Narrow Results

In this work, we present theoretical calculations of the structural, electronic, optical and thermoelectric properties of the perovskite oxides LaMO₃ (M = Ga or In) using density functional theory (DFT) with GGA–PBE approximation, as implemented in the ABINIT code. The cubic crystal structure of LaMO₃ (M = Ga or In) compounds changes and its volume increases when the Ga atom is replaced by an In atom. In addition, negative formation energies suggest the thermodynamic stability of the studied compounds. Electron charge densities reveal an ionic bond between La and O, while the bond between M and O appears covalent. Electronic properties showed the indirect semiconducting behavior of LaGaO₃ and LaInO₃ perovskites. The calculated indirect bandgaps $E_g$ (R–$\mathrm{\Gamma }$) are found to be 3.34eV for LaGaO₃ and 2.08eV for LaInO₃. In addition, optical characteristics are determined in terms of real ${\epsilon }_1$($\omega$) and imaginary ${\epsilon }_2$($\omega$) parts of dielectric constant $\epsilon (\omega )$, refractive index $n(\omega )$, absorption coefficient $\alpha (\omega )$, reflectivity $R(\omega )$, energy loss function $L(\omega )$, optical conductivity $\sigma (\omega )$ and transmittance $T(\omega )$ are also studied. Optical absorption of light energy has been observed in both the visible and ultraviolet ranges, increasing the importance of the studied materials for optoelectronic applications. Finally, the thermoelectric performance of LaMO₃ (M = Ga or In) materials has been explored using the Boltzmann transport theory implemented in the BoltzTraP software package.

Polyvinyl alcohol (PVA) has been used as a matrix to synthesize ZnS/PVA nanocomposite film on glass substrate by chemical method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Scanning electron microscopy (SEM) has been used for structural, morphological and compositional characterization. Optical properties have been studied by UV-Visible spectrophotometry and photoluminescence (PL) spectroscopy. Changing pH value from 4.8 to 0.8 decreases the particle size and correspondingly increases the band gap. The PL emission intensity also increases by decreasing the pH values.

The theoretical calculations indicate that the metal-doped boron nitride (BN) sheets are potential materials to store the hydrogen and tune the bandgap. It is all known that the BN sheet is a nonmagnetic wide-bandgap semiconductor. Using density function theory (DFT), the lattice parameters of Cr-doped BN sheets are optimized, which are still kept on two-dimensional (2D) planar geometry, and the bandgap and ${\mathrm{\text{H}}}_2$ storage are studied. The simulation results show that the ${\mathrm{\text{H}}}_2$ molecule can be easily absorbed by Cr-doped N in BN sheet. As the adsorption energy was greatly decreasing with the increasing number of Cr-doped N, B had an affinity for adsorption of ${\mathrm{\text{H}}}_2$. With the increase of Cr doping, the bandgap of Cr-doped BN sheet is decreasing. The bandgap decreases from 4.705 eV to 0.08 eV. So Cr-doped BN sheet is a promising material in storing ${\mathrm{\text{H}}}_2$ and tuning the bandgap.

Based on the concept of generalized phononic crystals (GPCs), a type of 1D cylindrical shell of generalized phononic crystals (CS-GPCs) where two kinds of homogeneous materials are arranged periodically along radial direction was proposed in this paper. On the basis of radial, torsional shear and axial shear vibrational equations of cylindrical shell, the total transfer matrix of mechanical state vector were set up respectively, and the bandgap phenomena of these three type waves were disclosed by using the method of transfer matrix eigenvalue of mechanical state vector instead of the previous localized factor analyses and Bloch theorem. The characteristics and forming mechanism of these bandgaps of CS-GPCs, together with the influences of several important structure and material parameters on them were investigated and discussed in detail. Our results showed that, similar to the plane wave bandgaps, 1D CS-GPCs can also possess radial, torsional shear and axial shear wave bandgaps within high frequency region that conforms to the Bragg scattering effect; moreover, the radial vibration of CS-GPCs can generate low frequency bandgap (the start frequency near 0 Hz), as a result of the double effects of wavefront expansion and Bragg scattering effect, wherein the wavefront effect can be the main factor and directly determine the existence of the low frequency bandgaps, while the Bragg scattering effect has obvious enhancement effect to the attenuation. Additionally, the geometrical and material parameters of units have significant influences on the wave bandgaps of CS-GPCs.

