Theoretical investigation of mechanical,optoelectronic and thermoelectric properties of BiGaO3 and BiInO3 compounds

The cubic BiGaO₃ and BiInO₃ perovskite oxides have been investigated for their structural stability, mechanical and opto-electronic properties by employing the density functional theory. The exchange-correlation effects have been modeled with Perdew–Burke–Ernzerhof generalized gradient approximation, while the improved evaluation of electronic properties has been achieved by using the Tran–Blaha modified form of semi-local Becke–Johnson functional. Structural properties are calculated and the thermodynamics stability of BiGaO₃ and BiInO₃ in the cubic perovskite structure has been established in terms of enthalpies of formation. Furthermore, elastic coefficients such as C_ij, bulk modules B, anisotropy factor, shear modulus G, Young’s modulus Y, Poisson’s ratio ν, and B/G ratio are predicted. Band structure calculations reveal that investigated compounds have an indirect band gap between the occupied O 2p and unoccupied Bi 6p orbitals. The optical response of BiGaO₃ and BiInO₃ are also inspected by computing the complex dielectric function, refractive index, absorption coefficient, extinction coefficient, reflectivity and optical conductivity for radiation with energy up to 16 eV. The important thermoelectric properties of the compounds are described in terms of thermal conductivity, electrical conductivity, Seebeck coefficient and figure of merit.     

High-pressure preparation and thermoelectric properties of Bi0.85Sb0.15

Nanocrystalline Bi_0.85Sb_0.15 powders were prepared by a novel mechanical alloying method. The bulk samples were formed by applying a pressure of 6 GPa at different pressing temperatures and times. Electrical conductivity, Seebeck coefficients, and thermal conductivity were measured in the temperature range 80–300 K. The Seebeck coefficient reaches a maximum value of −173 μV/K at 150 K. The largest figure of merit, 3.46 × 10⁻³ K⁻¹, achieved in this experiment is 50% higher than that of its single-crystal counterpart at 200 K.     

First principles study of Mg2X (X=Si,Ge, Sn,Pb): Elastic,optoelectronic and thermoelectric properties

Full potential linearized augmented plane wave method within the framework of density functional theory was applied to calculate the structural, elastic, electronic, thermoelectric and optical properties of Mg₂X (X=Si, Ge, Sn, Pb) compounds. Exchange-correlation effects were treated using generalized gradient approximation and modified Becke–Johnson technique. Calculated structural parameters were found in good agreement to the experimental data. With the pressure application, the lattice constant decreased while the bulk modulus increased. Brittleness and ductility of these compounds were interpreted via the calculated elastic constants. The optical properties like complex dielectric function, refractive index, reflectivity, and optical conductivity were investigated in the pressure range 0–10 GPa. Very high reflectivity in a wide energy range indicates the usefulness of these materials as a shield from high energy radiations. In addition, the thermopower of the materials was calculated as a function of the chemical potential at various temperatures. These materials are suitable for applications in optoelectronic and thermoelectric devices due to their high thermopower and narrow bandgap.     

Structural,mechanical and electronic properties of sodium based fluoroperovskites NaXF3 (X=Mg,Zn) from first-principle calculations

The structural stability, mechanical, electronic and thermodynamic properties of the cubic sodium based fluoro-perovskite NaXF₃ (X=Mg, Zn) have been studied using density functional theory (DFT). The generalized gradient approximation of Perdew–Burke and Ernzerhof (GGA-PBE) is used for modeling exchange-correlation effects. In addition, the alternative form of the GGA proposed by Engel and Vosko (GGA-EV) is also used to improve the electronic band structure calculations. The results show that both compounds are stable in the cubic Pm3m structure. From Poisson׳s ratio, it is inferred that cubic anti-perovskite NaXF₃ are ductile in nature and that bonding is predominantly of ionic in nature. The electronic band structure calculations and bonding properties show that anti-perovskites have an indirect energy band gap (M–Г) with a dominated ionic character. The thermal effects on thermal expansion coefficient, Debye temperature and Grüneisen parameter were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The calculations are found to be in good agreement with other results.     

Electronic,optical and bonding properties of MgYZ2 (Y=Si,Ge; Z=N,P) chalcopyrites from first principles

The electronic and optical properties of MgYZ₂ (Y=Si, Ge; Z=N, P) compounds are carried out using first-principle calculations within the density functional theory. The calculations show close correspondence to the available experimental data compared to the previous theoretical calculations. Band gap decreases by changing the cations Y from Si to Ge as well as Z from N to P in MgYZ₂. The N/P p-states contribute majorly in the density of states. Bonding nature of the herein studied compounds is predicted from the electron density plots. Optical response of these compounds is noted from the complex refractive index, reflectivity and optical conductivity. The direct band gap and the high reflectivity of these compounds in the visible and ultraviolet regions of electromagnetic energy spectrum ensure their applications in optoelectronic and photonic domains.     

