Antonio Ferreira da Silva

Strong polaronic effects on rutile TiO2 electronic band edges

Thin TiO2 films are prepared by dc magnetron sputtering as well as by the sol-gel technique, and the optical band edge absorption α(ω) is obtained from transmission spectroscopy. The electronic structure and optical properties are studied employing a first-principle linearized augmented plane-wave method within the local density approximation (LDA), improved by an on-site Coulomb self-interaction potential (LDA+USIC). We show that the correction potential, the polaronic screening, and the spin-orbit interaction are crucial for determining the TiO2 effective electron and hole masses. The dielectric function e(ω)=e1(ω)+ie2(ω) and the high-frequency constante(0⪡ω⪡Eg∕ℏ) show pronounced anisotropy. The electron-optical phonon interaction affectse(ω≈0) strongly.

Effects of the large distribution of CdS quantum dot sizes on the charge transfer interactions into TiO2 nanotubes for photocatalytic hydrogen generation

Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum dots have revealed a hydrogen generation improvement when added to TiO2 materials under visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the capping agent. This sensitization technique permits high loading and uniform distribution of CdS quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration remains almost unchanged (homogeneous), while the concentration relative to the sulfate anion decreases by more than 80% with respect to the initial value after ∼100 nm in depth. The presence of sulfate anions is due to the oxidation of sulfide and occurs in greater proportion in the material surface. This protection for air oxidation inside the nanotubular matrix seemingly protected the CdS for photocorrosion in sacrificial solution leading to good stability properties proved by long duration, stable photocurrent measurements. The effect of the size and the distribution of sizes of CdS quantum dots attached to TiO2 nanotubes on the photocatalytic hydrogen generation were investigated. The experimental results showed three different behaviors when the reaction time of CdS synthesis was increased in the sensitized samples, i.e. similar, deactivation and activation effects on the hydrogen production with regard to TiO2 nanotubes. The deactivation effect was related to two populations of sizes of CdS, where the population with a shorter band gap acts as a trap for the electrons photogenerated by the population with a larger band gap. Electron transfer from CdS quantum dots to TiO2 semiconductor nanotubes was proven by the results of UPS measurements combined with optical band gap measurements. This property facilitates an improvement of the visible-light hydrogen evolution rate from zero, for TiO2 nanotubes, to approximately 0.3 μmol cm(-2) h(-1) for TiO2 nanotubes sensitized with CdS quantum dots.

A fast sonochemical method to prepare 1D and 3D nanostructures of bismuth sulfide

In this work, a sonochemical method to synthesize nanostructures of bismuth sulfide in 1D and 3D framework was developed and compared with a synthetic route with heating under reflux. The sonochemical method showed to be faster and more efficient than refluxing method to obtain nanostructures with high morphological homogeneity. Form and quality of the nanocrystals were dependent on the type of solvent employed in the synthesis procedure. 3D flower-like superstructures were obtained when ethylene glycol was used as solvent, while 1D nanorods were obtained when a mixture of dimethyl sulfoxide and ethylene glycol was used as solvent.

Bio-inspired gold microtubes based on the morphology of filamentous fungi

This communication describes a general method for templating fungal filaments with gold nanoparticles that results in a gold replica of filaments after calcination of the biological template.

Magnetoresistance of doped silicon

We have performed longitudinal magnetoresistance measurements on heavily n-doped silicon for donor concentrations exceeding the critical value for the metal-nonmetal transition. The results are com ...

A Spectroscopic Study of Au/CeXZr1 XO2 Catalysts

The nature of gold species in Au/CeXZr1-XO2 catalysts was successfully investigated using the visible and near infrared photoacoustic spectroscopy and Fourier transform infrared spectroscopy of adsorbed carbon monoxide. It was found that the visible and near infrared photoacoustic spectroscopy is a powerful technique for investigating the changes of gold particle size due to addition of zirconium and the oxidation state of gold particles. It was also noted that the increase of zirconium in the solids makes gold incorporation more difficult due to the electronic shielding caused by zirconium, decreasing the probability of replacing cerium by gold. For the samples studied, the particle size ranged from 20-30 nm and the Au+, Au3+ and Auo species were found in the catalysts.

