V. N. Freire

Structural, electronic, and optical properties of ZrO2 from ab initio calculations

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 intera...

Three-dimensional self-consistent simulation of the charging time response in silicon nanocrystal flash memories

A numerical model to calculate the tunneling time from the channel of a metal–oxide–semiconductor device into a silicon nanocrystal embedded in SiO2 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.

Resveratrol prevents social deficits in animal model of autism induced by valproic acid.

Autism spectrum disorders (ASD) involve a complex interplay of both genetic and environmental risk factors, such as prenatal exposure to valproic acid (VPA). Considering the neuroprotective, antioxidant and anti-inflammatory effects of resveratrol (RSV), we investigated the influence of prenatal RSV treatment on social behaviors of a rodent model of autism induced by prenatal exposure to VPA. In the three-chambered apparatus test, the VPA group showed a reduced place preference conditioned by conspecific and no preference between exploring a wire-cage or a rat enclosed inside a wire cage, revealing sociability impairments. Prenatal administration of RSV prevented the VPA-induced social impairments evaluated in this study. A bioinformatics analysis was used to discard possible molecular interactions between VPA and RSV during administration. The interaction energy between RSV and VPA is weak and highly unstable, suggesting cellular effects instead of a single chemical process. In summary, the present study highlights a promising experimental strategy to evaluate new molecular targets possibly involved in the etiology of autism and developmental alterations implicated in neural and behavioral impairments in ASD.

Full-relativistic calculations of the SrTiO3 carrier effective masses and complex dielectric function

We perform fully relativistic band-structure calculations for cubic SrTiO3, which are used to obtain carrier effective masses and the frequency behavior of its complex dielectric function e(ω). The obtained values and anisotropy of the carrier effective masses are shown to be highly influenced by the relativistic contributions. In order to evaluate the static dielectric constant, the low-frequency behavior of e(ω) is obtained by taking into account also the optical phonon contributions to the imaginary part of e(ω), adopting a simplified classical oscillator dispersion model. It is found that the phonon contribution leads to about 240 times (at T=85 K) the value of the bare electronic contribution to the dielectric constant. The calculated temperature dependence of the dielectric constant is shown to be consistent with that observed in bulk SrTiO3 static permittivity measurements.

Coexistence of triclinic and monoclinic phases in WO3 ceramics

The coexistence of phases in WO3 ceramics sintered within the 600–1000 °C temperature range is investigated using Raman microprobe spectroscopy. The analysis of vibrational modes indicates the existence of triclinic (VI) P1 and monoclinic (V) P21/n phases mixing. The concentration ratio between WO3 microcrystals in the former and the latter phases is shown to depend on the ceramics sintering temperature, reaching a maximum value favouring the triclinic (VI) P1 phase at the 800 °C sintering temperature. Copyright

Velocity overshoot onset in nitride semiconductors

A theoretical study on the electron drift velocity and some nonequilibrium thermodynamic characteristics of wurtzite GaN, AlN, and InN is presented. It is based on a nonlinear quantum kinetic theory which provides a description of the dissipative phenomena developing in the system. The ultrafast time evolution of the electron drift velocity and quasitemperature is obtained, and overshoot effects are evidenced on both. The overshoot onsets are shown to occur at 20 kV/cm in GaN, 60 kV/cm in AlN, and 10 kV/cm in InN, electric field intensities which are considerably smaller than those that have been recently derived resorting to Monte Carlo simulations.

Structural and optoelectronic properties, and infrared spectrum of cubic BaSnO3 from first principles calculations

The electronic band structure, density of states, dielectric function, optical absorption, and infrared spectrum of cubic BaSnO3 were simulated using density functional theory, within both the local density and generalized gradient approximations, LDA and GGA, respectively. Dielectric optical permittivities and polarizabilities at ω=0 and ω=∞ were also estimated. Indirect band gaps E(R→Γ) of 1.01 eV (LDA) and 0.74 eV (GGA) were found, which are smaller than the experimental one (≈3.1 eV). A comparison of the calculated cubic BaSnO3 band gap with those of others stannates ASnO3 (A = Ca, Sr, Cd) already published highlights their dependence on each crystal profile. The cubic BaSnO3 effective masses of electrons and holes were computed by parabolic fittings along different directions at the conduction band minimum and valence band maximum, being anisotropic for both electrons and holes. The experimental band gap and calculated effective masses confirm the semiconductor character of cubic BaSnO3. Finally, the...

Anhydrous crystals of DNA bases are wide gap semiconductors.

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.

Electronic and optical properties of CaCO3 calcite, and excitons in Si@CaCO3 and CaCO3@SiO2 core-shell quantum dots

Density functional theory calculations of the electronic and optical properties of the CaCO3 calcite polymorph were performed within both the local density (LDA) and generalized gradient (GGA) approximations, respectively. The carriers effective masses are estimated, and the energy gap is shown to be indirect, with and (for comparison, the experimental value is 6.0 ? 0.35?eV). Two optical absorption regimes are predicted, and the dielectric function does not change considerably with the light polarization. The confinement features of excitons in Si@CaCO3 and CaCO3@SiO2 spherical core-shell quantum dots are also presented.

Nonlinear transport properties of III-nitrides in electric field

We consider the transport properties of polar direct-gap semiconductors in an electric field, specializing the numerical calculation of the general theory to the case of n-doped III-nitrides, in particular, GaN, AlN, and InN. The nonequilibrium thermodynamic state of these materials—characterized by the variables so-called quasitemperature, quasichemical potential, and drift velocity of the carriers, and the quasitemperatures of longitudinal optical and acoustical phonons—is studied. The evolution equations of these variables—which are highly nonlinear—are derived, and the transient regime and the ensuing steady state are analyzed. The nonlinear transport is characterized and its main properties are discussed. In one case comparison with a recent Monte Carlo calculation is made and good agreement is obtained. In this paper we mainly consider the ultrafast transient, and in the following paper the steady state.