Search

In this paper, we report on the deposition of zinc oxide thin films by the plasma enhanced chemical vapor deposition technique. The effects of oxygen flow rate on surface morphology, optic transmission, and gap energy are studied. The X-ray diffraction technique shows polycrystalline phases in the deposited material, the crystal structure is hexagonal, and the preferential orientation of crystal growth is in the (101) direction. The best optical transmission of zinc oxide is of the order of ~ 89%, and a wide optical gap of ~ 3.7 eV. To make a comparison with the present results and theoretical data, an ab initio calculation is carried out by the density functional theory method. To study the oxygen vacancies concentration effect on bandgap and optical transmission, we use three different configurations, namely ZnO0,97, ZnO0,94, and ZnO0,91. To confirm our experimental and calculated results, we compared them to those given in the literature. [ABSTRACT FROM AUTHOR]

In the present paper, inter-ionic reactions involving two cations and four anions were used to model the formation of neutral ion pairs. Moreover, density functional theory technique has been used to describe the physical, chemical, and vibrational properties of the ionic liquids, containing imidazolium-based cations and different anions at functional B3LYP with a def2-SVP basis set. The chemical parameters have been investigated for different ionic liquids along with frontier orbitals "highest occupied molecular orbital" and "lowest unoccupied molecular orbital" studies. All the optimized structures of the ionic liquids did not produce imaginary frequency (i.e., imaginary frequency is zero), which validates that the structures are energetically stable. The density functional theory method provides the stability sequence for ionic liquids: [BMIM][PF6] > [EMIM][PF6] > [BMIM][TFSI] > [EMIM][TFSI] > [EMIM][Cl] > [BMIM][Cl] > [EMIM][Br] > [BMIM][Br]. Herein, [BMIM][PF6] ionic liquid has highest energy gap (4.833 eV) that means that [BMIM][PF6] is the most stable. [ABSTRACT FROM AUTHOR]

This paper presents the results of density functional theory calculations for the (3,1) carbon nanotube ensuing in two structural configurations, representing two local energy minima. To be sure in this conclusion, we used two distinct density functional theory implementations and three different structure optimization algorithms. The interatomic distances of the two kinds of (3,1) carbon nanotubes do not differ too much, but the other properties are more pronounced. We found that the HOMO-LUMO energy gap of the lower energy configuration is 0.69 eV, whereas the second configuration possesses the 0.36 eV HOMO-LUMO gap. Both isomers have been obtained by optimization because the calculations started either with a different initial configuration or with different starting parameters critical for convergence, efficiency and/or accuracy. [ABSTRACT FROM AUTHOR]

Hydrogen bonded liquid crystal mixtures are derived from: (i) non-mesogenic compounds of diglycolic acid (DGA), (ii) mesogenic compounds of 4-amyloxybenzoic acid (5OBA) and (iii) 4-hexyloxy benzoic acid (6OBA). The existence of intermolecular H-bond is confirmed with FTIR and validated using the density functional theory studies. Polarized light microscopy and differential scanning calorimetry are used to observe the phase variations and transition temperature. In addition, enthalpy values of different mesogenic phases are determined. It is worth mentioning that the nematic mesogenic defects with S = ±1 and S = +1=2 are also analyzed. In order to discuss the induced higher order smectic F phase and its stability, the density functional theory study is applied. The band gap energy of the hydrogen bonded liquid crystal mixtures is found to be 5.1780 eV, based on the HUMO-LUMO approach. This induced thermochromic effect of hydrogen bonded liquid crystal mixtures is a useful phenomenon that can be utilized in optical device applications. In this paper, variation on liquid crystal parameters as well as the formation of H-bond between an acceptor and a donor, along with the extension of a chain length, are also reported. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the structural, elastic, electronic, and magnetic properties of Rh2CrGe full-Heusler alloy in the regular type (Cu2MnAl, prototype L21) phase by using the first-principle methods of density functional theory with local spin-density approximation. We have found that Rh2CrGe is stable in the ferromagnetic configuration. This result is confirmed by the calculated cohesive energies. The stability criteria show that Rh2CrGe is mechanically stable in L21 structure. The spin-polarized electronic structures depicted a metallic character. The calculated total magnetic moment of 3.78 μB is not in agreement to the value of 4 μB of the Slater-Pauling rule. Therefore the Rh2CrGe full-Heusler alloy is metallic ferromagnet in nature. [ABSTRACT FROM AUTHOR]

