Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.     

Electronic structures and effective masses of photogenerated carriers of CaZrTi2O7 photocatalyst: First-principles calculations

The band structures, density of states and effective masses of photogenerated carriers for CaZrTi₂O₇ photocatalyst were performed using first principles method with the virtual crystal approximation. The results indicated that CaZrTi₂O₇ has an indirect band gap of about 3.25 eV. The upper valence bands of CaZrTi₂O₇ are formed by O 2p states mixed with Ti 3d states, Zr 4d, 4p and 5s states, while the conduction bands are dominated by Ti 3d states, Zr 4d states and O 2p states. The calculated valence bands maximum (VBM) potential is located at 2.60 V (vs. normal hydrogen electrode (NHE)), while the conduction bands minimum (CBM) potential at ?0.65 V. Therefore, CaZrTi2O7 has the ability to split water to hydrogen and oxygen under UV light irradiation. The calculated minimum effective mass of electron in CBM is about 1.35 m₀, and the minimum effective mass of hole in VBM is about 1.23 m₀. The lighter effective masses facilitate the migration of photogenerated carriers and improve photocatalytic performance.     

Ab initio determination of atomic structure and energy of surface states of bare and hydrogen covered GaN (0001) surface — Existence of the Surface States Stark Effect (SSSE)

Density Functional Theory (DFT) calculations indicate that energetically stable structure of clean GaN(0001) surface posses (2 × 1) reconstruction, having every second row of Ga located near plane of N atoms, that gives rise to Ga-related dispersionless surface electronic state, already identified by angle resolved photoelectron spectroscopy (ARPES) measurements [S.S. Dhesi et al. Phys. Rev. B 56 (1997) 10271, L. Plucinski et al. Surf. Sci 507-10 (2002) 223, S. M. Widstrand et al. Surf. Sci. 584 (2005) 169]. The energy reduction in reconstruction proceeds via change of the hybridization of the occupied Ga surface states from sp³ to sp², transforming the empty states to p_z type. It is also shown that the electric subsurface field, modeled in new slab model which allows to simulate electric fields at the semiconductor surfaces [P. Kempisty et al., J. Appl. Phys. 106 (2009) 054901], strongly affects the energy of electronic states of GaN(0001) surfaces. The change of the field may shift the energy of surface states of bare and hydrogen covered GaN(0001) surface, by several eV with respect to the band states. The phenomenon, denoted as Surface States Stark Effect (SSSE), explains various band bending values, measured at differently doped n-type GaN(0001) surfaces. It is shown also that, for the adsorbate density up to one H atom for each Ga surface atom i.e. 1 monolayer coverage (1 ML), the hydrogen adatoms are located at the on-top positions, i.e. directly above Ga atoms. For these adsorbate densities, the H-related quantum surface state is located slightly below the valence band maximum (VBM) in the case of p-type GaN surface. For n-type GaN, the H-related surface state is located deeply in the valence band, about 2 eV below VBM. For higher, 1.25 ML hydrogen coverage, the two H adatoms create either surface attached H₂ ad-molecule (energetically stable) or triple bridge configuration is created (metastable). The H₂ ad-molecule is weekly attached to the surface, having the desorption energy barrier equal to 0.16 eV. For 1.25 ML coverage the DFT results were obtained for p-type GaN only. They show that in the ad-molecule case, a new surface electronic state arises which is located about 6.7 eV below VBM. In the case of the bridge configuration, the bridge related surface state is located closely to the conduction band minimum (CBM).     

Electronic and structural properties of Ti9XO20 (X=Ti,C, si,Ge, Sn and pb) clusters: A DFT study

The electronic and structural properties of Ti₉XO₂₀ (X=Ti, C, Si, Ge, Sn and Pb) clusters have been obtained in the density functional theory (DFT) framework. The changes in the bond length, binding energy, frontier orbitals, and electronic potential have been fully analyzed when one titanium atom in the (TiO₂)₁₀ cluster is replaced by elements with four valence electrons. When one titanium atom is substituted by one carbon atom, a charge excess among the guest and the surrounding oxygen atoms is generated, which is approximately 1.5 times that of the pristine case, and this structure has been shown to be the most stable among the studied systems. In addition, the Ti₁₀O₂₀–Cd₂ and Ti₉CO₂₀–Cd₂ clusters exhibit HOMO–LUMO gaps that have decreased by 0.58 and 2.12 eV, respectively, with respect to the bare cases.     

