J. A. K. Freire

Polycyclic aromatic hydrocarbons from asphalt binder: extraction and characterization

The asphalt binder, derived from petroleum, commonly employed in road paving contains PAHs (Polycyclic Aromatic Hydrocarbons) and a variety of other aliphatic and aromatic compounds. PAHs are pollutants that can induce health problems. This work aims to extract and characterize the PAHs in samples of asphalt binder and its concentration. Matrix solid phase dispersion was used as a technique for extraction of the PAHs. Characterization was performed employing gas chromatography and scanning tunneling microscopy. Fifteen of the priority PAHs listed by USEPA (United States Environmental Protection Agency), as well as coronene, were found in the binder samples.

5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol on gold: SAM Formation and electroactivity

5-(4-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) spontaneously adsorbs on gold forming SAMs (self-assembled monolayers) that, based on STM (Scanning Tunneling Microscopy) and electrochemical data, contain pinholes through which [Fe(CN)6]4- and [Ru(NH3)6]3+ probe molecules access the underlying gold electrode. For the former molecule, the dependence of the faradaic current on the electrolyte solution pH value allowed the evaluation of the surface pKa as 4.2. The thermodynamic parameters DHads and DGads for the Hpyt adsorption process could be described by the Langmuir model and were calculated as -20.01 and -39.39 kJ mol-1, respectively. Electrodic redox reaction of cytochrome c metalloprotein was accessed by using the Hpyt SAM with a heterogeneous electron transfer rate constant of 2.29 × 10-3 cm s-1.

The role of surface roughness on the electron confinement in semiconductor quantum rings

The electron energy spectrum of GaAs/Al0.30Ga0.70As quantum rings under applied magnetic fields is calculated, taking into account the existence of rough interfaces between materials. The Schrodinger equation, within the effective mass approximation, is solved in a realistic model, not limited to small perturbations. Our numerical results show that the existence of roughness on the ring surface modifies significantly the electron confinement energy, lifts the degeneracy of the electron angular momentum transition points of the Aharonov-Bohm oscillations, and in some special cases, it can even suppress the ground state energy oscillations.

Interface-related effects on confined excitons in GaAs/AlxGa1−xAs single quantum wells

Confined excitons in non-abrupt GaAs/Al x Ga 1-x As single quantum wells are studied. The graded interfaces are described taking into account fluctuations in their thickness a and positioning α with respect to the abrupt interface picture. Numerical results for confined (0, 0), (1, 1) and (0, 2) excitons in GaAs/Al 0.3 Ga 0.7 As quantum wells show that while the interfacial fluctuations produce small changes (<0.5 meV) in the exciton binding energies, the confined exciton energies can be red- or blue-shifted as much as 25 meV for wells with mean width of 50 A and 2 ML wide interfaces.

On the interplay between quantum confinement and dielectric mismatch in high-k based quantum wells

In this work we investigate the confinement properties of quantum systems with high dielectric mismatch between barriers and quantum well layers, focusing on the role of image charges. We demonstrate that the combination of high dielectric mismatch, band offset, and carrier’s effective mass strongly modifies the electronic and optical properties of those systems in such a way that the ideal confinement model is no longer suitable.

Large image potential effects in Si∕SrTiO3 and Si∕HfO2 two-dimensional quantum well structures

Charge image effects on the confinement properties of Si∕SrTiO3 and Si∕HfO2 two-dimensional quantum wells are studied. The combination of strong dielectric mismatch and band offset of the layers gives rise to structured confinement potentials, which can trap carriers close to the interfaces in Si∕SrTiO3 but not in Si∕HfO2 two-dimensional quantum wells. The charge image blueshifts strongly (a few hundred meV) the carrier recombination energy, comparable to the shift related to the well width shortening due to actual graded interfaces.

Statistical analysis of topographic images of nanoporous silicon and model surfaces

Abstract We investigated the quantitative description of nanoporous silicon morphology and its correlation with electrical and optical properties. We performed first-order as well as second-order statistical analysis of AFM images of nanoporous silicon fabricated by two different methods: reaction-induced-vapor-phase-stain-etch and electrochemical. We also simulated AFM images by generation of various model surfaces. From the height–height correlation plots, we were able to observe the effects of the growth method on RMS roughness parameter and verify that photoluminescence can be observed despite very small changes in silicon surface morphology. The analysis of the model surfaces showed that it is possible to reproduce the self-affine character of the porous surfaces by means of a linear combination of sine surfaces.

Optical properties of ellipsoidal CdSe quantum dots

The energy spectrum and optical absorption of confined carriers in ellipsoidal CdSe quantum dots are calculated. Our model relies on the effective mass approximation and Fermis golden rule for the electronic structure and optical properties, respectively. We demonstrate that the electronic structure and the interband optical absorption are highly dependent on the ellipsoid aspect ratio.

Energy levels in Si and SrTiO3-based quantum wells with charge image effects

In the present work we develop a theoretical study to analyze how the image charges effects can modify the electronic properties in Si and SrTiO3-based quantum wells. We have used the method based on the calculation of the image charge potential by solving Poisson equation in cylindrical coordinates. The numerical results show that the electron-heavy hole recombination energy can be shifted by more than 200 meV due to the combination of charge image and SiO2 (SrTiO3) interface thickness effects.

Dielectric mismatch effects on the electronic and optical properties of GaN∕HfO2 quantum wells

In this work, we demonstrate that the image charges effect induced by the dielectric mismatch strongly modifies the electronic structure of GaN∕HfO2 quantum wells (QWs) in such a way that the ideal QW confinement model is no longer suitable for the description of these systems. Particularly, two different confinement regimes were observed for narrow and wide QWs. In the former, electrons, light and heavy holes are spatially localized in the same region. In wide QWs, heavy holes are confined in the interfacial regions due to the strong attraction of the image charges, which does not occur for electrons and light holes. As a consequence, optical transitions involving electrons and heavy holes become less efficient in wide QWs.