Optoelectronic structure and related transport properties of BiCuSeO-based oxychalcogenides: First principle calculations

Recent experiments have revealed that the p-type BiCuSeO-based oxychalcogenides compounds exhibit a high thermoelectric figures of merit due to their very low lattice thermal conductivities and moderate Seebeck coefficient in the medium temperature range. In the present work, we reported on the optoelectronic and thermoelectric properties using the full potential linear augmented plane wave method and modified Becke-Johnson potential with spin-orbit coupling. The properties show that the BiCuSeO-based oxychalcogenides exhibit a semiconductor behavior with band gap values of 0.51, 0.45 and 0.41 eV for BiCuSO, BiCuSeO, and BiCuTeO, respectively. Due to their prominent role for thermoelectric applications, we combined Boltzmann transport theory to DFT results to compute the transport properties, mainly electronic conductivity, thermal conductivity, Seebeck coefficient and power factor. The present results show the dominance of BiCuTeO for thermoelectric application compared to the BiCuSO and BiCuSeO.     

Structural and optical characterization of thermally evaporated cadmium sulfide thin films

The article reports the structural and optical properties of vacuum‐evaporated cadmium sulfide (CdS) films with different thicknesses at room temperature. The structural investigations performed by means of X‐ray diffraction (XRD) technique have showed that all the films have the zinc‐blende structure, a face‐centered cubic form with lattice constants a = b = c = 5.82 Å and point group F4 3m. Crystallite sizes calculated from Scherrer relation are in the range of 173–345 Å. So far, because the optical parameters of the metastable cubic CdS have not been so well known, we apply spectroscopic ellipsometry to determine the thickness, optical constants and energy band gap of CdS thin film deposited by thermal evaporation onto opaque gold substrate, a perfect reflectivity and inert metal. As shown the measured spectral behavior of the optical constants and the band gap value of CdS thin film are in agreement with those obtained by the reflectance and transmittance methods. The energy band gap of CdS thin film determined from the spectral behavior of the absorption coefficient is about 2.46 eV. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical, optical, thermoelectric power, and dielectrical properties of organic semiconductor poly(1,12-bis(carbazolyl) dodecane) film

Electrical conductivity, optical, thermoelectric, and dielectrical properties of the poly(1,12-bis(carbazolyl) dodecane) film have been investigated. The activation energy for electrical conductivity and room-temperature electrical conductivity (at 25 degrees C) values were found to be 0.25 eV and 2.65 x 10-6 S/cm, respectively. The thermoelectric power results suggest that the conductivity is due to large polarons (i.e., the carriers in polymer move by hopping in the localized states at band gap edges). Electrical conductivity and thermoelectric power results confirm that the polymer is a p-type organic semiconductor. Optical absorption results suggest that the direct allowed transitions are dominant in the fundamental absorption edge in the polymer with optical band gap value of 2.72 eV. The refractive index dispersion of the polymer obeys the single oscillator model with oscillator energy (Eo = 3.06 eV) and dispersion energy (Ed = 17.82 eV) values. Alternating current conductivity results suggest that the hopping conductivity is dominant in the polymer. The dielectrical properties exhibit a non-Debye relaxation.     

Electrical transport properties of an organic semiconductor on polyaniline doped by boric acid

The electrical conductivity, thermoelectric power, and dielectric properties of polyaniline doped by boric acid (PANI‐B) have been investigated. The room temperature electrical conductivity of PANI‐B was found to be 1.02 × 10^?4 S cm^?1. The thermoelectric power factor for the polymer was found to be 0.64 µW m^?1 K^?2. The optical band gap of the PANI‐B was determined by optical absorption method, and the PANI‐B has a direct optical band gap of 3.71 eV. The alternating charge transport mechanism of the polymer is based on the correlated barrier hopping (CBH) model. The imaginary part of the dielectric modulus for the PANI‐B suggests a temperature dependent dielectric relaxation mechanism. Electrical conductivity and thermoelectric power results indicate that the PANI‐B is an organic semiconductor with thermally activated conduction mechanism. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural,elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn2P2

The present study explores the structural, elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn₂P₂ using a pseudopotential plane-wave method in the framework of density functional theory within the generalized gradient approximation. The calculated lattice constants and internal coordinates are in very good agreement with the experimental findings. Independent single-crystal elastic constants as well as numerical estimations of the bulk modulus, the shear modulus, Young's modulus, Poisson's ratio, Pugh's indicator of brittle/ductile behaviour and the Debye temperature for the corresponding polycrystalline phase were obtained. The elastic anisotropy of BaIn₂P₂ was investigated using three different indexes. The calculated electronic band structure and the total and site-projected l-decomposed densities of states reveal that this compound is a direct narrow-band-gap semiconductor. Under the influence of hydrostatic pressure, the direct D–D band gap transforms into an indirect B-D band gap at 4.08 GPa, then into a B–Γ band gap at 10.56 GPa. Optical macroscopic constants, namely, the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, absorption coefficient and energy-loss function, for polarized incident radiation along the [100], [010] and [001] directions were investigated.     