Experimental and theoretical studies on bis(glycine) lithium nitrate (BGLiN): A physico-chemical approach

Good quality and bulk size single crystal (size: 20×13×8 mm³) of bis(glycine) lithium nitrate (BGLiN) was grown by a slow evaporation solution technique from the aqueous solutions at constant temperature i.e. 27 °C using synthesized materials. Crystal system and lattice parameters were determined by single crystals as well as powder X-ray diffraction analysis. The lattice parameters of the titled compound are a=10.0223 Å, b=5.0343 Å, c=17.0510 Å, and V=860.312 Å³ and it crystallized in an orthorhombic system with space group Pca2₁ obtained by single crystal XRD. Elemental composition was confirmed by energy dispersive X-ray spectroscopic analysis. Optical absorption spectrum was recorded and various optical parameters such as optical transmission (~60%), and optical band gap (4.998 eV) were calculated. Photoluminescence study shows that the grown crystal is free from major defects. Crystalline perfection of the grown crystal was assessed and found good. Ground state optimized geometry has been obtained by using DFT with 6-31G(d,p) basis set. HOMO and LUMO energy gap was found to be 6.01 eV and dipole moment was 1.65 D.     

Synthesis and characterization of nano-peanuts of lead(II) coordination polymer [Pb(qcnh)(NO3)2]n with ultrasonic assistance: A new precursor for the preparation of pure-phase nano-sized PbO

A sonochemical method was used to synthesize nano-peanuts of a new lead(II) coordination 1D polymer, [Pb(qcnh)(NO₃)₂]_n (1), where qcnh = 2-quinolincarbaldehyde nicotinohydrazide. The compound was characterized by scanning electron microscopy (SEM), elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), and single crystal X-ray analysis. The X-ray structure revealed that the Pb(II) atom is coordinated by one oxygen and three nitrogen atoms from two qcnh ligands and five oxygen atoms from three nitrate ligands in an 8 + 1 fashion with a PbN₃O₆ donor set. One of the PdN distances in the vicinity of the central atom is a bit longer (Pb1N1 = 2.939(4) Å), which shows the effect of the 6s² lone electron pair localized within the valence shell of the lead(II) atom. PbO nanoparticles were obtained by thermolysis of 1 at 180 °C with oleic acid as a surfactant. The average diameter of the nanoparticles was estimated by XRD to be 28 nm. The morphology and size of the prepared PbO nanoparticles were further studied using SEM.     

Structure twinning,electronic and photoluminescence properties of yavapaiite-type orthophosphate BaTi(PO4)2

A ternary orthophosphate BaTi(PO₄)₂ has been prepared using a high temperature molten salt method and structurally determined by single crystal X-Ray diffraction analysis. It crystallizes in yavapaiite-type structure with monoclinic space group C2/m. The structure was refined by a non-merohedral twinning model with the twin law (−0.435 1.4350 −0.564 −0.435 0 0.097 −0.099 1). Band structure calculation using the density functional theory (DFT) method indicates that BaTi(PO₄)₂ has a direct bond gap of about 3.00 eV, which is well fitted with the experimental value of 2.95 eV. The photoluminescence excitation and emission spectra, decay curve, and the color coordinates for BaTi(PO₄)₂ were investigated. It can be efficiently excited by UV light (270 nm) and presents blue–green emission (centered at 506 nm), which may be attributed to the lattice defect emission.     

Improved calculation of band gap of Sr2Bi2O5 crystal using modified Becke–Johnson exchange potential

The electronic structure of Sr₂Bi₂O₅ is calculated by the scalar-relativistic full potential linearized augmented plane wave (FLAPW+lo) method using the modified Becke–Johnson potential combined with the local density approximation correlation (MBJ–LDA). Both the valence band maximum (VBM) and conduction band minimum (CBM) exist at the Γ-point, indicating that Sr₂Bi₂O₅ is a direct-band-gap material. The band gap is calculated to be 3.17 eV, which is very close to the experimental value. This result is in great contrast to the underestimation based on the GGA calculation. On the other hand, there is only a small difference in the effective masses of holes and electrons photogenerated near the VBM and CBM for the MBJ–LDA and GGA approaches. The optical properties of Sr₂Bi₂O₅ are calculated from the complex dielectric function ε(ω)=ε₁(ω)+iε₂(ω). A highly polarized peak is observed at 3.5 eV in the ε₂(ω) function. Furthermore, the absorption coefficient estimated from the MBJ–LDA is very similar to that from the experimental result.     

