Theoretical Density Of States Research Articles

Electronic band structure and optical properties of Li2In2GeSe6 crystal

We present results of a complex experimental and theoretical study of peculiarities of the electronic structure and optical constants of Li2In2GeSe6 compound crystallizing in monoclinic non-centrosymmetric space group Cc. In particular, we have measured the XPS spectra of core-level and valence electrons of atoms composing a high optical quality Li2In2GeSe6 crystal for both pristine and treated with middle-energy Ar+ ions surfaces. Furthermore, the XPS spectrum as measured for the valence-band range of the Li2In2GeSe6 crystal is compared with the total density of states (DOS) theoretically calculated within a density functional theory (DFT) framework as implemented in the augmented plane wave plus local orbitals (APW+lo) method. The present theoretical APW+lo results reveal a very good agreement regarding the shapes and energy positions of main features of the theoretical total DOS curve and the experimental XPS spectrum when the first-principles DFT calculations are made using the modified Becke-Johnson (mBJ) functional in the form of Tran-Blaha and involving the Hubbard correction parameter U treated for highly correlated In 4d electrons (TB-mBJ+U approach). This technique gives also band gap value of 2.26 eV, which is in good agreement with that experimentally measured for this compound. The present TB-mBJ calculations indicate that, at the top and in the upper region of the valence band, Se 4p states make the biggest contributions; the central region is formed mainly by Ge 4p states, while In 5s states dominate at the bottom and in the lower portion of the Li2In2GeSe6 valence band. With respect to peculiarities of filling the valence band by Se(Ge) 4p states, the present calculations are verified experimentally. Furthermore, the optical properties of Li2In2GeSe6 are studied theoretically in detail employing the TB-mBJ+U technique.

BiCuI4(Pyridine)5 a neutral ligand-supported compound of BiI3 and CuI

Reaction of equimolar pyridine (Py) solutions BiI3 and CuI produces the Py-supported 1:1 bimetallic complex BiCuI4(Py)5 in quantitative yield. The title complex is only the second neutral complex to feature Cu−I−Bi bridging and shows an octahedral trans-[BiI4(Py)2] unit joined to a distorted tetrahedral CuI(Py)3 unit by an iodide bridge. The complex shows low thermal stability, decomposing under modest heating or vacuum to produce a mixture of BiI3 and CuI. Thus, it is a potential entry/precursor to BiI3/CuI chemistry. Natural Localized Molecular Orbital (NLMO) calculations were performed to analyze the nature of the Cu−I−Bi bonds, revealing that the title compound lies on the cusp of being [Cu(Py)3]+[BiI4(Py)2]−. Diffuse reflectance spectroscopy measurements show a strong absorption band with an optical bandgap energy of 1.94 eV. Theoretical density of states (DOS) and time-dependent density functional theory (TD-DFT) experiments to map the electronic structure assign the primary electronic transition as a mixed halide/metal-to-ligand charge transfer between Cu−I−Bi donor orbitals and Py π* acceptor orbitals.

Comparison of Brillouin Light Scattering and Density of States in a Supported Layer: Analytical and Experimental Study

We provide a detailed analytical calculation of the Brillouin light scattering (BLS) intensity of a layer on a substrate, taking into account both photoelastic and moving boundary (ripple effect) mechanisms, and give a comparison between BLS intensity and density of states (DOS) to determine the dispersion curves of longitudinal guided modes in the supported layer. In particular, in the case where the mismatch between the elastic parameters of the substrate and the adsorbed layer is high, such as in a PMMA layer on a Si substrate, we derive closed-form expressions of BLS and DOS and demonstrate a simple relationship between these two quantities. A very good agreement between experimental and theoretical BLS spectra was found and compared to theoretical DOS spectra. In particular, we show that while the peaks in the DOS present a uniform behavior, the BLS spectra follows a sine cardinal (sinc) function shape around a given frequency fixed by the chosen laser wavelength. The theoretical calculation is performed within the framework of the Green’s function approach.

