J. T. Arantes

Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO

An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

Surface magnetization in non-doped ZnO nanostructures

We have investigated the magnetic properties of non-doped ZnO nanostructures by using ab initio total energy calculations. Contrary to many proposals that ferromagnetism in non-doped semiconductors should be induced by intrinsic point defects, we show that ferromagnetism in nanostructured materials should be mediated by extended defects such as surfaces and grain boundaries. This kind of defects creates delocalized, spin-polarized states that should be able to warrant long-range magnetic interactions.

Theoretical investigations of Ge nanowires grown along the [110] and [111] directions

An ab initio study of Ge nanowires (GeNWs) oriented along [110] and [111] directions is performed for H-passivated and non-passivated wires with diameters up to 3.5 nm. GeNWs grown in the [110] direction present a direct energy gap and in the [111] direction the energy gap is indirect, even for small diameters. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gap scales with diameter, d, as and d−1.1 for the [110] and [111] growth directions, respectively. Consequently for the same diameter GeNWs grown in the [110] direction present a smaller gap than nanowires grown in the [111] direction.

Experimental study of absorption and luminescence properties of Er3+ in lead silicate glass

Abstract The optical absorption and temperature dependence of photoluminescence (PL) in the temperature range of 10 to 300 K have been measured for Er3+-doped lead silicate glass. A strong PL band that originates from 4S13/2→4I15/2 transition centered at 562 nm was observed even at room temperature. A very weak temperature quenching compared with that of Er3+ in crystal Si and the peak wavelength remaining constant with temperature are the main characteristics of this PL emission band. They are explained by the competition of thermal population effect and multiphonon relaxation between the 4S3/2 and 2H11/2 levels of Er3+.

Effects of side-chain and electron exchange correlation on the band structure of perylene diimide liquid crystals: a density functional study.

The structural and electronic properties of perylene diimide liquid crystal PPEEB are studied using ab initio methods based on the density functional theory (DFT). Using available experimental crystallographic data as a guide, we propose a detailed structural model for the packing of solid PPEEB. We find that due to the localized nature of the band edge wave function, theoretical approaches beyond the standard method, such as hybrid functional (PBE0), are required to correctly characterize the band structure of this material. Moreover, unlike previous assumptions, we observe the formation of hydrogen bonds between the side chains of different molecules, which leads to a dispersion of the energy levels. This result indicates that the side chains of the molecular crystal not only are responsible for its structural conformation but also can be used for tuning the electronic and optical properties of these materials.

First principles calculations of as impurities in the presence of a 90º partial dislocation in Si

We investigated the interaction of As impurities with a 90o partial dislocation in Si. The calculations show that As atoms segregate to the dislocation core. The most stable site for the As atom is at the stacking fault side, which is favorable by 0.26 eV as compared to an As atom in a crystalline position. There is no charge transfer from the As impurity to the dislocation core when the impurity is far from it. The segregation of the impurity can be understood mainly due to structural effects. This result let us conclude that the experimentally observed negatively charged dislocations are due to impurities trapping at the core of the dislocation and not charge transfer from impurities away from the dislocation.

Interaction and Reaction of the Antioxidant Mn III [Meso-Tetrakis(4-NMethyI Pyridinium) Porphyrin] with the Apoptosis Reporter Lipid Phosphatidylserine

In the present study, the binding of the cationic MnTMPyP [meso-tetrakis (4-N-methyl pyridinium) porphyrin] to negatively charged membrane models containing phosphatidylcholine (PC)/ phosphatidylserine (PS) and the porphyrin reactivity with PS-derived peroxides (LOOH) were evaluated. This investigation is relevant due to the participation of PS in cell signaling and apoptosis. Addition of PC/PS liposomes to MnTMPyP solutions led to spectral changes of the porphyrin electronic absorption spectrum suggestive of electrostatic interactions with the negatively charged interface provided by PS. The binding of MnTMPyP to PC/PS liposomes was corroborated by the quenching of the fluorophore merocyanine 540. In addition total energy calculations of saturated and unsaturated PS based on the spinpolarized variant of KS-DFT were used to support some experimental data. Cyclic voltammetry analysis revealed that the association to PC/PS liposomes shifted the redox potential of the Mn/Mn (+87 mV vs NHE, in aqueous buffered solution) couple to a more positive value to (+110 mV vs NHE). This event favors the oxidation of O2 -• to O2 by the porphyrin. MnTMPyP was able to react with the lipid-derived peroxides as evidenced by spectral changes of the porphyrin consistent with the generation of a high valence state (Mn=O) of the catalyst. The spectral changes were not observed when biological lipids containing unsaturated PC and PS were replaced by the corresponding dipalmitoyl (DP)-

Dynamic covalent bond from first principles: Diarylbibenzofuranone structural, electronic, and oxidation studies

A structure that can self‐heal under standard conditions is a challenge faced nowadays and is one of the most promising areas in smart materials science. This can be achieved by dynamic bonds, of which diarylbibenzofuranone (DABBF) dynamic covalent bond is an appealing solution. In this report, we studied the DABBF bond formation against arylbenzofuranone (ABF) and O2 reaction (autoxidation). Our results show that the barrierless DABBF bond formation is preferred over autoxidation due to the charge transfer process that results in the weakly bonded superoxide. We calculated the electronic and structural properties using total energy density functional theory.