We have performed ab initio investigation of some physical properties of the perovskite TlMnX₃ (X = F, Cl) compounds using the full-potential linearized augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) is employed as exchange-correlation potential. The calculated lattice constant and bulk modulus agree with previous studies. Both compounds are found to be elastically stable. TlMnF₃ and TlMnCl₃ are classified as anisotropic and ductile compounds. The calculations of the band structure of the studied compounds showed the semiconductor behavior with the indirect (M–X) energy gap. Both compounds are classified as a ferromagnetic due to the integer value of the total magnetic moment of the compounds. The different optical spectra are calculated from the real and the imaginary parts of the dielectric function and connected to the electronic structure of the compounds. The static refractive index $n(0)$ is inversely proportional to the energy bandgap of the two compounds. Beneficial optics technology applications are predicted based on the optical spectra.

Phoxonic crystal (PXC) is a promising artificial periodic material for optomechanical systems and acousto-optical devices. The multi-objective topology optimization of dual phononic and photonic max relative bandgaps in a kind of two-dimensional (2D) PXC is investigated to find the regular pattern of topological configurations. In order to improve the efficiency, a multi-level substructure scheme is proposed to analyze phononic and photonic band structures, which is stable, efficient and less memory-consuming. The efficient and reliable numerical algorithm provides a powerful tool to optimize and design crystal devices. The results show that with the reduction of the relative phononic bandgap (PTBG), the central dielectric scatterer becomes smaller and the dielectric veins of cross-connections between different dielectric scatterers turn into the horizontal and vertical shape gradually. These characteristics can be of great value to the design and synthesis of new materials with different topological configurations for applications of the PXC.

The FP-LAPW method is utilized to investigate the elastic, optoelectronic and thermoelectric properties of ${\mathrm{\text{XTiO}}}_3$$(\mathrm{\text{X}}=\mathrm{\text{Ca, Sr}}$ and $\mathrm{\text{Ba}})$ within the GGA. The calculated lattice constants and bulk modulus are found in agreement with previous studies. The present oxide–perovskite compounds are characterized as elastically stable and anisotropic. ${\mathrm{\text{CaTiO}}}_3$ and ${\mathrm{\text{SrTiO}}}_3$ are categorized as ductile compounds, whereas the ${\mathrm{\text{BaTiO}}}_3$ compound is in the critical region between ductile and brittle. The DOS and the band structure calculations reveal indirect $(\mathrm{\text{M}}$–$\mathrm{\Gamma })$ energy bandgap for the present compounds. The hydrostatic pressure increases the energy bandgap and the width of the valence band. The character of the band structure does not change due to this pressure. The optical parameters are calculated in different radiation regions. Beneficial optics applications are predicted as revealed from the optical spectra. The transport properties are applied as a function of the variable temperatures or carrier concentration. It is found that the compounds under study are classified as a p-type semiconductor. The majority charge carriers responsible for conduction in these calculated compounds are holes rather than electrons.

Two alternating homogeneous materials are periodically introduced along the radial direction, forming a circular plate of radial phononic crystal (CPRPC). To illustrate the characteristics of the out-of-plane transverse wave and the in-plane longitudinal wave propagating along the radial direction, the transfer matrices are derived based on the basic wave equations of a thin circular plate in cylindrical coordinates. Localization factors are introduced to evaluate the average attenuation of the transverse and longitudinal waves in the structure, and corresponding bandgaps are obtained. Moreover, finite element method simulations, numerical analyses and the insertion loss method are combined to investigate the effects of the main parameters on these wave bandgaps. The results show that significant transverse and longitudinal wave bandgaps caused by the radial periodicity of the CPRPC exist, and the structural and material parameters have essential influences on them.

Based on first-principles plane-wave ultra-soft pseudopotential method, bandgaps of Ga${}_{1-x}$Al${}_x$N nanowires with different diameters and different Al constituents are calculated. After the optimization of the model, the bandgaps are achieved. According to the results, the bandgap of Ga${}_{1-x}$Al${}_x$N decreases with increasing diameter and finally, closed to that of the bulk. In addition, with increasing Al constituent, the bandgaps of Ga${}_{1-x}$Al${}_x$N nanowires increase. However, the amount of the increase is lower than that of the bulk Ga${}_{1-x}$Al${}_x$N with the increase of Al constituent.

In order to meet the design requirements of the high-performance antimonide-based optoelectronic devices, the spin–orbit splitting correction method for bandgaps of Sb-based multi-element alloys is proposed. Based on the analysis of band structure, a correction factor is introduced in the In${}_x$Ga${}_{1-x}$As${}_y$Sb${}_{1-y}$ bandgaps calculation with taking into account the spin–orbit coupling sufficiently. In addition, the In${}_x$Ga${}_{1-x}$As${}_y$Sb${}_{1-y}$ films with different compositions are grown on GaSb substrates by molecular beam epitaxy (MBE), and the corresponding bandgaps are obtained by photoluminescence (PL) to test the accuracy and reliability of this new method. The results show that the calculated values agree fairly well with the experimental results. To further verify this new method, the bandgaps of a series of experimental samples reported before are calculated. The error rate analysis reveals that the $\alpha$ of spin–orbit splitting correction method is decreased to 2%, almost one order of magnitude smaller than the common method. It means this new method can calculate the antimonide multi-element more accurately and has the merit of wide applicability. This work can give a reasonable interpretation for the reported results and beneficial to tailor the antimonides properties and optoelectronic devices.

