Jonder Morais

Lithium insertion and electrochromism in polycrystalline molybdenum oxide films

In this work, molybdenum oxide thin films were deposited by r.f. reactive sputtering of metallic molybdenum target in an Ar + O 2 atmosphere. Thin films with different compositions and crystal structures were obtained by varying the oxygen flow (o). All samples, with the exception of the MoO 2 film, showed an electrochromic behavior, due to the reversible Li + /e insertion or extraction process. The transmittance change (λ = 632.8 nm), for the best sample, was ∼ 70%. The film having the best optical behavior is mostly formed by the β-MoO 3 phase. The composition, optical and mechanical properties were determined and analyzed in as-grown and in Li intercalated films. The local order structure was analyzed by X-ray absorption spectroscopy measurements at the Mo L 2,3 -edges. The valence band was studied by Photoelectron Spectroscopy.

On the reactivity of carbon supported Pd nanoparticles during NO reduction: unraveling a metal-support redox interaction.

Pd nanoparticles (NPs) were successfully obtained by the reduction of PdCl2 with L-ascorbic acid, whose morphology was revealed by HRTEM to be a worm-like system, formed by linked crystallite clusters with an average short-axis diameter of 5.42 nm. In situ UV-vis absorption measurements were used to monitor their formation, while XPS and XRD characterization confirmed the NPs metallic state. A straightforward way to support the obtained Pd NPs on activated carbon (AC) was used to prepare a catalyst for NO decomposition reaction. The Pd/AC catalysts proved to be highly active in the temperature range of 323 to 673 K, and a redox mechanism is proposed, where the catalysts active sites are oxidized by NO and reduced by carbon, emitting CO2 and enhancing their capacity to absorb and dissociate NO.

Optical studies of InP/InAlAs/InP interface recombinations

Abstract We report on the photoluminescence characterization of a metalorganic vapour phase epitaxy grown InP/InAlAs/InP structure. The energy band alignment of the InAlAs/InP interface is of type II. The PL arising from the first grown interface (InAlAs grown on InP) is clearly seen at 1.2 eV. The localization was possible by scanning the laser beam on a angle-bevelled sample whose bevel crosses all the layers of the InP/InAlAs/InP structure. For the second interface, called the inverse interface (InP grown on InAlAs), a different PL behaviour is observed. The energy of the observed PL peak is 1.3 eV. It is very sensitive to the excitation power. From this behaviour, the Auger depth profiling measurements and the wedge transmission electron microscopy performed on this sample we conclude that this recombination does not originate from a type II interface band structure but from an InAs x P 1- x layer located at the inverse interface. This intermediate layer originates from the higher incorporation coefficient of As compared to that of P.

Pd–M/C (M = Pd, Cu, Pt) Electrocatalysts for Oxygen Reduction Reaction in Alkaline Medium: Correlating the Electronic Structure with Activity

The increasing global needs for clean and renewable energy have fostered the design of new and highly efficient materials for fuel cells applications. In this work, Pd-M (M = Pd, Cu, Pt) and Pt nanoparticles were prepared by a green synthesis method. The carbon-supported nanoparticles were evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium. A comprehensive electronic and structural characterization of these materials was achieved using X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Their electrochemical properties were investigated by cyclic voltammetry, while their activities for the ORR were characterized using steady-state polarization experiments. The results revealed that the bimetallic nanoparticles consist of highly crystalline nanoalloys with size around 5 nm, in which the charge transfer involving Pd and M atoms affects the activity of the electrocatalysts. Additionally, the samples with higher ORR activity are those whose d-band center is closer to the Fermi level.

