Narrow Results

Structural and electronic properties of perovskite KCaX₃ (X = F and Cl) compounds are investigated using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the Wien2k code. The exchange-correlation potential is treated by the generalized gradient approximation within the scheme of Perdew, Burke and Ernzerhof (GGA-PBE). Based on these calculations, it has been concluded that KCaX₃ compounds have indirect energy band-gap (Γ-R). Moreover, the theoretical investigation which has been carried out on the highly hydrostatic pressure dependence of the KCaX₃ electronic properties revealed a linear relationship between both the hydrostatic pressure and the energy band-gap. In addition, the electronic and bonding properties of the band structure, density of states (DOS) and electron charge density have been calculated and presented. Besides that, the dielectric function, refractive index and extinction coefficient are calculated. The origin of some of the peaks in the optical spectra is discussed in terms of the calculated electronic structure. Finally, the calculated structural properties are found to agree well with the available experimental and theoretical data.

Encapsulation of small clusters in fullerene cages provides a stable environment for their application in nanoscale functional devices. In this paper, first principles study of Ruthenium as an endohedral dopant in buckminsterfullerene has been carried out using density functional theory. Ruthenium atom has three stable dopant sites inside C₆₀, with three possible values of magnetic moment (4, 2 and 0 μB). The doping position of Ru atom can be seen to have an effect on HOMO–LUMO gap, formation energy, binding energy and magnetic moment of the fullerene cage. The interaction between Ru and C atoms in different conformations can be explained in terms of Mulliken analysis and density of states analysis. It is also possible to encapsulate more than one Ru atoms in the C₆₀ cage (Ru_n@C₆₀, n = 2–6); encapsulation up to six atoms has been analyzed, after which the process is energetically unfavorable. The geometry of the lowest energy structures, compared to the isolated Ru_n clusters, is found to change as a result of encapsulation (e.g., in Ru₃@C₆₀ and Ru₅@C₆₀). A reduction in magnetic moment of Ru clusters inside fullerene cage as compared to isolated clusters also occurs due to hybridization and confinement effects. The varied magnetic moments of Ru-encapsulated C₆₀ molecules reveal its applications in molecular magnetic devices and quantum peapods.

Optical properties of Zn_1-xMg_xS, Zn_1-xMg_xSe and Zn_1-xMg_x Te(0 ≤ x ≤ 1) ternary semiconductor alloys are calculated using the full potential linearized augmented plane wave within the density functional theory. The exchange correlation potential is treated by the generalized gradient approximation (GGA) within Perdew et al. scheme. The real and imaginary parts of the dielectric function ε(ω), the refractive index n(ω), the extinction coefficient k(ω), the optical absorption coefficient α(ω), the reflectivity R(ω) and the electron energy loss function (EELS) are calculated within random phase approximation (RPA). Our results are compared with the previous theoretical calculations and available experimental data. Moreover, the interband transitions responsible for the structures seen in the spectra are specified. It is shown that, the chalcogen p states as initial and Zn4s, Mg3s, chalcogen d states as final states perform the major role in optical transitions.

The phase stability and electronic properties in Al₃Ta compound are studied using the FP-LAPW method. In this approach, the generalized gradient approximation (GGA) is used for the exchange-correlation potential calculation. The total energy calculations show that the D0₂₂ structure is more stable than that of D0₂₃ and L1₂. The densities of states exhibit a pseudo gap near the Fermi level for all considered structures. By analyzing the electronic charge density we find a build-up of electrons in the interstitial region, and the bonds are directed from the Ta atoms to the Al atoms, which is the characteristic of covalent bonding. The temperature and pressure effects on the structural parameters, Debye temperature, Grüneisen parameter, heat capacities (C_v, C_p) and thermal expansion are predicted through the quasi-harmonic Debye model.

We have investigated theoretical Vickers hardness, thermodynamic and optical properties of four zirconium metal-based MAX phases Zr₂AC (A = Al, Si, P and S) for the first time in addition to revisiting the structural, elastic and electronic properties. First-principles calculations are employed based on density functional theory (DFT) by means of the plane-wave pseudopotential method. The theoretical Vickers hardness has been estimated via the calculation of Mulliken bond populations and electronic density of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and volume thermal expansion coefficient of all the compounds are derived from the quasi-harmonic Debye model. Further, the optical properties, e.g., dielectric functions, indices of refraction, absorption, energy loss function, reflectivity and optical conductivity of the nanolaminates have been calculated. The results are compared with available experiments and their various implications are discussed in detail. We have also shed light on the effect of different properties of Zr₂AC as the A-group atom moves from Al to S across the periodic table.

