Results 1 - 10 of 10
Results 1 - 10 of 10. Search took: 0.017 seconds
|Sort by: date | relevance|
[en] When the coatings are in nano-scale, the mechanical properties cannot be easily estimated by means of the conventional methods due to: tip shape, instrument resolution, roughness, and substrate effect. In this paper, we proposed a semi-empirical method to evaluate the mechanical properties of thin films based on the sputtering rate induced by bombardment of Ar"+ ion. The Ar"+ ion bombardment was induced by ion gun implemented in Auger electron spectroscopy (AES). This procedure has been applied on a series of coatings with different structure (carbon films) and a series of coating with a different density (ZnO thin films). The coatings were deposited on Silicon substrates by RF sputtering plasma. The results show that, as predicted by Insepov et al., there is a correlation between hardness and sputtering rate. Using reference materials and a simple power law equation the estimation of the nano-hardness using an Ar"+ beam is possible. - Highlights: • ZnO film and Carbon films were grown on silicon using PVD. • The growth temperature was room temperature. • The hardness of the coatings was estimated by means of nanoindentation. • Evaluation of resistance of materials to the mechanical damage induced by an Ar"+ ion gun (AES). • The hardness have been studied and a power law with the erosion rate has been found
[en] The wireless local area networks represent an alternative to custom sensors and dedicated surveillance systems for target indoor detection. The availability of the channel state information has opened the exploitation of the spatial and frequency diversity given by the orthogonal frequency division multiplexing. Such a fine-grained information can be used to solve the detection problem as an inverse scattering problem. The goal of the detection is to reconstruct the properties of the investigation domain, namely to estimate if the domain is empty or occupied by targets, starting from the measurement of the electromagnetic perturbation of the wireless channel. An innovative inversion strategy exploiting both the frequency and the spatial diversity of the channel state information is proposed. The target-dependent features are identified combining the Kruskal-Wallis test and the principal component analysis. The experimental validation points out the detection performance of the proposed method when applied to an existing wireless link of a WiFi architecture deployed in a real indoor scenario. False detection rates lower than 2 [%] have been obtained. (paper)
[en] Precise control of the various structural phases of TiO_2 at a low temperature is particularly important for practical applications. In this work, the deposition conditions for the growth of anatase and rutile phase at a low temperature (⩽300 °C) were optimized. TiO_2 films were deposited by radio frequency (RF) sputtering of a ceramic TiO_2 target in argon and argon-oxygen plasma (10 and 20% O_2) at room temperature. For the films deposited in pure Ar and 20% O_2, the growth temperature was varied from 25 to 400 °C. The plasma properties were investigated using optical emission spectroscopy (OES) in a wide range of values of gas composition (0–50% O_2 in Ar-O_2 mixture). The structural and chemical properties were characterized by means of x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The results indicate that O_2 addition to the Ar-O_2 gas mixture significantly changed the density of the plasma species (Ar, Ar"+, Ti, Ti"+ and O), which in turn influence the crystal structure and surface chemistry of the prepared films. Anatase phase was obtained for the films grown in Ar-O_2 plasma over the whole range of temperature. In contrast, the films deposited in argon discharge largely persist in amorphous phase at temperature ⩽200 °C and revealed the formation of single rutile phase at ⩾300 °C. The oxygen vacancies detected by XPS analysis for the films deposited in Ar plasma facilitate the growth of a rutile phase at low temperature (∼300 °C). Our results demonstrate that oxygen negative ions, oxygen vacancies and surface energy conditions at the substrate are the key parameters controlling the phase of the prepared films at low temperature. (paper)
[en] Thin carbon films with various thickness, deposited on different substrates (Si and poly-ethylene-terephthalate) at the same operating conditions in a ratio frequency plasma enhanced chemical vapor deposition system were characterized by Doppler broadening spectroscopy. The films and the substrates were depth profiled by a slow positron beam. The aim od these measurements was to study the open volume structure and the interface of the films. It was found that, independently from the substrate, the films were homogeneous and exhibited to some open volume distribution. On the contrary, the effective positron diffusion length in the Si substrate was found to change with the thickness of the carbon films. This behaviour was found to change with the thickness of the carbon films. This behaviour was interpreted as a change in the electric field at the carbon/silicon interface. (author)