Interacting Superparamagnetic Iron(II) Oxide Nanoparticles: Synthesis and Characterization in Ionic Liquids

Interacting superparamagnetic iron(II) oxide nanoparticles (NPs) with sizes of 5.3 ± 1.6 nm were prepared by simple decomposition of [Fe(COT)2] (COT = 1,3,5,7-cyclooctatetraene) with 5 bar of H2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI·NTf2) ionic liquid (IL). The static and dynamic magnetic characterization revealed a superparamagnetic behavior with weak dipolar interactions of these NPs. In situ structural studies by X-ray absorption spectroscopy demonstrated that they consist of nanostructured FeO. This approach is an appropriate method to prepare and stabilize nanostructured FeO particles, where the presence of an IL proved to be fundamental to suppress the aggregation and usual overoxidation of the FeO NPs.

Electronic structure of LixNiOy thin films

Abstract In this work, thin films of Li x NiO y were deposited by rf reactive magnetron sputtering, from a LiNiO 2 target. The composition of the films was analyzed by Rutherford backscattering spectroscopy. The electronic structure was analyzed by photoelectron spectroscopy, using either X-ray (Al Kα) or synchrotron light (120xa0eV). X-ray diffraction showed a clear presence of Li 2 CO 3 in the target material, after some deposition runs. The presence of superficial lithium carbonate was also evidenced, for all films.

AES sputter depth profiles applied to interface analysis of GaInAs/InP grown by atmospheric pressure MOCVD

Abstract Interfaces between InP and GaInAs layers, grown by atmospheric pressure MOCVD, have been studied using Auger electron spectroscopy and Ar+ sputtering. The abruptness of the interfaces in this kind of epitaxial growth depends on the microscopic morphology of the growing surface (roughness) and on the way in which the active gases are substituted when changing the composition of the layers. We have investigated the influence of interrupting the growth at hetero-interfaces on the quality of the interface. It is shown that the abruptness of the interface improved with longer growth interruptions.

Microanalysis of the surfaces of natural iron-based minerals by means of synchrotron radiation based experimental techniques

We investigated the surfaces of natural iron-based minerals that are magnetite, hematite, goethite, pyrite, pyrrhotite, chalcopyrite, bornite and vivianite, using synchrotron radiation based techniques. Most iron chalkogenides are very suitable for photoemission microscopy studies due to their low resistivity, which prevents from surface charging. The local compositions were studied employing photoemission microscopy in combination with X-ray absorption spectroscopy. Imaging of the sample in the near-edge region of the absorption edges was used to visualise the spatial distributions of the chemical phases on the surface. Distributions of trace elements are imaged with high chemical and lateral resolution.

Ohmic contacts formation on n‐InP

We have investigated the correlation between contact resistance, heat treatment, and micro structure in ohmic contacts on n‐InP. The samples consisted of three different structures: 50 nm‐Ni/200 nm‐AuGe/n‐InP, Au/100 nm‐Pt/100 nm‐Ti/50 nm‐Ni/n‐InP, and 200 nm‐Au/100 nm‐Pt/100 nm‐Ti/50 nm‐Ni/200 nm‐AuGe/n‐InP. After annealing the samples, depth profiles obtained by Auger electron spectroscopy and ion sputtering showed a tendency of the Ge to migrate from the Au–Ge alloy towards the Ni layer, as well as an accumulation of Ni at the semiconductor interface. A Ni–P phase is identified as being responsible for the ohmic character of the metal/InP interface. Specific contact resistivities were measured for the three different types of metallizations. We observed that the resistivity is sensitive to the annealing temperature and related to the amount of Ni at the metal/InP interface. The use of overlayers is suggested to avoid migration of the semiconductor components towards the contact surface.

Depth resolution for AES sputter profiles of GaAs/GaInAs strained superlattices

Abstract The effect of varying the energy (250–2000 eV) and angle of incidence (50° or 80°) of Ar ions used to sputter etch a surface was observed on depth profiles of strained GaAs/Ga0.81In0.19As superlattices studied by Auger electron spectroscopy (AES). The superlattices were grown by atmospheric-pressure metal-organic chemical vapor deposition (MOCVD) with a nominal layer thickness of 50 A. We show that using ion energies of around 500 eV and grazing incidence minimizes the artifacts due to the interaction between the ion beam and the surface.