We carried out ab initio calculations of structural, electronic and optical properties of Indium nitride (InN) compound in both zinc blende and wurtzite phases, using the full-potential linearized augmented plane wave method (FP-LAPW), within the framework of density functional theory (DFT). For the exchange and correlation potential, local density approximation (LDA) and generalized gradient approximation (GGA) were used. Moreover, the alternative form of GGA proposed by Engel and Vosko (EV-GGA) and modified Becke–Johnson schemes (mBJ) were also applied for band structure calculations. Ground state properties such as lattice parameter, bulk modulus and its pressure derivative are calculated. Results obtained for band structure of these compounds have been compared with experimental results as well as other first principle computations. Our results show good agreement with the available data. The calculated band structure shows a direct band gap Γ → Γ. In the optical properties section, several optical quantities are investigated; in particular we have deduced the interband transitions from the imaginary part of the dielectric function.

This study combines the use of the full potential linear-augmented plane wave method (FP-LAPW) within the framework of the density functional theory (DFT) and the optical matrix approach for modeling the multilayer assembly. A new class of heterostructures with sufficient number of alternating layers of rutile-TiO₂ (as a high index material) and α-Al₂O₃ (as a low index material) are proposed and their transmittance spectra are investigated. This study shows that the number of alternating layers, and the thickness and arrangement of them should be considered in making a heterostructured filter. The relation between heterostructure parameters and narrow-band-pass peaks of transmittance spectra is investigated. The proposed model seems to be successful in predicting the optical behavior of heterostructures and simulations agree well with the experimental observations. In addition, our model is very flexible and the effect of other parameters such as incident angle and light polarization can be easily investigated.

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.

Structures and stabilities of ${({\mathrm{\text{Os}}}_n\mathrm{\text{N}})}^{0,\pm }$ clusters have been systematically studied via using density functional theory (DFT) with generalized gradient approximation (GGA). The calculations show that the stable configurations of ${({\mathrm{\text{Os}}}_n\mathrm{\text{N}})}^{0,\pm }$ are such structures with one N atom bonded to the external of the basic constructions consisting of Os atoms. Meanwhile, ${({\mathrm{\text{Os}}}_n\mathrm{\text{N}})}^{0,\pm }$ clusters $(n=7-9)$ represent “magic number” effect, and 8 is the magic number. Additionally, the ground-state structures of ${({\mathrm{\text{Os}}}_8\mathrm{\text{N}})}^{0,\pm }$ clusters have the best stability, while that of ${\mathrm{\text{Os}}}_7\mathrm{\text{N}}$ cluster possesses the worst stability. The result of the study on the ionization potential (IP) and the electron affinity (EA) shows that there are not topological differences among the configurations of ${({\mathrm{\text{Os}}}_n\mathrm{\text{N}})}^{0,\pm }$$(n=7-11)$ clusters.

In this study, we investigated stable structures for a transition metal atom–boron (CrB) wheel-like clusters and compared them with their corresponding 3D counterparts by means of density functional theory (DFT). In addition, hydrogen storage capability of the wheel-like system was investigated. All simulations were performed at the B3LYP/TZVP level of theory. We set out a complete route to the formation of CrB wheel-like clusters. Our results showed that, some of the clusters, investigated in this work (CrB_n; n = 4, 6, 7, 8), either prefer to be in a 3D geometry rather than 2D quasi-planar or planar geometry. However, hydrogen doping has an interesting effect on both 2D quasi-planar and 3D geometries of this system. Simply it transforms the 3D structure, first, into a 2D quasi-planar, then a planar geometry. Furthermore, our results show that H–cluster interaction is too high for reversible hydrogen storage for these clusters.

The electronic structure, mechanical and thermodynamic properties of Fe₂VX, (with X = Al and Ga), have been studied self consistently by employing state-of-the-art full-potential linearized approach of augmented plane wave plus local orbitals (FP-LAPW + lo) method. The exchange-correlation potential is treated with the local density and generalized gradient approximations (LDA and GGA). Our predicted ground state properties such as lattice constants, bulk modulus and elastic constants appear more accurate when we employed the GGA rather than the LDA, and these results are in very good agreement with the available experimental and theoretical data. Further, thermodynamic properties of Fe₂VAl and Fe₂VGa are predicted with pressure and temperature in the ranges of 0–40 GPa and 0–1500 K using the quasi-harmonic Debye model. We have obtained successfully the variations of the heat capacities, primitive cell volume and volume expansion coefficient.