First principles calculations of the electronic structure and optical properties of (001), (011) and (111) Ga0.5Al0.5As surfaces

Using the first-principles plane-wave pseudo-potential method based on density function theory (DFT), the electronic structure and optical properties of Ga_0.5Al_0.5As (001), (011) and (111) surfaces are calculated. Result shows that (001) surface is reconstructed, (011) surface is not reconstructed but wrinkled, (111) surface is only relaxed. (111) is the most stable surface. (001) surface owns the lowest work function. Absorption coefficient and reflectivity of these surfaces are smaller than bulk, the transmittance of the surfaces are larger than the bulk, which is helpful for the incident light to excite photoelectrons. The decrease of the absorption coefficient and reflectivity at (001) surface are the largest. Calculation of electronic structure and optical properties predict that the (001) surface should have the strongest photoemission.     

Structural,electronic and optical properties of Ilmenite ATiO3(A=Fe,Co, Ni)

Ilmenite-type ATiO₃ (A=Fe, Co, Ni) crystals have been investigated via Generalized Gradient Approximation (GGA) in the scheme of Revised Perdew-Burke-Ernzerhof (RPBE) using the first-principles method. The band structures, densities of states, bond orders and charge populations, optical properties including the dielectric function ε(ω), absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), electron energy loss function L(ω) and reflectivity function R(ω), are calculated. The results show that the GGA-optimized geometries agree well with the experimental data. FeTiO₃ has a direct band gap, but both CoTiO₃ and NiTiO₃ exhibit indirect band gap. The analysis for densities of states and atomic charge populations exhibits that TiO bonds possess the stronger covalent bonding strength than AO bonds. The calculated optical properties along [100], [010] and [001] as well as polycrystalline directions demonstrate the significant optical anisotropy parallel and perpendicular to c-axis for ATiO₃. Finally, the origins of main peaks for optical spectra are presented based on electron transitions. Theoretical insights into the microscopic intrinsic properties of ATiO₃ should provide fundamental investigations for further understanding the Ilmenite ATiO₃ materials and improving their practical applications.     

Electronic structure,chemical bonding and optical properties of Di-2-pyrymidonium dichloride diiodide (C4H5ClIN2O) from first-principles

The electronic structure and electronic charge density of the monoclinic phase Di-2-pyrymidonium dichloride-di-iodide compound is studied by using the local density approximation (LDA) and Engel Vosko generalized gradient approximation (EVGGA). Using LDA for exchange correlation potential, we have optimized the atomic positions taken from the X-ray crystallographic data by minimization of the forces acting on the atoms. From the relaxed geometry the electronic structure, electronic charge density and the optical properties were determined. Band structures disclose that this compound has indirect energy band gap. The obtained energy band gap value using EVGGA (2.010 eV) is larger than that obtained within LDA (1.781 eV). To envision the chemical bonding nature between the composition of the investigated compound, the distribution of charge density was discussed in the (−1 0 1) crystallographic plane. The contour plot shows partial ionic and strong covalent bonding between C–O, N–C and C–H atoms. The optical properties of Di-2-pyrymidonium dichloride-di-iodide are obtained by the calculation of the dielectric function.     

Origin of the anomalous trends in band alignment of GaX/ZnGeX_2(X = N,P, As,Sb) heterojunctions

Utilizing first-principles band structure method, we studied the trends of electronic structures and band offsets of the common-anion heterojunctions GaX/ZnGeX_2(X = N, P, As, Sb). Here, ZnGeX_2 can be derived by atomic transmutation of two Ga atoms in GaX into one Zn atom and one Ge atom. The calculated results show that the valence band maximums(VBMs) of GaX are always lower in energy than that of ZnGeX_2, and the band offset decreases when the anion atomic number increases. The conduction band minimums(CBMs) of ZnGeX_2 are lower than that of GaX for X = P, As, and Sb, as expected. However, surprisingly, for ZnGeN2, its CBM is higher than GaN. We found that the coupling between anion p and cation d states plays a decisive role in determining the position of the valence band maximum, and the increased electronegativity of Ge relative to Ga explains the lower CBMs of ZnGeX_2 for X = P, As, and Sb. Meanwhile, due to the high ionicity, the strong coulomb interaction is the origin of the anomalous behavior for nitrides.     