An unusual Cu/Te hybridization in the Cu0.3ZrTe2 intercalation compound

The electronic structure of the novel intercalation compound Cu0.3ZrTe2 was analyzed using the X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy and DFT calculations. The resonant photoemission enhancement of the valence band features of Cu0.3ZrTe2 at the Zr 3p→4d and Cu 3p→3d photo absorption thresholds was studied. The tetrahedral coordination of the Cu atoms by the Te atoms was shown to be responsible for the strong chemical bonding of the Cu atoms within the ZrTe2 lattice through the Cu 3d/Te 5p hybridization. These findings were confirmed by theoretical density of states (DOS) calculations performed for CuxZrTe2 (x = 0.25 and 0.5) with the Cu atoms tetrahedrally and octahedrally coordinated by Te atoms. It was concluded that the substitution of atoms in the CuxTiSe2 system with heavier and polarizable ones (Se→Te, Ti→Zr), i.e. an increase in the lattice polarizability, destabilizes the octahedral coordination of the Cu atoms by the Se/Te ones. This limits the stability of the phases characterized by quantum states with a charge density wave and superconductivity.

Thermal scattering law data generation for hydrogen bound in zirconium hydride based on the phonon density of states from first-principles calculations

The thermal neutron scattering cross sections of zirconium hydride (ZrHx) is heavily affected by its lattice structure which can be represented by phonon density of states (DOS). In the past decades, many works tried to get parameterized phonon DOS of ZrHx by fitting certain types of experimental results. In the present work, we adopt the first-principles calculation to obtain the phonon DOS of ZrH1.5 in δ phase and ZrH2 in ε phase. The theoretical phonon DOS is used to calculate the thermal scattering cross sections which are then used in the neutronics simulations of several TRIGA reactors. The numerical results show that the phonon DOS obtained by first-principles calculations can produce more accurate scattering cross sections, and improve the neutronics results of TRIGA reactors compared with the phonon DOS models applied in ENDF/B and JEFF evaluated nuclear data libraries.

Van Hove Singularities and Trap States in Two-Dimensional CdSe Nanoplatelets.

Semiconductor nanoplatelets, which offer a compelling combination of the flatness of two-dimensional semiconductors and the inherent richness brought about by colloidal nanostructure synthesis, form an ideal and general testbed to investigate fundamental physical effects related to the dimensionality of semiconductors. With low temperature scanning tunnelling spectroscopy and tight binding calculations, we investigate the conduction band density of states of individual CdSe nanoplatelets. We find an occurrence of peaks instead of the typical steplike function associated with a quantum well, that rule out a free in-plane electron motion, in agreement with the theoretical density of states. This finding, along with the detection of deep trap states located on the edge facets, which also restrict the electron motion, provides a detailed picture of the actual lateral confinement in quantum wells with finite length and width.

First-principles calculation of Sb2Te3 topological insulator under pressure

The theoretical density of states (DOS) along with band structure (BS) has been evaluated for Topological insulator Sb2Te3 upon pressure. The density functional theory (DFT) calculation shows electronic topological transition (ETT) under the application of pressure. When pressure is applied conduction band and valence shifts significantly which is indicated as electronic transition of the Fermi surfaces. The band gap between conduction band and valence band reduced at Z-high symmetry point whereas it is observed to enhance at Γ-point with increase in pressure. Interestingly, we found semiconductor to metal transition near 2 GPa with LDA approximation. The overall change in the Fermi surfaces is found to follow other topological materials like Bi2Te3, Sb2Se3 and Bi2Se3. The topologically non-trivial surface state remains unchanged under pressure; however, the bulk band gap reduces.