[en] This work presents an innovative computational approach for the inversion of wideband ground penetrating radar ( GPR ) data. The retrieval of the dielectric characteristics of sparse scatterers buried in a lossy soil is performed by combining a multi-task Bayesian compressive sensing ( MT-BCS ) solver and a frequency hopping ( FH ) strategy. The developed methodology is able to benefit from the regularization capabilities of the MT-BCS as well as to exploit the multi-chromatic informative content of GPR measurements. A set of numerical results is reported in order to assess the effectiveness of the proposed GPR inverse scattering technique, as well as to compare it to a simpler single-task implementation. (paper)
[en] The focus of the present work is the study of carbon co-deposition effect on the optical and mechanical properties of zirconia films. Optical and dielectric constant, band gap and transition lifetime of such composite systems were determined, as well as their elasticity properties. The thin ZrO2-x-C films were sputter-deposited on silicon and polycarbonate, from a pure ZrO2 and graphite targets in a radio-frequency argon plasma. Besides the zirconia phase and crystalline parameter changes induced by carbon addition, the electronic properties to the films were significantly modified: a drastical optical gap lowering was observed along an increased electronic dielectric constant and refractive index. The invariance of the film elasticity modulus and the similarity of the optical transition lifetime values with those of pure amorphous carbon films indicate an immiscibility of the ceramic and carbon components of the film structure
[en] In 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.
[en] Parametrization 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
[en] This work reports on (i) the gas barrier properties of a-C:H films rf-sputtered in Ar-H2 plasmas from a graphite target on polyethylene terephthalate (PET) and (ii) the influence of the film chemical structure and defect properties on the gas permeability. The intrinsic permeabilities of the films to He, CO2, O2, N2 gases and H2O vapour were determined and found to be orders of magnitude lower than that of the bare PET. Indirect evidence was given to a solubility-diffusion process as the more probable permeation mechanism, over a gas flow through microdefects or gas transport through nanodefects by a Knudsen diffusion mechanism. The barrier capability of the films was found to scale as the gas molecular diameter within the He, CO2, O2 and N2 series, and inversely with the gas critical temperature for the CO2, O2, N2 and H2O series. A correlation between the film Urbach energy, Eu, and the gas permeability was established, except for H2O. Such findings further favour a bulk diffusion contributing mechanism to permeation over the gas state transport. Conversely, this Eu-permeability relation shed more light on the origin of the valence band tailing of the amorphous carbon electron structure
[en] Highlights: • Intrinsic, extrinsic and intrinsic-extrinsic co-doping in TiO2 is explored. • Intrinsic defects were induced in the film by the bombardment of high energetic species. • The as-grown films resistivity varied between 107 and 10−1 Ω cm with oxygen vacancies. • Lowest resistivity (7.4 × 10−4 Ω cm) was obtained for intrinsically-extrinsically co-doped TiO2 films. • The films' optical properties can also be controlled by controlling the density of defects. - Abstract: Oxide-based films and nanostructures have emerged as important materials for a wide range of applications such as photovoltaics, optoelectronics, gas sensing and electronics. 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 reports on the synthesis processes of TiO2 based transparent conductive films, their stoichiometry control and defect identifying, in relation with their electrical and optical properties. Un-doped and TiO2:Nb films were deposited by RF co-sputtering from TiO2 and Nb2O5 targets in Ar plasma. The chemical species present in the plasmas used in deposition process were investigated by optical emission spectroscopy, which was later on correlated with the defects structure of the films. Analysis by X-ray photoelectron spectroscopy shed more light on the nature of the vacancies and on the effect of the latter on the optical and electrical properties of the films. In terms of results, we measured electrical resistivity in the range 10−2–10−3 Ω cm for the intrinsically and extrinsically doped films (films doped with oxygen vacancies and Nb+5 respectively) while the lowest resistivity was obtained for intrinsically-extrinsically co-doped TiO2 films (7.4 × 10−4 Ω cm). The films transparency was also actively determined by the defects in the lattice and highly transparent films (65–85% in the visible range) were obtained by controlling the density of defects. The approach adopted in this work for the generation of oxygen vacancies could be useful for other oxide-based films, where the oxygen vacancies-dependent properties are crucial, for room temperature ferromagnetism and photocatalytic applications.