F.L. Freire

Structural and mechanical characterization of fluorinated amorphous-carbon films deposited by plasma decomposition of CF4–CH4 gas mixtures

Fluorinated amorphous-carbon films (a-C:F:H) were deposited by low-power rf capacitively coupled plasma-enhanced chemical-vapor deposition using CH4–CF4 gas mixtures. Different series of films were deposited, changing one parameter at a time: the CF4 partial pressure from 0% to 100%, the self-bias voltage from −50 to −700 V, and the total deposition pressure from 5 to 15 Pa. The composition was determined by ion-beam analysis (IBA): Rutherford backscattering spectrometry, elastic recoil detection analysis, and nuclear reaction analysis. The atomic density of the films was evaluated by combining the IBA results with the thickness value measured by stylus profilometry. Film structure was investigated by infrared transmission and Raman scattering spectroscopies. The internal stress and Vickers hardness were also measured. For a fixed self-bias, the increase of the CF4 partial pressure leads to a higher fluorine incorporation and the decrease of both hardness and internal stress. The film microstructure chang...

Film growth and relationship between microstructure and mechanical properties of a-C:H:F films deposited by PECVD

Abstract Results of a systematic investigation on the effects of some deposition parameters (partial pressure of CF4 and self-bias voltage) on the microstructure, mechanical and tribological properties of a-C:H:F films are presented. The films were deposited by r.f.-PECVD using CH4–CF4 mixtures. The film composition was measured by ion beam analysis and, combining these results with the film thickness, the film density was determined. The structural arrangement was probed by Raman spectroscopy and the chemical bonding was investigated by infrared absorption and X-ray photoelectron spectroscopies. The hardness was measured by microindentation and the internal stress was determined by measuring the changing of the substrate curvature after the film deposition. The friction coefficient was measured by lateral force microscopy. The results indicate that the properties of a-C:H:F films are controlled by the ionic bombarding during the film growth. For a fixed self-bias, the increase of the CF4 partial pressure leads to a transition from diamond-like to a polymer-like structure, to a higher fluorine incorporation and to a decrease of both hardness and internal stress. The friction coefficient decreases too. The fluorine incorporation also increases with the increase of the self-bias and was associated to higher plasma decomposition. Fluorine-poor polymer-like films were deposited at low self-bias (−50 V). In both situations, fluorine incorporation occurs at the expenses of the hydrogen content and the reduction of the energy of the bombarding species results in less dense and soft films with a polymer-like structure.

Effects of the reaction atmosphere composition on the synthesis of single and multiwalled nitrogen-doped nanotubes

Single and multiwalled nitrogen-doped carbon nanotubes were grown by chemical vapor deposition varying the feedstock composition between pure acetonitrile and ethanol/acetonitrile mixtures. The advantage of using CN sources that develop close vapor pressure values has been used in order to elucidate the effects of the reaction atmosphere in the synthesis of N-doped nanotubes. Our findings show that the morphology of the nanotube material depends strongly on the composition of the reaction atmosphere. When carrying out the experiments in an atmosphere solely determined by the vapor pressure of the feedstock components, improved homogeneity is achieved with pure CN sources or low concentration of the foreign solute. Under these conditions the temperature has strong influence in the diameter distribution.

Hard amorphous carbon–fluorine films deposited by PECVD using C2H2–CF4 gas mixtures as precursor atmospheres

Abstract An investigation of the structural arrangement and mechanical properties of a-C:F:H films obtained by plasma enhanced chemical vapor deposition (PECVD) using C 2 H 2 –CF 4 gas mixtures as precursor atmospheres was carried out. The results indicate that fluorine incorporation increases the hydrophobicity of the films and relaxes the internal stress, while reducing the hardness also. It was shown that it is possible to obtain fluorinated carbon films with hydrophobicity comparable to that of polytetrafluoroethylene (TFE) but with much higher hardness, that is in the range of a few GPa.

Interdiffusion and reaction in the FeAl bilayer: I: Rutherford backscattering analysis of furnace-annealed samples

Abstract This is the first of a series of articles concerning the experimental investigation of the interdiffusion processes and solid phase reactions occurring in an FeAl bilayer subjected to furnace annealing, ion beam mixing and the simultaneous combination of these two types of metallurgical treatment. In this article, we describe the results of the Rutherford backscattering analyses performed on FeAl bilayered samples, subjected to high vaccum annealing at a temperature between 290 and 870 K. We demonstrate that, if the FeAl interface is oxide free, pronounced interdiffusion starts at a temperature as low as 570 K, and solid phase reaction leading to intermetallic phase formation starts at 600 K. From the experimental data, it is possible to extract two diffusivities D 1 =2.2×10 −7 exp(− 0.95eV kT )cm 2 s −1 and D 2 =1.7×10 −8 exp(− 0.33eV kT )cm 2 s −1 The reaction kinetics at 600 K were also determined from the experimental results, and the reaction rate constant K at this temperature is 53 μg2 cm4 min−1.

