V. Sessa

Local indentation modulus characterization of diamondlike carbon films by atomic force acoustic microscopy two contact resonance frequencies imaging technique

Two contact resonance frequencies atomic force acoustic microscopy imaging technique has been used to evaluate local indentation modulus of a diamondlike carbon film deposited on a molybdenum foil by laser ablation from glassy carbon target. Acoustic images were obtained by measuring both first and second contact resonance frequency at each point of the scanned area, and then numerically evaluating local contact stiffness and reconstructing an indentation modulus bidimensional pattern. The wide difference of the indentation modulus values allows to detect the presence of residual glassy carbon agglomerates in the diamondlike carbon film.

Carbon nanotubes and nanowires grown from spherical carbon nano-particles

Abstract In this Letter, we report a new method for the fabrication of carbon nanotubes (CNT) and nanowires where amorphous hydrogenated carbon nano-particles were used as precursor, without metal catalyst addition. In particular, depending on the process parameters, we obtained single walled nanotubes (SWNTs) with mean diameter 1.2 nm and carbon nanowires with mean diameter 250 nm. A discussion on the possible growth mechanism is also reported.

Self-assembled carbon nanotubes grown without catalyst from nanosized carbon particles adsorbed on silicon

Carbon nanotubes films have been prepared by low-velocity spraying of carbon nanosized particles on heated Si substrate. Studies reveal that by properly choosing the deposition temperature, well-aligned carbon nanotubes are self-assembled from the particles without a catalyst. Raman scattering and reflection high-energy electron diffraction show that the tubes are bundles of single-wall nanotubes.

Laser‐induced structural modifications of glassy carbon surfaces

Structural modifications induced by pulsed laser irradiations in the surface layers of glassy carbon have been monitored by reflection high energy electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy. The glassy carbon samples were irradiated by 30 superimposed laser pulses (λ=6.943 nm). The energy density (100–500 mJ/cm2 per pulse) delivered to the material and the repetition rate of the laser (0.05 Hz) have been chosen so that the temperature increase of the irradiated surface layers was below the melting point of the glassy carbon. The combined use of the analysis techniques indicated that the beginning of the solid state processes, leading to microstructural modifications of the surface layers, occurs at energy density of 300 mJ/cm2. An increase of the average crystalline size of graphitic clusters occurs upon radiation performed at fluences of 300 and 400 mJ/cm2, whereas at higher energy density the material undergoes complete amorphization. The analysis of chemical state ...

Residual stress in polycrystalline diamond/Ti6Al4V systems

Abstract Polycrystalline diamond coatings were deposited on Ti6Al4V alloy by HF-CVD, at fixed temperature (650 C) for different deposition times. During the process, thick titanium carbide layers were formed at the metal/diamond interface. X-ray diffraction (XRD) methods were used to assess coating quality, phase composition, texture, and residual macrostress of the diamond/TiC/Ti system. For a better evaluation of the residual stress present in each phase, three independent measurements were performed with synchrotron radiation (SR-XRD). The measured residual strain could be interpreted in terms of a simple axially uniform residual stress model: σ11 = σ22, σ33 = 0, σij = 0 (i ≠ j). Irrespective of film thickness, the residual stress was very intense, compressive both in the diamond layer (approx. −6.5 GPa) and in TiC (approx. −1.4 GPa), and tensile in Ti6Al4V (approx. 70 MPa). The high residual strain in the diamond layer affected the results of texture measurements using the traditional pole figure method; more reliable results were obtained by measuring the integrated intensity, rather than peak maximum intensity, as a function of tilting angles.

Experimental evidence of different crystalline forms in chemical vapor deposited diamond films

Diamond films have been obtained on Ta polycrystalline substrates from mixtures of methane and hydrogen by the hot‐filament chemical vapor deposition technique. The structural characteristics of the polycrystalline deposits have been investigated by reflection high energy electron diffraction (RHEED), while the surface morphologies have been observed by scanning electron microscopy or carbon replica transmission electron microscopy. For one of the films, the formation of thermal spikes during the deposition process yielded a structure giving a RHEED pattern with d spacings and intensities not corresponding to the already identified carbon and diamond phases. On the base of the RHEED pattern the observed phase has been identified as a face‐centered‐cubic lattice, belonging to the space group F43m and ascribed to a so‐called X‐diamond polytype.

Growth of single-walled carbon nanotubes by a novel technique using nanosized graphite as carbon source

Abstract Single-walled carbon nanotubes (SWCN) have been produced treating nanosized graphitic powders with atomic hydrogen fluxes in a purpose-designed reactor. The Fe-catalyzed synthesis process yielded rather dense networks of nanotubes forming continuous layers adherent to the substrates. Raman spectroscopy in the 140–250 cm −1 region, high-resolution transmission electron microscopy and reflection electron diffraction in selected area configuration have been used to define the structural details of the nanotubes. The analysis evidenced a nanotubes population belonging prevalently to the ( n ,0) achiral symmetry subclass. This new experimental approach for SWCNs growth provides promising conditions for maximizing yields and generating selected tubular configurations.

Temperature-dependent orientation of diamond films on titanium and structural evolution of interfacial layers

X‐ray diffraction analytical techniques have been used to investigate the influence of the deposition temperature (650–850 °C) on the composition and microstructure of the transition layers formed at the interface between titanium substrates and diamond thin films. The diamond coatings were produced by hot‐filament chemical vapor deposition using a 1% methane/hydrogen mixture. X‐ray diffraction analysis, performed both through θ–2θ scans and at grazing incidence, allowed investigation of the crystallographic properties and of the structural evolution of the various phases (TiC, TiH2, α‐Ti) generated inside the intermediate reaction layers. The temperature‐dependent orientation of diamond crystallites is discussed with reference to the complex structure of these interfacial layers.

Modulation of charge transport in diamond-based layers

Doping of diamond by substitutional insertion of metallic species or production of diamond/metals nanocomposite layers has been obtained by a hybrid chemical vapor deposition based technique. The potential of such an approach makes it possible to obtain a wide class of purposely designed diamond-based structures characterized by specific properties of charge transport. Reflection high-energy electron diffraction, scanning electron microscopy and x-ray dispersive spectrometry have been used to study the structural and compositional characteristics of some Nd-, W- and Ti-containing diamond films. The peculiar electrical properties conferred to the host diamond layers by the insertion of various metals have been investigated in the range of 25–500 K by performing Hall effect and conductivity measurements. The mechanism of charge transport and the electrical properties of these materials are found to be mainly governed by organization of the metallic species, which can be in different forms, such as dispersio...

Electrochemical behaviour of electrodes assembled with Ti-containing diamond films

Abstract High quality polycrystalline diamond films containing Ti dispersions have been produced by means of a hybrid CVD/powder flowing technique and used to assemble electrochemical probes. The electrochemical response was evaluated by performing cyclic voltammetry and fixed potential amperometry of both inorganic [Fe(CN)64−], and organic (ascorbic acid, epinephrine) compounds. The electro-analytical responses of these diamond-based electrodes have been compared with those of conventional glassy carbon and pyrolytic graphite electrodes. In view of their rather good performances, the Ti-containing diamond-based electrodes appear to be suitable for analytical purposes, and especially promising for applications in systems requiring absolute biocompatibility.