Marcelo Barbalho Pereira

Silver migration at the surface of ion-exchange waveguides: a plasmonic template

The formation and evolution of metallic-silver nanoparticles capped with silver oxide, in the surface of Ag-doped waveguides produced by ion-exchange, were characterized. The samples were exposed to air atmosphere for periods lasting until 35 days and their aging process was investigated by optical and Atomic Force Microscopy (AFM) measurements. The results evidence migration of the Ag+ cations from inside the glass to the surface at room temperature, followed by aggregation of the silver nanoparticles (NPs) and oxidation, creating a nanometric-thick layer over the waveguide surface. This layer was employed for surface-enhanced Raman scattering (SERS) signal and for the fabrication of holographic diffraction gratings (HDG), which are presented as application examples of this material as a new plasmonic template.

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.

Simple modeling of plasmon resonances in Ag/SiO2 nanocomposite monolayers.

Normal incidence transmittance and reflectance spectra of sputtered nanocomposite monolayer films of Ag in SiO2, buried and unburied, showed significant redshifted plasmon resonances from 410 to 455 nm, which could be well interpreted with a simple model that starts from the Maxwell Garnett theory and the Kreibig extension of the Drude-Lorentz equation, but with a further extension related to the dipolar interaction between the metal particles distributed on a surface.

Refractive index control in bicomponent polymer films for integrated thermo-optical applications

Optical properties of transparent polymer thin films, produced by spin-coating on silicon and constituted of polycarbonate (PC), poly(methyl methacrylate) (PMMA), and PC/PMMA, were investigated with regard to integrated thermo-optical (TO) device applications. Refractive index dependences on wavelength, temperature, and film composition were measured by spectroscopic ellipsometry with a dedicated autocontrolled heater setup, in the ranges of 400 to 800 nm, 25 to 85 °C and 0 to 100 wt % PC, respectively, with determination of Cauchy and Lorentz-Lorenz parameters. Within these intervals, thermomechanical compatibility and pronounced index contrast of around 0.12 between PC and PMMA, as well as their TO coefficients one order of magnitude higher than that of silica, allow convenient tailoring for specific TO requirements. In addition, wide-range fine-tuning of refractive index variation is found to be facilitated by the weak dependence of isothermal dispersion curves and TO coefficients on film composition.

Glass window coatings for sunlight heat reflection and co-utilization.

Buildings that simultaneously provide natural illumination and thermal comfort for all seasons have met with increasing demand as conventional resource limitations are realized. In this context, organic and metal-dielectric coatings are tested, and a simple, coated double-glazed window with solar blinds is conceived that includes passive infrared (IR) reflection, active illumination control, and integration to the building envelope. As a result, a proper spectrally selective coating is applied to produce a low-emissivity solar window with climate-adaptive co-utilization of the reflected IR.

Silver Nanoparticle Thin Films Deposited on Glass Surface Using an Ionic Silsesquioxane as Stabilizer and as Crosslinking Agent

Thin films containing silver nanoparticles homogeneously dispersed, with narrow size distribution below 10 nm, were synthesized on flat glass surface, by using an ionic silsesquioxane as stabilizer and crosslinking agent. The films can be prepared without previous functionalization of substrate surfaces and without addition of other components. The films were heat treated up to 200 oC and characterized by ultraviolet-visible, transmission electron microscopy, atomic force microscopy, thermogravimetric analysis and ellipsometry. The films were thermally stable when heated up to 200 oC, presenting the same thickness, and maintaining both optical and morphological properties of silver nanoparticles. The antibacterial activity of the films, containing the silver nanoparticles, was evaluated against Staphylococcus aureus by using the film applicator coating method, showing an excellent performance even after the third cycle of sterilization.

Photosensitised Degradation of Organic Dyes by Visible Light Using Riboflavin Adsorbed on the Surface of TiO2 Nanotubes

Visible light-assisted degradation of indigo carmine (IC), methylene blue (MB) and methyl orange (MO) aqueous solutions has been achieved on the surface of TiO2 nanotube (NT) arrays impregnated with riboflavin (RF). Diluted RF water solutions in the presence of RF-TiO2 NTs irradiated with UV light produced O2, CO and CO2 as main gas products. On the contrary, the same solutions irradiated with visible light evolved O2 as a main product. This in situ O2 generation under visible light absorption allows the degradation of the dyes without the necessity to bubble air or oxygen in the reaction system The photocatalytic degradation of MO, MB and IC can be described by a pseudo-first-order kinetic model obtaining ca. 100% degradation of MB, MO and IC in less than 3 h of visible light illumination. The results provided here are highly promising in view of various photocatalytic applications of the prepared RF-TiO2 NTs catalysts by two environmentally friendly compounds in the degradation of pollutants using solar radiation.

Schottky barrier height of Ni/TiO2/4H-SiC metal-insulator-semiconductor diodes

Ni/TiO2/4H-SiC diodes were analysed through measurements of current–voltage curves varying the temperature. The Schottky Barrier Height (SBH) which increased with temperature was studied by simulation of the Thermionic Emission Model, considering Ni/SiC Schottky structures with an insulator layer between the metal and semiconductor. This model shows that a new method of calculation should be applied to diodes that have a metal–insulator–semiconductor structure. Misleading results for SBH are obtained if the thin insulator layer is not considered. When applying the suggested method to the Ni/TiO2/4H-SiC diodes it was necessary to consider not only the deposited TiO2 layer, but also a second dielectric layer of native SiCxOy at the surface of SiC. By measuring I–V–T curves for two samples with different thicknesses of TiO2, the suggested method allows one to estimate the thicknesses of both dielectric layers: TiO2 and SiOxCy.

Effect of cage configuration in structural and optical properties of tin films grown by cathodic cage discharge

Cathodic cage discharge was developed recently in order to eliminate phenomena as edge effect and overheating, which occurs during conventional processes. In this study, the effect of cage configuration in active species during the deposition process and optical properties of TiN film were studied. TiN compound was chosen because its optical properties are very sensitive to slight variations in microstructure and film thickness, becoming a good monitoring tool in fabrication process control. Cages were made of titanium and have different holes numbers and holes diameter. Electrical efficiency of the system and optical properties of TiN films were strongly influenced by experimental conditions. It was found that with more holes at the top of cage, deposition rate and crystallinity were higher, if compared to cages with a small number of holes at the top. On the other hand, the opposite behavior was observed when more holes were located at the sidewall of cage.

Spectral polarimetry technique as a complementary tool to ellipsometry of dielectric films

Ellipsometry is a highly sensitive optical technique for coating characterization but usually presents multiple solutions in many cases. To prevent these, a method with addition of a spectral polarimetric technique is proposed. An initial film dispersion curve, independently of its physical thickness, is then provided using the same setup as spectral ellipsometry and at the same sample position, which later is used for thickness determination and dispersion refinement with increase of reliability of results. Characterization of thin TiO2 films with one and two ellipsometric solutions is shown to corroborate the validity of the proposed method.