Fluorine incorporation into amorphous hydrogenated carbon films deposited by plasma-enhanced chemical vapor deposition : structural modifications investigated by X-ray photoelectron spectrometry and Raman spectroscopy

Abstract Fluorinated hydrogenated amorphous carbon films were grown by plasma-enhanced chemical vapor deposition (PECVD). Two sets of films were deposited. For the first set, the self-bias voltage (VB) was kept constant and equal to −350 V, while the CF4 partial pressure in CH4/CF4 gas mixtures was changed from 0 to 80%. For the second one, a fixed 2:1 CF4/CH4 proportion in the precursor atmosphere was employed while VB was varied from −50 V to −500 V. The carbon chemical environment was investigated by X-ray photoelectron spectroscopy (XPS), while structural changes in the carbon matrix were probed by Raman spectroscopy. The increase of both the CF4 partial pressure and the |VB| leads to an increase of the fluorine concentration within the film. While in the first case the increase of the fluorine content leads to a relative increase of the CFn XPS bands, in the second one the relative intensity of the CFn bands are nearly the same within the range of VB values investigated. In particular, the CF2 band grows for higher CF4 partial pressures, while it is negligible in the self-bias study. The analysis of the Raman spectra shows that a progressive structural modification from a diamond-like towards a polymer-like material occurs for higher CF4 partial pressures as well as for lower |VB|. This trend is accompanied by a noticeable broadening of the C1s XPS peak on the side of lower binding energies.

Characterization of phosphorus-doped multiwalled carbon nanotubes

Phosphorus-doped multiwalled carbon nanotubes (P-MWNTs) have been successfully synthesized by spray pyrolysis methods using a solution of ferrocene and triphenylphosphine in toluene. Electron microscopy images reveal corrugated tubes with a special morphology, similar to a carbon necklace. P-MWNTs are shorter compared to undoped tubes grown in the same conditions using ferrocene and toluene as precursors. Raman spectroscopy characterization suggests the formation of more defective tubes as the phosphorus in the precursor solution was increased. X-ray photoelectron spectroscopy (XPS) revealing the chemical environment of the phosphorus atoms clearly indicates the presence of substitutional phosphorus in the nanotubes.

Oxygen transport and GeO2 stability during thermal oxidation of Ge

Oxygen transport during thermal oxidation of Ge and desorption of the formed Ge oxide are investigated. Higher oxidation temperatures and lower oxygen pressures promote GeO desorption. An appreciable fraction of oxidized Ge desorbs during the growth of a GeO2 layer. The interplay between oxygen desorption and incorporation results in the exchange of O originally present in GeO2 by O from the gas phase throughout the oxide layer. This process is mediated by O vacancies generated at the GeO2/Ge interface. The formation of a substoichiometric oxide is shown to have direct relation with the GeO desorption.

Synthesis and characterization of silver nanoparticle-multiwalled carbon nanotube composites

Multiwalled carbon nanotubes (MWCNTs) grown by spray pyrolysis have been decorated with silver nanoparticles prepared via the silver mirror reaction. Good dispersion of silver nanostructures was obtained on the surface of MWCNTs, resulting in an efficient and simple wet chemistry method for increasing the reactivity of the carbon nanotubes surfaces. High-resolution transmission electron microscopy showed the orientations of the crystallography planes of the anchored silver nanoparticles and revealed their size distribution. Raman spectroscopy results confirm that the composite material preserves the integrity of the MWCNTs. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were also employed for sample characterization.

Multiwalled carbon nanotubes decorated with cobalt oxide nanoparticles

Multiwalled carbon nanotubes (MWCNTs) synthesized by spray pyrolysis were decorated with cobalt oxide nanoparticles using a simple synthesis route. This wet chemistry method yielded nanoparticles randomly anchored to the surface of the nanotubes by decomposition of cobalt nitrate hexahydrate diluted in acetone. Electron microscopy analysis indicated that dispersed particles were formed on the MWCNTs walls. The average size increased with the increasing concentration of cobalt nitrate in acetone in the precursor mixture. TEM images indicated that nanoparticles were strongly attached to the tube walls. The Raman spectroscopy results suggested that the MWCNT structure was slightly damaged after the nanoparticle growth.