V. Micheli

Structural and compositional study of B–C–N films produced by laser ablation of B4C targets in N2 atmosphere

Abstract In this work, we report on a structural and compositional characterization of B–C–N thin films deposited by laser reactive ablation of a B4C target, in low-pressure (5 Pa) nitrogen atmosphere. For target ablation, a KrF excimer laser (λ=248 nm, τ=20 ns) has been used, at the fluences of 6 and 12 J/cm2. Films have been deposited on silicon 〈100〉 substrates at room temperature. Scanning electron miroscopy (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and Fourier transform-infrared spectroscopy (FT-IR) characterization techniques were used to analyze the composition and the structure of the deposited films. The film results to be a mixture of sp2/sp3 BN and sp2/sp3 nitrogenated C phases. The concentration of the different BN phases depends on the used laser fluence for the deposition of the film.

Intrinsic defects and their influence on the chemical and optical properties of TiO2−x films

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.

Mechanical and structural properties of Ni-C films obtained by RF sputtering

Abstract This work reports on the structural and mechanical properties of Ni-C films, deposited on silicon substrates by RF magnetron sputtering of a plasma-sprayed Ni-C target. X-ray diffraction was used to determine the phase structure, while the atomic composition and the chemical states were given by X-ray photoelectron and Auger electron spectroscopies. The internal stress, Youngs modulus, hardness and scratch resistance were investigated, as a function of the RF electric power. The microstructure and the mechanical properties were found to be governed by the kinetic energy transferred to the growing film. In low energy conditions, the Ni-C films behaved mechanically as diamond-like carbons, and the microstructure coincided with that of an amorphous C-fcc Ni composite. Inversely, in high energy growth conditions, the films were composed of dispersed Ni3C small crystallites in an amorphous Ni-C mixture. Such a microstructure was characterized by low mechanical properties.

Growth of titanium dioxide films by cluster supersonic beams for VOC sensing applications

We developed and tested gas sensing devices based on TiO/sub 2/ nano-crystalline films produced at room temperature by the novel growth method of cluster beam deposition. The devices show a very good response to ethanol, methanol, and propanol and an overall performance comparable or better to the best devices reported in literature, based on films grown with other techniques. The major advantage of our growth method is that there is no need of the thermal annealing or doping processes usually required to improve sensitivity and reliability of gas sensing devices based on nanostructured thin film. The sensors dynamic response gives a maximum sensitivity for ethanol at about 250/spl deg/C. As the morphological (AFM) and structural (XRD) characterizations of the films show, the high performance of our sensors could only be achieved because their nano-crystalline structure was well controlled by the properties of the cluster precursors in the supersonic beam. We envisage possible further developments in terms of sensitivity and selectivity of gas sensing devices based on films grown by cluster beam deposition.

Low temperature growth study of nano-crystalline TiO2 thin films deposited by RF sputtering

Precise control of the various structural phases of TiO2 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. TiO2 films were deposited by radio frequency (RF) sputtering of a ceramic TiO2 target in argon and argon-oxygen plasma (10 and 20% O2) at room temperature. For the films deposited in pure Ar and 20% O2, 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% O2 in Ar-O2 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 O2 addition to the Ar-O2 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-O2 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.

Carbon effect on the phase structure and the hardness of RF sputtered zirconia films

Abstract Thin films of zirconia–carbon (ZrO2−x–C) were deposited by RF magnetron sputtering, on both polycarbonate and cemented-WC substrates. Two separate targets of monoclinic ZrO2 and graphite were used and a composition range was selected from 0 to ∼30 at.% carbon. The C-containing films resulted in a composite structure, which consisted of a zirconia phase coexisting with an amorphous carbon phase. A tetragonal zirconia phase was found in films containing 10–30 at.% carbon, while films of pure zirconia grew in the monoclinic phase. The tetragonal phase of zirconia coincided with the presence of compressive internal stresses in the films, which increased with increasing C content. As a consequence of the microstructure induced by the presence of carbon, the ZrO2−x–C films resulted in a stronger surface hardening of the polymeric substrates than the pure zirconia films did.