The effects of symmetries on the bandgap in a newly designed hybrid phononic crystal plate composed of rubber slab and epoxy resin stub are studied for better controlling of bandgaps. The point group symmetry is changed by changing the orientation of the stub. The translation group symmetry is changed by changing the side length and the height of adjacent stubs. Results show that the point group symmetry and translation group symmetry can be important factors for controlling of the bandgaps of phononic crystal. Wider bandgap is obtained by suitable orientation of the stub. Lower bandgap appears when the differences between the adjacent stubs become bigger in supercell.

First-principle calculation was carried out to systematically investigate carbon monoxide (CO) adsorption on pristine and cobalt (Co)-doped phosphorenes (Co-bP). Whether or not CO is adsorped, pristine phosphorene is a direct-band-gap semiconductor. However, the bandgap of Co-bP experiences direct-to-indirect transition after CO molecule adsorption, which will affect optical absorption considerably, implying that Co doping can enhance the sensitivity of phosphorene as a CO gas sensor. Moreover, Co doping can improve an adsorption energy of CO to 1.31 eV, as compared with pristine phosphorene (0.12 eV), also indicating that Co-bP is energetically favorable for CO gas sensing.

In this paper, we investigate theoretically the transmission properties of one-dimensional quasi-periodic photonic crystals that containing nanocomposite material in the IR wavelength regions. Our structure is particularly designed using the Fibonacci role. Here, the nanocomposite material is composed of nanoparticles of Ag that are randomly immersed in a host dielectric material of SiO₂. Numerical results are mainly investigated based on the well-known characteristic matrix method. The numerical results show the appearance of many photonic bandgaps due to the multiple periodicities of our structure. Furthermore, the effects of the parameters of the nanocomposite such as the volume fraction, the refractive index of the dielectric material and the size of the nanoparticles have distinct effects on the transmittance characteristics of our structure. Wherefore, the proposed structure could be considered the cornerstone for many applications such as multichannel filters and optical switches.

We have performed the first-principles density functional theory (DFT) and DFT$+$U calculations on the electronic and optical properties of CaO: Eu${}^{+2}$ (SrO: Eu${}^{+2}$) phosphors compounds. Herein, we have focused on the polarization of the electronic structures, i.e., the energy bandgap and the density of states. All electrons were treated within the most common exchange and correlation functional called generalized gradient approximation plus optimized effective Hubbard parameter U as GGA$+$U. GGA$+$U is a very effective tool for describing the electronic band energy upto considerable accuracy. Hence, we have opted for the arbitrary values of U as 3.0, 4.0, 5.0 and 7.0 eV to treat the strongly correlated electrons for obtaining the matching result with the experimental one. However, GGA$+$U is highly expensive in terms of computation due to interaction of d or f electrons. The result shows that the appearance of Eu-4f states at the valance band maximum of the spin-up causes a substantial impact on the electronic properties of the studied compounds. The value of energy bandgap is smaller in case of spin up as compared to spin down case. In case of majority spin, the energy gap of 2.224 (2.14) eV belongs to the Eu-4f orbitals and governs the CBM. The partial densities of states (PDOS) structure displays a strong hybridization that may be pointed to the formation of covalent bonds. The calculated and the measured values are in good agreement with each other. In the study of optical properties of the compound, the optical spectral structure shows a lossless region and uniaxial anisotropy. The value of uniaxial anisotropy is positive at static limit and its value is negative above this value.

In this study, zinc selenide (ZnSe) thin films were produced on glass substrate by using chemical bath deposition (CBD) method at 80${}^{\circ }$C, from aqueous solutions of zinc sulphate and sodium selenosulphide, which were produced using solid selenium as the selenium source. The optical and structural properties of ZnSe thin films were investigated at room-temperature. The pH of the chemical bath, in which ZnSe thin films were immersed, were changed between pH:8–11. Optical properties of the films, including extinction coefficient, refractive index, reflectance, absorbance, transmittance, dielectric constants and optical density values were calculated using absorbance and transmittance measurements determined using a Hach Lange 500 spectrophotometer, in 300–1100 nm wavelength range. Optical bandgap values were obtained from transmittance and absorbance spectra ranged between 2.12 and 2.49 eV. According to XRD results, it was found that the films have polycrystalline structure and they exhibited different film thicknesses depending on phase and pH changes.