Energetically stable Ge diamondoids are used as building blocks to investigate Ge nanocrystal properties using density functional theory (DFT). Energy gap, HOMO and LUMO of Ge diamondoids vary according to confinement theory with shape fluctuations. Ge diamondoid vibrational force constants reach 2.18 mDyne/Å which is between that of bulk silicon and tin. Ge–Ge vibrational frequencies and reduced masses reach 357.47 cm${}^{-1}$ and 41.93 amu, respectively, which are higher than the values of bulk Ge. Size variation of UV–Vis shows that the maximum optical peak moves from 163 nm to nearly 290 nm as the size of Ge diamondoids and molecules increases. The higher peak value approaches that of experimental Ge quantum dots at 300 nm. NMR spectra of Ge diamondoids are analyzed as a function of diamondoid sizes. Our results show that all investigated spectroscopic tools are sensitive to molecular or nanocrystals size. NMR is particularly good size indicator. Natural bond orbital (NBO) population analysis shows present diamondoid bondings differ from ideal $s{p}^3$ bonding. The bonding for Ge valence orbitals is in the range $(4s^{1.24}4p^{2.60})-(4s^{1.27}4p^{2.73})$ depending on distance between Ge atom and diamondoid surface. Highest Ge diamondoids vibrational longitudinal optical (LO) mode is blue shifted with respect to experimental bulk value which is the opposite case for C and Si. H surface atom effects on electronic and vibrational properties are discussed.

The electronic, thermoelectric, optical, and magnetic properties of the samarium aluminate (SmAlO₃) compound is studied using the spin-polarized full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). The exchange and correlation potential is treated with the generalized gradient approximation (GGA) and the Coulomb repulsion ($U=0.51$ Ry) has been calculated theoretically and was used for the GGA$+U$ based approximated electronic structures. Additionally, the modified Becke–Johnson (mBJ) potential was also utilized along with the GGA$+U$ approach for the calculation of the band gap. On the other hand, the optical properties were analyzed with the mBJ$+U$ results and the thermoelectric properties were explained on the basis of the electronic structures and density of states (DOS) with a thermoelectric efficiency of 0.66 at 300 K. The minimum reflectivity at 1.13 eV (which was equal to 1.097 $\mu$m) was found to be in agreement with the experimental results. Further refinements in the electronic structures were obtained by adding the spin–orbit coupling (SOC) interactions to the GGA$+U$ approach, which was then combined with the mBJ approximations. Hence, a conclusion using the combined mBJ$+U+$SOC study indicates that the SmAlO₃ compound is a potential candidate for both thermoelectric as well as magnetic devices.

This paper communicates the structural, electronic and optical properties of L-alanine, monofluoro and difluoro substituted alanines using density functional calculations. These compounds exist in orthorhombic crystal structure and the calculated structural parameters such as lattice constants, bond angles and bond lengths are in agreement with the experimental results. L-alanine is an indirect band gap insulator, while its fluorine substituted compounds (monofluoroalanine and difluoroalanine) are direct band gap insulators. The substitution causes reduction in the band gap and hence these optically tailored direct wide band gap materials have enhanced optical properties in the ultraviolet (UV) region of electromagnetic spectrum. Therefore, optical properties like dielectric function, refractive index, reflectivity and energy loss function are also investigated. These compounds have almost isotropic nature in the lower frequency range while at higher energies, they have a significant anisotropic nature.

In previous researches it is demonstrated that reactivity and sensitivity of boron nitride nanotubes (BNNTs) toward gas molecules can be modified by impurity. In this work, oxygen defect for three nitrogen sites was used to study the adsorption of NO molecule through the surface of boroxol ring of oxygen doped BNNT (7,0) with different adsorption patterns, including side-on and end-on. All calculations are performed using the DFT-B3LYP/6-31G${}^{\ast }$ level of theory, and their electronic energies are corrected by gCP and D3 correction terms. High binding energies indicate that NO molecule undergoes chemical adsorption with large charge transfer from the tube which can significantly change electronic properties of the tube. Density of state (DOS) and partial DOS (PDOS) analyses revealed that adsorption of NO molecule on the boroxol ring position is covalent in nature with significant effect on the electronic properties of tube. The Laplacian of electron density, Lagrangian kinetic energy density, Hamiltonian kinetic energy density and potential energy density at bond critical points between the tube and NO indicate that the interaction between the tube and NO molecule is covalent in nature. Topological analysis of the electron localization function shows that electrons in the new formed bonds are approximately localized, meaning that the nature of adsorption process is chemical covalent. The studied nanotube is a suitable candidate to filter and eliminate NO gas molecule.

