Electronic Band Structure Properties Research Articles

First Principles Calculations of Electronic Band Structure and Optical Properties of Cr-Doped ZnO

Electronic band structure and optical properties of Cr-doped ZnO were studied using the density functional method within the generalized-gradient approximation. Three configurations with the substitution of Zn by one and two Cr atoms in different positions were considered. For the pure ZnO, the Fermi level locates at the valence band maximum, while it shifts to the conduction band and exhibits metal-like characteristic after Cr atoms are introduced into the ZnO supercell. The calculated optical properties indicate that the optical energy gap is increased after Cr doping. More importantly, strong absorption in the visible-light region is found, which originates from the intraband transition of the Cr 3d bands and the conduction bands. Our calculations provide electronic structure evidence that, in addition to usage as short-wavelength optoelectronic devices, the Cr-doped ZnO system could be a potential candidate for photoelectrochemical application due to the increase in its photocatalytic activity.

FIRST PRINCIPLE CALCULATION OF ELECTRONIC BAND STRUCTURE AND OPTICAL PROPERTIES OF KIO3

The electronic band structure and optical properties of the ferroelectric single crystal KIO 3 have been investigated using the density functional methods. The calculated band structure for KIO 3 evidences that the crystal has a direct band gap with a value of 2.83 eV. The structural optimization has been performed. The real and imaginary parts of dielectric function, energy-loss function for volume and surface, and refractive index are calculated along the crystallographic axes.

Electronic band structure and linear optical properties of paraelectricKIO3

The electronic energy band structure, partial (PDOS) and total densityof states (DOS), and linear optical properties of the paraelectricKIO3 single crystal are calculated using density functional theory (DFT) in its localdensity approximation (LDA). The calculated band structure for paraelectricKIO3 indicates that the crystal has a direct bandgap. Using LDA and generalized gradient approximation(GGA), structural optimization has been performed. The optical spectra of the paraelectricKIO3 in the photon energy range up to 30 eV are investigated under the scissor approximation. Thereal and imaginary parts of the frequency-dependent linear dielectric function, as well asrelated quantities such as energy-loss function, refractive index and effective number of valenceelectrons, are calculated. The calculated structural optimization and bandgap of the paraelectricKIO3 have been compared with experimental data and have been found to be in good agreementwith the experimental results.

Ab-initio study of the electronic structure and optical properties of KNO3 in the ferroelectric phase

The electronic band structure and optical properties of the ferroelectric phase III of KNO3 have been investigated by the first-principle pseudopotential method using the density functional theory under the generalized gradient approximation (GGA) and local density approximation (LDA). The calculated band structure shows that the ferroelectric phase III of KNO3 has a direct band gap with a value of 1.76 eV at the point of the first Brillouin zone (BZ). The structural optimization of KNO3 has been performed using the GGA and LDA. The results of the structure optimization have been compared with the experimental results and have been found to be in good agreement with these results. Moreover, the linear photon-energy-dependent dielectric functions and some optical constants such as energy-loss functions for volume and surface, extinction, reflectivity and absorption coefficients, refractive index and effective number of valance electrons per unit cell participating in the interband transitions have been calculated.

Synthesis and characterization of aurivillius phase Bi5Ti3FeO15 layered perovskite for visible light photocatalysis

An aurivillius phase Layered perovskite Bi5FeTi3O15 system was synthesized for first time by the solid-state reaction method yielding a single phase Bi5FeTi3O15 at only 1030°C. The electronic band structure and optical properties as well as the crystallization behavior of the materials were studied by using UV-DRS and XRD. The as-prepared particles those were the agglomerates of 25 nm crystals, exhibited the band gap of 2.38 eV (525 nm). Their visible light (λ ≥ 420 nm) photocatalytic activity as studied by the photo-decomposition of isopropyl alcohol (IPA) was found to be much higher than that of well-known TiO2-xNx photocatalyst.

Crystal orientation effects on the piezoelectric field of strained zinc-blende quantum-well structures

A three-layered zinc-blende quantum-well structure is analyzed subject to both static and dynamic conditions for different crystal growth directions taking into account piezoelectric effects and lattice mismatch. It is found that the strain component ${S}_{zz}$ in the quantum-well region strongly depends on the crystal growth direction and that a piezoelectric strain contribution exists in zinc blende as in wurtzite, albeit smaller. It is also found in the absence of loss effects that resonance frequencies, giving large strains in the structure, depend strongly on the crystal growth direction. Due to the higher symmetry of the zinc-blende structure, we find in a one-dimensional model that piezoelectric effects do not affect strain values for zinc-blende structures grown along the [001] direction in contrast to the corresponding wurtzite result. However, zinc-blende structures grown along a general crystal direction show important changes in strain and the electric distribution due to piezoelectric effects. The findings indicate the quantitative importance of a fully coupled model even for zinc blende, in particular when discussing electronic band structure and optoelectronic properties.