An ab-initio study of structural,opto-electronic and thermoelectric aspects of zinc sulfide and zinc telluride

In this study, structural, electronic, optical and thermoelectric aspects of Zinc Sulfide (ZnS) and Zinc Telluride (ZnTe) have been explored in detail. These calculations have been done by utilizing FP-LAPW method via Density Functional Theory (DFT). In order to attain accurate band gaps, opto-electronic properties are evaluated with modified Becke Johnson potential (mBJ). From band structure plots, both ZnS and ZnTe reveals direct (Γ_v–Γ_C) band gap semiconductors in nature with bandgap value equal to 3.5 and 2.3 eV while in Density Of States (DOS) major influence is observed due to p states of S/Te and d state of Zn. Prominent variation of optical responses such as high values of imaginary dielectric constants 𝜀1 (ω) and n (ω) refractive index suggests that ZnS and ZnTe are applicant materials for future photonics and microelectronic devices. The thermoelectric aspects were explored by Boltz Trap code to determine electrical and thermal conductivities, Seebeck coefficients, power factors and figure of merit. The figure of merits is closer to 1 while compared with p-type ZnS and ZnTe, n-type ZnS and ZnTe has good thermoelectric properties, which are attributed to low thermal conductivity of the hole and larger effective mass. The goal of this research is to investigate not only the detailed physical aspects but also to provide an overview of its future applications in optoelectronics, displays, sensors and microelectronic industry.     

First principles calculations of electronic and optical properties of Ag2S

A theoretical study of structural, electronic and optical properties of Ag₂S is presented using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the modified Becke Johnson (MBJ) potential coupled with Local Density Approximation (LDA) was used for the exchange-correlation potential calculation. Ground state properties are determined for the bulk material in monoclinic phase. Band structure reveals that this compound is a direct energy band gap semiconductor. MBJLDA results for the band gap of this compound are much better than those obtained using LDA, Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and Engel–Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gap and corresponding experimental values as compared to other calculations. Optical constants including the dielectric function, refractive index, extinction coefficient, electron energy loss function, reflectivity and absorption coefficient are obtained and discussed.     

Effect of Pressure on Electronic Structures and Optical Properties of Rocksalt InN

The electronic structures and optical properties of rocksalt indium nitride (InN) under pres-sure were studied using the first-principles calculation by considering the exchange and cor-relation potentials with the generalized gradient approximation. The calculated lattice con-stant shows good agreement with the experimental value. It is interestingly found that the band gap energy E_g at the Γ or X point remarkably increases with increasing pressure, but E_g at the L point does not increase obviously. The pressure coefficient of E_g is calculated to be 44 meV/GPa at the Γ point. Moreover, the optical properties of rocksalt InN were calculated and discussed based on the calculated band structures and electronic density of states.     

CdO及CdxZn1-xO化合物的结构、能量和电子性能分析

采用基于密度泛函理论的第一性原理的平面波超软赝势计算方法, 研究了纤锌矿结构的CdxZn1-xO化合物以及CdO在纤锌矿结构、岩盐结构和闪锌矿结构的基态电子特性和体结构, 分析了CdO的稳定性. 通过对比纤锌矿结构、岩盐结构和闪锌矿结构CdO的内聚能, 发现岩盐结构和纤锌矿结构CdO的稳定性好, 闪锌矿结构相对较差; 通过对CdxZn1-xO化合物在不同Cd组分下的电子结构计算, 得到了较好的禁带宽度拟合结果, 能带弯曲参量B=1.02 eV; 通过形成能与组分关系的分析, 我们认为当Cd的组分x=0.4左右时, CdxZn1-xO化合物最不稳定, 容易出现相分离现象.     

Preparation and characterization of nanorods Sb doped CdO films by sol–gel technique

Pure and antimony (Sb) doped CdO films were grown using sol–gel spin coating technique. The structural properties of the films were investigated using atomic force microscopy. The structure of CdO film is converted from microrods to nanorods with Sb dopant. The analysis of optical absorption revealed that optical bandgap of the films changes with doping. The optical bandgap for 0.1, 0.5, 1.0, and 2.0% Sb doped CdO was determined to be 2.28, 2.30, 2.56, and 2.42 eV, respectively. Other optical constants such as refractive index, extinction coefficient, and dielectric constants were calculated using the optical data. The refractive index dispersion of the films obeys the single oscillator model. The volume and surface energy loss functions were calculated and observed to increase with increase in the photon energy.     

