No abstract

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.

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 create delocalized, spin polarized states that should be able to warrant longrange magnetic interactions.

We have investigated, using ab initio total energy density-functional calculations, the electronic, structural, and magnetic properties of Mn doped InP nanowires. The nanowires have been constructed along the ͓111͔ direction and the dangling bonds on the surface have been saturated by hydrogen atoms. We found that the most energetically favorable position for the Mn atom is near the surface. When the Mn atoms are in "bulklike" positions, the magnetic Mn-Mn coupling in the nanowire is ferromagnetic. In these cases, our results are consistent with a host-p-Mn-d coupling mechanism. On the other hand, if at least one of the Mn atoms is located near the surface, the coupling is antiferromagnetic.

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.

We have examined the properties of CdS-ZnS and ZnS-CdS core-shell nanocrystals over a range of shell thicknesses using a real-space pseudopotential-density functional theory approach.The effect of structural relaxation was shown to be important as it leads to significant changes in the band gap and frontier orbital localizations. It was also predicted that strains at the core-shell interface are only affected by addition of the first few shell-layers, with subsequent layers producing small changes in the strain configuration. This strain saturation gives rise to a "thin" shell regime where both confinement and strain effects contribute to the evolution of the band gap, and a "thick" shell regime where band gap variations from bulk values are strongly dependent on confinement effects but approximately constant with respect to strain.

Ca 2+-overload contributes to the oxidation of mitochondrial membrane lipids and associated events such as the permeability transition pore (MPTP) opening. Numerous experimental studies about the Ca 2+ /cardiolipin (CL) interaction are reported in the literature, but there are few studies in conjunction with theoretical approaches based on ab initio calculations. In the present study, the lipid fraction of the inner mitochondrial membrane was modeled as POPC/CL large unilamellar vesicles (LUVs). POPC/CL and, comparatively, POPC, and CL LUVs were challenged by singlet molecular oxygen using the anionic porphyrin TPPS4 as a photosensitizer and by free radicals produced by Fe 2+-citrate. Calcium ion favored both types of lipid oxidation in a lipid composition-dependent manner. In membranes containing predominantly or exclusively POPC, Ca 2+ increased the oxidation at later reaction times while the oxidation of CL membranes was exacerbated at the early times of reaction. Considering that Ca 2+ interaction affects the lipid structure and packing, density functional theory (DFT) calculations were applied to the Ca 2+ association with totally and partially protonated and deprotonated CL, in the presence of water. The interaction of totally and partially protonated CL head groups with Ca 2+ decreased the intramolecular P-P distance and increased the hydrophobic volume of the acyl chains. Consistently with the theoretically predicted effect of Ca 2+ on CL, in the absence of pro-oxidants, giant unilamellar vesicles (GUVs) challenged by Ca 2+ formed buds and many internal vesicles. Therefore, Ca 2+ induces changes in CL packing and increases the susceptibility of CL to the oxidation promoted by free radicals and excited species.

ZnO nanocrystals are studied using theoretical calculations based on the density functional theory. The two main effects related to the reduced size of the nanocrystals are investigated: quantum confinement and a large surface:volume ratio. The effects of quantum confinement are studied by saturating the surface dangling bonds of the nanocrystals with hypothetical H atoms. To understand the effects of the surfaces of the nanocrystals, all saturation is removed and the system is relaxed to its minimum energy position. Several different surface motifs are reported, which should be observed experimentally. Spin-polarized calculations are performed in the nonsaturated nanocrystals, leading to different magnetic moments. We propose that this magnetic moment can be responsible for the intrinsic magnetism observed in ZnO nanostructures.