Optical absorption parameters of amorphous carbon films from Forouhi–Bloomer and Tauc–Lorentz models: a comparative study

International audienceParametrization models of optical constants, namely Tauc-Lorentz (TL), Forouhi-Bloomer (FB) and modified FB models, were applied to the interband absorption of amorphous carbon films. The optical constants were determined by means of transmittance and reflectance measurements in the visible range. The studied films were prepared by rf sputtering and characterized for their chemical properties. The analytical models were also applied to other optical data published in the literature pertaining to films produced by various deposition techniques. The different approaches used to determine important physical parameters of the interband transition yielded different results. A figure-of-merit was introduced to check the applicability of the models and the results showed that FB modified for an energy dependence of the dipole matrix element adequately represents the interband transition in the amorphous carbons. Further, the modified FB model shows a relative superiority over the TL ones for concerning the determination of the band gap energy, as it is the only one to be validated by an independent, though indirect, gap measurement by x-ray photoelectron spectroscopy. Finally, the application of the modified FB model allowed us to establish some important correlations between film structure and optical absorption properties

Intrinsic defects and their influence on the chemical and optical properties of TiO2x films

International audienceIn this work, TiO2 films produced by rf sputtering of a TiO2 target in argon and argon–oxygen plasmas were studied. The oxygen content in the feed gas was varied in a range 3–20%. The chemical composition and structure of films were characterized by Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy (XPS) and x-ray diffraction. Important information about the intrinsic defects of the films and their effects on the optical properties as well as a scheme of the energy band structure of the films could be derived from a combined use of optical spectroscopy and XPS

Graphene as Barrier to Prevent Volume Increment of Air Bubbles over Silicone Polymer in Aqueous Environment

The interaction of air bubbles with surfaces immersed in water is of fundamental importance in many fields of application ranging from energy to biology. However, many aspects of this topic such as the stability of surfaces in contact with bubbles remain unexplored. For this reason, in this work, we investigate the interaction of air bubbles with different kinds of dispersive surfaces immersed in water. The surfaces studied were polydimethylsiloxane (PDMS), graphite, and single layer graphene/PDMS composite. X-ray photoelectron spectroscopy (XPS) analysis allows determining the elemental surface composition, while Raman spectroscopy was used to assess the effectiveness of graphene monolayer transfer on PDMS. Atomic force microscopy (AFM) was used to study the surface modification of samples immersed in water. The surface wettability has been investigated by contact angle measurements, and the stability of the gas bubbles was determined by captive contact angle (CCA) measurements. CCA measurements show that the air bubble on graphite surface exhibits a stable behavior while, surprisingly, the volume of the air bubble on PDMS increases as a function of immersion time (bubble dynamic evolution). Indeed, the air bubble volume on the PDMS rises by increasing immersion time in water. The experimental results indicate that the dynamic evolution of air bubble in contact with PDMS is related to the rearrangement of surface polymer chains via the migration of the polar groups. On the contrary, when a graphene monolayer is present on PDMS, it acts as an absolute barrier suppressing the dynamic evolution of the bubble and preserving the optical transparency of PDMS

Influence of Nd3+ doping on the structural and near-IR photoluminescence properties of nanostructured TiO2 films

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Study of the early stage growth of carbon-based films and application to multi-nanolayers plasma synthesis

The plasma deposition processes and solid-state growth mechanisms of two kinds of amorphous carbon films, one harder than the other, and nano-layered hard/soft nanocomposite carbon films will be presented. Two plasma processes were used for the film synthesis: rf sputtering and plasma assisted chemical vapor deposition. The mechanical properties and gas barrier functionality of such materials will also be discussed. By means of residual stress measurements and angle resolved x-ray photoelectron spectroscopy applied to the films in their early growth stages, the film growth mechanism could be identified in order to better understand the single film growth itself and, further, to control the internal stress of composite structures for protective coatings. The sputtered film growth occurred via the high mobility Wolmer-Veber mechanism. The soft film growth was found to start by a layer-by-layer mechanism, inducing a compression stress building in the very first growth stages, which tends to relax when going to higher thicknesses through a change of the growth mechanism from a layer-by-layer to a columnar growth one. The gas barrier properties of the hard films on polyethylene terephtalate (PET) were studied. The film intrinsic permeabilities to He, CO2, O2, N2 gases and H2O vapor were determined and found to be orders of magnitude lower than that of PET. The permeation mechanism was found to be based more likely on a solubility-diffusion process than on a gas flow through microdefects or gas transport through nanodefects in the films. However, these films suffer from adhesion weakness, which limits their application as protective coatings. By combining the two plasma processes in sequential film depositions, composite films consisting of periodically stacked hard and soft amorphous carbon layers were produced. They were depth-profiled by means of Auger electron spectroscopy and analysed by transmission electron microscopy. Positive effects of nanolayering on film mechanical stability are evidenced. The role of interfaces and the stored stress at these interfaces is discussed

Defects probing and effects in oxide films and nanostructures

Oxide-based films and nanostructures have emerged as important and promising materials for a wide range of applications such as photovoltaics, photocatalysis, optoelectronics, gas sensing and electronics. A common feature unifies these application fields which is related to the structural defects of these materials and strongly affects their functionality. To develop an appropriate understanding of the properties of these oxides, it is necessary to address the material preparation methods and defect probing issues. This work deals with oxide films and nanostructures synthesis processes, their stoichiometry control and defect identifying, in relation with their electronic, electrical and optical properties. Titanium, zinc and zirconium oxide-based materials are considered. X-ray diffraction, x-ray photoelectron, positron annihilation and UV-Vis-NIR spectroscopies are employed for the structure and defect study

Analog Circuit Fault Classification and Data Reduction Using PCA-ANFIS Technique Aided by K-means Clustering Approach

The paper work aims to extract effectively the fault feature information of analog integrated circuits and to improve the performance of a fault classification process. Thus, a fault classification method based on principal component analysis (PCA) and adaptive neuro fuzzy inference system classifier (ANFIS) preprocessed by K-means clustering (KMC) is proposed. To effectively extract and select fault features the traditional signal processing based on sampling technique conducts to different signature parameters. A stimulus pulse signal applied to the circuit under test (CUT) allowed us to get a reference output response. Respecting both specific sampling interval and step, the fault free and the faulty output responses are sampled to create amplitude sample features that will serve the fault classification process. The PCA employed for data reduction has lessened the computational complexity and obtaining the optimal features. Thus more than 75% of data volume decreased without loss of original information. The principal components extracted by this reduction data method have been input into ANFIS aided by KMC to obtain the best fault diagnosis results. The experimental results show a score of 100% diagnostic accuracies for the CUTs. Therefore, our approach has achieved best fault classification precision comparing to other research works