Synthesis and characterization of sulfur-containing polyacetylene derivative: Synthesis of poly(phenyl propargyl sulfide) and its electro-optical properties

Sulfur-containing conjugated polymer was synthesized by the polymerization of phenyl propargyl sulfide by transition metal catalysts such as PdCl₂, RuCl₃, (NBD)PdCl₂, WCl₆, and MoCl₅. The polymerization proceeded well in homogeneous manner to give a moderate yield of polymer. The chemical structure of poly(phenyl propargyl sulfide) was characterized by NMR (¹H–, ¹³C–), IR, and UV–visible spectroscopy, and elemental analysis to have the conjugated polymer backbone with the designed moieties. The FT-IR spectra of the polymer did not show the acetylenic CC bond stretching (2119 cm⁻¹) and acetylenic C–H bond stretching (3293 cm⁻¹) frequencies of the monomer. The thin polymer film exhibited reversible electrochemical behaviors between the doping and undoping peaks. Poly(phenyl propargyl sulfide) showed the characteristic UV–visible absorption band at 360 nm and blue PL spectrum at 460 nm, corresponding to the photon energy of 2.70 eV. The energy band gap of poly(PPS) was estimated to be 2.77 eV from the analysis of the absorption edge.     

First-principles study of structural and electronic properties of Cd3P2

First-principles linear combination of atomic orbitals method within the framework of density functional theory is applied to study structural and electronic properties of tetragonal and cubic phases of Cd₃P₂. The equilibrium lattice constants and bulk moduli deduced from Murnaghan equation of state for the two structures are in good agreement with the experiment. Enthalpy–pressure curves do not show possibility of pressure induced structural phase transitions between the two structures up to 80 GPa. Electronic band structures and Mulliken population analysis for the two structures are presented. It is found that tetragonal Cd₃P₂ has direct band gap 1.38 eV while cubic structure shows indirect band gap of 0.35 eV. The branch point energies for the tetragonal structure lie below the conduction band while for cubic structure it lies in the conduction band. Mulliken population analysis shows that occupancies in 5sp, 6sp and 5d states of Cd and 3sp and 4sp states of P are largely affected on bond formation.     

Electronic band structure calculation of GaNAsBi alloys and effective mass study

Electronic band structures of GaN_xAs_1−x−yBi_y dilute nitrides–bismides have been determined theoretically within the framework of the band anticrossing (BAC) model and k ⋅ p method. We have developed computer codes based on our extended BAC model, denoted (16 × 16), in which the dimension of the used states basis was equal to 16. We have investigated the band gap and the spin orbit splitting as a function of Bi composition for alloys lattice matched to GaAs. We have found that the substitution of As element by N and Bi impurities leads to a significant reduction of band gap energy by roughly 198 meV/%Bi. Meanwhile, spin orbit splitting increases by 56 meV/%Bi regardless N content. There is an excellent agreement between the model predictions and experiment reported in the literature. In addition, alloys compositions and oscillator strengths of transition energies have been calculated for GaNAsBi alloys which represent active zone of temperature insensitive (1.55 μm and 1.3 μm) wavelength laser diodes intended for optical fiber communications. A crossover at about 0.6 eV has occurred between E_g and Δ_so of GaN_.039As_.893Bi_.068. When the quaternary is lattice mismatched to GaAs, resonance energy increases with Bi content if N content decreases. On the other hand, effective mass behavior of carriers at Γ point has been discussed with respect to alloy composition, k-directions and lattice mismatch.     

