E. Pascual

Comparison of different advanced oxidation processes for phenol degradation.

Advanced Oxidation Processes (O3, O3/H2O2, UV, UV/O3, UV/H2O2, O3/UV/H2O2, Fe2+ /H2O2 and photocatalysis) for degradation of phenol in aqueous solution have been studied in earlier works. In this paper, a comparison of these techniques is undertaken: pH influence, kinetic constants, stoichiometric coefficient and optimum oxidant/pollutant ratio. Of the tested processes, Fenton reagent was found to the fastest one for phenol degradation. However, lower costs were obtained with ozonation. In the ozone combinations, the best results were achieved with single ozonation. As for the UV processes, UV/H2O2 showed the highest degradation rate.

Mueller matrix microscope with a dual continuous rotating compensator setup and digital demodulation

In this paper we describe a new Mueller matrix (MM) microscope that generalizes and makes quantitative the polarized light microscopy technique. In this instrument all the elements of the MU are simultaneously determined from the analysis in the frequency domain of the time-dependent intensity of the light beam at every pixel of the camera. The variations in intensity are created by the two compensators continuously rotating at different angular frequencies. A typical measurement is completed in a little over one minute and it can be applied to any visible wavelength. Some examples are presented to demonstrate the capabilities of the instrument.

Effect of ion bombardment on the properties of B4C thin films deposited by RF sputtering

Abstract Boron carbide is a good material for very hard coatings mechanical applications. Sintered bulk B 4 C is one of the hardest known materials (40 GPa), with a high Youngs modulus and a very high chemical and thermal stability. In B 4 C film deposition, ion bombardment during film growth can deeply affect the material properties. Films were deposited by tuned RF magnetron sputtering from a sintered B 4 C target, under different conditions of ion bombardment, corresponding to substrate biases varying from +15 to −80 V. Homogeneous and stochiometric B 4 C films were obtained. Their mechanical properties: microhardness, Youngs modulus, internal stress and adhesion have been measured by the dynamical nanoindentation method, by the beam bending method and by the microscratch method, respectively. As ion energy is increased, the stress of the films and the critical load increases, while both microhardness and Youngs modulus have maximum values of 30 and 350 GPa respectively, for a bombarding ion energy of 50 eV.

Optical and structural characterization of hydrogenated amorphous silicon carbide thin films prepared by r.f. plasma chemical vapour deposition

Abstract The optical, vibrational and structural properties of hydrogenated amorphous silicon carbide (a-Si1−xCx:H) thin films were studied as a function of the material composition and the precursor gas ratio. Thus, a series of a-Si1−xCx:H thin films was prepared on c-Si, NiCr and Corning 7059 substrates by r.f. plasma chemical vapour deposition, from different gas mixtures (0.25 to 0.025 [SiH 4 ] [CH 4 ] gas ratios), at 50 W r.f. power, 20 Pa pressure and 300 °C substrate temperature. Spectroscopic (UV-VIS) ellipsometry provided the spectral dependencies of the optical constants and the gap energy of the films. The composition of the films was analysed by X-ray photoelectron spectroscopy and the vibrational properties were studied using Fourier transform infrared and Raman spectroscopies.

Effect of a balanced concentration of hydrogen on graphene CVD growth

The extraordinary properties of graphene make it one of the most interesting materials for future applications. Chemical vapor deposition (CVD) is the synthetic method that permits obtaining large areas of monolayer graphene. To achieve this, it is important to find the appropriate conditions for each experimental system. In our CVD reactor working at low pressure, important factors appear to be the pretreatment of the copper substrate, considering both its cleaning and its annealing before the growing process. The carbon precursor/hydrogen flow ratio and its modification during the growth are significant in order to obtain large area graphene crystals with few defects. In this work, we have focused on the study of the methane and the hydrogen flows to control the production of single layer graphene (SLG) and its growth time. In particular, we observe that hydrogen concentration increases during a usual growing process (keeping stable the methane/hydrogen flow ratio) resulting in etched domains. In order to balance this increase, a modification of the hydrogen flow results in the growth of smooth hexagonal SLG domains. This is a result of the etching effect that hydrogen performs on the growing graphene. It is essential, therefore, to study the moderated presence of hydrogen.

Optical security verification by synthesizing thin films with unique polarimetric signatures.

This Letter reports the production and optical polarimetric verification of codes based on thin-film technology for security applications. Because thin-film structures display distinctive polarization signatures, this data is used to authenticate the message encoded. Samples are analyzed using an imaging ellipsometer able to measure the 16 components of the Mueller matrix. As a result, the behavior of the thin film under polarized light becomes completely characterized. This information is utilized to distinguish among true and false codes by means of correlation. Without the imaging optics the components of the Mueller matrix become noise-like distributions and, consequently, the message encoded is no longer available. Then, a set of Stokes vectors are generated numerically for any polarization state of the illuminating beam and thus, machine learning techniques can be used to perform classification. We show that successful authentication is possible using the k-nearest neighbors algorithm in thin-films codes that have been anisotropically phase-encoded with pseudorandom phase code.

