A.H. Reshak

Macroscopic Polarization Enhancement Promoting Photo- and Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation

Efficient photo- and piezoelectric-induced molecular oxygen activation are both achieved by macroscopic polarization enhancement on a noncentrosymmetric piezoelectric semiconductor BiOIO3 . The replacement of V5+ ions for I5+ in IO3 polyhedra gives rise to strengthened macroscopic polarization of BiOIO3 , which facilitates the charge separation in the photocatalytic and piezoelectric catalytic process, and renders largely promoted photo- and piezoelectric induced reactive oxygen species (ROS) evolution, such as superoxide radicals (. O2- ) and hydroxyl radicals (. OH). This work advances piezoelectricity as a new route to efficient ROS generation, and also discloses macroscopic polarization engineering on improvement of multi-responsive catalysis.

Investigation of the linear and nonlinear optical susceptibilities of KTiOPO4 single crystals: theory and experiment.

Experimental and theoretical studies of the linear and nonlinear optical susceptibilities for single crystals of potassium titanyl phosphate KTiOPO₄ are reported. The state-of-the-art full potential linear augmented plane wave method, based on the density functional theory, was applied for the theoretical investigation. The calculated direct energy band gap at Γ, using the Engel-Vosko exchange correlation functional, is found to be 3.1 eV. This is in excellent agreement with the band gap obtained from the experimental optical absorption spectra (3.2 eV). We have calculated the complex dielectric susceptibility ε(ω)dispersion, its zero-frequency limit ε₁(0) and the birefringence of KTiOPO₄. The calculated birefringence at the zero-frequency limit Δn(0) is equal to about 0.07 and Δn(ω) at 1.165 eV (λ = 1064 nm) is 0.074. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ₁₁₃((2))(ω), χ₂₃₂((2))(ω), χ₃₁₁((2))(ω), χ₃₂₂((2))(ω), and χ₃₃₃((2))(ω). The intra- and interband contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components |χ(ijk)((2))(ω)| at λ = 1064 nm for all the five tensor components are compared with those obtained from our measurements performed by nanosecond Nd:YAG laser at the fundamental wavelength (λ = 1064 nm). Our calculations show reasonably good agreement with our experimental nonlinear optical data and the results obtained by other authors. The calculated the microscopic second order hyperpolarizability, β₃₃₃, vector component along the dipole moment direction for the dominant component χ₃₃₃((2))(ω) is found to be 31.6 × 10⁻³⁰ esu, at λ = 1064 nm.

Thermoelectric properties for AA- and AB-stacking of a carbon nitride polymorph (C3N4)

The transport properties, and electronic structure, for C3N4 were investigated, and it was found that the valence band maximum (VBM) and the conduction band minimum (CBM) of AA-stacking of C3N4 are located at the A point of the Brillouin zone (BZ), resulting in a direct band gap of approximately 0.69 eV (LDA), 0.870 eV (GGA), 1.237 eV (EV-GGA), and 2.589 eV (mBJ). For AB-stacking, the VBM and the CBM are situated at the Γ point of the BZ, maintaining a direct band gap of approximately 1.204 eV (LDA), 1.357 eV (GGA), 1.680 eV (EV-GGA), and 2.990 eV (mBJ). The calculated electronic band structure was used to determine the thermoelectric properties such as electrical conductivity (σ/τ), electronic thermal conductivity (κe/τ), Seebeck coefficient (S), power factor (P), and figure of merit (ZT) using the semi-classical Boltzmann theory as incorporated in the BoltzTraP code. The maximum values of σ for AA-stacking are 3.4 × 1020 (Ω m s)−1 and 1.05 × 1020 (Ω m s)−1, which are achieved at ±0.17 μ(eV) for p-type and n-type materials at 300 and 600 K, respectively. For AB-stacking, the maximum values are 3.8 × 1020 and 3.7 × 1020 (Ω m s)−1 at 300 and 600 K for p-type, whereas they are 2.8 × 1020 (Ω m s)−1 at 300 and 600 K for n-type; these values are achieved at ±0.2 μ(eV) for p-type and n-type. One can see that AA-stacking exhibits the highest S values of approximately 99.0 (μV K−1) for p-type and 98.0 (μV K−1) for n-type, while AB-stacking exhibits the maximum S values of approximately 270 (μV K−1) for p-type and 280 (μV K−1) for n-type. The critical points of S for p-type and n-type are ±0.03 μ(eV) and ±0.08 μ(eV) for AA- and AB-stacking of C3N4, respectively. The values of the critical points are the range where AA- and AB-stacking of C3N4 is expected to exhibit good thermoelectric properties, and beyond these critical points, S is zero. AA-stacking shows the minimum values of the κe between ±0.03 μ(eV), while they are ±0.08 μ(eV) for AB-stacking. Therefore, in these regions, AA- and AB-stacking are expected to exhibit maximum efficiency. The highest peaks of ZT (equal to unity) are confined between ±0.05 μ(eV) for AA-stacking and ±0.1 μ(eV) for AB-stacking.