First-principles DFT computation and X-ray spectroscopy study of the electronic band structure and optical constants of Cu2HgGeS4

We present results of a complex study of the electronic structure and optical constants from experimental and theoretical points of view of quaternary copper mercury germanium sulfide, Cu2HgGeS4. Partly, based on measurements employing X-ray photoelectron (XP) spectroscopy, we evaluate binding energy of core-level electrons for atoms constituting the sulfide under study using a high-quality Cu2HgGeS4 crystal. Furthermore, the energy distribution of the electronic valence states in Cu2HgGeS4 is evaluated on measurements of the valence band XP spectrum of the crystal under study. To find the best correspondence of the shape of the valence band XP spectrum to theoretical total density of states (DOS) curve, different approaches for exchange-correlation (XC) potential are explored in the present work. We find that the best correlation of the shapes of the theoretical total DOS curve and the experimental valence band XP spectrum is obtained when the first-principles calculation within a framework of density functional theory (DFT) are made using modified Becke-Johnson (mBJ) functional and treating also the correction parameter U (Hubbard parameter) and effect of spin-orbit (SO) coupling (mBJ + U + SO technique). Based on the present mBJ + U + SO calculation, we explore in detail projected DOS curves, bands dispersion, and the main optical constants. The projected DOS data are found to be in excellent agreement with X-ray emission bands measured for the main contributors to the valence-band region of the compound under study. The present experimental and theoretical results allow for statement that Cu2HgGeS4 is a promising photovoltaic absorber material.

Accurate surface band bending determination on Ga-polar n-type GaN films by fitting x-ray valence band photoemission spectrum

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.

Hole-induced electronic and optical transitions in La1−xSrxFeO3 epitaxial thin films

We have investigated the electronic and optical properties of epitaxial La1-xSrxFeO3 for x from 0 to 1 prepared by molecular beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward to the valence band maximum. Both Fe 2p and O 1s spectra broaden to higher binding energy with increasing x, consistent with delocalization of Sr-induced holes in the Fe 3d/O 2p hybridized valence band. Combining X-ray valence band photoemission and O K-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level, resulting from hole doping. In-plane transport measurements confirm that the material becomes a p-type semiconductor at lower doping levels and exhibits a insulator-to-metal transition at x equal to 1. Sub-gap optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

Electronic band structure and basic optical constants of TlGaSn2Se6, a promising NLO semiconductor: First-principles calculations under DFT framework

We present results of band-structure computation under framework of density functional theory (DFT) of total and partial densities of states (DOS) of quaternary thallium gallium tin selenide, TlGaSn2Se6, a novel and promising material for application in non-linear optics. The present computation data are obtained employing different approaches for exchange-correlation (XC) potential to uncover the best fit of theoretical DOS curves to the experimental valence band (VB) spectrum of TlGaSn2Se6 compound as measured by X-ray photoelectron spectroscopy. Our findings show that the best fit of the theoretical and experimental data are detected in the case of application for XC potential of modified Becke-Johnson (MBJ) approach and involving in the computation procedure of the spin-orbit (SO) effect and the correction parameter U for highly correlated electrons of d symmetry. Based on our findings, we calculate band structure and primary optical constants of the quaternary thallium gallium tin selenide. The calculations indicate that this selenide is an indirect gap semiconductor in which the main input at the top und upper constituent of the TlGaSn2Se6 VB comes from Se 4p states, its central part is dominated by contributions of Sn 5p states. Further, Tl 6 s and Ga 4 s states are the most substantial contributors to the lower VB portion, while the VB bottom is prevailed by contributions of Sn 5 s states.

Electronic structure and optical properties of LiGa0.5In0.5Se2 single crystal, a nonlinear optical mid-IR material

Measurements of X-ray photoelectron core-level and valence-band spectra for pristine and irradiated with Ar+ ions surfaces of LiGa0.5In0.5Se2 single crystal, novel nonlinear optical mid-IR selenide grown by a modified vertical Bridgman-Stockbarger technique, are reported. Electronic structure of LiGa0.5In0.5Se2 is elucidated from theoretical and experimental points of view. Notably, total and partial densities of states (DOSs) of the LiGa0.5In0.5Se2 compound are calculated based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method. In accordance with the DFT calculations, the principal contributors to the valence band are the Se 4p states, making the main input at the top and in the upper part of the band, while its bottom is dominated by contributions of the valence s states associated with Ga and In atoms. The theoretical total DOS curve peculiarities are found to be in excellent agreement with the shape of the X-ray photoelectron valence-band spectrum of the LiGa0.5In0.5Se2 single crystal. The bottom of the conduction band of LiGa0.5In0.5Se2 is formed mainly by contributions of the unoccupied Ga 4s and In 5s states in almost equal proportion, with somewhat smaller contributions of the unoccupied Se 4p states as well. Our calculations indicate that the LiGa0.5In0.5Se2 compound is a direct gap semiconductor. The principal optical constants of LiGa0.5In0.5Se2 are calculated in the present work.

