Electronic structure and transport of Ca3Co2O6 and Ca3CoNiO6

We have calculated the band structure of Ca₃Co₂O₆ and Ca₃CoNiO₆ by using the self-consistent full-potential linearized augmented plane-wave method within density function theory and the generalized gradient approximation for the exchange and correlation potential. The spin-orbit interaction is incorporated in the calculations using a second variational procedure. The relation of these band structure calculations to thermoelectric transport is discussed. The results illustrate that transport is highly anisotropic with much larger mobility in the a-b plane than out of the a-b plane, and the introduction of Ni in Ca₃Co₂O₆ alters its electronic structure and its thermoelectric transport properties.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

Band structure and photocatalytic properties of perovskite-type compound Ca2NiWO6 for water splitting

An oxide semiconductor Ca₂NiWO₆, with double-perovskite crystal structure, was synthesized by solid-state reaction method. The compound Ca₂NiWO₆ was characterized by X-ray diffraction, UV-visible diffuse reflectance, and photoluminescence. The photocatalytic properties of the compound for water splitting were investigated under UV and visible light irradiation. The results showed H₂ evolution was not observed over the compound under visible light irradiation (λ>420 nm) with a 300 W xenon arc lamp when using methanol (CH₃OH) as electron donor, although the compound was responsive to visible light region. Based on the experimental results, a possible band structure was proposed through theoretical calculation of the electronic structure by using the full potential-linearized augmented plane wave (F-LAPW). The band structure and photocatalytic properties were attributed to the special crystal and electronic structures. Due to the oxygen vacancies in the compound, which worked as electron-hole recombination centers, the photocatalytic activity of the compound was low.     

Optical properties of new photovoltaic materials: AgCuO2 and Ag2Cu2O3

AgCuO₂ and Ag₂Cu₂O₃ are new types of semiconductor materials. A theoretical study is presented for both the electronic and optical properties of these new photovoltaic materials in the framework of density functional theory (DFT). The calculated cohesive energy is −3.606 eV/atom and −3.723 eV/atom for Ag₂Cu₂O₃ and AgCuO₂, respectively. Electronic calculations indicate that AgCuO₂ is a small band gap semiconductor and Ag₂Cu₂O₃ is metallic in nature. The valency state of Cu is divalent in Ag₂Cu₂O₃ and trivalent in AgCuO₂. The largest absorption coefficient of CuO₂ is 332 244, which is significantly greater than that of CuInSe₂, CdTe, GaAs, etc.     

Optical constants of Cu(In0.7Ga0.3)5Se8 and Cu(In0.4Ga0.6)5Se8 crystals

Variable angle spectroscopic ellipsometry has been applied to characterize the optical constants of bulk Cu(In_0.7Ga_0.3)₅Se₈ and Cu(In_0.4Ga_0.6)₅Se₈ crystals grown by the Bridgman method. The spectra were measured at room temperature over the energy range 0.8-4.4 eV. Adachi’s model was used to calculate the dielectric functions as well as the spectral dependence of complex refractive index, absorption coefficient, and normal-incidence reflectivity. The calculated data are in good agreement with the experimental ones over the entire range of photon energies. The parameters such as strength, threshold energy, and broadening, corresponding to the E₀, E_1A, and E_1B interband transitions, have been determined using the simulated annealing algorithm.     

Temperature dependence of the Urbach energy in ordered defect compounds Cu-III3-VI5 and Cu-III5-VI8

Two previous models used with success in Cu-III-VI₂ semiconductors have been employed to study the temperature dependence of the Urbach energy in ordered compounds Cu-III₃-VI₅ and Cu-III₅-VI₈. The model which contains two variable parameters seems to explain better the data over the whole temperature range studied. However, the ordered vacancy or the donor acceptor defect pair in the cation sublattice provides new features in these compounds that need further study.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Chiral optical absorption bands in [ ]Cd1−x Mnx Ga2S4 semiconducting compound

[ ]Cd_1−x Mn_x Ga₂S₄ is a semimagnetic semiconductor and it has revealed an exceptional property namely ‘optical activity‘. Therefore, a spectroscopic investigation of chiral absorption bands has been carried out with the view to examine the role of d*-d states of manganese atoms. It has been found that inner transitions of Mn⁺⁺ dominate the spectral region with a special feature, indicating that these transitions show the presence of a substantial contribution from the magnetic dipole moment which rotates the electric vector of the incident polarized radiation. The origin is associated to the lack of a symmetry center caused by the ordered vacancies in this defect compound.     

Electronic structure and chemical bonding of phosphorus-contained sulfides InPS4, TI3PS4, and Sn2P2S6

The electronic structure of phosphorus-contained sulfides InPS₄, Tl₃PS₄, and Sn₂P₂S₆ was investigated experimentally with X-ray spectroscopy and theoretically by quantum mechanical calculations. The partial densities of electron states calculated with the ab initio multiple scattering FEFF8 code correspond well to their experimental analogues—the X-ray K- and L_2,3-spectra of sulfur and phosphorus. The good agreement between theory and experiment was also achieved for K-absorption spectra of S and P in the investigated sulfides. In spite of the difference in the crystallographic structure of InPS₄, TI₃PS₄, and Sn₂P₂S₆ that influence the form of K-absorption spectra, the electronic structure of their valence bands are rather similar. This is due to the strong interaction of the P and S atoms, which are the nearest neighbors in the compounds studied. The electron densities of p- and s-states of phosphorus are shifted by about 3 eV to lower energies in comparison to the analogous electron states of sulfur. This is connected with the greater electro-negativity of sulfur, and is confirmed by the calculated electron charge transfer from P to S.     