First-principles study of atomic and electronic structures of kaolinite in soft rock

Kaolinite is a kind of clay mineral which often causes large deformations in soft-rock tunnel engineering and thus causes safety issues.To deal with these engineering safety issues,the physical/chemical properties of the kaolinite should be studied from basic viewpoints.By using the density-functional theory,in this paper,the atomic and the electronic structures of the kaolinite are studied within the local-density approximation(LDA).It is found that the kaolinite has a large indirect band gap with the conduction band minimum(CBM) and the valence band maximum(VBM) being at the Γ and the B points,respectively.The chemical bonding between the cation and the oxygen anion in kaolinite is mainly ionic,accompanied by a minor covalent component.It is pointed that the VBM and the CBM of kaolinite consist of oxygen 2p and cation s states,respectively.The bond lengths between different cations and anions,as well as of the different OH groups,are also compared.     

Theoretical investigations of NiTiSn and CoVSn compounds

The structural, elastic and electronic properties of NiTiSn and CoVSn half-Heusler compounds have been calculated using the full-potential linear muffin-tin orbital (FP-LMTO) method. The computed equilibrium lattice constants are in excellent agreement with the available experimental and theoretical data. The elastic constants C_ij are calculated using the total energy variation with strain technique. The polycrystalline elastic moduli (namely: the shear modulus, Young's modulus, Poisson's ratio, Lamé's coefficients, sound velocities and the Debye temperature) were derived from the obtained single-crystal elastic constants. The ductility mechanism for the studied compounds is discussed via the elastic constants C_ij and their related parameters. The electronic band structure calculations show that the conduction band minimum (CBM) is located at the X point for both compounds, whereas the valence band maximum (VBM) is located at the Г point for NiTiSn and at the L point for CoVSn, resulting in indirect energy band gaps of 0.46 and 0.75 eV for NiTiSn and CoVSn, respectively. The pressure and volume dependences of the energy band gaps have been calculated.     

Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb₂ZnCl₄. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.     

Band offsets at ZnO/SiC heterojunction: Heterointerface in band alignment

Pulsed laser deposited ZnO layers on 6H-SiC substrates showed the six-fold symmetry, indicating a two-dimensional epitaxial growth mode. X-ray photoelectron spectroscopy was employed to study the valence band discontinuity and interface formation in the ZnO/6H-SiC heterojunction. The valence band offset was measured to be 1.38 ± 0.28 eV, leading to a conduction band offset value of 1.01 ± 0.28 eV. The resulting band lineup in epitaxial ZnO/6H-SiC heterojunction is determined to be of staggered-type alignment.     

Lattice structures and electronic properties of MO/MoSe2 interface from first-principles calculations

Using first-principles plane-wave calculations within density functional theory, we theoretically studied the atomic structure, bonding energy and electronic properties of the perfect Mo (110)/MoSe₂ (100) interface with a lattice mismatch less than 4.2%. Compared with the perfect structure, the interface is somewhat relaxed, and its atomic positions and bond lengths change slightly. The calculated interface bonding energy is about −1.2 J/m², indicating that this interface is very stable. The MoSe₂ layer on the interface has some interface states near the Fermi level, the interface states are mainly caused by Mo 4d orbitals, while the Se atom almost have no contribution. On the interface, Mo-5s and Se-4p orbitals hybridize at about −6.5 to −5.0 eV, and Mo-4d and Se-4p orbitals hybridize at about −5.0 to −1.0 eV. These hybridizations greatly improve the bonding ability of Mo and Se atom in the interface. By Bader charge analysis, we find electron redistribution near the interface which promotes the bonding of the Mo and MoSe₂ layer.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

Photoemission spectroscopy at MOVPE-prepared InGaAs(100) surface reconstructions

Angular resolved ultraviolet photoemission spectroscopy at BESSY was employed to study the electronic structure of the three different, (4 × 3)-, (2 × 4)-, and (4 × 2)-surface reconstructions of In_0.53 Ga_0.47As, which was grown lattice-matched to InP(100). The surfaces have been prepared using metal organic vapor phase epitaxy (MOVPE). For spectroscopy, a dedicated transfer system was employed and samples were transferred contamination-free from the MOVPE reactor to UHV-based analysis tools. For the different surface reconstructions, the Γ ? Δ ? X direction was scanned while varying the photon energy between 10 eV and 28 eV. We observed two surface states in the photoelectron spectra on all of these surface reconstructions in addition to the bulk derived valence band emissions. Different binding energies of the surface states originating from different surface band bending were detected and described.