An Investigation of Quantum Dot Super Lattice Use in Nonvolatile Memory and Transistors

Site-specific self-assembled colloidal quantum dots (QDs) will deposit in two layers only on p-type substrate to form a QD superlattice (QDSL). The QDSL structure has been integrated into the floating gate of a nonvolatile memory component and has demonstrated promising results in multi-bit storage, ease of fabrication, and memory retention. Additionally, multi-valued logic devices and circuits have been created by using QDSL structures which demonstrated ternary and quaternary logic. With increasing use of site-specific self-assembled QDSLs, fundamental understanding of silicon and germanium QDSL charge storage capability, self-assembly on specific surfaces, uniform distribution, and mini-band formation has to be understood for successful implementation in devices. In this work, we investigate the differences in electron charge storage by building metal-oxide semiconductor (MOS) capacitors and using capacitance and voltage measurements to quantify the storage capabilities. The self-assembly process and distribution density of the QDSL is done by obtaining atomic force microscopy (AFM) results on line samples. Additionally, we present a summary of the theoretical density of states in each of the QDSLs.

Specific heat, Electrical resistivity and Electronic band structure properties of noncentrosymmetric Th7Fe3 superconductor

Noncentrosymmetric superconductor Th7Fe3 has been investigated by means of specific heat, electrical resisitivity measurements and electronic properties calculations. Sudden drop in the resistivity at 2.05 ± 0.15 K and specific heat jump at 1.98 ± 0.02 K are observed, rendering the superconducting transition. A model of two BCS-type gaps appears to describe the zero-magnetic-field specific heat better than those based on the isotropic BCS theory or anisotropic functions. A positive curvature of the upper critical field Hc2(Tc) and nonlinear field dependence of the Sommerfeld coefficient at 0.4 K qualitatively support the two-gap scenario, which predicts Hc2(0) = 13 kOe. The theoretical densities of states and electronic band structures (EBS) around the Fermi energy show a mixture of Th 6d- and Fe 3d-electrons bands, being responsible for the superconductivity. Furthermore, the EBS and Fermi surfaces disclose significantly anisotropic splitting associated with asymmetric spin-orbit coupling (ASOC). The ASOC sets up also multiband structure, which presumably favours a multigap superconductivity. Electron Localization Function reveals the existence of both metallic and covalent bonds, the latter may have different strengths depending on the regions close to the Fe or Th atoms. The superconducting, electronic properties and implications of asymmetric spin-orbit coupling associated with noncentrosymmetric structure are discussed.

Electronic structure and magnetic anisotropy of L1-FePt thin film studied by hard x-ray photoemission spectroscopy and first-principles calculations

We have studied the electronic structure of the L10 ordered FePt thin film by hard x-ray photoemission spectroscopy (HAXPES), cluster model, and first-principles calculations to investigate the relationship between the electronic structure and perpendicular magneto-crystalline anisotropy (MCA). The Fe 2p core-level HAXPES spectrum of the ordered film revealed the strong electron correlation in the Fe 3d states and the hybridization between the Fe 3d and Pt 5d states. By comparing the experimental valence band structure with the theoretical density of states, the strong electron correlation in the Fe 3d states modifies the valence band electronic structure of the L10 ordered FePt thin film through the Fe 3d-Pt 5d hybridization. These results strongly suggest that the strong electron correlation effect in the Fe 3d states and the Fe 3d-Pt 5d hybridization as well as the spin-orbit interaction in the Pt 5d states play important roles in the perpendicular MCA for L10-FePt.

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental-Theoretical Study.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

Valence Band Control of Metal Silicide Films via Stoichiometry.