Structural analysis and valence variation in ytterbium pnictides YbX (X=P,As) under high pressure

We have studied the pressure induced structural phase transition, pressure volume relationship and valence variation in ytterbium pnictide compounds (YbX; X=P, As). An improved interaction potential model (IIPM) (including the polarizability effect) has been applied. We have also studied the trivalent and intermediate valence in ytterbium compounds and predicted the phase transition pressures, volume collapses for two different valences. The variations of the second-order elastic constants with pressure and stability condition have shown systematic trends in YbX compounds similar to those observed in other compounds of NaCl structure. The pressure volume relationship curves show that ytterbium pnictide compounds undergo NaCl to CsCl type structural transition. The present study also confirms that these compounds behave similar to the rare earth cerium pnictide compounds and they are trivalent in nature.     

CH3NH3PbI3和CH3NH3MnI3的电子结构和光学性质研究

采用第一性原理计算,研究了有机金属卤化物钙钛矿CH₃NH₃PbI₃和CH₃NH₃MnI₃的电子结构、磁性和光吸收。CH₃NH₃PbI₃和CH₃NH₃MnI₃都是具有直接带隙半导体,CH₃NH₃MnI₃磁基态为G型反铁磁序(G-AFM)。CH₃NH₃MnI₃在G-AFM状态下的带隙值为1.668 eV;当系统处于FM态时,多数自旋通道的带隙为0.696 eV,少数自旋通道的带隙为2.148 eV。结果表明,具有FM态的CH₃NH₃MnI₃的光激发电子将迅速熔化局域磁序。最后计算了CH₃NH₃PbI₃和CH₃NH₃MnI₃的光学特性,结果表明具有铁磁态的CH₃NH₃MnI₃(FM)表现出较强的红外光吸收。     

Highly efficient n-(Bi, Sb)2Te3 thermoelectric materials for temperatures below 200 K

The possibility of using an n-type Bi_2?x Sb_xTe₃ solid solution in thermoelectric refrigerators at T<200 K is considered. It is shown that, if the material under consideration is optimized for the above temperature region, the temperature dependence of the Seebeck coefficient α becomes less pronounced, and the crystal-lattice thermal conductivity κ_L decreases as compared to what is observed in a conventional n-Bi₂Te_3?y Se_y solid solution. These factors and a high mobility of charge carriers μ₀ bring about an increase in the parameter β ～ ZT, where Z is the thermoelectric efficiency.     

The Shubnikov-de Haas effect and thermoelectric properties of Tl-doped Sb2Te3 and Bi2Se3

Semiconductors - The influence of doping with Tl on the Shubnikov-de Haas effect at T = 4.2 K in magnetic fields up to 38 T in p-Sb2−x Tl x Te3 (x = 0, 0.005, 0.015, and 0.05) and...     

Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

Fabrication of Microstructured thermoelectric Bi2Te3 thin films by seed layer assisted electrodeposition

In this work, bismuth telluride (Bi₂Te₃) thin films have been fabricated on Bi₂Te₃/ITO substrates by constant potential electrochemical deposition at room temperature. Bi₂Te₃ seed layers with different thicknesses (2 nm, 4 nm and 6 nm) were deposited onto ITO substrates using molecular beam epitaxy (MBE) method. The SEM images show that the morphology of Bi₂Te₃ thin films can be controlled not only by the deposition potential, but also the thickness of seed layer. Moreover, the morphologies of Bi₂Te₃ thin films with different thickness of seed layers tend to be similar and contain two-layer structure along the vertical direction after prolonged deposition time. Due to the two layers structure, Bi₂Te₃ thin films have shown different electrical conductivity performances. At room temperature, Bi₂Te₃ thin films with 4 nm-thick seed layer possess the maximum electrical conductivity value of 617.9 s cm^-1.     

Ni掺杂CoSb3化合物的热电性能及机理研究

采用机械合金化结合冷等静压的方法制备了CoSb3和Co3.5Ni0.5Sb12化合物,并测量了其热电性能。利用基于密度泛函理论赝势平面波的方法对Ni掺杂前后的CoSb3的能带结构和态密度进行了计算,实验和理论计算结果表明:CoSb3的费密面位于导带和价带之间,其电阻率随温度的升高而降低,为非简并半导体;Co3.5Ni0.5Sb12的费密面进入导带,其电阻率随温度的升高而增大,为n型简并半导体;本实验条件下,Co3.5Ni0.5Sb12化合物的功率因子在550 K时出现最大值2 292.92μW/(m.K2),是未掺杂CoSb3化合物最大值的12倍。     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.