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The structural, electronic, and optical properties of homogeneous and inhomogeneous Ca 1 − x Mg x O alloys are studied with the density functional theory (DFT) under the assumption of polymorphism. Large supercells (2 × 2 × 2 and above) with different lattice symmetries and varying Mg molar fraction x were constructed, representing distinct solid phases that may coexist in micro/nanodomains of inhomogeneous alloys. We demonstrate that these polymorphs exhibit rich phenomenology like similar formation enthalpies for a given concentration x , but different electronic and optical properties. For example, Ca 0.5 Mg 0.5 O may have crystallites with four possible lattice symmetries using a 2 × 2 × 2 supercell for its description, with bandgaps varying between 3.26 eV (direct) and 4.46 eV (indirect). The DFT-simulated X-ray diffraction shows that polymorphism causes broadening and shift of the diffraction peaks. We also performed a detailed calculation of the bandgaps, optical absorption, and dielectric constants as a function of x for each polymorph, and they exhibit a structured bandgap behavior with maxima and minima in the 0.0 < x < 1.0 range, which is in marked contrast to Vegard's law rule of mixtures. We also report a direct-to-indirect bandgap transition occurring between 0.072 ≤ x ≤ 0.1094 for the F M 3 ¯ M symmetry. [ABSTRACT FROM AUTHOR]

The study examines the role and consequences of the nonstoichiometric Si[O.sub.2] interfacial layers in the oxide-semiconductor interface of Si/Si[O.sub.2] on the electrical characterization of the metal-oxide semiconductor (MOS) devices. The analysis reveals that a decrease in the capacitance for high voltages and the errors faced during the electrical measurements lead to the nonabrupt layers.

Ethionamide (ETH) is a high-profile drug for the treatment of patients with multidrug-resistant Mycobacterium tuberculosis and, in order to produce its inhibitory effects, it needs to be bioactivated by monooxygenase EthA. This process is under the control of the transcriptional repressors EthR and EthR2, so that their inhibition results in the boosting of ethionamide activation. Herein, through crystallographic data and computer simulations, we calculated the interaction binding energies of four inhibitors with improved in vitro potency, namely BDM76060 (PDB ID: 6HS1), BDM72201 (PDB ID: 6HRX), BDM76150 (PDB ID: 6HS2) and BDM72719 (PDB ID: 6HRY), in complexes with the transcriptional repressor EthR2, using density functional theory (DFT) within the molecular fractionation with conjugated caps (MFCC) approach. It was observed that these ligands share the same binding site within a 10.0 Å radius of the EthR2 protein; however, their structural particularities have a significant impact on the global energies of systems. The BDM72201 and BDM72719 components are weakly attached to the binding site, while BDM76060 and BDM76150 components produce stronger bonds, corroborating with experimental studies demonstrating that BDM76060 and BDM76150 are more successful in producing inhibitory effects. BDM76060 and BDM76150 have many functional groups that increase the contact surface with the protein and attract a more significant number of amino acid residues, being able to produce polarities that generate stronger interactions. In the current scenario of a growing number of cases of bacterial resistance, the obtained data can be used to guide clinical trials of these inhibitors and other inhibitors that act on the alternative EthR2 pathway, focusing on improving the activity of ethionamide, its effectiveness, a reduction in the treatment time and exposure to cytotoxic effects. [ABSTRACT FROM AUTHOR]

The axial sliding of carbon-based normal nanorings (NCNs) and one-turn Möbius nanorings (MCNs) along inner coaxial carbon nanotubes with hydrogen-terminated edges is described. Classical molecular dynamics simulations were carried out to investigate their sliding considering C300H100 NCN and MCN nanostructures and single wall armchair carbon nanotubes (SW CNTs) with chiralities (16, 16), (18, 18), and (20, 20). The NCN:CNT system exhibited an oscillatory movement with an initial frequency of about 10 gigahertz decaying slowly with time as the vibra-tional energy is transferred to radial and torsional modes of the nanoring, while the amplitude remained practically constant except for the (16, 16)-CNT. The MCN:CNT systems, on the other hand, initially oscillated as the NCN:CNT, but a strong damping regime ensued due to the strong interaction of the twisted nanoring with the CNT tips, effectively stopping the oscillation after 400 ps for all MCN:CNT structures investigated. [ABSTRACT FROM AUTHOR]

Results of optical absorption measurements are presented together with calculated structural, electronic, and optical properties for the anhydrous monoclinic L-asparagine crystal. Density functional theory (DFT) within the generalized gradient approximation (GGA) including dispersion effects (TS, Grimme) was employed to perform the calculations. The optical absorption measurements revealed that the anhydrous monoclinic L-asparagine crystal is a wide band gap material with 4.95 eV main gap energy. DFT-GGA+TS simulations, on the other hand, produced structural parameters in very good agreement with X-ray data. The lattice parameter differences Δa, Δb, Δc between theory and experiment were as small as 0.020, 0.051, and 0.022 Å, respectively. The calculated band gap energy is smaller than the experimental data by about 15%, with a 4.23 eV indirect band gap corresponding to Z→Γ and Z→β transitions. Three other indirect band gaps of 4.30 eV, 4.32 eV, and 4.36 eV are assigned to α3→Γ, α1→Γ, and α2→Γ transitions, respectively. Δ-sol computations, on the other hand, predict a main band gap of 5.00 eV, just 50 meV above the experimental value. Electronic wavefunctions mainly originating from O 2p–carboxyl, C 2p–side chain, and C 2p–carboxyl orbitals contribute most significantly to the highest valence and lowest conduction energy bands, respectively. By varying the lattice parameters from their converged equilibrium values, we show that the unit cell is less stiff along the b direction than for the a and c directions. Effective mass calculations suggest that hole transport behavior is more anisotropic than electron transport, but the mass values allow for some charge mobility except along a direction perpendicular to the molecular layers of L-asparagine which form the crystal, so anhydrous monoclinic L-asparagine crystals could behave as wide gap semiconductors. Finally, the calculations point to a high degree of optical anisotropy for the absorption and complex dielectric function, with more structured curves for incident light polarized along the 100 and 101 directions. [ABSTRACT FROM AUTHOR]