Enhanced thermoelectric performance of hydrothermally synthesized polycrystalline Te-doped SnSe

In this study,large-scale Te-doped polycrystalline SnSe nanopowders were synthesized by a facile hydrothermal approach and the effect of Te doping on the thermoelectric properties of SnSe was fully investigated.It is found that the carrier concentration increases due to the reduction of band gap by alloying with Te,which contributes to significant enhancement of electrical conductivity especially at room temperature.Combined with the moderated Seebeck coefficient,a high power factor of 4.59μW cm ~1 K ~2 is obtained at 773 K.Furthermore,the lattice the rmal conductivity is greatly reduced upon Te substitution owing to the atomic point defect scattering.Benefiting from the synergistically optimized both electrical-and thermal-transport properties by Te-doping,thermoelectric performance of polycrystalline SnSe is enhanced in the whole temperature range with a maximum ZT of-0.79 at a relatively low temperature(773 K) for SnSe_(0.85)Te_(0.15).This study provides a low-cost and simple lowtemperature method to mass production of SnSe with high thermoelectric performance for practical applications     

Improved optical and electrical properties of sol–gel-derived boron-doped zinc oxide thin films

Sol–gel spin-coating was used to grow zinc oxide (ZnO) thin films doped with 0–2.5 at.% B on quartz substrates. The structural, optical, and electrical properties of the thin films were investigated using field-emission scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), ultraviolet–visible spectroscopy, and van der Pauw Hall-effect measurements. All the thin films had deposited well onto the quartz substrates and exhibited granular morphology. The average crystallite size, lattice constants, residual stress, and lengths of the bonds in the crystal lattice of the thin films were calculated from the XRD data. The PL spectra showed near-band-edge (NBE) and deep-level emissions, and B doping varied the PL properties and increased the efficiency of the NBE emission. The optical transmittance spectra for the undoped ZnO and boron-doped zinc oxide (BZO) thin films show that the optical transmittance of the BZO thin films was significantly higher than that of the undoped ZnO thin films in the visible region of the spectra and that the absorption edge of the BZO thin films was blue-shifted. In addition, doping the ZnO thin films with B significantly varied the absorption coefficient, optical band gap, Urbach energy, refractive index, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator strength, average oscillator wavelength, dielectric constant, and optical conductivity of the BZO thin films. The Hall-effect data suggested that B doping also improved the electrical properties such as the carrier concentration, mobility, and resistivity of the thin films.     

Thinking Like a Chemist: Intuition in Thermoelectric Materials

The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.     

High-pressure study of lithium azide from density-functional calculations

The structural, electronic, optical, and vibrational properties of LiN(3) under high pressure have been studied using plane wave pseudopotentials within the generalized gradient approximation for the exchange and correlation functional. The calculated lattice parameters agree quite well with experiments. The calculated bulk modulus value is found to be 23.23 GPa, which is in good agreement with the experimental value of 20.5 GPa. Our calculations reproduce well the trends in high-pressure behavior of the structural parameters. The present results show that the compressibility of LiN(3) crystal is anisotropic and the crystallographic b-axis is more compressible when compared to a- and c-axes, which is also consistent with experiment. Elastic constants are predicted, which still awaits experimental confirmation. The computed elastic constants clearly show that LiN(3) is a mechanically stable system and the calculated elastic constants follow the order C(33) > C(11) > C(22), implying that the LiN(3) lattice is stiffer along the c-axis and relatively weaker along the b-axis. Under the application of pressure the magnitude of the electronic band gap value decreases, indicating that the system has the tendency to become semiconductor at high pressures. The optical properties such as refractive index, absorption spectra, and photoconductivity along the three crystallographic directions have been calculated at ambient as well as at high pressures. The calculated refractive index shows that the system is optically anisotropic and the anisotropy increases with an increase in pressure. The observed peaks in the absorption and photoconductivity spectra are found to shift toward the higher energy region as pressure increases, which implies that in LiN(3) decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN(3) at ambient conditions as well as at high pressures are calculated from which we predict that the response of the lattice modes toward pressure is relatively high when compared to the internal modes of the azide ion.     