Electronic Properties of Intrinsic and Heavily Doped 3C-, nH-SiC (n=2, 4, 6) and III-N (III=B, Al, Ga, In)

This chapter reviews the latest calculations of the electronic properties of intrinsic and heavily doped 3C-, n H-SiC (n = 2,4,6) and zb-, wz-III-N (III = B, Al, Ga, In). It presents the first-principles, fully relativistic full-potential linearized augmented plane wave (FPLAPW) calculation of the electronic band structure, the effective electron and hole masses, and the dielectric constants for intrinsic materials masses that is vital for the understanding of the electronic transport properties in these semiconductors. It focuses on the non-parabolicities of the energy bands near the band edges. The local density approximation (LDA) bandgap error can be corrected by a quasi-particle energy shift that has an effect on the high-frequency dielectric constant. This chapter calculates the static dielectric constant assuming constant optical phonon frequency dispersion. The anisotropy of the dielectric function is small in wz SiC and III-N. The effective electron and hole masses both as the bare effective masses and by including the polaron interaction has been presented in the chapter. It is crucial to take into account the spin-orbit interaction for determining the effective hole masses even in light semiconductor materials.

Engineering the electron g-factor anisotropy in double quantum wells

We investigate the quantum tunneling effects on the electron g factor and its main anisotropy in semiconductor double quantum wells (DQWs). With respect to single QWs, these structures introduce a new degree of freedom, given by the tunnel coupling parameter, and can be very helpful in the g-factor engineering. The eight-band effective-mass Hamiltonian for electrons in III-V double QWs and in the presence of an external magnetic field (applied both in the QW plane and along the growth direction) is solved for the g factor within first-order perturbation theory. We then calculate the g-factor anisotropy as a function of the QW width and the inter-well barrier length in typical AlGaAs/GaAs DQWs. The obtained results reproduce exactly the well-known single QW results in the corresponding limits, i.e. Lb=0 and very large, and explicitly show how the well-width dependence of the g-factor anisotropy changes with Lb, interpolating between these two limiting single QWs, with well width 2Lw and Lw respectively.

Ultraestrutura celular e expressão de proteínas de leveduras hanseniaspora sob efeito do estresse etanólico

O objetivo deste estudo foi avaliar a resposta de Hanseniaspora opuntiae (Ho41) e H. guilliermondii (Hg43) ao estresse etanolico, observando a ultraestrutura e o perfil de expressao proteica em concentracoes crescentes de etanol. A ultraestrutura foi analisada por microscopia eletronica de varredura (MEV) e a expressao proteica, pelo perfil eletroforetico (SDS-PAGE). Na analise microscopica, as cepas em meio Yeast Malt Agar sem etanol mostraram celulas jovens com morfologia apiculada, brotamento bilateral e polos distais concavos. Com o inicio do estresse, a 3% de etanol, as celulas apresentaram multiplas cicatrizes em forma de aneis e, com 6%, alteracoes na integridade da parede celular, plasmolise e ativacao da autolise. Na analise eletroforetica, observou-se, tanto para Ho41 quanto para Hg43, aumento na expressao de um peptideo de 100 kDa, com aumento do etanol no meio, indicando ser uma proteina de choque termico (HSP). As HSPs vem sendo patenteadas como marcadores de organismos de interesse biotecnologico, ja que as condicoes necessarias para obtencao de bioprodutos muitas vezes requerem cultivo sob estresse. Neste contexto, esta proteina pode ser indicada como marcador molecular para bioprospeccao ou melhoramento genetico de cepas nao-saccharomyces mais resistentes aos processos de fermentacao, na fabricacao de vinhos.