The unique electronic and mechanical properties of metal silicide films render them interesting for advanced materials in plasmonic devices, batteries, field-emitters, thermoelectric devices, transistors, and nanoelectromechanical switches. However, enabling their use requires precisely controlling their electronic structure. Using platinum silicide (PtxSi) as a model silicide, we demonstrate that the electronic structure of PtxSi thin films (1 ≤ x ≤ 3) can be tuned between metallic and semimetallic by changing the stoichiometry. Increasing the silicon content in PtxSi decreases the carrier density according to valence band X-ray photoelectron spectroscopy and theoretical density of states (DOS) calculations. Among all PtxSi phases, Pt3Si offers the highest DOS due to the modest shift of the Pt5d manifold away from the Fermi edge by only 0.5 eV compared to Pt, rendering it promising for applications. These results, demonstrating tunability of the electronic structure of thin metal silicide films, suggest that metal silicides can be designed to achieve application-specific electronic properties.

Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses

We studied atomic and electronic structures of binary Cu-Zr metallic glasses (MGs) using combined experimental and computational methods including X-ray absorption fine structure spectroscopy, electrical resistivity, thermoelectric power (TEP) measurements, molecular dynamics (MD) simulations, and ab-initio calculations. The results of MD simulations and extended X-ray absorption fine structure analysis indicate that atomic order of Cu-Zr MGs and can be described in terms of interpenetrating icosahedral-like clusters involving five-fold symmetry. MD configurations were used as an input for calculations of theoretical electronic density of states (DOS) functions which exhibits good agreement with the experimental X-ray absorption near-edge spectra. We found no indication of minimum of DOS at Fermi energy predicted by Mott's nearly free electron (NFE) model for glass-forming alloys. The theoretical DOS was subsequently used to test Mott's model describing the temperature variation of electrical resistivity and thermoelectric power of transition metal-based MGs. We demonstrate that the measured temperature variations of electrical resistivity and TEP remain in a contradiction with this model. On the other hand, the experimental temperature dependence of electrical resistivity can be explained by incipient localization of conduction electrons. It is shown that weak localization model works up to relatively high temperatures when localization is destroyed by phonons. Our results indicate that electron transport properties of Cu-Zr MGs are dominated by localization effects rather than by electronic structure. We suggest that NFE model fails to explain a relatively high glass-forming ability of binary Cu-Zr alloys.

Band energy levels and compositions of CdS-based solid solution and their relation with photocatalytic activities

CdS-based solid solution (AgIn)xCd2(1−x)S2 (x = 0, 0.02, 0.05, 0.1, 0.2, 1.0) was prepared through hydrothermal reaction. The physicochemical and optical properties of the as-prepared solid solution samples were characterized by X-ray diffraction (XRD), diffuse reflectance UV-visible absorption spectroscopy (DRS) and N2 adsorption–desorption isotherms. Through controlling the composition of CdS solid solution, its band-gap energy (Eg) can be easily tuned from ca. 2.4 to 1.8 eV. The band edge energy levels of all samples were determined through the measurement of the open circuit potential (OCP) and onset potential of photocurrent (Eonset) for their respective electrodes. The decrease in Eg of the solid solutions is attributed to the downward and upward shift of conduction and valence band potentials, respectively. It was found that In 5s5p and Cd 5s5p states mainly contribute to the bottom of conduction band, and Ag 4d and S 3p states mainly contribute to the top of valence band as confirmed by theoretical density of states (DOS) calculation. To test the photocatalytic activity of the prepared solid solution materials, the evolution of H2 and the reduction of polyoxometalate (POM: PMo12O403−) under visible light irradiation were used as probe reactions. The photocatalysis test results were well correlated with the band structures determined from the photoelectrochemical measurements and DOS calculation. The relative positioning between the band-edge levels and the substrate conversion potentials was the most critical and this can be controlled with the solid solution-based photocatalysts.

A photoelectron spectroscopy study of the electronic structure evolution in CuInSe2-related compounds at changing copper content

Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.