N. Mattoso

Ferromagnetism induced by oxygen and cerium vacancies above the percolation limit in CeO2

We studied the structural, chemical and magnetic properties of non-doped ceria (CeO(2)) thin films electrodeposited on silicon substrates. Experimental results confirm that the observed room temperature ferromagnetism is driven by both cerium and oxygen vacancies. We investigated ceria films presenting vacancy concentrations well above the percolation limit. Irradiation experiments with neon ions were employed to generate highly oxygen defective CeO(2-δ) structures. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectroscopy were used to estimate the concentration of Ce(3+) sites in the films, which can reach up to 50% of Ce(3+) replacing Ce(4+), compared to a stoichiometric CeO(2) structure. Despite the increment of structural disorder, we observe that the saturation magnetization continuously increases with Ce(3+) concentration. Our experiments demonstrate that the ferromagnetism observed in ceria thin films, highly disordered and oxygen-deficient, preserving the fluorite-type structure only in a nanometer scale, remains intrinsically stable at room temperature.

Structure, Composition, and Morphology of Electrodeposited Co x Fe1 − x Alloys

We have investigated the structure, composition, and morphology of Co x Fe 1-x alloys prepared on copper substrates under potentiostatic electrodeposition conditions currently used to produce composition modulated alloys. Co x Fe 1-x alloys with x = 1 to 0 have been obtained by simple control of the Co 2+ : Fe 2+ ratio in the bath solutions using the characteristic behavior of the codeposition of the metals. The alloy composition was determined by energy dispersive spectroscopy. X-ray analysis of the structure of deposits indicates texture with coexistence of phases. For higher iron concentrations the alloys are mostly body-centered-cubic with (110) preferred orientation. For higher cobalt concentrations the alloys become mostly hexagonal with (0001) preferred orientation. Scanning electron microscopy was used to analyze the deposit morphology. Except for alloy concentrations of about x = 0.7, where a nonuniform character of deposits with partial cobalt segregation has been observed, the Co-Fe alloys have a rather uniform composition and morphology in the cathodic potentials used for electrodeposition

Mechanical properties of layered InSe and GaSe single crystals

The mechanical properties of InSe and GaSe single crystals have been studied by means of nanoindentation tests. Both bulk crystals are well ordered and present a predominant γ-type interlayer stacking sequence as determined by x-ray diffraction and transmission electron microscopy measurements. The course of plastic deformation induced in the crystals by application of a definite shear stress through the penetration of a Berkovich tip indicates that the deformation occurs predominantly by pop-in events along easy slip directions having a fairly elastic character between displacements. Hardness anisotropy along crystal axes is clearly seen and the measured elastic modulus presents a discrepancy smaller than 5% in comparison with theoretical calculations performed using previous experimental values of the elastic constants.

Covalent grafting of phenylphosphonate groups onto layered silica derived from in situ-leached chrysotile fibers

The reaction of natural chrysotile fibers with phenylphosphonic acid leads to a new grafted material. The layered material, with an interplanar basal distance of 15.2 A, was characterized by powder X-ray diffraction, thermal analysis, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and energy dispersive X-ray analysis. The experimental data are consistent with the grafting of phenylphosphonate groups to the surface of the layered silica sheets, obtained by the in situ acidic leaching of brucite sheets, from chrysotile.

Structural and Morphological Characterization of the PP-0559 Kaolinite from the Brazilian Amazon Region

We report a structural and morphological characterization of the natural kaolinite PP-0559, from the Brazilian Amazon region, with the aim to contribute to the understanding of this clay mineral in environmental redox reactions. The triclinic structure was confirmed by transmission electron microscopy and electron diffraction studies. It exhibits oriented crystals, with diameters between 0.2 and 2 mm and about 50 nm of thickness. The anisotropic crystals after orientation were studied by electron paramagnetic resonance as a function of the applied magnetic field. The main EPR absorption lines of the substitutional Fe+3 impurity and radiation induced paramagnetic centers were studied as possible oxidizing centers of kaolinite.

Giant magnetoresistance in electrodeposited Co87Fe13/Cu compositionally modulated alloys

In this paper we report on the giant magnetoresistance effect and oscillatory exchange coupling in electrodeposited Co87Fe13/Cu compositionally modulated alloys. The alloys are compositionally modulated over nanometre length scales with ferromagnetic Co87Fe13 (Cu) alloy layers and non-magnetic Cu layers. Co87Fe13/Cu multilayered thin films were electrodeposited from a single sulfate plating solution containing all metallic ions of interest under computer-controlled potentiostat switching on (111)-textured Cu buffers previously evaporated on Si(111). Electrodeposits present a polycrystalline character with coexistence of face-centred cubic (fcc) and hexagonal close-packed (hcp) structures. Magnetoresistance effects as high as 8% were observed at room temperature. The dependence of the magnetoresistance effect on Cu spacer layer thickness shows an oscillation period around 1.0 nm.

Structure, Composition, and Morphology of Electrodeposited Co0.9Fe0.1 ( Cu ) Alloys

The successful use of the electrochemical process to produce compositional heterogeneous Co{sub 0.9}Fe{sub 0.1}(Cu) alloys is shown. Structure, composition, and morphology of the heterogeneous alloyed films, produced by the potentiostatic method, has been investigated by means of different techniques. Energy dispersive spectroscopy analyses reveal that the total amount of Co and Fe in the electrodeposits is strongly dependent on the Cu concentration in the plating solution at the potentiodynamic range investigated. The Co:Fe ratio does not change drastically due to Cu incorporation in the electrodeposits. It was also observed that by increasing the Cu content in the electrodeposits a granular morphology is obtained. Transmission electron microscopy was used to characterize the micromorphology and the heterogeneous composition of the alloys. The polycrystalline character of electrodeposits with a predominant face-centered cubic structure was characterized by using selected area electron diffraction and x-ray diffraction measurements.

Electrodeposition of Nanocrystalline CeO2 on Si(001)

We report the cathodic electrodeposition of cerium oxide thin films from cerium-chloride aqueous solutions on p-type Si(001) substrates under illumination. Transmission electron microscopy coupled with electron diffraction analyses show that the deposits exhibited a nanocrystalline structure with a significant amorphous fraction when a hydrogen peroxide additive is used. X-ray photoelectron spectroscopy analyses were used to evaluate the surface stoichiometry, revealing a significant reduction of Ce(III) replacing Ce(IV) ionic sites for deposits prepared from solutions containing hydrogen peroxide as an additive. Adherent, uniform, and transparent CeO 2 films with controllable stroichiometry can be deposited on Si, which is what renders these deposits versatile candidates for technological applications.

Electrodeposition of ZnO-Fe Granular Films

ZnO-Fe granular films have been cathodically deposited from aqueous sulfate solutions on Si(001) substrates. Electron microscopy analyses reveal a network of metallic nanometer-size Fe particles dispersed in a polycrystalline ZnO matrix without evidence of intermediate compound formation as Zn 1 - x Fe x O or iron oxides.

The epitaxial growth of evaporated Cu/CaF2 bilayers on Si(111)

Successful and unexpected epitaxial growth of Cu/CaF2 bilayers on hydrogen terminated Si(111) wafers by thermal evaporation is reported. The bilayers were characterized with conventional x‐ray diffraction experiments, grazing angle incidence x‐ray diffraction experiments, rocking curves, and χ scans. The growth mode of Cu films on CaF2 epitaxially grown on Si(111) is completely different from that of the Cu film grown directly on Si(111).