A theoretical study of the structural,thermoelectric, and spin-orbit coupling influenced optoelectronic properties of CsTmCl3 halide perovskite

This first principles study explores the structural, electronic, optical, and thermoelectric properties of the CsTmCl₃ halide perovskite using density functional theory. The structural and thermoelectric properties are calculated without considering the spin-orbit coupling (SOC), while both the electronic and optical properties are calculated with and without the SOC effect. A comparison of the results obtained with and without SOC reveals that inclusion of the SOC effect reduces the band gap from 1.18 to 0.99 eV due to shifting of the Tm-d states toward the Fermi level. However, direct nature of the band gap remains the same in both the cases. The effect of SOC on the optical properties is, however, only visible in shifting of the third characteristic peak to lower energies. Strong optical absorption in the visible and ultraviolet regions shows effectiveness of CsTmCl₃ in the optical devices working in these regions. Moreover, the calculated transport properties reveal CsTmCl₃ as a useful thermoelectric material at room temperature.     

t-Si64: A Novel Silicon Allotrope

Utilizing first principle calculations, a novel Si₆₄ silicon allotrope in the I4₁/amd space group with tetragonal symmetry (denoted as t-Si₆₄ below) is proposed in this work. In addition, also its structural, anisotropic mechanical, and electronic properties along with its minimum thermal conductivity κ_min were predicted. The mechanical and thermodynamic stability of t-Si₆₄ were evaluated by means of elastic constants and phonon spectra. The electronic band structure indicates that t-Si₆₄ is an indirect band gap semiconductor with a band gap: 0.67 eV (primitive cell) compared to a direct band gap of 0.70 eV with respect to a conventional cell. The minimum thermal conductivity of t-Si₆₄ (0.74 W cm⁻¹ K⁻¹) is much smaller than that of diamond silicon (1.13 W cm⁻¹ K⁻¹). Therefore, Si−Ge alloys in the I4₁/amd space group are potential thermoelectric materials.     

Structural,elastic, electronic and optical properties of the cubic perovskite BiAlO3

We report first-principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BiAlO₃ using the pseudopotential plane waves method within the local density approximation. The calculated structural parameters are in good agreement with previous calculations. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffness is found. Band structures show that BiAlO₃ has an indirect band gap between the occupied O 2p and unoccupied Bi 6p states. The density of states and Mulliken charge populations analysis shows that Al–O and Bi–O bonds are covalent with a strong hybridization. The variation of the gap versus pressure is well fitted to a quadratic function and an indirect to direct band gap transition occurs at 15.5 GPa. Furthermore, in order to understand the optical properties of BiAlO₃, the dielectric function, absorption coefficient, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 30 eV.     

Full potential study of the elastic, electronic, and optical properties of spinels MgIn2S4 and CdIn2S4 under pressure effect

The structural, elastic, electronic, and optical properties of cubic spinel MgIn₂S₄ and CdIn₂S₄ compounds have been calculated using a full relativistic version of the full-potential linearized-augmented plane wave with the mixed basis FP/APW+lo method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel-Vosko GGA formalism is also applied to optimize the corresponding potential for band structure calculations. The ground state properties, including the lattice constants, the internal parameter, the bulk modulus, and the pressure derivative of the bulk modulus are in reasonable agreement with the available data. Using the total energy-strain technique, we have determined the full set of first-order elastic constants C_ij and their pressure dependence, which have not been calculated or measured yet. The shear modulus, Young’s modulus, and Poisson’s ratio are calculated for polycrystalline XIn₂S₄ aggregates. The Debye temperature is estimated from the average sound velocity. Electronic band structures show a direct band gap (Г-Г) for MgIn₂S₄ and an indirect band gap (K-Г) for CdIn₂S₄. The calculated band gaps with EVGGA show a significant improvement over the GGA. The optical constants, including the dielectric function ε(ω), the refractive index n(ω), the reflectivity R(ω), and the energy loss function L(ω) were calculated for radiation up to 30 eV.     

Ab-initio study of electronic,optical, thermal,and transport properties of Cr4AlB6

Theoretical investigation of different physical parameters of Cr₄AlB₆ have been done within the framework of density functional theory. Cr₄AlB₆ is a no band gap material. Its Cr-3d states contributes the most at the Fermi level. Thermal properties are investigated using quasi-harmonic Debye model as implemented in Gibbs code for different values of pressure and temperature. Study of transport property suggests that its electrical conductivity increases nonlinearly with increase in temperature but the relative change in its value is very low whereas its thermal conductivity increases linearly with the increase in temperature and relative increase in thermal conductivity is very high. The behavior of Cr₄AlB₆ is anisotropic and property is ceramic. It has potential applications in making ceramic capacitors. Its reflectivity is high in low energy region. It suggests that material can be used as coating material for far-infrared radiation. Study of the transport property suggests that because of very high value of thermal conductivity, it can be used for heat sink applications.     

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

Comprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p-type cubic AgSnSbTe₃ which shows an innately ultra-low lattice thermal conductivity (κ_lat) of 0.47–0.27 Wm⁻¹ K⁻¹ and a high electrical conductivity (1238 – 800 S cm⁻¹) in the temperature range 294–723 K. We investigated the origin of the low κ_lat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe₃. Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence-band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe₃.