-C3N4 layer. It is identified that the inhomogeneous onsite energies between interlayer and the band edge modulations are induced by the inhomogeneous charge redistribution between interlayer caused by graphene. Further, the initial dynamic reaction processes of oxygen atoms in g-C3N4/graphene heterojunctions also confirm the significant role of graphene on the surface chemical reactions.

Spectral properties of RuAl2 and RuGa2 compounds: Ellipsometric analysis

The optical properties of RuAl2 and RuGa2 intermetallic compounds have been investigated in the spectral range of 0.22–14 μm. The nature of interband light absorption has been interpreted based on a comparative analysis of calculated and experimental frequency dependences of optical conductivity. The data obtained confirm the existence of pseudogaps with a width of ~0.8 eV localized at the Fermi level in the electron densities of states of these materials, which was predicted in previous energy-band calculations.

Implementation of density functional theory method on object-oriented programming (C++) to calculate energy band structure using the projector augmented wave (PAW)

This study implemented DFT method into the C++ programming language with object-oriented programming rules (expressive software). The use of expressive software results in getting a simple programming structure, which is similar to mathematical formula. This will facilitate the scientific community to develop the software. We validate our software by calculating the energy band structure of Silica, Carbon, and Germanium with FCC structure using the Projector Augmented Wave (PAW) method then compare the results to Quantum Espresso calculation’s results. This study shows that the accuracy of the software is 85% compared to Quantum Espresso.

Cage disorder and gas encapsulation as routes to tailor properties of inorganic clathrates

Inorganic clathrates with the type II crystal structure are of interest as potential materials for high temperature thermoelectric applications. In this study we present ab initio calculations for the electronic and phonon properties of several Sn type II clathrate compositions with partial Ga substitution on the framework, empty cage Sn136, and compounds of Sn136 filled with inert Xe atoms. It is found that cage disorder due to atomic substitution and guest encapsulation affect the fundamental characteristics of these materials in profound ways. We determine that the stability of these materials is enhanced by the presence of guests and lack of direct GaGa bonds in disordered clathrates. Inert Xe atoms provide a unique opportunity to preserve the overall electronic structure of Sn136 and take advantage of the loosely bound guest rattling for enhanced phonon scattering. The calculated energy bands and density of states, as well as phonon band structure and mode Gruneisen parameter, enable further analysis of type II Sn clathrates and reveal interesting structure-property relations.

A Studing Of the Optical Properties Of ZnO thin film doped (ZnO)1-x(SeO2)x By (Sol-Gel spin coating)

In This research prepeared Pure (ZnO) thin film was done by using (spin coating Sol-Gel ) method and doped(ZnO)1-x(SeO2)xas (x= 2.5%)molar ratio from the (SeO2)to get the(ZnO)1-x(SeO2) and annealing temperature(400,500,600)C°. the measurement of the Optical properities absorbance and transmittance of (ZnO)، (ZnO)1-x(SeO2) were studies by using the (Uv-Visible ) spectrophotometer in the wave length rang (300-1100nm), Calculated energy band gap of thin films with study the effect of temperature on prepeared thin film .

Unified band-theoretic description of structural, electronic, and magnetic properties of vanadium dioxide phases

The debate about whether the insulating phases of vanadium dioxide (VO2) can be described by band theory or must be described by a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases, but have not yet successfully accounted for the observed magnetic orderings. Strongly-correlated theories have had limited quantitative success. Here we report that, by using hard pseudopotentials and an optimized hybrid exchange functional, the energy gaps and magnetic orderings of both monoclinic VO2 phases and the metallic nature of the high-temperature rutile phase are consistent with available experimental data, obviating an explicit role for strong correlations. We also report a potential candidate for the newly-found metallic monoclinic phase and present a detailed magnetic structure of the M2 monoclinic phase.

Fermi Surface and Magnetic Properties in Ferromagnet CoS2 and Paramagnet CoSe2 with the Pyrite-type Cubic Structure

We succeeded in growing high-quality single crystals of pyrite-type cubic compounds CoSe2 and CoS2 using a transport agent of CoBr2 and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and the de Haas–van Alphen (dHvA) effect. We confirmed that CoSe2 is an exchange-enhanced paramagnet revealing a broad maximum at around 50 K in the temperature dependence of the magnetic susceptibility. The electronic specific heat coefficient is moderately large, γ = 18 mJ/(K2·mol). On the other hand, CoS2 is a ferromagnet with a Curie temperature TC = 122 K and an ordered moment μs = 0.93 μB/Co. The γ value of 21 mJ/(K2·mol) in CoS2 is slightly larger than that of CoSe2. A large ordered moment, together with a large γ value, is a characteristic feature in CoS2 because CoS2 is a half-metallic spin state in the ferromagnetic state. Correspondingly, we detected a main dHvA branch with a large cyclotron effective mass of 13m0 in the dHvA experiments. The detected dHvA branches in CoS2 and CoSe2 are discussed on the basis of the results of energy band calculations, revealing a broken four-fold-symmetry in the angular dependence of the dHvA frequency.

Enhanced Photocatalytic Performance under Visible and Near-Infrared Irradiation of Cu1.8Se/Cu₃Se₂ Composite via a Phase Junction.

A novel Cu1.8Se/Cu3Se2 composite photocatalyst was prepared by the simple precipitation method. This composite possesses a wide photoabsorption until the range of near-infrared light, and exhibits significantly enhanced photocatalytic activity for methyl orange degradation under visible and near-infrared light irradiation compared with bare Cu1.8Se and Cu3Se2. The mechanism of this outstanding photocatalytic behavior can be explained by the calculated energy band positions. The efficient charge separation via a phase junction of Cu1.8Se/Cu3Se2 composite would make a great contribution to its much-enhanced photocatalytic efficiency.