Electronic structure and related properties of the ferroelectric crystal triglycine sulfate

The electronic band structure and related physical properties of the crystal triglycine sulfate (TGS), (NH 2CH 2COOH) 3·H 2SO 4, in the ferroelectric phase P2 1 have been calculated using the first principles Linear Combination of Atomic Orbitals (LCAO) code CRYSTAL’06 at the B3LYP level of theory. The interactions between the quasi-separate three glycine groups and the sulfate complex have been studied by the analysis of the density of states of the crystal. The glycine 2 group was found to be the zwitterion in the optimized structure. Ten coefficients of the elastic stiffness tensor c kl , four coefficients of the tensor of the elastoelectrical effect e ki , and the spontaneous polarization P s of TGS have been calculated for the first time and have been found to agree well with experimental data. Extrema are found in the elastoelectric coefficient e 22 as a function of the strain ε 2, and in the spontaneous polarization P s as a function of the unit cell volume.

InAsN/GaSb/InAsN ‘W’ quantum well laser for mid-infrared emission: from electronic structure to threshold current density calculations

The electronic band-structure and optical gain properties of dilute-nitride InAsN/GaSb/InAsN type-II ‘W’ quantum wells based mid-infrared laser diodes on InAs substrate are numerically investigated with an accurate 10-bands kp model. The laser diodes are designed to operate at 3.3 µm at room temperature. The dispersion relations, the optical gain and the threshold current density including computation of non-radiative Auger recombination rate are calculated. For typical injected carrier concentration of 1.5 × 1018 cm−3 at 300 K, peak gain values of the order of 1000 cm−1 are reached and a modal gain value equal to 70 cm−1 can be achieved. By evaluating reasonable optical losses, a threshold current density around 500 A cm−2 is expected, showing that dilute-nitride InAsN/GaSb/InAsN ‘W’ quantum wells are suitable for mid-infrared laser operation at room temperature.

Electronic Band Structure and Optical Properties of Srn+1TinO3n+1 Ruddlesden–Popper Homologous Series

State-of-the-art calculations of electronic band structures, density of states and frequency-dependent optical properties have been reported for Srn+1TinO3n+1 (n=1, 2, 3, ∞) compounds. These materials possess indirect wide energy band gaps. The frequency dependent optical properties of n=1,2,3 compounds show considerable anisotropy and positive birefringence. The conduction band minimum is originates from Ti-d states, while the valence band maximum is governed by O-p states. The bandwidth of the Ti-d states is responsible for the decrease in the energy band gap as n changes from 1 to 2, 3, and ∞. We have analyzed the degree of hybridization on the basis of the ratio of the orbital overlapping within the muffin tin sphere.

Ab initio investigation of perovskite and post-perovskite CaPtO 3

For the recently synthesized post-perovskite (PPV) CaPtO 3 we investigate the electronic band structure properties in connection with hypothetical perovskite PV–CaPtO 3 within density functional theory using pseudo-potential and all electron methods. Both systems are found insulating in agreement with a tetravalent Pt(IV) low spin d 6 state. The equilibrium values of the volume and energy point to a stabilization of the PPV versus PV oxide with a smaller volume in agreement with other homologue systems within Earth lower mantle such as PPV–MgSiO 3. This is also addressed from the analysis of the chemical bonding focusing on the change of the Pt–O bonding upon going from PV and PPV systems whereby the covalent Pt–O bonds are differentiated and reinforced within PPV system with respect to PV.

Electronic Energy Band Structure of the Double Perovskite Ba2MnWO6

The electronic and magnetic structures of the double perovskite oxide Ba 2MnWO6 (BMW) were determined by employing the density functional theory within the generalized gradient approximation (GGA) + U approach. BMW is considered a prototype double perovskite due to its high degree of B-site ordering and is a good case study for making a comparison between computations and experiments. By adjusting the U-parameter, the electronic energy band structure and magnetic properties, which were consistent with the experimental results, were obtained. These computations revealed that the valence bands are mainly formed from Mn 3d and O 2p states, while the conduction bands are derived from W 5d and O 2p states. The localized bands composed from Mn 3d states are located in the bandgap. The results imply that the formation of polarons in the conduction band initiate the resonance Raman modes observed as a series of equidistant peaks.

Threshold effect in the energy loss of hydrogen and helium ions transmitted in channeling conditions in gold single crystal

The energy loss of hydrogen and helium ions in the low energy range (<10keV/u) in single crystal gold targets is investigated experimentally. We found that the stopping power for helium ions shows a deviation from the proportionality with ion velocity predicted theoretically. This behavior has been also observed for protons in previous experiments. Additionally, we found that for a small range of velocity the energy loss for helium ions has the same magnitude as for protons for the same velocity. This last finding is in agreement with early theoretical prediction for the ion energy loss interacting with low electron densities, beyond of metallic densities, in the frame of the free electron gas model and the density functional theory. Both effects are explained due to two particular phenomena: particle channeling in monocrystalline targets, where the inhomogeneity of the spatial electron density distribution plays a fundamental role and the well-known threshold effect in the stopping power which is explained considering the electronic band structure properties of metallic targets.