Ovary fragments from six sexually mature cats were vitrified in the presence or absence of betaine or ascorbic acid, loaded (7.4 or 74 µM betaine; 20 or 200 µM ascorbic acid) or not (1 mM betaine or 0.3 mM ascorbic acid) into CaCO3 microparticles, and assessed for follicular morphology, oxidative stress and mitochondrial activity Feline ovarian tissue was successfully preserved after vitrification in the presence of 74 µM betaine loaded in CaCO3 microparticles, as confirmed by morphological analysis and the density of preantral follicles and stromal cells, as well as by the increased mitochondrial activity and decreased production of reactive oxygen species. It is well known that feline ovarian follicles are sensitive to cryopreservation procedures. Betaine microencapsulated in CaCO3 appears as a compound to improve the survival of feline preantral follicles submitted to vitrification within the ovarian tissue. Additionally, microencapsulated betaine was able to increase mitochondrial activity without induce oxidative stress. [ABSTRACT FROM AUTHOR]

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results. [ABSTRACT FROM AUTHOR]

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists Nt varying from 0 to 7 (the case Nt=1 corresponds to a half-twist Möbius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO-4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO→LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions. [ABSTRACT FROM AUTHOR]

Confinement features of excitons in Si-SiO2-Si and Si-CaCO3-Si spherical nanoparticles (NPs) with a core–double-shell structure are studied in this work. In order to compute carrier and exciton ground-state energies, we estimate electron and hole effective masses in CaCO3 calcite using first-principles calculations. A comparison is made between the recombination energies and oscillator strengths of excitons in SiO2 and CaCO3 NPs as a function of diameter and barrier (middle shell) thicknesses. The recombination energy spans the energy range of the visible spectrum by changing the barrier width. The excitonic oscillator strength is orders of magnitude larger for Si-CaCO3-Si NPs in comparison to Si-SiO2-Si NPs. The results allow us to suggest that Si-CaCO3-Si nanoparticles emitting light in any wavelength between the infrared and ultraviolet can be fabricated, and are more efficient than SiO2-based ones. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the role of nonstochiometric SiOx suboxides in the oxide-semiconductor interface of Si/SiO2 metal-oxide-semiconductor (MOS) devices. We show that nonabrupt SiOx layers and fluctuations in their localization imply: (i) Capacitance degradation for voltages above threshold, (ii) variations in the threshold voltage of the order of 20 mV, and (iii) errors in the determination of the effective oxide thickness by means of electrical measurements. [ABSTRACT FROM AUTHOR]

Structural, electronic, and optical properties for the cubic, tetragonal, and monoclinic crystalline phases of ZrO2, as derived from ab initio full-relativistic calculations, are presented. The electronic structure calculations were carried out by means of the all-electron full-potential linear augmented plane wave method, within the framework of the density functional theory and the local density approximation. The calculated carrier effective masses are shown to be highly anisotropic. The results obtained for the real and imaginary parts of the dielectric function, the reflectivity, and the refraction index show good agreement with the available experimental results. In order to obtain the static dielectric constant of ZrO2, we added to the electronic part the optical phonon contribution, which leads to values of ε1(0)≃29.5,26.2,21.9, respectively, along the xx, yy, and zz directions, for the monoclinic phase, in excellent accordance with experiment. Relativistic effects, including the spin-orbit interaction, are demonstrated to be important for a better evaluation of the effective mass values and in the detailed structure of the frequency dependent complex dielectric function. [ABSTRACT FROM AUTHOR]

We consider the nonequilibrium thermodynamic state of carriers in III-nitrides, and calculate the mobility of holes in cubic GaN layers under electric fields of low intensity. The contribution of different scattering mechanisms to the mobility is analyzed, and the relevance of each one is characterized. Satisfactory agreement with recently published experimental data is obtained.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A numerical model to calculate the tunneling time from the channel of a metal-oxidesemiconductor device into a silicon nanocrystal embedded in SiO[SUB2] is presented. Self-consistent simulations of the Kohn-Sham and Poisson equations are performed to study the role of the size, shape, and barrier thickness of a quantum dot (QD). We found that the charging process is very sensitive to the shape of the QD, resulting in changes of several orders of magnitude in the electron transfer and retention times. [ABSTRACT FROM AUTHOR]

The complex mobility of hot electrons in 3C– and 6H–SiC at 300, 673, and 1073 K is calculated through the numerical solution of balance transport equations within a nonparabolic band picture. The electric field, applied along the hexagonal c axis or [111] direction in the cubic material, is taken as the sum of a strong variable dc (200–800 kV/cm) and a weak (20 kV/cm) ac component whose frequency is varied in the 0.1–100 THz range. The real electron mobility presents a characteristic maximum peaking around 6–10 THz, while the imaginary electron mobility presents a characteristic maximum and minimum peaking around 20–40 THz and 2–5 THz, respectively. At the highest lattice temperature explored, the electron complex mobilities for both hexagonal and cubic SiC polytypes have nearly similar frequency dependent patterns. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]