N. Lisi

Carbon Nanotube Growth on PAN‐ and Pitch‐Based Carbon Fibres by HFCVD

Abstract Carbon nanotubes (CNT) were grown on carbon fibres, both PAN‐ and pitch‐based, by hot filament chemical vapour deposition (HFCVD) using H2 and CH4 as precursors. Nickel clusters were electrodeposited on the fibre surfaces to catalyse the growth, and uniform CNT coatings were obtained on both PAN‐ and pitch‐based carbon fibres. Ni cluster features varied, depending on the deposition parameters, showing on average larger dimensions and denser distribution on pitch fibres. Multi‐walled CNTs with smooth walls and low impurity content were grown. The morphological features, both before and after the growth process, were characterised by SEM. This novel material based on carbon fibres coated with CNT, shows a potential for applications in polymeric matrix composites.

X-ray contact microscopy using an excimer laser plasma source with different target materials and laser pulse durations

Soft X‐ray contact microscopy (SXCM), using a pulsed X‐ray source, offers the possibility of imaging the ultrastructure of living biological systems at sub‐100 nm resolution. We have developed a table‐top pulsed plasma X‐ray source for this application, generated by a large‐volume XeCl laser, achieving a good conversion efficiency to ‘water‐window’ X‐rays (hν≈280–530 eV).

Development and characterisation of an XeCl excimer laser-generated soft-X-ray plasma source and its applications

SummaryA large-aperture, long-pulse XeCl excimer laser has been used to generate a soft-X-ray plasma source. Several laser optical configurations have been employed to optimise X-ray emission, including positive-branch unstable resonators and injection by seeding the gain region of the laser with a small commercial excimer laser, resulting in power densities in the range 1012–1014 W cm−2. The characteristics of the plasma source for each different laser configuration and for different target materials are investigated. The most suitable source conditions (spectral energy distribution, time duration, etc.) for specific applications are discussed.

Nitrogen-doped graphene films from chemical vapor deposition of pyridine: Influence of process parameters on the electrical and optical properties

Summary Graphene films were produced by chemical vapor deposition (CVD) of pyridine on copper substrates. Pyridine-CVD is expected to lead to doped graphene by the insertion of nitrogen atoms in the growing sp2 carbon lattice, possibly improving the properties of graphene as a transparent conductive film. We here report on the influence that the CVD parameters (i.e., temperature and gas flow) have on the morphology, transmittance, and electrical conductivity of the graphene films grown with pyridine. A temperature range between 930 and 1070 °C was explored and the results were compared to those of pristine graphene grown by ethanol-CVD under the same process conditions. The films were characterized by atomic force microscopy, Raman and X-ray photoemission spectroscopy. The optical transmittance and electrical conductivity of the films were measured to evaluate their performance as transparent conductive electrodes. Graphene films grown by pyridine reached an electrical conductivity of 14.3 × 105 S/m. Such a high conductivity seems to be associated with the electronic doping induced by substitutional nitrogen atoms. In particular, at 930 °C the nitrogen/carbon ratio of pyridine-grown graphene reaches 3%, and its electrical conductivity is 40% higher than that of pristine graphene grown from ethanol-CVD.

Cyclododecane as support material for clean and facile transfer of large-area few-layer graphene

The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge. We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements. Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the cyclododecane transfer method and outperformed reference cells made by standard methods.

Space and time resolved discharge evolution of a large volume X-ray triggered XeCl laser system

The spatial distribution and the temporal development of the net gain have been measured in a ten liter active volume switchless discharge XeCl laser. The experimental results are compared with both zero-dimensional and two-dimensional kinetic code predictions. The comparison between the results of the kinetic codes and of some measurements relevant to the time-dependent discharge homogeneity allows a deeper insight into the influence of the streamer evolution on the discharge characteristics.

Features of plasma produced by excimer laser at low intensities

A plasma, created at interaction of short-wavelength excimer laser radiation with flat targets was investigated (tlas = 12 ns, λlas = 0.308 μm, qlas = 4 – 8 × 1012 W/cm2) with the help of various x-ray spectroscopic methods. The comparison of shapes and intensities of some observable spectral lines of H-, He and Li-like ions of Na, Mg and Al with results of model calculations has allowed to determine space distributions of laser plasma parameters up to distances of 0.4 mm from the target surface. Comparison of obtained results with theoretical models of absorption of short-wavelength radiation in a plasma shows, that the absorption of short-wavelength laser radiation in a plasma (at considered values of laser flux density) is executed due to inverse bremsstrahlung process in the areas with Ne < Ne, crit..

Some design limitations for large-aperture high-energy per pulse excimer lasers

SummarySome limitation problems for gas discharge excimer lasers, when scaled to a high pulsed energy output with high repetition rate are discussed. As an example, we present some experimental results obtained with an X-ray preionized (10×10×100) cm3 active volume, low-repetition-rate-operated gas discharge XeCl laser system.

Time-resolved divergence measurement of an excimer laser beam by the knife-edge technique

Abstract We present a simple method to measure the time-evolution of the divergence of pulsed laser beams, joining the time-resolution capability of photodetectors and the space-resolved information of the knife-edge technique. We measured the time-integrated and the time-resolved divergence of a long pulsewidth, high output energy XeCl laser ( λ = 308 nm), equipped with positive branch unstable resonators having either magnification factor M = 5 or M = 9. In both cases, the time-resolved beam divergence was smaller than the time-integrated one for most of the 100 ns full-width-at-half-maximum laser pulse duration. A minimum divergence of (40 × 23) μ rad 2 at 86.5% energy content and a peak radiance of 9 × 10 14 W/cm 2 /ster were achieved a few ns before the peak of the laser pulse, for the case of M = 9. The corresponding time-integrated values were (56 × 38) μ rad 2 and 5 × 10 14 W/cm 2 /ster, respectively.

Modeling of the he-like magnesium spectral lines radiation from the plasma created by xecl and nd-glass lasers

Abstract Resonant and intercombination spectral line formation of He-like magnesium is analyzed both experimentally and numerically. Simulations of the laser-produced plasma evolution in ID approximation made using the code gidra show fairly good agreement with experimental data both close and far from the target. It is shown that in plasma created by XeCl laser at flux density 8 × 1012 W/cm2 the peak of electron temperature is placed downstream from the critical surface at density significantly smaller than critical, and radiation in both resonant and recombination lines is also produced by the plasma region with density below critical. Simulations also show significant line radiation at large distances (1–2 mm) from the target as it was observed in experiments. This secondary peak is produced by a compression wave forming near the plasma front. In contrast, radiation in these lines in plasma created by Nd-glass laser at flux density 5 × 1013 W/cm2 comes from the plasma region placed deeper than the critical surface and has no tail at large distances. Spectral line radiation reabsorbtion in the plasma was found to be responsible for the observed relative intensity of the resonant and intercombination lines.