Mechanical characterization of thin TiO2 films by means of microelectromechanical systems-based cantilevers

The measurement of mechanical parameters by means of microcantilever structures offers a reliable and accurate alternative to traditional methods, especially when dealing with thin films, which are extensively used in microfabrication technology and nanotechnology. In this work, microelectromechanical systems (MEMS)-based piezoresistive cantilevers were realized and used for the determination of Youngs modulus and residual stress of thin titanium dioxide (TiO(2)) deposited by sputtering from a TiO(2) target using a rf plasma discharge. Films were deposited at different thicknesses, ranging from a few to a hundred nanometers. Dedicated silicon microcantilevers were designed through an optimization of geometrical parameters with the development of analytical as well as numerical models. Youngs modulus and residual stress of sputtered TiO(2) films were assessed by using both mechanical characterization based on scanning profilometers and piezoresistive sensing elements integrated in the silicon cantilevers. Results of MEMS-based characterization were combined with the tribological and morphological properties measured by microscratch test and x-ray diffraction analysis.

Super-Hydrophilic PDMS and PET Surfaces for Microfluidic Devices

In this work the effect of air plasmas on wettability of Polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) was studied. These polymers are widely used materials in the fabrication of microfluidic devices. The microfluidic system fabricated from native PET and PDMS requires active pumping mechanism, due to a low hydrophilic surface behavior. To render hydrophilic and increase the capillary flow into the device, plasma treatments can be used. Air plasma treatment is an interesting technology for microfluidic fields due to simplicity of use and low cost. This study describes the effect of the working plasma pressure on wettability of polymers. The polymers were treated by RF plasma and the wettability was studied by means of sessile contact angle. The results established that the air plasma can increase the wettability of both polymers. Moreover we demonstrated that by optimizing the working pressure a superhydrophilic surface (with a contact angle less than 5°) can be obtained. The findings suggest that air plasma treatments are a suitable technology to enhance polymers surface wetting performance for microfluidic devices.

Chemical and microstructural characterisation of silicon-containing carbon films

Abstract The chemical and microstructural modifications of carbon films, induced by silicon incorporation during the film growth, were studied on films deposited by rf magnetron sputtering of a graphite-silicon target. Silicon incorporation produced Si–C bondings in the films and an increased defect density in the carbon matrix structure. Besides, increasing the silicon content resulted in a significant decrease of the π-plasmon loss peak intensity in the C1s binding energy region in the X-ray photoelectron spectra, along with a lineshape sharpening for the σ–s component of the valence band spectra. This specific electronic structure is best discussed in terms of σ–π system perturbations rather than in terms of an increased sp 3 /sp 2 hybridisation ratio in the films.

Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen Bubbles with Graphite Surface in Water

The interaction of the confined gas with solid surface immersed in water is a common theme of many important fields such as self-cleaning surface, gas storage, and sensing. For that reason, we investigated the gas-graphite interaction in the water medium. The graphite surface was prepared by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). The surface chemistry and morphology were studied by X-ray photoelectron spectroscopy, profilometry, and atomic force microscopy. The surface energy of HOPG was estimated by contact angle measurements using the Owens-Wendt method. The interaction of gases (Ar, He, H2, N2, and air) with graphite was studied by a captive bubble method, in which the gas bubble was in contact with the exfoliated graphite surface in water media. The experimental data were corroborated by molecular dynamics simulations and density functional theory calculations. The surface energy of HOPG equaled to 52.8 mJ/m2 and more of 95% of the surface energy was attributed to dispersion interactions. The results on gas-surface interaction indicated that HOPG surface had gasphilic behavior for helium and hydrogen, while gasphobic behavior for argon and nitrogen. The results showed that the variation of the gas contact angle was related to the balance between the gas-surface and gas-gas interaction potentials. For helium and hydrogen the gas-surface interaction was particularly high compared to gas-gas interaction and this promoted the favorable interaction with graphite surface.