Infrared and UV-visible ellipsometric study of WO3 electrochromic thin films

Abstract Infrared Fourier Transform phase-modulated and UV-visible ellipsometry techniques have been applied to study the colouring/bleaching process of WO 3 electrochromic thin films. The optical absorption of these films can be changed markedly in a reversible process induced by hydrogen incorporation. The colouring/bleaching process was carried out by using the sample as one electrode of an electrochemical cell filled with an electrolytic solution of H 2 SO 4 . The spectroscopic ellipsometric angles Ψ and Δ were obtained ex situ by an infrared Fourier Transform phase-modulated ellipsometer working in the wavenumber range from 800 to 4000 cm −1 and a rotating analyzer ellipsometer in the UV-visible range (1.7–4.8 eV). Significant optical absorption and vibrational changes associated with the colouring/bleaching process which follow the hydrogen incorporation in the film were observed in the UV-visible range from 1.7 to 3.8 eV and in the IR range at 1605 cm −1 and from 3000 to 3800 cm −1 . The strong changes can be seen by changes in the spectroscopic dielectric function. For the coloured WO 3 layer, the main absorption contribution associated with the colour effect was found at 6032 cm −1 , outside the analysed region.

Optical emission spectroscopy of rf glow discharges of methane–silane mixtures

Abstract Optical emission spectroscopy (OES) measurements of radio frequency (rf) SiH 4 –CH 4 plasmas were carried out during the growth of a-Si 1−x C x :H thin films in order to correlate the plasma emission characteristics with the composition of the deposited thin films. The relative flow fraction of SiH 4 in the gas mixture was varied from 0.02 to 0.4 at three different values of rf power (30 W, 50 W and 70 W). Real time OES measurements were performed using a diode array-based optical multi-channel analyzer working in the visible range (300–800 nm). The main features of OES spectra corresponded to SiH * (414 nm) and CH * (430 nm) emissions whose intensities were strongly dependent on the gas feed mixture composition and on the rf power supplied to the discharge. The gas mixture composition dependence of the SiH * and CH * emission intensities where similar to those of [Si] and [C] contents in the films as measured by X-ray photoelectron spectroscopy. In particular, a linear relationship was found between the ratio of the SiH * to CH * emission intensities and the [Si]:[C] composition ratio in the films.

Spectral ellipsometric and compositional characterization of hydrogenated amorphous silicon carbide thin films

Abstract A spectroscopic (UV-visible) ellipsometric analysis of hydrogenated amorphous silicon carbide (a-Si 1−x C x : H) thin films, grown by plasma-enhanced chemical vapour deposition, is presented. The films were analysed by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to determine their composition. In this study, a wide range of x values, from 0.16 to 0.59, was covered. These values depend mainly on the composition of the precursor gas. The spectroscopic ellipsometric analysis of the samples, by a multilayer model, shows that all the a-Si 1−x C x : H films studied consist of a homogeneous layer, on a c-Si substrate, with an overlayer (effective medium mixture of the same material and 41% voids). This analysis also provides values of the dielectric function of the layer material.

Ion energy distributions in bipolar pulsed-dc discharges of methane measured at the biased cathode

The ion fluxes and ion energy distributions (IED) corresponding to discharges in methane (CH4) were measured in time-averaged mode with a compact retarding field energy analyser (RFEA). The RFEA was placed on a biased electrode at room temperature, which was powered by either radiofrequency (13.56 MHz) or asymmetric bipolar pulsed-dc (250 kHz) signals. The shape of the resulting IED showed the relevant populations of ions bombarding the cathode at discharge parameters typical in the material processing technology: working pressures ranging from 1 to 10 Pa and cathode bias voltages between 100 and 200 V. High-energy peaks in the IED were detected at low pressures, whereas low-energy populations became progressively dominant at higher pressures. This effect is attributed to the transition from collisionless to collisional regimes of the cathode sheath as the pressure increases. On the other hand, pulsed-dc plasmas showed broader IED than RF discharges. This fact is connected to the different working frequencies and the intense peak voltages (up to 450 V) driven by the pulsed power supply. This work improves our understanding in plasma processes at the cathode level, which are of crucial importance for the growth and processing of materials requiring controlled ion bombardment. Examples of industrial applications with these requirements are plasma cleaning, ion etching processes during fabrication of microelectronic devices and plasma-enhanced chemical vapour deposition of hard coatings (diamond-like carbon, carbides and nitrides).