Theoretical investigation of the electronic properties, and first and second harmonic generation for cadmium chalcogenide

We present the results of the ab initio theoretical study of the electronic properties, and first and second harmonic generation for CdX compounds with zinc-blende structure performed using the full potential linearized augmented plane wave method. Our calculations show that these compounds have similar structures. The valence band maximum and the conduction band minimum are located at Gamma, resulting in a direct energy gap. The energy gap of these compounds decreases when S is replaced by Se and Se by Te, in agreement with the experimental data and previous theoretical work. This can be attributed to the increase in the bandwidth of the conduction bands. The optical spectra are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. Our calculations for the linear optical properties show excellent agreement with the available experimental data.

Fe2MnSixGe1−x: influence thermoelectric properties of varying the germanium content

The semi-classical Boltzmann theory, as implemented in the BoltzTraP code, was used to study the influence of varying the germanium content on the thermoelectric properties of the Heusler compounds, Fe2MnSi and Fe2MnGe. The electrical conductivity (σ/τ), the Seebeck coefficient (S), the electronic power factor (S2σ), the electronic thermal conductivity (κe), the electronic heat capacity cel(Tel), and the Hall coefficient (RH), as a function of temperature at certain values of chemical potential (μ) with constant relaxation time (τ), were evaluated on the basis of the calculated band structure using the standard Boltzmann kinetic transport theory and the rigid band approach. The increase/reduction in the electrical conductivity (σ = neμ) of Fe2MnSixGe1−x alloys is attributed to the density of charge carriers (n) and their mobility (μ = eτ/me). The S for Fe2MnGe is negative over the entire temperature range, which represents the n-type concentration. Whereas Fe2MnSi shows a positive S up to 250 K and then drops to negative values, which confirms the existence of the p-type concentration between 100–250 K. Fe2MnSi0.25Ge0.75/Fe2MnSi0.5Ge0.5/Fe2MnSi0.75Ge0.25 possess positive S up to 270/230/320 K and then drop to negative values. The power factor of Fe2MnGe rapidly increases with increasing temperature, while for Fe2MnSi it is zero up to 300 K, and then rapidly increases with increasing temperature. The S2σ of Fe2MnSi0.25Ge0.75 is zero between 250–350 K, whereas Fe2MnSi0.5Ge0.5 possesses a zero S2σ of up to 320 K. Fe2MnSi0.75Ge0.25 has a zero S2σ between 200 and 500 K. The electronic thermal conductivity (κe) and the electronic heat capacity cel(Tel) increases with increasing temperature. The parent compounds (Fe2MnGe and Fe2MnSi) show the highest positive value of the Hall coefficient RH at 100 K, and then drop to negative values at 260 K. On the other hand, the RH for Fe2MnSi0.25Ge0.75, Fe2MnSi0.5Ge0.5 and Fe2MnSi0.75Ge0.25 alloys exhibit negative RH along the temperature scale. The behavior of RH is attributed to the concentration of the charge carriers and their mobility.

Acentric nonlinear optical 2,4-dihydroxyl hydrazone isomorphic crystals with large linear, nonlinear optical susceptibilities and hyperpolarizability.

A systematic ab initio study of the linear, nonlinear optical susceptibilities, and hyperpolarizability of noncentrosymmetric-monoclinic 2,4-dihydroxyl hydrazone isomorphic crystals (DHNPH) within density functional theory in the local density approximation (LDA), general gradient approximation (GGA), the Engel-Vosko generalized gradient approximation (EV-GGA) and modified Becke-Johnson potential (mBJ) has been performed. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence are studied. Using scissors’ corrected mBJ we find a large uniaxial dielectric anisotropy (-0.56) resulting in a significant birefringence (0.61). We also find that 2,4- DHNPH possess large second harmonic generation. The calculated second order susceptibility tensor components for the static limit |χ(111)(2)(0)| and |χ(111)(2)(ω)| at λ=1.9 μm (0.651 eV) and at λ = 1.064 μm (1.165 eV) are 53, 91, and 209 pm/V, respectively. A remarkable finding, applying the scissors’ correction has a profound effect on value, magnitude and sign of χ(ijk)(2)(ω). In additional we have calculated the microscopic hyperpolarizability, β(111), vector component along the principal dipole moment directions for the dominant component. We find that the value of β(111) equal to 47× 10(-30) esu, in good agreement with the measured value (48.2× 10(-30) esu).