Effect of annealing temperature on the optical and electrical properties of amorphous As45.2Te46.6In8.2 thin films

The optical absorption of the As-prepared and annealed As_45.2Te_46.6In_8.2 thin films are studied. Films annealed at temperatures higher than 453 K show a decrease in the optical energy gap (E_o). The value of E_o increases from 1.9 to 2.43 eV with increasing thickness of the As-prepared films from 60 to 140 nm. The effect of thickness on high frequency dielectric constant (?_∞) and carrier concentration (N) is also studied. The crystalline structures of the As_45.2Te_46.6In_8.2 thin films resulting from heat treatment of the As-prepared film at different elevated temperatures is studied by X-ray diffraction. An amorphous-crystalline transformation is observed after annealing at temperatures higher than 453 K. The electrical conductivity at low temperatures is found due to the electrons transport by hopping among the localized states near the Fermi level. With annealing the films at temperatures higher than 473 K (the crystallization onset temperature) for 1 h, the electrical conductivity increases and the activation energy decreases, which can be attributed to the amorphous-crystalline transformations.     

Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2

Spectroscopic ellipsometry measurements of CuInSe₂ (CIS) and CuIn_1−xGa_xSe₂ (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α-phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ?₂ in the spectral region, 1-3 eV. This reduction can be explained in terms of the Cu-3d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn_1−xGa_xSe₂ (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn_1−xGa_xSe₂. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.     

Exciton spectra, valence band splitting, and energy band structure of CuGaXIn1−XS2 and CuGaXIn1−XSe2 crystals

Exciton spectra are studied in CuGa_XIn_1−XS₂ solid solutions by means of photoreflectivity and wavelength modulation spectroscopy at liquid nitrogen temperature. The exciton parameters, dielectric constants, and free carrier effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The crystal field and spin orbit valence band splitting is calculated as a function of X taking into account the energy position of excitonic lines. The energy band structure of CuGa_XIn_1−XS₂ and CuGa_XIn_1−XSe₂ compounds is derived from optical spectra at photon energies higher than the fundamental band gap. The energies of optical transitions are tabulated for X values from 0 to 1.     

The effect of (00l) crystal plane orientation on the thermoelectric properties of Bi2Te3 thin film

Highly (00l)-oriented pure Bi₂Te₃ films with in-plane layered grown columnar nanostructure have been fabricated by a simple magnetron co-sputtering method. Compared with ordinary Bi₂Te₃ film and bulk materials, the electrical conductivity and Seebeck coefficient of such films have been greatly increased simultaneously due to raised carrier mobility and electron scattering parameter, while the thermal conductivity has been decreased due to phonon scattering by grain boundaries between columnar grains and interfaces between each layers. The power factor has reached as large as 33.7 μW cm⁻¹ K⁻², and the out-of-plane thermal conductivity is reduced to 0.86 W m⁻¹ K⁻¹. Our results confirm that tailoring nanoscale structures inside thermoelectric films effectively enhances their performances.     

Preparation of Cu2ZnSnS4 thin films by hybrid sputtering

In order to fabricate Cu₂ZnSnS₄ thin films, hybrid sputtering system with two sputter sources and two effusion cells is used. The Cu₂ZnSnS₄ films are fabricated by the sequential deposition of metal elements and annealing in S flux, varying the substrate temperature. The Cu₂ZnSnS₄ films with stoichiometric composition are obtained at the substrate temperature up to 400 °C, whereas the film composition becomes quite Zn-pool at the substrate temperature above 450 °C. The Cu₂ZnSnS₄ film shows p-type conductivity, and the optical absorption coefficient and the band gap of the Cu₂ZnSnS₄ film prepared in this experiment are suitable for fabricating a thin film solar cell.     

Optical and thermoelectric characterizations of electroplated n-Bi2(Te0.9Se0.1)3

Bismuth selenotelluride (Bi₂(Te_0.9Se_0.1)₃) films were electrodeposited at constant current density from acidic aqueous solutions with Arabic gum in order to produce thin films for miniaturized thermoelectric devices. X-ray fluorescence spectroscopy determined film compositions. X-ray diffraction pattern shows that the films as deposited are polycrystalline, isostructural to Bi₂Te₃ and covered by crystallites. Mueller-matrix analysis reveals that the electroplated layers are optically like an isotropic medium. Their pseudo-dielectric functions were determined using mid-infrared spectroscopic ellipsometry. Tauc-Lorentz combined with Drude dispersion relations were successfully used. The energy band gap E_g was found to be about 0.15 eV. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. From Seebeck coefficient measurement, films exhibit n-type charge carrier and the value of thermoelectric power is about −40 μV/K.     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.