Ab initioMolecular and Solid State Studies of the FeII Spin Cross-over System [Fe(btz)2(NCS)2] (btz = 2.2’-bis-4.5-dihydrothiazine)

Ab initio computations within the density functional theory are reported for the spin cross-over complex [Fe(btz)2(NCS)2] (btz = 2.2'-bis-4.5-dihydrothiazine), where 3d6 FeII is characterized by high-spin (HS t2g 4, eg 2) and low-spin (LS t2g 6, eg 0) states. Results of infrared and Raman spectra for the isolated molecule are complemented for the crystalline solid with a full account of the electronic band structure properties: the density of states assessing the crystal field effects and the chemical bonding, assigning a specific role to the Fe-N interactions within the coordination sphere of FeII

LiNbO3ground- and excited-state properties from first-principles calculations

The atomic and electronic structure, zone center phonon frequencies, and optical absorption of $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ are calculated from first principles. The structural and vibrational properties predicted from density functional theory are in good agreement with experiment and earlier theoretical work. The electronic band-structure and optical properties are found to be very sensitive to quasiparticle and electron-hole attraction effects, which are included here using the $GW$ approach and by solving the Bethe-Salpeter equation, respectively. We predict the fundamental gap to be more than $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ larger than the $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ frequently cited for ferrolectric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ and calculate optical absorption spectra in good agreement with experiment.

Band structure and lattice vibration properties of III-P ternary alloys

Electronic band structure, optical and vibrational properties of zinc-blende GaP x Sb 1− x and GaAs 1− x P x ternary alloys are obtained from pseudo-potential calculations. Comparisons are made with the available experimental values and with data obtained in previous theoretical studies. These comparisons show generally good agreement between the present results and experiment. The direct and indirect band-gap energies, the transverse effective charge, and the longitudinal and transversal optical phonon energies show a non-linear behavior with varying the composition x. The ionicity of the materials of interest is discussed in terms of the antisymmetric gap.

Electronic structure and optical properties ofZnX(X=O,S, Se, Te): A density functional study

Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and $\mathrm{LDA}+U$ approaches. Calculations of the optical spectra have been performed for the energy range $0--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

Ab initio calculations of the electronic structure and linear optical properties, includingself-energy effects, for paraelectric SbSI

The electronic energy band structure and linear optical properties of the ferroelectricsemiconductor SbSI in the paraelectric phase are calculated by an ab initio pseudopotentialmethod using density functional theory in the local density approximation. Thecalculated electronic band structure shows that SbSI has an indirect band gap of1.45 eV and that the smallest direct gap is at the S point of the Brillouin zone(1.56 eV). The total density of states has been analysed. The linear energy dependentdielectric functions and some optical constants such as the absorption coefficient,extinction coefficient, refractive index, energy-loss function, reflectivity and opticalconductivity, including self-energy effects, are calculated. The effective number ofvalence electrons and the effective optical dielectric constant are also calculated.

First-principles studies on the conductive and ferromagnetic properties of [Mn(ins)(μ 1,1-N 3)(CH 3OH)] 2

First-principles calculations have been performed to study the electronic band structure and ferromagnetic properties of [Mn(ins)(μ 1, 1-N 3)(CH 3OH)] 2. The calculations were based on the density functional theory (DFT) and the full potential linearized augmented plane wave (FPLAPW) method. The total and partial density of states (DOS), the spin magnetic moments and the electronic band structure were calculated. The results reveal that the compound shows semiconductive properties. The spin magnetic moment per molecule is about 8.08 μ B, which mainly comes from Mn 3+, and is in good agreement with the experimental results.

Threshold effect in the energy loss of slow protons and deuterons channeled in Au crystals

Measurements of the energy loss of protons and deuterons channeled in a very thin single-crystal foil of gold were performed, covering the range of very low velocities. The experimental results provide clear evidence of the deviation of the energy loss from the proportionality with ion velocity predicted theoretically, showing a transition between two well-defined regimes. We explain this behavior by a theoretical analysis that takes into account the electronic band structure properties of the medium, separating the contribution of the conduction band (described as a free Fermi gas) from the contribution of the nearly free $d$ electrons of gold, which are affected by a threshold behavior due to the shift of the density of states of this band with respect to the Fermi level. The theoretical model yields a very good description of the experimental findings.

PbTe Colloidal Nanocrystals: Synthesis, Characterization, and Multiple Exciton Generation

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.