Electronic band structure of AgCd2GaS4: theory and experiment

We report theoretical calculations of the band structure of AgCd2GaS4 using the full-potential linear augmented plane wave method and experimental measurements of the valence band x-ray photoelectron spectroscopy. We find that the valence band maximum and the conduction band minimum are located at the ? point of the Brillouin zone resulting in a direct energy gap of 1.0?eV compared to our measured experimental value of 2.15?eV. Our analysis of the partial density of states shows that there is a weak covalent interaction between Ag and Ga atoms and between Ag and Cd atoms, and a substantial covalent interaction between Ag and S atoms. Thus the Ga?Ag and Cd?Ag bonds are basically of ionic character, and Ag?S bonds are of covalent character. The theoretical results of the density of states are in agreement with the valence band x-ray photoelectron spectroscopy measurements with respect to spectral peak positions. We have analyzed the calculated density of states and find a strong/weak hybridization between the Ag, Cd, Ga and S states in the valence and conduction bands.

Dispersion of linear and non-linear optical susceptibilities for amino acid 2-aminopropanoic CH3CH(NH2)COOH single crystals: experimental and theoretical investigations

A comprehensive experimental and theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities for amino acidL-alanine single crystals is reported. The state-of-the-art full potential linear augmented plane wave method, within a framework of the density functional theory was applied. The atomic positions from X-ray diffraction have been optimized so that the force on each atom is around 1 mRy au−1. This relaxed geometry has been used for the theoretical calculations. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence of amino acidL-alanine single crystals were studied. The crystal exhibits a large uniaxial dielectric anisotropy resulting in a significant birefringence. The calculated birefringence at static limit is 0.072 and 0.074 at λ = 1064 nm (corresponding to 1.165 eV) in good agreement with the measured value (0.073). We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ(2)123(ω), χ(2)231(ω) and χ(2)312(ω). The calculated second order susceptibility tensor components |χ(2)123(ω)|, |χ(2)231(ω)|, and |χ(2)312(ω)| at λ = 1064 nm are compared with those obtained from our measurements performed using the 25 ps Nd:YAG pulsed laser at λ = 1064 nm. Our calculations are in reasonably good agreement with our experimental data. In addition we have calculated the microscopic second order hyperpolarizability, β123, vector component along the principal dipole moment directions for the dominant component χ(2)123(ω) and it is found to be 0.21 × 10−21 pm V−1 in the static limit and 0.27 × 10−21 pm V−1 at 1.165 eV (λ = 1064 nm) in comparison with our measured value (0.31 × 10−21 pm V−1) at λ = 1064 nm. Additional study of the second order susceptibilities versus the external laser treatment is performed.

Electronic, linear, and nonlinear optical properties of III-V indium compound semiconductors

We have made an extensive theoretical study of the electronic, linear, and nonlinear optical properties of the III-V indium compound semiconductors InX (X=P, As, and Sb) with the use of full potential linear augmented plane wave method. The results for the band structure, density of states, and the frequency-dependent linear and nonlinear optical responses are presented here and compared with available experimental data. Good agreement is found. Our calculations show that these compounds have similar electronic structures. The valence band maximum and the conduction band minimum are located at Gamma resulting in a direct energy gap. The energy band gap of these compounds decreases when P is replaced by As and As by Sb. This can be attributed to the increase in bandwidth of the conduction bands. The linear and nonlinear optical spectra are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. The calculated linear optical properties show very good agreement with the available experimental data. We find that the intra-and interband contributions of the second-harmonic generation increase when moving from P to As to Sb. The smaller energy band gap compounds have larger values of chi(123) ((2))(0) in agreement with the experimental measurements and other theoretical calculations.

Influence of replacing Si by Ge in the chalcogenide quaternary sulfides Ag2In2Si(Ge)S6 on the chemical bonding, linear and nonlinear optical susceptibilities, and hyperpolarizability.

The linear and nonlinear optical properties of Ag2In2SiS6 and Ag2In2GeS6 are calculated so as to obtain further insight into the electronic properties. The influence of using different exchange correlation potentials and the effect of replacing Si by Ge on the geometry, chemical bonding, and on the optical properties are presented. There is notable increasing in the energy band gap when moving from LDA to GGA, EVGGA then to mBJ. The effect of replacing Si by Ge atom causes a geometric change, which leads to large changes in the linear as well as the nonlinear optical susceptibilities. For the linear optical properties, it causes to increase the amplitude of the left-hand hump of ε(2)(average)(ω) as well as a small shift of the main peak to lower energies. We have evaluated ε(1)(average)(0) and find that a smaller energy gap yields a larger ε1(0) value. From the calculated refractive indices we obtained the birefringence, which is important for second harmonic generation (SHG) and optical parametric oscillation (OPO) as it is defined by the phase-matching condition. The second-order nonlinear optical susceptibilities, namely, the SHG are investigated for χ(111)(2)(ω), χ(122)(2)(ω), χ(133)(2)(ω), χ(221)(2)(ω), and χ(331)(2)(ω). We find that χ(111)(2)(ω) is the dominant component. The microscopic second order hyperpolarizability, β111, for the dominant component χ(111)(2)(ω) was obtained. We should emphasize that replacing Si by Ge enhances the linear and nonlinear optical susceptibilities so that Ag2In2GeS6 shows higher values of the linear and nonlinear optical susceptibilities and β111 in comparison to Ag2In2SiS6.