The ${\mathrm{\text{Cu}}}_2{\mathrm{\text{ZnSnS}}}_4$, ${\mathrm{\text{Cu}}}_2{\mathrm{\text{ZnSnSe}}}_4$ and ${\mathrm{\text{Cu}}}_2{\mathrm{\text{ZnSnTe}}}_4$ and their alloys have been frequently investigated experimentally owing to their suitable bandgap for the solar cell applications. For the first time, density functional theory is applied to explore the structural, electronic and optical properties of ${\mathrm{\text{Cu}}}_2\mathrm{\text{ZnSn}}({\mathrm{\text{S}}}_{1-x}{\mathrm{\text{Te}}}_x)$ and ${\mathrm{\text{Cu}}}_2\mathrm{\text{ZnSn}}({\mathrm{\text{Se}}}_{1-x}{\mathrm{\text{Te}}}_x)$$(x=0,0.25,0.5,0.75,1)$. The energy minimization procedure reveals that the Kesterite phase is stable compared to the Stannite structure. Lattice constants of the compounds are in good agreement with the previous experimental results. The alloys have direct bandgaps which decrease by increasing the concentration of Te. The chemical bonding among the cations and anion is dominantly covalent. Electronic bandgap dependent optical properties like absorption coefficient and optical conductivity are studied in detail. The materials show strong response in the visible region of energy spectrum indicating the usefulness of these materials for optoelectronic devices.

We report the structural, electronic, optical, and thermoelectric properties of the five cubic alkali-earth transition-metals AZn${}_{13}$ (A-Na, K, Ca, Sr, Ba) using density functional theory. Structural properties, electronic structures and optical behaviors are calculated explicitly via highly accurate contemporary full potential-linearized augmented plane wave (FP-LAPW) method. The investigated ground state data of these materials is quite close to the experimental information. The modified Becke–Johnson (mBJ) predicts the intermetallic nature of AZn${}_{13}$ (A-Na, K, Ca, Sr, Ba) materials. The complex dielectric function of these intermetallic compounds has been calculated and the observed noticeable peaks are examined through mBJ. With the help of complex dielectric function, the other important optical parameters like reflectivities, conductivities and refractive indices of AZn${}_{13}$ (A-Na, K, Ca, Sr, Ba) have been calculated as a function of energy. The optical response suggests that AZn${}_{13}$ (A-Na, K, Ca, Sr, Ba) compounds can be used for the optoelectronic devices. Further, the thermoelectric properties have been calculated through BoltzTraP program, the calculated values for different thermoelectric parameters recommend that these AZn${}_{13}$ (A-Na, K, Ca, Sr, Ba) materials are the suitable candidates for thermoelectric applications.

Mechanical and electronic properties of oxygen plasma-treated graphene sheets are investigated using density functional theory (DFT). Oxygen plasma-treated graphene is modeled using a graphene sheet with adsorbed epoxide functional groups (C–O–C) on its one side. The most stable configurations of such oxidized graphene sheets with different O/C ratios ranging from 12.5% to 50% are then calculated. In the special case of O/C = 50% (fully oxidized surface), both single- and double-sided oxidation cases are considered. The elastic and electronic properties of the energetically most favorable configurations are evaluated under the tensile and compressive loads in harmonic range. For structures with high O/C ratios (O/C $\ge$ 25%), the elastic constants (modulus of elasticity and bulk modulus) are significantly smaller than those of graphene while for low O/C ratios (O/C $\le$ 12.5%), these quantities are almost equal to the elastic constants of pristine graphene. We also found that the electronic bandgap of the oxidized sheets is increased under tensile loading.

First-principles study of elastic, electronic and optical properties of full-Heusler Co₂V(Al, Ge, Ga and Si) compounds are calculated through density functional theory (DFT) to obtain and compare the mentioned properties. Equilibrium lattice constants of these compounds are in good agreement with other works. Electronic calculations are shown full spin polarization at Fermi level for all compounds, so in the down spin, indirect bandgap is calculated as 0.33, 0.6, 0.2 and 0.8 eV for Co₂V(Al, Ge, Ga and Si), respectively. The integer amounts of the magnetic moments are compatible with Slater–Pauling role. The optical treatment of Co₂VGa is different from three other compounds. All mentioned compounds have metallic behavior by 22 eV plasmonic frequency. The imaginary part of the dielectric function for the up spin indicates that the main optical transitions occurred in this spin mode. Moreover, the elastic results show that the Co₂VGa does not have elastic stability, but the other three compounds have fully elastic stability and the Co₂V(Al, Ge and Si) belong to the hardness of materials.

Density functional theory (DFT) and generalized gradient approximation (GGA) have been employed to study origins of the intrinsic $n$-type electrical conductivity in the zinc oxide. Hubbard-like term has been introduced to provide a better description for the Zn 3$d$ electrons. Two intrinsic point defects, namely oxygen vacancy and hydrogen impurity, were taken into consideration. Results on conductivity are analyzed using density of states patterns for different configurations of defects. Microstructure and local magnetic moments are studied as well. The obtained results clearly indicate that oxygen vacancy does not and cannot be responsible for the intrinsic $n$-type electrical conductivity whereas inserted hydrogen atoms tend to lose its only valence electron, which in turn becomes a free electron contributing towards the $n$-type conductivity.