Rafael Álvarez

Study of CeO2 XPS spectra by factor analysis: reduction of CeO2

Abstract The evaluation of the reduction state of CeO2 by XPS is still controversial because of the presence of more than 10 peaks in the Ce(3d) photoemission spectra due to Ce3+ and Ce4+, some of them at similar binding energies. This makes difficult conventional fitting using nonlinear least-square (NLLSF) methods with Gaussian–Lorentzian curves that require the knowledge of the number of bands. In the present work, we have tried to overcome this problem using factor analysis (FA), a method that has been applied with success to other spectroscopic problems. In order to obtain confident results for the individual shapes of the components, (the basis for a correct quantification) we have combined target testing (TT) and iterative target transformation factor analysis (ITTFA). We have shown that the combined use of these two methods results in a more accurate evaluation (ca. ±2%) of the reduction state of partially reduced CeO2. The election of the “real factors” corresponding to Ce4+ and Ce3+ species is discussed in detail.

An Abel inversion method for radially resolved measurements in the axial injection torch

Abstract A method for the radial analysis of the plasma torch produced by the axial injection torch device at atmospheric pressure is proposed. The method uses a fast high-resolution acquisition set-up including an intensified charge coupled device and an image rotation system, as well as a new data processing procedure based on the Abel inversion technique that provides the radial profile of thin plasmas. Use of the Fernandez-Palop smoothing technique significantly reduces distortion in the data profile. The accuracy of the proposed method was assessed by applying it to the estimation of the populations of HeI excited states.

An XPS study of the Ar+-induced reduction of Ni2+ in NiO and Ni-Si oxide systems

The reduction effect on the Ni2+ species induced by 3.5 keV Ar+ bombardment of NiO particles dispersed on SiO2 and on NiSiO3 as well as on two NiO powder samples with different grain sizes has been quantitatively studied by XPS. In all the cases reduction of the Ni2+ species to metallic Ni° has been observed. However, it is also shown that Ar+-bombardment-induced reduction of Ni2+ is strongly enhanced by the presence of Si4+ cations, which on the other hand remain stable.

Tilt angle control of nanocolumns grown by glancing angle sputtering at variable argon pressures

We show that the tilt angle of nanostructures obtained by glancing angle sputtering is finely tuned by selecting the adequate argon pressure. At low pressures, a ballistic deposition regime dominates, yielding high directional atoms that form tilted nanocolumns. High pressures lead to a diffusive regime which gives rise to vertical columnar growth. Monte Carlo simulations reproduce the experimental results indicating that the loss of directionality of the sputtered particles in the gas phase, together with the self-shadowing mechanism at the surface, are the main processes responsible for the development of the columns.

Theoretical and experimental characterization of TiO2 thin films deposited at oblique angles

The microstructural features of amorphous TiO2 thin films grown by the electron beam physical vapour deposition technique at oblique angles have been experimentally and theoretically studied. The microstructural features of the deposited films were characterized by considering both the column tilt angle and the increase in the column thickness with height. A Monte Carlo model of film growth has been developed that takes into account surface shadowing, short-range interaction between the deposition species and the film surface, as well as the angular broadening of the deposition flux when arriving at the substrate. The good match between simulations and experimental results indicates the importance of these factors in the growth and microstructural development of thin films deposited at oblique angles.

Growth regimes of porous gold thin films deposited by magnetron sputtering at oblique incidence: from compact to columnar microstructures

Growth regimes of gold thin films deposited by magnetron sputtering at oblique angles and low temperatures are studied from both theoretical and experimental points of view. Thin films were deposited in a broad range of experimental conditions by varying the substrate tilt angle and background pressure, and were analyzed by field emission scanning electron microscopy and grazing-incidence small-angle x-ray scattering techniques. Results indicate that the morphological features of the films strongly depend on the experimental conditions, but can be categorized within four generic microstructures, each of them defined by a different bulk geometrical pattern, pore percolation depth and connectivity. With the help of a growth model, a microstructure phase diagram has been constructed where the main features of the films are depicted as a function of experimentally controllable quantities, finding a good agreement with the experimental results in all the studied cases.

On the microstructure of thin films grown by an isotropically directed deposition flux

The influence of isotropically directed deposition flux on the formation of the thin film microstructure at low temperatures is studied. For this purpose we have deposited TiO2 thin films by two different deposition techniques: reactive magnetron sputtering, in two different experimental configurations, and plasma enhanced chemical vapor deposition. The obtained results indicate that films grown under conditions where deposition particles do not possess a clear directionality, and in the absence of a relevant plasma/film interaction, present similar refractive indices no matter the deposition technique employed. The film morphology is also similar and consists of a granular surface topography and a columnarlike structure in the bulk whose diameter increases almost linearly with the film thickness. The deposition has been simulated by means of a Monte Carlo model, taking into account the main processes during growth. The agreement between simulations and experimental results indicates that the obtained mic...

Morphological evolution of pulsed laser deposited ZrO2 thin films

Morphological evolution of ZrO2 thin films deposited during pulsed laser deposition of Zr in O2 atmosphere has been experimentally studied at two different film deposition temperatures, 300 and 873 K. The roughness exponent, α, the growth exponent, β, the coarsening exponent, 1/z, and the exponent defining the evolution of the characteristic wavelength of the surface, p, for depositions at 300 K amounted to β=1.0±0.1, α=0.4±0.1, 1/z=0.34±0.03, and p=0.49±0.03, whereas for depositions carried out at 873 K amounted to β=0.3±0.3, α=0.4±0.2, and 1/z=0.0±0.2. Experimental error becomes important due to the flat morphology of the films inherent to the deposition technique. The change in the surface topography with the film temperature has been studied with the help of a simple Monte Carlo model which indicates the existence of two different growth regimes: a shadowing dominated growth, occurring at low temperatures, characterized by calculated values β=1.00±0.04, α=0.50±0.04, p=0.46±0.01, and 1/z=0.35±0.02 and ...

Nanocolumnar growth of thin films deposited at oblique angles: Beyond the tangent rule

The growth of nanostructured physical vapor deposited thin films at oblique angles is becoming a hot topic for the development of a large variety of applications. Up to now, empirical relations, such as the so-called tangent rule, have been uncritically applied to account for the development of the nanostructure of these thin films even when they do not accurately reproduce most experimental results. In the present paper, the growth of thin films at oblique angles is analyzed under the premises of a recently proposed surface trapping mechanism. The authors demonstrate that this process mediates the effective shadowing area and determines the relation between the incident angle of the deposition flux and the tilt angle of the columnar thin film nanostructures. The analysis of experimental data for a large variety of materials obtained in our laboratory and taken from the literature supports the existence of a connection between the surface trapping efficiency and the metallic character of the deposited materials. The implications of these predictive conclusions for the development of new applications based on oblique angle deposited thin films are discussed.

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.