Angel R. Landa-Canovas

In this work, we have studied structural and magnetic properties of LaFeO₃ as a function of the particle size d, from bulk (d >> 1 µm) to nanoscale (d ≈ 30 nm). A large number of twins were observed for large particles that disappear for small particle sizes. This could be related to the softening of the FeO₆ distortion as particle size decreases. It was observed that the bulk sample showed spin canting that disappeared for d ~ 125 nm and can be associated with the smoothening of the orthorhombic distortion. On the other hand, for d < 60 nm, the surface/volume ratio became high and, despite the high crystallinity of the nanoparticle, a notable exchange effect bias appeared, originated by two magnetic interactions: spin glass and antiferromagnetism. This exchange bias interaction was originated by the formation of a “magnetic core–shell”: the broken bonds at the surface atoms give place to a spin glass behavior, whereas the inner atoms maintain the antiferromagnetic G-type order. The LaFeO₃ bulk material was synthesized by the ceramic method, whereas the LaFeO₃ nanoparticles were synthesized by the sol-gel method; the particle size was varied by annealing the samples at different temperatures. The physical properties of the materials have been investigated by XRD, HRTEM, TGA, and AC and DC magnetometry.

Nanocomposite films consisting of gold nanoparticles embedded in an yttria-stabilized zirconia matrix (Au–YSZ) have been synthesized with different gold loadings by reactive magnetron sputtering followed by ex situ annealing in air or laser interference patterning (LIP) treatment. It is shown that the electrical conductivity of the nanocomposite films can be modified to a large extent by changing the gold loading, by thermal annealing, or by LIP. The structural and microstructural analyses evidenced the segregation of metallic gold in crystalline form for all synthesis conditions and treatments applied. Thermal annealing above 400 °C is observed to trigger the growth of pre-existing nanoparticles in the volume of the films. Moreover, pronounced segregation of gold to the film surface is observed for Au/(Au + Zr + Y) ratios above 0.40, which may prevent the use of thermal annealing to functionalize gold-rich Au–YSZ coatings. In contrast, significant modifications of the microstructure were detected within the interference spot (spot size close to 2 × 2 mm) of LIP treatments only for the regions corresponding to constructive interference. As a consequence, besides its already demonstrated ability to modify the friction behavior of Au–YSZ films, the LIP treatment enables local tailoring of their electrical resistivity. The combination of these characteristics can be of great interest for sliding electrical contacts.

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

SbVO₄ series of compounds could be used as oxidation catalysts of industrial interest yielding 20% cheaper acrylonitrile by the ammoxidation of propane compared to current method producing 8×10⁶ ton/year based on propylene as starting reactant.

ZnO is a promising material for use in solar cell applications which require antireflective coatings and transparent conducting materials in front contacts. Its resistivity can be reduced by appropriate doping with different group III elements, which act as donors, without sacrificing optical transmission. Besides, it constitutes a non-toxic and cheap alternative to ITO (In₂O₃:SnO₂), which is nowadays commonly used as transparent conducting oxide (TCO) in optoelectronic devices. Al(III) is one of the cheapest elements on nature, and has been therefore one of the first candidates to be used as a dopant. However recent studies demonstrate that for similar amounts of dopant the use of Ga largely improves both the optical and electrical properties of the films. The interface between Si (used as substrate) and the corresponding TCO is a critical part of Si-based solar cells, since the conversion efficiency may be profoundly altered by the presence of recombination centers formed at the interface. In this work we present a comparison of Si/ZnO, Si/ZnO:Ga and Si/ZnO:Al interfaces (with [dopant]/[Zn] = 2%) by using HRTEM and associated spectroscopies. Resistivity values of the films are: ρ(ZnO) = 4.4 × 10^-2 Ω × cm; ρ(Ga:ZnO) = 6.3 × 10^-4 Ω × cm; ρ(Al:ZnO) = 2.9 × 10^-3 Ω × cm. and the films optical transmitance is larger than 80% along the visible spectrum.

δ-Bi2O3, a material with a fluorite-type structure, is one of the best solid-state oxygen-ion conductors. It is a high-temperature form that cannot be quenched to room temperature. However, doping with small amounts of transition metal oxides preserves the δ-Bi2O3 structure at low temperature and retains its anionic conduction properties. The Bi2O3–MoO3 materials are interesting because of their functional properties, chiefly as catalysts and as good ionic conductors. All the phases in this system are related to the fluorite structure except Bi2MoO6 which shows an Aurivillius-type structure.