Eronides Felisberto da Silva

A novel fluorinated Eu(III) β-diketone complex as thin film for optical device applications

Abstract We discuss the synthesis and spectroscopic characteristics of a thin film (∼30–90 nm) based on lanthanide europium (III) complexes as the emitter layers, to shift the UV portion of light spectrum into the visible region. The complex presents high quantum efficiency (∼65%), is highly volatile and thermodynamically stable. In addition, the thin film is used as an alternative antireflection coating on a silicon solar cell, allowing for an improvement of about 21% on cell efficiency. The high absorption and luminescence properties in the UV–visible region and its compatibility with device fabrication processes make this material of great potential for use in advanced optical device technologies.

Potential of a simplified measurement scheme and device structure for a low cost label-free point-of-care capacitive biosensor

A simplified measurement scheme and device structure aiming at developing a low cost, label-free, point-of-care capacitive biosensor were investigated. The detection principle is the increase of low frequency capacitance between two planar Al electrodes observed after antibody-antigen interaction. The electrodes, deposited on oxidized Si wafers, were covered with an antibody layer, with and without using self-assembled thiol monolayer. Immunoglobulin G (IgG) and cardiac troponin T (TnT) were used as analytes to asses this proposal. The device was able to detect successfully TnT levels in the range 0.07 to 6.83ng/mL in human serum from patients with cardiac diseases and in the range 0.01ng/mL to 5ng/mL for TnT in phosphate buffer saline. An equivalent circuit model able to reproduce the general behavior of experimental capacitance versus frequency curves was presented. The investigated features that have potential to reduce costs and simplify measurements were: use of single, low frequency (1kHz) measurement signal, within the range of low cost portable capacitance meters; employment of a lower cost electrode material, aluminum, instead of gold electrodes; and use of simple and miniaturized planar two-electrodes arrangement, thus making a portable system for point-of-care applications.

Molecular UV dosimeters of lanthanide complex thin films: AFM as a function of ultraviolet exposure

Abstract In a previous work we developed a molecular dosimeter of high sensitivity and selectivity to UV cumulative measurements, using as the active part a thin film produced by thermal evaporation of europium complexes. The Eu 3+ emission ( 5 D 0 → 7 F 2 ) is monitored as a function of UV dose, and a memory effect allows dosimetry measurements. All measurements were quantified by an integration sphere coupled to a radiometer that provides absolute dose measurements in J/cm 2 and may be related to the MED (minimum erythemal dose) of 28 mJ/cm 2 as a reference. In this work, the preliminary results about the photodegradation mechanism of the complexes at molecular level is presented through AFM images. We have observed that the organic ligands, which absorb the UV radiation and transfer to the ion emitters, may be photofragmenting and therefore interfering in the luminescence emission of these ions. This ligand photofragmentation process justifies the selectivity presented by the dosimeter. On the other hand, the non-dependence of the film morphology as a function of UV radiation exposure indicates a different behavior from that which we had suspected before, indicating a non-ablative process, which is the contrary to what was previously suggested.

Simulation of the early stages of thin film growth

To gain a better understanding of the silicon oxidation process, we perform numerical simulation of thermal thin-film growth. It is shown that the oxidation rate in the early stages of growth is governed by two processes: the rapid initial formation of the oxidation front and its subsequent diffusion. The resulting oxidation rate provides a rather good description of the experimental data with the minimum number of variable parameters, suggesting that the effect of external parameters (such as temperature and pressure) can be explained in terms of scaling concepts. The results of the simulation are also in agreement with the fitting of experimental data to a power law (where is the measured film thickness and t the oxidation time) predicted by a simple model for thin film growth.

Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(VO)0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μB/cell, for Fe and Co. Two metastable states, with 0 and 4 μB/cell were found for Fe, and a single value, 3 μB/cell, for Co. The spin-crossover energies (ES) were calculated. The values are ES0/2 = 107 meV and ES4/2 = 25 meV for Fe. For Co, ES3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and ES change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 μB/cell instead of 2 μB/cell, and the energy ES2/4 is 30 meV. For cobalt, the ground state is then found with 3 μB/cell and the metastable state with 1 μB/cell. The spin-crossover energy ES1/3 is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.

Correlation between dopant reduction and interfacial defects in low-energy x-ray-irradiated MOS capacitors

We discuss effects associated with dopant reduction and interface defects generated in p-type metal-oxide semiconductor (MOS) capacitors exposed to doses of low-energy x-ray radiation up to 500 Mrad (Si). The dependence on dose, size, oxide charge and interface trapped charge density suggests that the dopant-reduction behaviour is influenced by the oxide and interface charges and their behaviour during and after irradiation. In particular, the interfacial defect generation dynamics suggests that mechanisms associated with tunnelling of carriers at the interface may be involved.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices.

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)nsuperlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)nSLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)nSLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Spin-polarization effects in homogeneous and non-homogeneous diluted magnetic semiconductor heterostructures

Spin polarization is a key characteristic in developing spintronic devices. Diluted magnetic heterostructures (DMH), where subsequent layers of conventional and diluted magnetic semiconductors (DMS) are alternate, are one of the possible ways to obtain it. Si being the basis of modern electronics, Si or other group-IV DMH can be used to build spintronic devices directly integrated with conventional ones. In this work we study the physical properties and the spin-polarization effects of p-type DMH based in group-IV semiconductors (Si, Ge, SiGe, and SiC), by performing self-consistent [Formula: see text] calculations in the local spin density approximation. We show that high spin polarization can be maintained in these structures below certain values of the carrier concentrations. Full spin polarization is attained in the low carrier concentration regime for carrier concentrations in the DMS layer up to approximately 2.0 x 10(19) cm(-3) for Si and up to approximately 6.0 x 10(19) cm(-3) for SiC. Partial, but still important spin polarization can be achieved for all studied group-IV DMH, with the exception of Ge for carrier concentrations up to 6.0 x 10(19) cm(-3). The role played by the effective masses and the energy splitting of the spin-orbit split-off hole bands is also discussed throughout the paper.

Strong exciton energy blue shift in annealed Si/SiO2 single quantum wells

Abstract A theoretical study is presented on how the annealing-induced interfacial transition region changes the confined ground state exciton in Si/SiO2 single quantum wells (QWs). The interface thickness and the mean well width confinement depend on the time and temperature of annealing, as well as on the diffusion coefficient of oxygen in silicon. It is shown that an annealing-related interface width increase of a few Angstroms can strongly blue shift (hundreds of milli-electron volts) the confined ground state exciton energy in Si/SiO2 single quantum wells. The results allow to suggest that annealing processes can be used to tune (from red to blue) the light emission in Si/SiO2 single quantum wells.

Lattice dynamics of Ga1−xMnxN and Ga1−xMnxAs by first-principle calculations

In this work, we present theoretical results, using first-principle methods associated to the virtual crystal approximation model, for the vibrational mode frequencies of both the Ga1−xMnxN (in both cubic and hexagonal structures) and the Ga1−xMnxAs alloys, with the Mn contents in the range of 0% to 20%. The dependence of the calculated phonon frequencies with the Mn content was analyzed, and the results indicate that the phonon frequencies decrease with the increasing of Mn composition, leading to the false impression that they obey the Vegard rule in some cases. Moreover, the hexagonal Ga1−xMnxN alloys are elastically unstable for Mn concentrations at the order of 20%, which explains in part the experimentally observed deterioration of these alloys. These findings can be used in future technologies as a guide for the synthesis of spintronic nanostructured devices, such as nanowires, based on these materials.