First-principles calculations of the structural, electronic, optical and thermal properties of chalcopyrite CuXTe₂ (X=Al, Ga, In) have been performed within density functional theory using the full-potential linearized augmented plane wave (FP-LAPW) method, by employing for the exchange and correlation potential the approximations WC-GGA and mBJ-GGA. The effect of X cations replacement on the structural, electronic band structure, density of states and optical properties were highlighted and explained. Our results are in good agreement with the previous theoretical and experimental data. As far as we know, for the first time we find the effects of temperature and pressure on thermal parameters of CuAlTe₂ and CuGaTe₂ compounds. Thermal properties are very useful for optimizing crystal growth, and predict photovoltaic applications on extreme thermodynamic conditions.

Based on the density functional theory (DFT) and using the generalized gradient approximation (GGA) and GGA-mBJ approximations, the electronic and optical properties of the bulk and mono-layer Molybdenum sulphoselenide MoS${}_x$Se${}_{(2-x)}$ ($0\le x\le 2$) are calculated. In both bulk and mono-layer cases, the energy gap decreases by increasing the density of the Se atom, so that the reduction in the mono-layer case is more observable than the bulk case. Also, the variation of the exchange-correlation (XC) potential has the maximum and minimum effects on the bulk MoS₂ and the mono-layer MoSe₂ cases, respectively. The static dielectric function of MoS₂ in the ${𝜀}_1(\omega )$ as well as its optical conductivity is less than the other two compounds. In the mono-layer case along $x$-direction, the first response to the incident photon belongs to MoS₂ in the infrared region, while the other two compounds respond in the edge of the visible area. The intensity of the MoSSe response, unlike its bulk case, is less than the other compounds. Along $z$-direction, the first response is observed in the visible area. The remarkable point is that the optical stability in the bulk case is more than that in the mono-layer case.

A new zigzag lattice phononic crystal with holes is designed. Nondominated sorting genetic algorithm-II (NSGA-II) is used for the optimization of the newly designed phononic crystal (PC). Results indicate that geometrical parameters are key factors as well as density and elastic modulus for the determination of the bandgaps (BGs). The width of the BG of the optimized PC with holes can be increased three times higher than the initial design without holes.

Cobalt (Co) doped magnesium hydroxide Mg(OH)₂ nanoparticles are synthesized by a surfactant-free co-participation method. Scanning electron microscopy (SEM) images show nanometer size Mg(OH)₂ particles in spherically shaped particle-like morphology. Synthesis of these Mg(OH)₂ nanocrystals involves the formation of monomeric MgOH${}^{+}$ ions as the precursor for the Mg(OH)₂ nuclei which finally evolves in spherical particle-like morphology. X-ray diffraction (XRD) confirms the hexagonal crystal structure of the samples. With increasing Co concentration, the absorption spectra of the samples show narrowing of the bandgap from 5.47 eV (for pure Mg(OH₂)) to 5.26 eV (for 10% Co-doped Mg(OH₂)) effect is attributed to changes in the interaction potentials between Co and the host Mg(OH)₂ lattice due to dopant-induced lattice distortion and the presence of a mixed valance Co${}^{2+}$/Co${}^{3+}$ state.

Undoped and manganese (Mn)-doped zinc oxide (ZnO) thin films have been deposited onto glass substrates at 300${}^{\circ }$C using a low cost spray pyrolysis technique. Structural, optical and electrical properties of the as-deposited films have been investigated. Scanning electron microscopy images show the existence of clusters with well-defined nucleation centers consisting of highly dense ganglia-like fibers over a large area around the nucleation center. Chemical compositions of the ZnO and Mn-doped ZnO thin films are studied by using energy dispersive X-ray (EDX) analysis. X-ray diffraction spectra depict that the films have polycrystalline wurtzite structure. The average crystallite sizes are calculated in the range of 8–16 nm by Williamson–Hall method and found in good agreement with Scherer method. Optical transmittance of the films is about 80% in the visible region. Bandgap energy is tuned to 2.83 eV from 3.10 eV with increasing Mn doping. Electrical resistivity at room-temperature decreases significantly with increasing Mn doping as well as increasing temperature from 300–440 K. The activation energies in the temperature ranges 300–350 K and 350–440 K are found to be in the range of 0.25–0.16 eV and 0.35–0.59 eV, respectively. Hall Effect measurements show that the thin films have negative Hall co-efficient indicating $n$-type conductivity at room-temperature. Carrier concentration is found to be of the order of 10${}^{18}$ cm${}^{-3}$.