In this paper, we present the first principle calculation of the different properties of the full Heusler compounds Cd2LaB, Cd2PrB, and Cd2CeB. Our calculations are based on density functional theory, using the full potential muffin-tin linear orbital method implemented in the LmtART code. For the determination of the exchange and correlation potential, we used the local density approximation and local spin density approximation. The elastic constants are also calculated. The total magnetic moment of Cd2LaB, Cd2PrB, and Cd2CeB using local spin density approximation are 3.334, 3.482, and 3.473 μB at ambient pressure, respectively. We also present the thermal effects using the quasi-harmonic Debye model, in which the vibrations of the network are taken into account. The effects of temperature and pressure on structural parameters, heat capacity, entropy, coefficient of thermal expansion, and the Debye temperatures are determined for unbalanced Gibbs functions. [ABSTRACT FROM AUTHOR]

The I2-II-IV-VI4 series of quaternary chalcogenides semiconductors have drawn wide interest for their potential application as solar-cell absorbers. In this paper, we present a study of electronic and optical properties of the equilibrium kesterite structure (KS) of Cu2ZnGeS4 (CZGS) calculated by means of the full potential linearized augmented plane wave method. For this purpose, we used the Wien2k code based on the density functional theory with the generalized gradient approximation (GGA), modified Becke-Johnson exchange potential (mBJ) and the Hubbard potential U. The TB-mBJ is used alone or combined with U (TB-mBJ, and TB-mBJ+U). The results are compared with other theoretical results and the available experimental ones. The obtained results show that the top of the valence band is mainly composed by the Cu d orbital while the bottom of the conduction band is a mixture of Ge s and S p states. It was found that KS-CZGS has a direct band gap of 0.3, 1.8 and 2.0 eV using GGA, GGA+TB-mBJ and GGA+TB-mBJ+U, respectively. It is observed that states exist above the Fermi level (0 eV) with a width of 0.3 and 0.2 eV when we use GGA and GGA+TB-mBJ, while these states come down under the Fermi level when using GGA+TB-mBJ+U. The optical properties are calculated and an almost isotropic behavior is found in contrast to the reported properties of the stannite phase. This behavior can be linked to the charge density and the structure. [ABSTRACT FROM AUTHOR]

The paper presents the results of the plane wave density functional theory calculations of newly predicted Y2FeSi full-Heusler alloy with optimized lattice constants. We have explored two most likely structures (AlCu2Mn and CuHg2Ti prototypes) with different atomic arrangements and including collinear magnetic ordering. The results show that the classic L21 structure is more favourable in terms of energy than the second one studied in this work. Moreover, for both crystalline structures ferromagnetic arrangement is more favourable, although in the second structure the difference in energy between ferromagnetic and nonmagnetic state is small. Study did not revealed band gap in any spin channel, yet the spin polarization in majority channel at the Fermi level is significant (σ = 83%). The calculated structural lattice a parameter for the energetically lowest structure is a = 13:0996 a0. For the lattice constant range of 12.2 to 13.2 a0 the total magnetic moment varies significantly from 0.86 μB to 1.74 μB per formula unit. [ABSTRACT FROM AUTHOR]

In this paper, we studied the relaxation of (001) surface of BaTiO3 and SrTiO3 slabs with two termination surfaces and with 3, 5, and 7 layers thickness for each cases, using density functional theory and generalized gradient approximation for exchange-correlation functional and pseudo potential method. We calculated the slab energy and the rate of expansion and contraction of the layers and compare them for different thicknesses. Band structure and density of states for these slabs and for BaTiO3 and SrTiO3 bulk were computed to find out the variation of band gap with respect to slab thickness. It is found that in comparison with bulk, in TiO2 slabs of both materials gap size decreases while in SrO and BaO slabs it increases. [ABSTRACT FROM AUTHOR]

An algebraic model of coupled anharmonic oscillators is introduced, capable of describing the stretching vibrations of medium-size molecules. This model is applied to the calculation of O-H vibrational modes of water molecules. In this paper, we have reported the stretching and bending vibrational energy levels of water molecule using the algebraic and density functional theory method. The results obtained by theoretical models show good agreement with the experimental values. [ABSTRACT FROM AUTHOR]

In this paper we investigate fundamental gaps of three octametallic Cr-based molecular rings Cr8F8(Piv)16, Cr7NiF8(Piv)16 and Cr7CdF8(Piv)16 using the SIESTA package. We find that for the ground-state antiferromagnetic configurations, the gap of the homometallic ring is significantly higher than those of the heterometallic rings. In addition, the HOMO and LUMO orbitals are plotted and discussed. [ABSTRACT FROM AUTHOR]